[SPACE NOTES] Articles on 1-Ceres
May. 6th, 2003 06:52 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
rfmcdonaldDetailed Hubble Images of the Surface of Asteroid Ceres
In a paper set to appear in the January 2002 issue of the Astronomical Journal, astronomers report Hubble Space Telescope images taken of asteroid 1 Ceres that show surface detail at a resolution of ~50 km. Among the surface features they have observed is a large, ~250km diameter surface feature for which they propose the name "Piazzi'' (after the asteroid's discoverer). It is presently uncertain if this feature is due to a crater, albedo variegation, or other effect.
InfoTrac Web: Gen'l Reference Ctr (Magazine Index).
Source: Science, March 20, 1992 v255 n5051 p1551(3).
Title: Evidence for ammonium-bearing minerals on Ceres.
Author: Trude V.V. King, R.N. Clark, W.M. Calvin, D.M. Sherman and R.H. Brown
Author's Abstract: COPYRIGHT 1992 American Association for the Advancement of Science. Due to publisher request, Science cannot be reproduced until 360 days after the original publication date.
Spectra obtained from recent telescopic observation of 1-Ceres and laboratory measurements and theoretical calculations of three component mixtures of Ceres
analog material suggest that an ammoniated phyllosilicate is present on the surface of the asteroid, rather than [H.sub.2O] frost as bad been previously reported. The presence of an ammoniated phyllosilicate, most likely ammoniated saponite, on the surface of Ceres implies that secondary temperatures could not have exceeded 400 kelvin.
THE SURFACE MINERALOGY OF THE asteroids reflects the initial composition of the parent planetesimal and the effects of endogenic and exogenic processes
[1]. Knowledge of the mineralogy provides information on physical-chemical processes related to asteroid evolution. Ceres is the largest asteroid in the asteroid belt, appears to have suffered aqueous alteration, and may retain its primordial mass [2]. Samples of Ceres are not represented in the terrestrial meteorite collections, apparently because of the atmospheric entry selection against weak and fast-moving meteoroids (1), and therefore it must be studied with remote sensing techniques.
Spectra of 1-Ceres show a broad absorption feature near 3 [mu]m that is characteristic of [H.sub.2O]. These data suggest that the surface of the asteroid is enriched in phyllosilicates. A narrow absorption feature near 3.07 [mu]m in the Ceres spectrum has been interpreted to imply that there is also a small but significant amount of [H.sub.2O] frost on the surface [3]. To evaluate this interpretation and characterize further the mineralogy, we analyzed recent observations of Ceres in the 2.8- to 2.4-[mu]m wavelength region.
Spectra of Ceres were obtained in December 1989 and January 1990 (total of four nights of observations) from the National Aeronautics and Space Administration (NASA) Infrared Telescope Facility at Mauna Kea (4). We removed the thermal component from the Ceres spectrum before analysis by determining the overall disk temperature and then by adjusting the wavelength independent emissivity ([epsilon]) and normalization constant (n) from the standard thermal model to match the geometric albedo of Ceres [5]. The effective surface temperature for the 2.8- to 4.0-[mu]m wavelength region of the integrated surface area of Ceres is 231.5 K. The thermal component contributed less than 0.3% of the total signal at 3.0 [mu]m and 24% at 4.0 [mu]m. The reflectance level of the composite Ceres spectrum ranges from 0.04 to 0.05. The thermally corrected spectrum of Ceres was compared to spectra from the U.S. Geological Survey digital spectral library. The 3.07- [mu]m absorption feature cannot be attributed to [OH.sub.2], [H.sub.2O], [CO.sub.3], [SO.sub.4], or other ions containing oxygen in geological materials.
The composite Ceres spectrum was compared to [NH.sub.4]-bearing mineral species including naturally occurring buddingtonite ([NH.sub.4]-feldspar) and an [NH.sub.4]-bearing illitesmectite, as well as laboratory-prepared samples of [NH.sub.4]-substituted dioctahedral nontronite (Ng-1), a Ca-NA montmorillonite (SWy-1), a ferruginous smectite (SWa-1), and a trioctahedral saponite (SAPCa-1) [6)] Although all these [NH.sub.4]-bearing minerals have an absorption feature near 3.07 [mu]m, the feature in the Ceres spectrum is most similar to an [NH.sub.4]-bearing saponite.
Isolated [NH.sub.4.sup.+] belongs to the [T.sub.d] symmetry group. The triply degenerate [v.sub.3] and [v.sub.4] modes are infrared active. The [v.sub.3] mode produces three absorptions in the wavelength region from 3.0 to 3.6 [mu]m. When it is adsorbed on the surface of the clay minerals, the [NH.sub.4.sup+] ion is distorted because of local surface charge. Thus, there is a shift in the wavelength of the [NH.sub.4.sup.+] absorptions in ammonia-bearing minerals relative to the free [NH.sub.4.sup.+] ion. The shift is dependent on the crystal structure of the mineral and is thus indicative of the mineralogy.
Saponite is a trioctahedral smectite that commonly forms from hydrothermal alteration or weathering of basic rocks [7]. Saponite has been reported to be present as an aqueous alteration product in CV and CI carbonaceous chondrites and may be present in CM carbonaceous chondrites (8).
Comparison of the ammoniated saponite and saponite show that the strongest [NH.sub.4.sup+] [v.sub.3] fundamental vibration occurs at 3.05 [mu]m in the ammoniated clay and that it is stronger than the fundamental OH absorption, which occurs near 2.7 [mu]m. The other two [v.sub.3] absorptions are also seen
in the ammoniated saponite, at 3.30 and 3.54 [mu]m, but are weaker than the absorption feature at 3.05 [mu]m and cannot be detected in the spectra of
low-albedo objects like Ceres with the current signal-to-noise ratio.
The high-resolution telescopic data indicate that the center of the [NH.sub.4.sup.+] absorption on Ceres occurs at 3.07 [+ or -] 0.02 [mu]m, compared to 3.05 [mu]m for our laboratory sample. The width of the Ceres absorption is 0.127 [+ or -] 0.08 [mu]m, whereas that of the laboratory sample is 0.145 [mu]m. The depth of the 3.07-[mu]m absorption on Ceres in approximately 10% (Fig.1), and because Ceres is so dark this is indicative of a very strong absorption, stronger than the bound-water absorption features on Ceres.
The 3.07-[mu]m absorption feature on Ceres has been attributed to the presence of [H.sub.2O] ice [3, 9]. The [H.sub.2O] ice grains on the surface of Ceres have been estimated to be approximately 0.01 [mu]m thick if distributed across the entire surface and could not exceed 0.3 [mu]m thick because the absorption feature is not saturated (3). However, such a thin layer of ice would not be stable against sublimation for more than a few days (10) Two possible explanations for the presence of [H.sub.O] frost were given: (i) it is stable in the interlayer position of the phyllosilicate as bound ice or (ii) it is replenished from regolith material by migration of the absorbed [H.sub.2O] in the phyllosilicates. Analytical models of the [H.sub.2O] regime of Ceres predict that [H.sub.2O] ice is not stable anywhere on the optical surface of Ceres [10].
Theoretical calculations of [H.sub.2O] ice spectra at many grain sizes based on radiative transfer reflectance theory [11, 12] show that the 3.07-[mu]m absorption on Ceres is much too narrow to be caused by the presence of [H.sub.2O] ice (Fig. 2). For calculated spectra of [H.sub.2O] ice at grain diameters of 1,2, and 10 [mu]m, the width of the 3.1-[mu]m fundamental [H.sub.2O] ice absorption ranges from 2.8 to 4 times the width of the 3.07-[mu]m absorption in the spectrum of Ceres. In addition, the center of the absorption feature for [H.sub.2O] ice occurs at longer wavelengths than the 3.07-[mu]m feature in the spectrum of Ceres. Thus, [NH.sub.4.sup.+]-bearing minerals rather than [H.sub.2O] ice are most consistent with the 3.07-[mu]m absorption feature.
To understand better the origin of the spectrum of Ceres, we theoretically computed and physically mixed components to mimic the spectrum of Ceres. We computed spectra using the derived optical constants of ammoniated saponite, a black component (low-albedo mixture, 8% reflectance at 2.8 [mu]m for a 1-[mu]m grain size) and a gray component (medium albedo, 28% reflectance at 2.8 [mu]m for a 1-[mu]m grain size). These components were selected to represent assumed albedo levels of surface constituents of Ceres (carbon, organic phases, and mafic silicates). The best spectral agreement was achieved with a mixture of 7% ammoniated saponite by weight (grain size of 2 [mu]m), plus 52% of the black component (grain size of 5 [mu]m), and 41% of the gray component (grain size of 10 [mu]m) (Fig. 1). This computed spectrum produced an absorption feature at 3.07 [mu]m with a depth of 12% versus 10% for Ceres (Fig. 3). The reflectance level of the calculated spectrum is less than 1% lower than the reflectance level of the Ceres spectrum ( 0.04 at 3.0 [mu]m).
Spectra were also calculated in which [H.sub.2O] ice at grain sizes of 1 and 0.3 [mu]m was substituted for ammoniated saponite in the three-component mixtures in various proportions. All mixtures with [H.sub.2O] ice produced spectra with absorption features that were too wide and centered at longer wavelengths than the 3.07-[mu]m Ceres absorption feature (Fig. 1). Increasing or decreasing the grain size of the [H.sub.2O] ice component did not produce a spectrum similar to that of Ceres (12).
A physical mixture of ammoniated saponite and carbon black (8% by weight) produced a spectrum that resembles the 2.8- to 3.4-[mu]m wavelength region of Ceres (Fig. 3). However, the reflectance level of the laboratory mixture is too low compared to the Ceres spectrum if the strength of the 3.07-[mu]m feature is correct. This result suggest that the laboratory mixture contains more [NH.sub.4] than does Ceres.
Either nonhomogeneous or equilibrium condensation of the solar nebula would provide mechanisms for incorporating [NH.sub.4] in Ceres. In nonhomogeneous condensation, ammonium salts and [NH.sub.3] ice condense at temperatures below 200 K and form the outermost layer of a planet [13]. The equilibrium condensation model, however, provides a more appealing method for explaining the origin of ammonia on Ceres. In equilibrium condensation, nebular material
with solar elemental composition (14) is thought to begin condensing at 1600 K with the formation of refractory oxides and continue until [H.sub.2O] ices and
clathrates formed at temperatures of less than 300 K. Kinetic models (13, 14) predict that only limited amounts of [NH.sub.3] and [CH.sub.4] could be made
over the lifetime of the nebula. Thus, there is an enrichment in CO, [CO.sub.2], and [N.sub.2] greater than the equilibrium amount. At nebula temperatures near 150 K, the temperature at which [H.sub.2O] ice condenses, ammonium bicarbonate ([HN.sub.4HCO.sub.3]) and ammonium carmate ([NH.sub.4COOONH.sub.2]) may condense because excess CO, [CO.sub.2], and [CH.sub.4] are present in the gas phase [14].
The formation of phyllosilicates, by hydration of anhydrous materials, would not occur in the nebula at temperatures greater than 300 to 350 K. Recent theoretical work [15] suggests that the nebula hydration reaction is kinetically inhibited and that phyllosilicates are secondary alteration products. Thus, fluids to initiate the secondary alteration processes need to be derived internally after condensation and accretion or by impact-generated melting. For small bodies with confining pressures of less than 1 kbar, heating will produce fluids enriched in [NH.sub.4.sup.+], [Na.sup.+], [K.sup.+], and [Ca.sup.2+] and salts of [Co.sub.3.sup.2-] and
[SO.sub.4.sup.2-] (13). This ammoniated water inferred to react with anhydrous silicates and produce the ammoniated hydrous mineral(s) observed in spectrum of Ceres.
The identification of an ammonium-bearing mineral species, most likely ammoniated saponite, on the surface of Ceres implies that the secondary temperatures cannot have exceeded 400 K. Studies on the stabilities of ammoniated phyllosilicates indicate that the deammoniation of most samples begins at temperatures of 400 K [16]. The deammoniation of a phyllosilicate is spectrally detected by decreased intensity and wavelength shifts of the fundamental NH absorptions. The wavelength position of the [NH.sub.4.sup.+] absorption on Ceres is similar to that of samples that have not been heated to greater than 400 K. This observation implies that Ceres has experienced minimal thermal reprocessing. Difficulties in obtaining high resolution data with a high signal-to-noise ratio, such as that obtainable with the CGAS
spectrometer, for small asteroids with low albedos may be inhibiting identification of additional ammonium-bearing asteroids.
REFERENCES AND NOTES
[ 1.] M. J. Gaffey et al., in Asteroids II, R. P. Binzel, T. Gehrels, M. S. Matthews, Eds. (Univ. of Arizona Press, Tucson, 1989), pp. 98-127.
[ 2.] D. R. Davis et al., in ibid., pp. 805-826.
[ 3.] L. A. Lebofsky et al., Icarus 48, 453 (1981).
( 4.) We obtained data using the Cooled Grating Array Spectrometer (CGAS) with a 32 linear array InSb detector and the D grating with a resolution of 0.013 [mu]m. Observing conditions were good during the nights the data were obtained. The reference standards included the moon and the solar analog star Lambda Auriga. Full terrestrial atmospheric extinction corrections were applied to the data [as described in R. N. Clark et al., J. Geophys. Res. 95, 14463 (1990); R. N. Clark and T. B. McCord, Publ. Astrom. Soc. Pac. 91, 571 (1979)].
[ 5.] M. A. Feierberg et al., Geochim. Cosmochim. Acta 95, 971 (1981).
[ 6.] F. Van Olphen and J. J. Fripiat, Data Handbook for Clay Materials and Other Nonmetallic Minerals (Pergamon, New York, 1979).
[ 7.] C. E. Weaver and L. D. Pollard, The Chemistry of Clay Minerals (Elsevier,, New York, 1975).
[ 8.] M. Zolensky and H. Y. McSween, Jr., in Meteorites and the Early Solar System; J. F. Kerridge and M. S. Matthews (Univ. of Arizona Press, Tucson, 1988), pp. 114-143.
[ 9.] T. D. Jones, thesis, University of Arizona (1988).
[10.] F. P. Fanale and J. R. Salvail, Icarus 82, 97 (1989).
[11.] B. Hapke, J. Geophy. Res. 86, 3939 (1981).
[12.] W. M. Calvin and R. N. Clark, Icarus 89, 305
(1991). [13.] J. S. Lewis and R. G. Prinn, Planets and Their Atmospheres (Academic Press, New York, 1984).
[14.] J. S. Lewis and R. G. Prinn, Astrophys. J. 238, 357 (1980).
[15.] R. G. Prinn and M. B. Fegley, Annu. Rev. Earth Planet. Sci. 15, 171 (1987).
[16.] B. Chourabi and J. J. Fripiat, Clays Clay Minerals 29, 260 (1981). [17.]
This work was supported by NASA Planetary Geology and Geophysics Program (W17077 to T.V.V.K. and W15805 to R.N.C.). T.V.V.K., R.N.C., W.M.C., and R.H.B. were Visiting Astronomers at NASA's Infrared Telescope Facility.
13 November 1991; accepted 3 February 1992
InfoTrac Web: Gen'l Reference Ctr (Magazine Index).
Source: Science, Nov 10, 1989 v246 n4931 p790(3).
Title: Phyllosilicilate absorption features in main-belt and outer-belt asteroid reflectance spectra.
Author: Faith Vilas and Michael J. Gaffey
Phyllosilicate Absorption Features in Main-Belt and Outer-Belt Asteroid Reflectance Spectra
PRIMITIVE METEORITES OF TYPES 1 and 2 are assumed to be the result of the melting of ice and subsequent aqueous alteration of rocky materials comprising their original parent bodies [1]. Laboratory reflectance spectra of meteorites that appear to have undergone aqueous alteration and terrestrial rock samples
that are products of aqueous alteration show subtle absorption features in the visible and near-infrared spectral regions (2); however, telescopic reflectance spectra of aseroids labeled "primitive" have been considered featureless in the same spectral regions [3]. In this study, we searched 26 high-quality spectra of primitive (C-, P-, D-, F-, and Gl-class) [4] asteroids for weak features in the visible and near-infrared spectral regions. The asteroid sample presented here is composed primarily of the dark asteroids located beyond the outer edge of the main asteroid belt (the outer belt). The main emphasis of the observing program from which these spectra were culled was to understand the nature of asteroids located betweenl the main belt and Jupiter's orbit. We also observed some asteroids classed as P or D in the main belt to search for compositional differences between these objects and the outer-belt P and D asteroids [5].
Spectra of asteroids and Hardorp solar analog stars [6] were acquired during the years 1984 through 1987. All asteroid data were reduced to relative reflectance spectra scaled to 1.0 around 0.7 [micrometer] [5]. Spectra were selected for this study on the basis of the following criteria: (i) low peak-to-peak noise within a spectrum; (ii) good observing conditions during the nights when these data were obtained; and (iii) comparison of individual spectra with maps of the telluric atmospheric absorptions that could affect spectral shape if the extinction correction insufficiently removed some atmospheric absorptions. In interpretations of the data for asteroids 1162, 1512, and 2357 we have taken account of residual telluric water vapor features in these spectra. For the rest of the asteroids included in this study, such artifacts either were not present or were not sufficiently strong to affect the interpretation [7]. Each spectrum was treated as a continuum with discrete
absorption features superimposed on it. For each object, a linear least-squares fit to the spectral data points defined a simple linear continuum, which was then divided into each individual spectrum, thus removing the sloped continuum and allowing the intercomparison of residual spectral features. The residual features for various groupings of asteroids are shown in Figs. 1 through 4 and can be compared with residual features from laboratory spectra of terrestrial phyllosilicates and carbonaceous chondrite meteorites [8].
These diverse features may represent the effect of some asteroidal property (real features), or they could be artifacts from the stellar calibration sources or from the observing or data-reduction procedures. The reality of the slopes and features in the asteroid spectra was tested by a number of criteria. The standard stars can be eliminated as a source of significant features because asteroid data reduced through the same standard exhibit a wide variety of spectral slopes and residual features. No one feature is present in all of the asteroid spectra calibrated with any single standard star, and similar features are seen in asteroid spectra calibrated with different standard stars. If features were not common to all objects observed during any single night and if the observational or reduction procedures provided no explanation for a feature, then we concluded that the features derived primarily from the surface mineralogy of the asteroid.
The strongest of the residual features (Fig. 2) has a maximum absorption intensity of approximately 5%. Most are considerably weaker. Features with intensities below 1% have generally been discounted, without future prejudice, in this study. Thus, a significant subset of these dark asteroids are featureless by this criterion, even though there are variations in their spectra that subsequent study may prove to be real. Asteroid 1 Ceres is the exception to this general rule. This spectrum is a composite of 31 spectra of Ceres obtained at an air mass of 1.00, each having a signal-to-noise ratio greater than 100:1. The weak features in this spectrum (Fig. 1) are most probably real and deserve additional study.
The CI1 and CM2 meteorites exhibit a suite of spectral features that are generally similar to those seen in these asteroids. The asteroid features are generally one-half to one-third as strong as those seen in the meteorites, with a few important exceptions discussed below. Although all of these features appear to be relatively weak, they must represent intense absorptions to be present in spectra of objects having such low albedos (amount of visible light reflected by the surface material) and therefore certainly arise from strongly featured mineral species. Only a relatively limited suite of viable candidate species meet these criteria. The strongest asteroidal features, seen in mainbelt asteroid 102 Miriam (Fig. 1) and Cybele asteroid 1467 Mashona (Fig. 2), are very similar, both in shape and in intensity, to those in the
spectra of CM2 (carbonaceous) chondrites Murchison, Mighei, and Nogoya. The features in 102 Miriam and 1467 Mashona as well as those in the meteorites are consistent wth the absorption features in iron-bearing serpentines (antigorite) and chlorites [2]. The features in 877 Walkure (Fig. 1) could represent absorptions by phyllosilicate phases perhaps in combination with iron oxides such as goethite [9].
The CM2 and CI1 chondrites are considered to be the products of varying degrees of aqueous alteration within their parent bodies [1]. One proposed alteration sequence starts with a primarily anhydrous parent material similar to the CV3 chondritic assemblage. Metal is altered to form iron-rich tochilinite (an Fe-Ni-S-OH mineral). Olivine alters to magnesian serpentine, and tochilinite reacts to form cronstedtite (an FE.sup.3+ 1:1 layer phyllosilicate), which subsequently reacts to form iron-rich serpentine, sulfides, and magnetite.
If tochilinite is the major opaque phase in the assemblage, this alteration sequence should first produce a large decrease in albedo. If the initial assemblage had additional, strongly absorbing species such as kerogens orother organic compounds, the initial albedo would be low and only a small albedo decrease would occur. In such anhydrous parent assemblages, the absorbing material would largely overwhelm the relatively weak features of anhydrous silicates causing the spectral curve to be featureless and dominated by any spectral slope imparted by the strongly absorbing phase. The albedo should slowly increase as alteration proceeds, removing the tochilinite phase and depleting disseminated organic components. Phyllosilicate features should appear and be weakest at the initial stages of alteration both because of the competing high background absorbance (the low albedo) and because of their relatively low iron content. As alteration proceeds, the phyllosilicate features should become more pronounced as a result of the lower background absorbance (higher albedo) and their increasing iron content. Continued
aqueous alteration in moderately reducing environments (as might be expected in the presence of small amounts of organic matter) should eventually leach iron from the phyllosilicates and sequester an increasingly higher proportion of this element in magnetite. If this magnetite grows to grains larger than a few tenths of a micrometer in size, it becomes much less effective either as a darkening agent or as a source of spectral features. At this stage of alteration, the albedo is substantial (perhaps 7 to 15%) and phyllosilicate features are very weak.
Four spectra of main-belt asteroids (1 Ceres, 102 Miriam, 368 Haidea, and 877 Walkure) are included in the present study (Fig. 1). Although this is a limited set, their relative spectral and albedo [10] differences are consistent with the alteration sequence outlined above. Asteroid 368 Haidea has the lowest albedo (p.sub.v [+ or -] = 0.032 [+ or -] 0.002) and weak features and is spectrally similar to CM2 chondrite Cold Bokkeveld. Asteroid 102 Miriam has a substantially higher albedo (p.sub.v [+ or -] SE = 0.049 [+ or -] 0.002) and the strongest features and is spectrally similar to CM2 chondrites Murchison, Nogoya, and Mighei. Asteroid 1 Ceres has a much higher albedo (p.sub.v [+ or -] SE = 0.10 [+ or -] 0.01) and very weak features, a combination that precludes a significant iron-bearing phyllosilicate component
[11] and that is expected under conditions of alteration different from those seen in the CM2-CI1 suite (perhaps an additional alteration stage), with no meteoritic analogs for, or samples from, Ceres yet detected in our meteorite collections. The weak features in the Ceres curve should provide clues to the identity of the major phases, presumably dominated by phyllosilicates formed under conditions of intense, low-temperature, aqueous alteration such as members of the kaolin or smectite groups. Asteroid 877 Walkure has an albedo (p.sub.v [+ or -] SE = 0.047 [+ or -] 0.004) comparable to that of 102 Miriam and exhibits relatively strong but distinct spectral features, some similar to spectral features of CM2 chondrite Murray, which suggest an intermediate stage of alteration, perhaps somewhat less than that of 102 Miriam or perhaps along a different redox path or with different fluid: rock ratios. The data on this limited set of dark, main-belt asteroids suggest that all have been aqueously altered to some degree and that intense alteration is relatively common.
In addition to the main-belt asteroids, fourteen Cybele, four Hilda, and four Trojan asteroids were included in this study. Ten of these asteroids (Cybele asteroids 76, 87, 566, 643, 733, and 1167 and Trojan asteroids 884, 1172, 2357, and 2674) have spectra with no absorption features that passed our acceptance criteria, implying that these asteroids have not undergone any aqueous alteration. Of these ten objects, 733 Mocia has the highest albedo (p.sub.v [+ or -] SE = 0.049 [+ or -] 0.009); the other nine have albedos in the range of 0.029 to 0.042. All of the measured D-class asteroids beyond the main belt are included in this featureless group. This is in agreement with recent water of hydration absorption results at 3.0 [micrometer] and the proposed models for solar system evolution [12].
The remaining eight Cybele asteroids (Fig. 3) and the Hilda asteroids (Fig. 4) show a diversity of spectral features, the nature of which is not yet completely understood. Although some meteoritic analogs are evident, the diversity suggests that many of these assemblages are not represented in our meteorite collections. The CM2 specimens Nogoyo, Mighei, and Murchison provide very good matches both in absorption band position and in intensity to the spectrum of Cybele asteroid 1467 Mashona. The band position and intensity are indicative of a serpentine-type phyllosilicate having a well-ordered crystal structure. Asteroids 466 Tisiphone, 528 Rezia, and 940 Kordula (Cybele group objects, Fig. 3) and 153 Hilda and 1512 Oulu (Hilda group objects, Fig. 4) show a somewhat similar spectral pattern, although weaker and less well defined. These may represent assemblages that have undergone less but still substantial aqueous alteration, but the significance of the differences is unclear. Asteroids 225 Henrietta and 570 Kythera (Fig. 3) have features that are not well matched by any single phyllosilicate or iron oxide for which
spectra are currently available although they are similar to components seen in spectra.
It appears that the processes that produced the most aqueous alteration among the CM2 assemblages were active to approximately the same extent on objects such as 1467 Mashona and 102 Miriam, and to a lesser extent on other objects in the mainbelt, Cybele, and Hilda groups. Features in asteroids such as 368 Haidea area similar in band position to but less intense than those present in the Cold Bokkeveld and Murray spectra. The intensity difference may reflect a real mineralogical difference or a regolith process (coarse particle size versus fine). The featureless or nearly featureless spectra imply either a significant depletion of iron from the phyllosilicates (producing weak features that are easily masked by the dark phases) or the presence of an anhydrous silicate assemblage with the correspondingly weak features. It seems unlikely that a higher abundance of the dark absorbing phases, such as organics, can be the sole explanation for the absence of spectral features because that would imply substantially lower albedos than measured for these objects.
The pattern of features and the inferred postaccretionary aqueous alteration of their parent planetesimals is consistent with several recent models and observations that point to a selective heating mechanism that has a steep decline in efficiency with increasing heliocentric distance [13]. In the Trojan asteroids, temperatures in the planetesimal interiors did not rise above the water-ice solidus (if it is assumed that ice was the primary water source for the aqueous alteration). In the main belt, most or all bodies exceeded this temperature. In the Cybele asteroids [mean semimajor axis [alpha] = 3.4 astronomical units (AU)] and the Hilda asteroids ([alpha] = 4.0 AU), somewhat more than half of the bodies exceeded this temperature to produce aqueous alteration.
REFERENCES AND NOTES
[1] K. Tomeoka and P. Buseck, Geochim. Cosmochim. Acta 49, 2149 (1985).
[2] T. V. V. King, thesis, University of Hawaii (1986); M. J. Gaffey, Lunar Planet Sci. Conf. 11, 312 (1980).
[3] T. V. Johnson and F. P. Fanale, J. Geophys. Res. 78, 8507 (1973); L. A. Lebofsky, Astron. J. 85, 573 (1980).
[4] Asteroid classifications used here are those by D. J. Tholen, thesis, University of Arizona (1984).
[5] F. Vilas and B. A. Smith, Icarus 64, 503 (1985); F. Vilas and L. A. McFadden, Am. Astron. Soc. Publ. 19, 825 (1987).
[6] J. Hardorp, Astron. Astrophys. 120, 529 (1980).
[7] Individual spectra, which have dispersions of 8 to 11 [Angstrom], were smoothed with a running a mean average of 300 [Angstrom] spectral width to suppress pixel-to-pixel noise and to enhance broad, weak features. Obviously spurious data points (for example, those due to incomplete correction of the O.sub.2 A band near 0.76 [micrometer]) were removed before smoothing.
[8] The removal of a sloped continuum has a distorting effect on the residual features. In general, this procedure will tend to weaken the low albedo edge (generally the shorter wavelength side) of the feature relative to its actual absorbance. The magnitude of this effect correlates directly, but not linearly, with spectral slope and inversely with albedo. The net effect for steeply sloped spectra (the slope indicating that an absorbing agent is operating more effectively at lower wavelengths) is to shift the effective centers of features toward longer wavelengths and to under-represent the short-wavelength components in compound absorption bands. These effects have been considered in a qualitative fashion in the present discussion.
[9] R. V. Morris et al., J. Geophys. Res. 90, 3126 (1985).
[10] Infrared Astronomical Satellite asteroid albedos were used throughout this study; D. L. Matson, Jet Propulsion Laboratory Document D-3698 (1986).
[11] M. J. Gaffey, Meteoritics 13, 471 (1978); M. A. Feierberg, L. A. Lebofsky, H. P. Larson, Geochim. Cosmochim. Acta 45, 971 (1981).
[12] L. A. Lebofsky, T. D. Jones, P. D. Owensby, M. A. Feierberg, G. J. Consolmagno, Icarus, in press; T. D. Jones, thesis, University of Arizona (1988).
[13] F. Herbert, Icarus 78, 402 (1989); F. Herbert, C. P. Sonnett, M. J. Gaffey, in preparation; M. J. Gaffey, in preparation.
[14] F.V. was supported by the National Aeronautics and Space Administration (NASA) Planetary Astronomy program. M.J.G. was supported under the NASA Planetary Geology and Geophysics Program (grant NAGW-642). F.V. was a visiting astronomer at the Cerro Tololo Inter-American Observatory, operated by the
National Optical Astronomy Observatories for the National Science Foundation.
F. Vilas, National Aeronautics and Space Administration Johnson Space Center, Space Science Branch, Houston, TX 77058.
M. J. Gaffey, Geology Department, Rensselaer Polytechnic Institute, Troy, NY 12181.
InfoTrac Web: Gen'l Reference Ctr (Magazine Index).
Source: Astronomy, June 1995 v23 n6 p30(8).
Title: The Sun's fab four. (asteroids Ceres, Pallas, Juno, and Vesta; includes related articles)
Author: Alan Stern
Abstract: Recent observations of four of the larger bodies in the asteroid belt between Mars and Jupiter suggest that these minor planets have evolutionary histories similar to full-fledged planets. An observational history of Ceres, Vesta, Pallas, and Juno is given, along with tips on viewing them.
The soft-edged, lopsided object glows feebly in this blurry inset photo from HST. Its features are indistinct. This could almost be the Moon as seen by the naked eye, but it isn't. The object is Vesta, a 500-kilometer-diameter ball of rock circling the Sun halfway to Jupiter.
The new Hubble photo hints at what's remarkable about Vesta: Those light and dark areas may correspond to areas like the highlands and maria on the Moon. If true, that would give Vesta a kind of evolutionary history scientists once thought only full-sized planets had.
Nor is Vesta alone. It is but one of perhaps a million small, rocky bodies that orbit the Sun between Mars and Jupiter. Thousands more lie inside the orbit of Mars, while another set orbit along with Jupiter. These bodies, collectively called the asteroids, are so numerous they were once called "the vermin of the skies." Given their numbers, it's no surprise that the tiniest asteroids captured the lion's share of asteroid studies. However, scattered among these Lilliputian tribes lie a few giants, worlds that truly deserve the term minor planet. Vesta is one such, as are its cousins Ceres, Pallas, and Juno. And these larger bodies are now coming into their own as worlds to study.
Two streams of solar system research converge in the asteroid belt. The older arose from classical astronomy and seeks to understand the origin of the planets. The second, a child of the space age, is using the latest telescopes and techniques to delve into the individual histories of these bodies. Where the two streams intersect, scientists are at last unfolding a marvelously rich and detailed view of the solar system.
Asteroids were not always scorned as vermin. in fact, in 1801 when astronomer Giuseppe Piazzi discovered the first, which he named Ceres, it neatly filled an embarrassing gap. Astronomers had noticed the big jump between the orbits of Mars and Jupiter and had postulated a missing planet to fill the space.
Ceres' orbit fit well.
Then in 1802, Heinrich Olbers discovered a second, fainter object orbiting between Mars and Jupiter; he named it Pallas. Confronted with two objects to fill the gap, astronomers' thoughts shifted from missing planets to shattered ones. Could the original missing planet have somehow broken apart, they wondered? This seemed confirmed as intensified searches for additional bodies turned up Juno in 1804, Vesta in 1807, and Astraea in 1845. All lay within the gap between Mars and Jupiter.
But the pace of discovery was slow when astronomers worked at the telescope only by eye. With the advent of astronomical photography in the late 19th century, the discovery rate shot upward and hasn't slowed since. Planetary scientists have now cataloged more than 6,000 asteroids, and they estimate that between 100,000 and 1,000,000 additional bodies (with diameters between I km and about 40 km) remain to be found.
Such numbers can overwhelm. But from them a general picture of the asteroid belt has taken shape, and some curious features emerge. Interestingly, the belt isn't just a random mix of bodies; instead, it's stratified according to distance from the Sun. For example, the inner one-third of the belt has mainly asteroids with surfaces that are relatively reflective, somewhat reddish, and rich in iron-bearing minerals. The outer two-thirds of the belt is dominated by darker asteroids that resemble carbonaceous chondrite meteorites. Less populous classes also exist, including at least one believed to derive from Vesta itself. These objects appear to be covered in basaltic lava. This is the hallmark of planetary evolution and a telltale sign that whatever Vesta is, it is not geologically simple.
Asteroid statistics point out another anomaly. The total mass in the asteroid belt is surprisingly small - only about a tenth the mass of Earth's Moon. Moreover, of all the mass of asteroids a kilometer or larger in size, fully 40 percent reside in just the "Fab Four" - Ceres, Pallas, Juno, and Vesta - alone. Ceres, Pallas, and Vesta are also the only asteroids with diameters larger than 500 km. These, the very largest asteroids, form a clearly distinct class that seems to have "broken away" from the general asteroid population. How could this have occured?
To answer this question, planetary scientists first recognize that the present-day population of the asteroid belt is the result of both nature and nurture - that is, the population results from the belt's origin and its later evolution.
Three processes shaped the sizes of asteroids: growth through collisions among bodies that encounter one another at low enough speeds to combine; erosion (or catastrophic shattering) that occurs when bodies collide at high speeds; and orbital changes primarily forced by the gravity of massive Jupiter, lying just beyond the outer edge of the belt.
As with the formation of the planets themselves, asteroids grew initially in a bottom-up process: larger bodies came from smaller ones. The critical factor is that average collision speeds were low enough that collisions mainly resulted in growth, rather than destruction.
So far, so good. What complicates the process (and makes it more interesting) is Jupiter. This giant planet profoundly influenced the belt in two ways. First, Jupiter's gravitational stirring made asteroid orbits more eccentric. This sped up impact velocities so that most collisions produced fragmentation rather than growth. in fact, computer models of asteroid belt evolution indicate that essentially all the bodies smaller than about 350 km in diameter are the result of catastrophic, high-speed collisions, and that they are significantly younger than the age of the solar system.
Second, Jupiter's influence also created some special regions in the asteroid belt called resonances. These amplify the effects of gravitational perturbations: In some resonances, orbits become unstable; as a result, bodies can be ejected from the asteroid belt. Jack Wisdom of the Massachusetts institute of Technology showed in the early 1980s that this mechanism delivers many meteorites and some asteroids into Earth-crossing orbits.
More recently, George Wetherill of the Carnegie Institution in Washington, D.C., has uncovered a subtle interplay between collisions, gravitational perturbations, and Jovian orbit resonances that deeply affected the early evolution of the asteroid belt.
In his simulations, Wetherill found that collisions can nudge objects into resonances so efficiently that most of the bodies in the early asteroid belt were probably removed, primarily through ejection into Jupiter-crossing orbits. As the asteroid belt cleared out, collisions became infrequent, as they are today. With fewer and fewer objects nudged into unstable resonances, the rate of ejection decreased. Scientists find Wetherill's model attractive because it offers a natural explanation for the meager total mass of the asteroid belt.
Wetherill's results also imply that in its early days, the asteroid belt may have formed more objects in the Vesta to Ceres size-class. Most of these mega-bodies were probably ejected from the belt to suffer collisions with jupiter or were tossed out of the solar system by it. Thus it would seem that Ceres, Pallas, Juno, and Vesta grew large by accreting smaller objects in low-speed collisions, perhaps just as the belt was clearing out. Then, thanks to their size - and having escaped perturbation into unstable resonances - they survived to the present.
In the billions of years that have rolled by while these four objects orbited the Sun, little has changed on them. They have no detectable atmospheres. Few have satellites. And their surfaces remain today as they have been since they cooled after formation - cold! - with temperatures of -100[degrees] C (about 170 K) being typical.
Yes, some of their myriad brethren have bashed and battered them. Yes, solar radiation and cosmic rays have weathered their surfaces. And yes, their orbits have moved around a bit as collisions and perturbations worked their subtle synergy.
But with so little to change them, the Fab Four have remained almost-timeless relics of a distant past. This cannot be said for the terrestrial planets, which have evolved greatly from their formative state. Nor can it be said for most of the objects in today's asteroid belt, which are simple shards left from collisions that occurred only yesterday, considering the age of the solar system. Of all those. million-plus objects in the belt, just a handful - namely those larger than about 350 km in diameter - have survived largely intact since the early days of the solar system.
As four of the solar system's largest asteroids, Ceres, Pallas, Juno, and Vesta preserve a bit of the past that is otherwise mostly lost. Scientists look to these roundish little worlds on the frosty border between the inner and outer solar system in hopes that they will one day become scientific beachheads, perhaps even staging points, for the long jump humans will ultimately take across the cold, yawning vacuum to Jupiter.
Visualizing Vesta
Vesta's surface is so unlike that of any other asteroid, it's been placed into a class of its own. Not surprisingly, this uniqueness makes it a rather attractive target for many astronomers. Last fall, Ben Zellner of Georgia Southern University used the Hubble Space Telescope's Wide Field and Planetary Camera 2 to take pictures of Vesta as it completed one rotation. The images show much of Vesta's surface to a resolution of 70 km or better.) In them we see bright and dark features temptingly similar to regions on our Moon. This leads to the intriguing possibility that geologists may one day have a smaller cousin to our Moon to study and compare.
And that day may be not be far off. A proposal made by Cornell University's Joseph Veverka, NASA's Jet Propulsion Laboratory, and Martin Marietta Aerospace puts forth an unmanned mission to Vesta arriving in 2003. Imaging cameras would map areas of its surface to 10 meters, with some on-board instruments "tasting" the asteroid's chemical composition from a 250 km orbit.
What exactly makes Vesta so interesting?
For one thing, of all the large asteroids, Vesta has the most reflective surface. Its average visible-wavelength albedo, or efficiency at reflecting light, is 38 percent. Compared to the Moon's average albedo of 7 percent, Vesta is brilliant. In fact, even though at Vesta's distance sunlight is reduced to just 15 percent of the strength experienced on Earth, an astronaut on its surface would see a scene about as bright as on Earth's Moon. And the similarity wouldn't end there.
Like areas on the Moon humans have seen, Vesta's surface is covered in a fine, pulverized powder about 6 centimeters deep. Vesta is also composed of volcanic basalts created when its surface was melted, perhaps by some nearly catastrophic collision early in its history.
Vesta's intriguing characteristics led it to become one of the first asteroids compositionally fingerprinted. Over 20 years ago, Tom McCord (now at the University of Hawaii) developed a method called reflectance spectroscopy that uses light broken down into its component colors. With the technique, McCord accurately determined Vesta's mineral content.
Similar observations pioneered by Michael Gaffey of Rensselaer Polytechnic Institute, along with recent images taken by Hubble, reveal other features analogues to lunar maria. Some of these regions have minerals called pyroxenes and feldspars, typically found in the Earth's mantle. Perhaps craters punched through Vesta's basaltic crust to excavate material from its upper mantle.
Complementing this picture are actual pieces of Vesta believed to have have fallen to Earth as meteorites called basaltic achondrites. Careful work by astronomers like Richard Binzel of MIT reveal the way these chips off Vesta came to Earth.
A string of tiny meteoroids blasted off Vesta fell into orbits susceptible to the influences of Jupiter's gravity, which then perturbed them into Earth-crossing orbits. As a result, these basaltic achondrites, which have reflectance spectra, identical to the spectra of Vesta, fall to Earth on a regular basis. Whether this debris found its way to Earth via Jovian resonances or was blasted directly off Vesta's crust is uncertain. It's likely that both channels make the Vesta-to-Earth meteorite conveyor work.
Close-up on Ceres
The solar system's largest asteroid may be more like a planet than an asteroid: Its shape is spherical, and like Mars, this ruddy-colored minor planet might harbor water at its poles.
An astronaut orbiting Ceres would see a yellowish or perhaps slightly orange-tinged surface pockmarked and battered by craters. Studies made from Earth suggest the surface might be clay-like, consisting of soil that has water chemically locked into it and minerals similar to those found in meteorites called carbonaceous chondrites.
Groundbased evidence also indicates it's unlikely our astronaut would see dramatic differences in Ceres' surface color, save at the minor planet's poles. Several tantalizing lines of evidence point to the possibility that Ceres' poles may harbor a cache of frozen water, either mixed in the planet's clay-like soil or, perhaps, lying encrusted on the surface in frozen ponds as a patchy veneer.
While the evidence for water ice on Ceres is not definitive, it is highly suggestive. Larry Lebofsky of the University of Arizona and his colleagues have repeatedly identified the distinctive spectral fingerprint of water ice. Others, including Michael A'Hearn of the University of Maryland and Paul Feldman of Johns Hopkins University, have found evidence for hydroxide (OH), the most common product of water vapor ([H.sub.2]O), above Ceres' surface.
These and other findings have led to studies of the stability of water ice on Ceres. In the most complete study to date, modelers Fraser Fanale and James Salvail of the University of Hawaii believe that water frost would be stable just under, and perhaps even on, the cool surfaces of Ceres' poles. For no other asteroid is the possibility of water ice on its surface known to be so strong.
Moving in orbit around Ceres, an astronaut would notice its surface reflects less light that the lunar highlands - by almost 40 percent, and he might confirm groundbased radar and thermal measurements that indicate a thin, powdery surface overlies a much deeper, more compact layer.
Several orbits later, our astronaut would quickly come to the conclusion that change comes to Ceres only over a long time and through random meteorite impacts and incessant bombardment from cosmic rays, sunlight, and solar wind. We can only speculate what weathered vistas of mountains, rilles, valleys, or scraps might greet a visitor to Ceres. Our still-growing databank isn't very large.
Just because it's small doesn't mean that Ceres isn't interesting.
And, finally, a picture:
In a paper set to appear in the January 2002 issue of the Astronomical Journal, astronomers report Hubble Space Telescope images taken of asteroid 1 Ceres that show surface detail at a resolution of ~50 km. Among the surface features they have observed is a large, ~250km diameter surface feature for which they propose the name "Piazzi'' (after the asteroid's discoverer). It is presently uncertain if this feature is due to a crater, albedo variegation, or other effect.
InfoTrac Web: Gen'l Reference Ctr (Magazine Index).
Source: Science, March 20, 1992 v255 n5051 p1551(3).
Title: Evidence for ammonium-bearing minerals on Ceres.
Author: Trude V.V. King, R.N. Clark, W.M. Calvin, D.M. Sherman and R.H. Brown
Author's Abstract: COPYRIGHT 1992 American Association for the Advancement of Science. Due to publisher request, Science cannot be reproduced until 360 days after the original publication date.
Spectra obtained from recent telescopic observation of 1-Ceres and laboratory measurements and theoretical calculations of three component mixtures of Ceres
analog material suggest that an ammoniated phyllosilicate is present on the surface of the asteroid, rather than [H.sub.2O] frost as bad been previously reported. The presence of an ammoniated phyllosilicate, most likely ammoniated saponite, on the surface of Ceres implies that secondary temperatures could not have exceeded 400 kelvin.
THE SURFACE MINERALOGY OF THE asteroids reflects the initial composition of the parent planetesimal and the effects of endogenic and exogenic processes
[1]. Knowledge of the mineralogy provides information on physical-chemical processes related to asteroid evolution. Ceres is the largest asteroid in the asteroid belt, appears to have suffered aqueous alteration, and may retain its primordial mass [2]. Samples of Ceres are not represented in the terrestrial meteorite collections, apparently because of the atmospheric entry selection against weak and fast-moving meteoroids (1), and therefore it must be studied with remote sensing techniques.
Spectra of 1-Ceres show a broad absorption feature near 3 [mu]m that is characteristic of [H.sub.2O]. These data suggest that the surface of the asteroid is enriched in phyllosilicates. A narrow absorption feature near 3.07 [mu]m in the Ceres spectrum has been interpreted to imply that there is also a small but significant amount of [H.sub.2O] frost on the surface [3]. To evaluate this interpretation and characterize further the mineralogy, we analyzed recent observations of Ceres in the 2.8- to 2.4-[mu]m wavelength region.
Spectra of Ceres were obtained in December 1989 and January 1990 (total of four nights of observations) from the National Aeronautics and Space Administration (NASA) Infrared Telescope Facility at Mauna Kea (4). We removed the thermal component from the Ceres spectrum before analysis by determining the overall disk temperature and then by adjusting the wavelength independent emissivity ([epsilon]) and normalization constant (n) from the standard thermal model to match the geometric albedo of Ceres [5]. The effective surface temperature for the 2.8- to 4.0-[mu]m wavelength region of the integrated surface area of Ceres is 231.5 K. The thermal component contributed less than 0.3% of the total signal at 3.0 [mu]m and 24% at 4.0 [mu]m. The reflectance level of the composite Ceres spectrum ranges from 0.04 to 0.05. The thermally corrected spectrum of Ceres was compared to spectra from the U.S. Geological Survey digital spectral library. The 3.07- [mu]m absorption feature cannot be attributed to [OH.sub.2], [H.sub.2O], [CO.sub.3], [SO.sub.4], or other ions containing oxygen in geological materials.
The composite Ceres spectrum was compared to [NH.sub.4]-bearing mineral species including naturally occurring buddingtonite ([NH.sub.4]-feldspar) and an [NH.sub.4]-bearing illitesmectite, as well as laboratory-prepared samples of [NH.sub.4]-substituted dioctahedral nontronite (Ng-1), a Ca-NA montmorillonite (SWy-1), a ferruginous smectite (SWa-1), and a trioctahedral saponite (SAPCa-1) [6)] Although all these [NH.sub.4]-bearing minerals have an absorption feature near 3.07 [mu]m, the feature in the Ceres spectrum is most similar to an [NH.sub.4]-bearing saponite.
Isolated [NH.sub.4.sup.+] belongs to the [T.sub.d] symmetry group. The triply degenerate [v.sub.3] and [v.sub.4] modes are infrared active. The [v.sub.3] mode produces three absorptions in the wavelength region from 3.0 to 3.6 [mu]m. When it is adsorbed on the surface of the clay minerals, the [NH.sub.4.sup+] ion is distorted because of local surface charge. Thus, there is a shift in the wavelength of the [NH.sub.4.sup.+] absorptions in ammonia-bearing minerals relative to the free [NH.sub.4.sup.+] ion. The shift is dependent on the crystal structure of the mineral and is thus indicative of the mineralogy.
Saponite is a trioctahedral smectite that commonly forms from hydrothermal alteration or weathering of basic rocks [7]. Saponite has been reported to be present as an aqueous alteration product in CV and CI carbonaceous chondrites and may be present in CM carbonaceous chondrites (8).
Comparison of the ammoniated saponite and saponite show that the strongest [NH.sub.4.sup+] [v.sub.3] fundamental vibration occurs at 3.05 [mu]m in the ammoniated clay and that it is stronger than the fundamental OH absorption, which occurs near 2.7 [mu]m. The other two [v.sub.3] absorptions are also seen
in the ammoniated saponite, at 3.30 and 3.54 [mu]m, but are weaker than the absorption feature at 3.05 [mu]m and cannot be detected in the spectra of
low-albedo objects like Ceres with the current signal-to-noise ratio.
The high-resolution telescopic data indicate that the center of the [NH.sub.4.sup.+] absorption on Ceres occurs at 3.07 [+ or -] 0.02 [mu]m, compared to 3.05 [mu]m for our laboratory sample. The width of the Ceres absorption is 0.127 [+ or -] 0.08 [mu]m, whereas that of the laboratory sample is 0.145 [mu]m. The depth of the 3.07-[mu]m absorption on Ceres in approximately 10% (Fig.1), and because Ceres is so dark this is indicative of a very strong absorption, stronger than the bound-water absorption features on Ceres.
The 3.07-[mu]m absorption feature on Ceres has been attributed to the presence of [H.sub.2O] ice [3, 9]. The [H.sub.2O] ice grains on the surface of Ceres have been estimated to be approximately 0.01 [mu]m thick if distributed across the entire surface and could not exceed 0.3 [mu]m thick because the absorption feature is not saturated (3). However, such a thin layer of ice would not be stable against sublimation for more than a few days (10) Two possible explanations for the presence of [H.sub.O] frost were given: (i) it is stable in the interlayer position of the phyllosilicate as bound ice or (ii) it is replenished from regolith material by migration of the absorbed [H.sub.2O] in the phyllosilicates. Analytical models of the [H.sub.2O] regime of Ceres predict that [H.sub.2O] ice is not stable anywhere on the optical surface of Ceres [10].
Theoretical calculations of [H.sub.2O] ice spectra at many grain sizes based on radiative transfer reflectance theory [11, 12] show that the 3.07-[mu]m absorption on Ceres is much too narrow to be caused by the presence of [H.sub.2O] ice (Fig. 2). For calculated spectra of [H.sub.2O] ice at grain diameters of 1,2, and 10 [mu]m, the width of the 3.1-[mu]m fundamental [H.sub.2O] ice absorption ranges from 2.8 to 4 times the width of the 3.07-[mu]m absorption in the spectrum of Ceres. In addition, the center of the absorption feature for [H.sub.2O] ice occurs at longer wavelengths than the 3.07-[mu]m feature in the spectrum of Ceres. Thus, [NH.sub.4.sup.+]-bearing minerals rather than [H.sub.2O] ice are most consistent with the 3.07-[mu]m absorption feature.
To understand better the origin of the spectrum of Ceres, we theoretically computed and physically mixed components to mimic the spectrum of Ceres. We computed spectra using the derived optical constants of ammoniated saponite, a black component (low-albedo mixture, 8% reflectance at 2.8 [mu]m for a 1-[mu]m grain size) and a gray component (medium albedo, 28% reflectance at 2.8 [mu]m for a 1-[mu]m grain size). These components were selected to represent assumed albedo levels of surface constituents of Ceres (carbon, organic phases, and mafic silicates). The best spectral agreement was achieved with a mixture of 7% ammoniated saponite by weight (grain size of 2 [mu]m), plus 52% of the black component (grain size of 5 [mu]m), and 41% of the gray component (grain size of 10 [mu]m) (Fig. 1). This computed spectrum produced an absorption feature at 3.07 [mu]m with a depth of 12% versus 10% for Ceres (Fig. 3). The reflectance level of the calculated spectrum is less than 1% lower than the reflectance level of the Ceres spectrum ( 0.04 at 3.0 [mu]m).
Spectra were also calculated in which [H.sub.2O] ice at grain sizes of 1 and 0.3 [mu]m was substituted for ammoniated saponite in the three-component mixtures in various proportions. All mixtures with [H.sub.2O] ice produced spectra with absorption features that were too wide and centered at longer wavelengths than the 3.07-[mu]m Ceres absorption feature (Fig. 1). Increasing or decreasing the grain size of the [H.sub.2O] ice component did not produce a spectrum similar to that of Ceres (12).
A physical mixture of ammoniated saponite and carbon black (8% by weight) produced a spectrum that resembles the 2.8- to 3.4-[mu]m wavelength region of Ceres (Fig. 3). However, the reflectance level of the laboratory mixture is too low compared to the Ceres spectrum if the strength of the 3.07-[mu]m feature is correct. This result suggest that the laboratory mixture contains more [NH.sub.4] than does Ceres.
Either nonhomogeneous or equilibrium condensation of the solar nebula would provide mechanisms for incorporating [NH.sub.4] in Ceres. In nonhomogeneous condensation, ammonium salts and [NH.sub.3] ice condense at temperatures below 200 K and form the outermost layer of a planet [13]. The equilibrium condensation model, however, provides a more appealing method for explaining the origin of ammonia on Ceres. In equilibrium condensation, nebular material
with solar elemental composition (14) is thought to begin condensing at 1600 K with the formation of refractory oxides and continue until [H.sub.2O] ices and
clathrates formed at temperatures of less than 300 K. Kinetic models (13, 14) predict that only limited amounts of [NH.sub.3] and [CH.sub.4] could be made
over the lifetime of the nebula. Thus, there is an enrichment in CO, [CO.sub.2], and [N.sub.2] greater than the equilibrium amount. At nebula temperatures near 150 K, the temperature at which [H.sub.2O] ice condenses, ammonium bicarbonate ([HN.sub.4HCO.sub.3]) and ammonium carmate ([NH.sub.4COOONH.sub.2]) may condense because excess CO, [CO.sub.2], and [CH.sub.4] are present in the gas phase [14].
The formation of phyllosilicates, by hydration of anhydrous materials, would not occur in the nebula at temperatures greater than 300 to 350 K. Recent theoretical work [15] suggests that the nebula hydration reaction is kinetically inhibited and that phyllosilicates are secondary alteration products. Thus, fluids to initiate the secondary alteration processes need to be derived internally after condensation and accretion or by impact-generated melting. For small bodies with confining pressures of less than 1 kbar, heating will produce fluids enriched in [NH.sub.4.sup.+], [Na.sup.+], [K.sup.+], and [Ca.sup.2+] and salts of [Co.sub.3.sup.2-] and
[SO.sub.4.sup.2-] (13). This ammoniated water inferred to react with anhydrous silicates and produce the ammoniated hydrous mineral(s) observed in spectrum of Ceres.
The identification of an ammonium-bearing mineral species, most likely ammoniated saponite, on the surface of Ceres implies that the secondary temperatures cannot have exceeded 400 K. Studies on the stabilities of ammoniated phyllosilicates indicate that the deammoniation of most samples begins at temperatures of 400 K [16]. The deammoniation of a phyllosilicate is spectrally detected by decreased intensity and wavelength shifts of the fundamental NH absorptions. The wavelength position of the [NH.sub.4.sup.+] absorption on Ceres is similar to that of samples that have not been heated to greater than 400 K. This observation implies that Ceres has experienced minimal thermal reprocessing. Difficulties in obtaining high resolution data with a high signal-to-noise ratio, such as that obtainable with the CGAS
spectrometer, for small asteroids with low albedos may be inhibiting identification of additional ammonium-bearing asteroids.
REFERENCES AND NOTES
[ 1.] M. J. Gaffey et al., in Asteroids II, R. P. Binzel, T. Gehrels, M. S. Matthews, Eds. (Univ. of Arizona Press, Tucson, 1989), pp. 98-127.
[ 2.] D. R. Davis et al., in ibid., pp. 805-826.
[ 3.] L. A. Lebofsky et al., Icarus 48, 453 (1981).
( 4.) We obtained data using the Cooled Grating Array Spectrometer (CGAS) with a 32 linear array InSb detector and the D grating with a resolution of 0.013 [mu]m. Observing conditions were good during the nights the data were obtained. The reference standards included the moon and the solar analog star Lambda Auriga. Full terrestrial atmospheric extinction corrections were applied to the data [as described in R. N. Clark et al., J. Geophys. Res. 95, 14463 (1990); R. N. Clark and T. B. McCord, Publ. Astrom. Soc. Pac. 91, 571 (1979)].
[ 5.] M. A. Feierberg et al., Geochim. Cosmochim. Acta 95, 971 (1981).
[ 6.] F. Van Olphen and J. J. Fripiat, Data Handbook for Clay Materials and Other Nonmetallic Minerals (Pergamon, New York, 1979).
[ 7.] C. E. Weaver and L. D. Pollard, The Chemistry of Clay Minerals (Elsevier,, New York, 1975).
[ 8.] M. Zolensky and H. Y. McSween, Jr., in Meteorites and the Early Solar System; J. F. Kerridge and M. S. Matthews (Univ. of Arizona Press, Tucson, 1988), pp. 114-143.
[ 9.] T. D. Jones, thesis, University of Arizona (1988).
[10.] F. P. Fanale and J. R. Salvail, Icarus 82, 97 (1989).
[11.] B. Hapke, J. Geophy. Res. 86, 3939 (1981).
[12.] W. M. Calvin and R. N. Clark, Icarus 89, 305
(1991). [13.] J. S. Lewis and R. G. Prinn, Planets and Their Atmospheres (Academic Press, New York, 1984).
[14.] J. S. Lewis and R. G. Prinn, Astrophys. J. 238, 357 (1980).
[15.] R. G. Prinn and M. B. Fegley, Annu. Rev. Earth Planet. Sci. 15, 171 (1987).
[16.] B. Chourabi and J. J. Fripiat, Clays Clay Minerals 29, 260 (1981). [17.]
This work was supported by NASA Planetary Geology and Geophysics Program (W17077 to T.V.V.K. and W15805 to R.N.C.). T.V.V.K., R.N.C., W.M.C., and R.H.B. were Visiting Astronomers at NASA's Infrared Telescope Facility.
13 November 1991; accepted 3 February 1992
InfoTrac Web: Gen'l Reference Ctr (Magazine Index).
Source: Science, Nov 10, 1989 v246 n4931 p790(3).
Title: Phyllosilicilate absorption features in main-belt and outer-belt asteroid reflectance spectra.
Author: Faith Vilas and Michael J. Gaffey
Phyllosilicate Absorption Features in Main-Belt and Outer-Belt Asteroid Reflectance Spectra
PRIMITIVE METEORITES OF TYPES 1 and 2 are assumed to be the result of the melting of ice and subsequent aqueous alteration of rocky materials comprising their original parent bodies [1]. Laboratory reflectance spectra of meteorites that appear to have undergone aqueous alteration and terrestrial rock samples
that are products of aqueous alteration show subtle absorption features in the visible and near-infrared spectral regions (2); however, telescopic reflectance spectra of aseroids labeled "primitive" have been considered featureless in the same spectral regions [3]. In this study, we searched 26 high-quality spectra of primitive (C-, P-, D-, F-, and Gl-class) [4] asteroids for weak features in the visible and near-infrared spectral regions. The asteroid sample presented here is composed primarily of the dark asteroids located beyond the outer edge of the main asteroid belt (the outer belt). The main emphasis of the observing program from which these spectra were culled was to understand the nature of asteroids located betweenl the main belt and Jupiter's orbit. We also observed some asteroids classed as P or D in the main belt to search for compositional differences between these objects and the outer-belt P and D asteroids [5].
Spectra of asteroids and Hardorp solar analog stars [6] were acquired during the years 1984 through 1987. All asteroid data were reduced to relative reflectance spectra scaled to 1.0 around 0.7 [micrometer] [5]. Spectra were selected for this study on the basis of the following criteria: (i) low peak-to-peak noise within a spectrum; (ii) good observing conditions during the nights when these data were obtained; and (iii) comparison of individual spectra with maps of the telluric atmospheric absorptions that could affect spectral shape if the extinction correction insufficiently removed some atmospheric absorptions. In interpretations of the data for asteroids 1162, 1512, and 2357 we have taken account of residual telluric water vapor features in these spectra. For the rest of the asteroids included in this study, such artifacts either were not present or were not sufficiently strong to affect the interpretation [7]. Each spectrum was treated as a continuum with discrete
absorption features superimposed on it. For each object, a linear least-squares fit to the spectral data points defined a simple linear continuum, which was then divided into each individual spectrum, thus removing the sloped continuum and allowing the intercomparison of residual spectral features. The residual features for various groupings of asteroids are shown in Figs. 1 through 4 and can be compared with residual features from laboratory spectra of terrestrial phyllosilicates and carbonaceous chondrite meteorites [8].
These diverse features may represent the effect of some asteroidal property (real features), or they could be artifacts from the stellar calibration sources or from the observing or data-reduction procedures. The reality of the slopes and features in the asteroid spectra was tested by a number of criteria. The standard stars can be eliminated as a source of significant features because asteroid data reduced through the same standard exhibit a wide variety of spectral slopes and residual features. No one feature is present in all of the asteroid spectra calibrated with any single standard star, and similar features are seen in asteroid spectra calibrated with different standard stars. If features were not common to all objects observed during any single night and if the observational or reduction procedures provided no explanation for a feature, then we concluded that the features derived primarily from the surface mineralogy of the asteroid.
The strongest of the residual features (Fig. 2) has a maximum absorption intensity of approximately 5%. Most are considerably weaker. Features with intensities below 1% have generally been discounted, without future prejudice, in this study. Thus, a significant subset of these dark asteroids are featureless by this criterion, even though there are variations in their spectra that subsequent study may prove to be real. Asteroid 1 Ceres is the exception to this general rule. This spectrum is a composite of 31 spectra of Ceres obtained at an air mass of 1.00, each having a signal-to-noise ratio greater than 100:1. The weak features in this spectrum (Fig. 1) are most probably real and deserve additional study.
The CI1 and CM2 meteorites exhibit a suite of spectral features that are generally similar to those seen in these asteroids. The asteroid features are generally one-half to one-third as strong as those seen in the meteorites, with a few important exceptions discussed below. Although all of these features appear to be relatively weak, they must represent intense absorptions to be present in spectra of objects having such low albedos (amount of visible light reflected by the surface material) and therefore certainly arise from strongly featured mineral species. Only a relatively limited suite of viable candidate species meet these criteria. The strongest asteroidal features, seen in mainbelt asteroid 102 Miriam (Fig. 1) and Cybele asteroid 1467 Mashona (Fig. 2), are very similar, both in shape and in intensity, to those in the
spectra of CM2 (carbonaceous) chondrites Murchison, Mighei, and Nogoya. The features in 102 Miriam and 1467 Mashona as well as those in the meteorites are consistent wth the absorption features in iron-bearing serpentines (antigorite) and chlorites [2]. The features in 877 Walkure (Fig. 1) could represent absorptions by phyllosilicate phases perhaps in combination with iron oxides such as goethite [9].
The CM2 and CI1 chondrites are considered to be the products of varying degrees of aqueous alteration within their parent bodies [1]. One proposed alteration sequence starts with a primarily anhydrous parent material similar to the CV3 chondritic assemblage. Metal is altered to form iron-rich tochilinite (an Fe-Ni-S-OH mineral). Olivine alters to magnesian serpentine, and tochilinite reacts to form cronstedtite (an FE.sup.3+ 1:1 layer phyllosilicate), which subsequently reacts to form iron-rich serpentine, sulfides, and magnetite.
If tochilinite is the major opaque phase in the assemblage, this alteration sequence should first produce a large decrease in albedo. If the initial assemblage had additional, strongly absorbing species such as kerogens orother organic compounds, the initial albedo would be low and only a small albedo decrease would occur. In such anhydrous parent assemblages, the absorbing material would largely overwhelm the relatively weak features of anhydrous silicates causing the spectral curve to be featureless and dominated by any spectral slope imparted by the strongly absorbing phase. The albedo should slowly increase as alteration proceeds, removing the tochilinite phase and depleting disseminated organic components. Phyllosilicate features should appear and be weakest at the initial stages of alteration both because of the competing high background absorbance (the low albedo) and because of their relatively low iron content. As alteration proceeds, the phyllosilicate features should become more pronounced as a result of the lower background absorbance (higher albedo) and their increasing iron content. Continued
aqueous alteration in moderately reducing environments (as might be expected in the presence of small amounts of organic matter) should eventually leach iron from the phyllosilicates and sequester an increasingly higher proportion of this element in magnetite. If this magnetite grows to grains larger than a few tenths of a micrometer in size, it becomes much less effective either as a darkening agent or as a source of spectral features. At this stage of alteration, the albedo is substantial (perhaps 7 to 15%) and phyllosilicate features are very weak.
Four spectra of main-belt asteroids (1 Ceres, 102 Miriam, 368 Haidea, and 877 Walkure) are included in the present study (Fig. 1). Although this is a limited set, their relative spectral and albedo [10] differences are consistent with the alteration sequence outlined above. Asteroid 368 Haidea has the lowest albedo (p.sub.v [+ or -] = 0.032 [+ or -] 0.002) and weak features and is spectrally similar to CM2 chondrite Cold Bokkeveld. Asteroid 102 Miriam has a substantially higher albedo (p.sub.v [+ or -] SE = 0.049 [+ or -] 0.002) and the strongest features and is spectrally similar to CM2 chondrites Murchison, Nogoya, and Mighei. Asteroid 1 Ceres has a much higher albedo (p.sub.v [+ or -] SE = 0.10 [+ or -] 0.01) and very weak features, a combination that precludes a significant iron-bearing phyllosilicate component
[11] and that is expected under conditions of alteration different from those seen in the CM2-CI1 suite (perhaps an additional alteration stage), with no meteoritic analogs for, or samples from, Ceres yet detected in our meteorite collections. The weak features in the Ceres curve should provide clues to the identity of the major phases, presumably dominated by phyllosilicates formed under conditions of intense, low-temperature, aqueous alteration such as members of the kaolin or smectite groups. Asteroid 877 Walkure has an albedo (p.sub.v [+ or -] SE = 0.047 [+ or -] 0.004) comparable to that of 102 Miriam and exhibits relatively strong but distinct spectral features, some similar to spectral features of CM2 chondrite Murray, which suggest an intermediate stage of alteration, perhaps somewhat less than that of 102 Miriam or perhaps along a different redox path or with different fluid: rock ratios. The data on this limited set of dark, main-belt asteroids suggest that all have been aqueously altered to some degree and that intense alteration is relatively common.
In addition to the main-belt asteroids, fourteen Cybele, four Hilda, and four Trojan asteroids were included in this study. Ten of these asteroids (Cybele asteroids 76, 87, 566, 643, 733, and 1167 and Trojan asteroids 884, 1172, 2357, and 2674) have spectra with no absorption features that passed our acceptance criteria, implying that these asteroids have not undergone any aqueous alteration. Of these ten objects, 733 Mocia has the highest albedo (p.sub.v [+ or -] SE = 0.049 [+ or -] 0.009); the other nine have albedos in the range of 0.029 to 0.042. All of the measured D-class asteroids beyond the main belt are included in this featureless group. This is in agreement with recent water of hydration absorption results at 3.0 [micrometer] and the proposed models for solar system evolution [12].
The remaining eight Cybele asteroids (Fig. 3) and the Hilda asteroids (Fig. 4) show a diversity of spectral features, the nature of which is not yet completely understood. Although some meteoritic analogs are evident, the diversity suggests that many of these assemblages are not represented in our meteorite collections. The CM2 specimens Nogoyo, Mighei, and Murchison provide very good matches both in absorption band position and in intensity to the spectrum of Cybele asteroid 1467 Mashona. The band position and intensity are indicative of a serpentine-type phyllosilicate having a well-ordered crystal structure. Asteroids 466 Tisiphone, 528 Rezia, and 940 Kordula (Cybele group objects, Fig. 3) and 153 Hilda and 1512 Oulu (Hilda group objects, Fig. 4) show a somewhat similar spectral pattern, although weaker and less well defined. These may represent assemblages that have undergone less but still substantial aqueous alteration, but the significance of the differences is unclear. Asteroids 225 Henrietta and 570 Kythera (Fig. 3) have features that are not well matched by any single phyllosilicate or iron oxide for which
spectra are currently available although they are similar to components seen in spectra.
It appears that the processes that produced the most aqueous alteration among the CM2 assemblages were active to approximately the same extent on objects such as 1467 Mashona and 102 Miriam, and to a lesser extent on other objects in the mainbelt, Cybele, and Hilda groups. Features in asteroids such as 368 Haidea area similar in band position to but less intense than those present in the Cold Bokkeveld and Murray spectra. The intensity difference may reflect a real mineralogical difference or a regolith process (coarse particle size versus fine). The featureless or nearly featureless spectra imply either a significant depletion of iron from the phyllosilicates (producing weak features that are easily masked by the dark phases) or the presence of an anhydrous silicate assemblage with the correspondingly weak features. It seems unlikely that a higher abundance of the dark absorbing phases, such as organics, can be the sole explanation for the absence of spectral features because that would imply substantially lower albedos than measured for these objects.
The pattern of features and the inferred postaccretionary aqueous alteration of their parent planetesimals is consistent with several recent models and observations that point to a selective heating mechanism that has a steep decline in efficiency with increasing heliocentric distance [13]. In the Trojan asteroids, temperatures in the planetesimal interiors did not rise above the water-ice solidus (if it is assumed that ice was the primary water source for the aqueous alteration). In the main belt, most or all bodies exceeded this temperature. In the Cybele asteroids [mean semimajor axis [alpha] = 3.4 astronomical units (AU)] and the Hilda asteroids ([alpha] = 4.0 AU), somewhat more than half of the bodies exceeded this temperature to produce aqueous alteration.
REFERENCES AND NOTES
[1] K. Tomeoka and P. Buseck, Geochim. Cosmochim. Acta 49, 2149 (1985).
[2] T. V. V. King, thesis, University of Hawaii (1986); M. J. Gaffey, Lunar Planet Sci. Conf. 11, 312 (1980).
[3] T. V. Johnson and F. P. Fanale, J. Geophys. Res. 78, 8507 (1973); L. A. Lebofsky, Astron. J. 85, 573 (1980).
[4] Asteroid classifications used here are those by D. J. Tholen, thesis, University of Arizona (1984).
[5] F. Vilas and B. A. Smith, Icarus 64, 503 (1985); F. Vilas and L. A. McFadden, Am. Astron. Soc. Publ. 19, 825 (1987).
[6] J. Hardorp, Astron. Astrophys. 120, 529 (1980).
[7] Individual spectra, which have dispersions of 8 to 11 [Angstrom], were smoothed with a running a mean average of 300 [Angstrom] spectral width to suppress pixel-to-pixel noise and to enhance broad, weak features. Obviously spurious data points (for example, those due to incomplete correction of the O.sub.2 A band near 0.76 [micrometer]) were removed before smoothing.
[8] The removal of a sloped continuum has a distorting effect on the residual features. In general, this procedure will tend to weaken the low albedo edge (generally the shorter wavelength side) of the feature relative to its actual absorbance. The magnitude of this effect correlates directly, but not linearly, with spectral slope and inversely with albedo. The net effect for steeply sloped spectra (the slope indicating that an absorbing agent is operating more effectively at lower wavelengths) is to shift the effective centers of features toward longer wavelengths and to under-represent the short-wavelength components in compound absorption bands. These effects have been considered in a qualitative fashion in the present discussion.
[9] R. V. Morris et al., J. Geophys. Res. 90, 3126 (1985).
[10] Infrared Astronomical Satellite asteroid albedos were used throughout this study; D. L. Matson, Jet Propulsion Laboratory Document D-3698 (1986).
[11] M. J. Gaffey, Meteoritics 13, 471 (1978); M. A. Feierberg, L. A. Lebofsky, H. P. Larson, Geochim. Cosmochim. Acta 45, 971 (1981).
[12] L. A. Lebofsky, T. D. Jones, P. D. Owensby, M. A. Feierberg, G. J. Consolmagno, Icarus, in press; T. D. Jones, thesis, University of Arizona (1988).
[13] F. Herbert, Icarus 78, 402 (1989); F. Herbert, C. P. Sonnett, M. J. Gaffey, in preparation; M. J. Gaffey, in preparation.
[14] F.V. was supported by the National Aeronautics and Space Administration (NASA) Planetary Astronomy program. M.J.G. was supported under the NASA Planetary Geology and Geophysics Program (grant NAGW-642). F.V. was a visiting astronomer at the Cerro Tololo Inter-American Observatory, operated by the
National Optical Astronomy Observatories for the National Science Foundation.
F. Vilas, National Aeronautics and Space Administration Johnson Space Center, Space Science Branch, Houston, TX 77058.
M. J. Gaffey, Geology Department, Rensselaer Polytechnic Institute, Troy, NY 12181.
InfoTrac Web: Gen'l Reference Ctr (Magazine Index).
Source: Astronomy, June 1995 v23 n6 p30(8).
Title: The Sun's fab four. (asteroids Ceres, Pallas, Juno, and Vesta; includes related articles)
Author: Alan Stern
Abstract: Recent observations of four of the larger bodies in the asteroid belt between Mars and Jupiter suggest that these minor planets have evolutionary histories similar to full-fledged planets. An observational history of Ceres, Vesta, Pallas, and Juno is given, along with tips on viewing them.
The soft-edged, lopsided object glows feebly in this blurry inset photo from HST. Its features are indistinct. This could almost be the Moon as seen by the naked eye, but it isn't. The object is Vesta, a 500-kilometer-diameter ball of rock circling the Sun halfway to Jupiter.
The new Hubble photo hints at what's remarkable about Vesta: Those light and dark areas may correspond to areas like the highlands and maria on the Moon. If true, that would give Vesta a kind of evolutionary history scientists once thought only full-sized planets had.
Nor is Vesta alone. It is but one of perhaps a million small, rocky bodies that orbit the Sun between Mars and Jupiter. Thousands more lie inside the orbit of Mars, while another set orbit along with Jupiter. These bodies, collectively called the asteroids, are so numerous they were once called "the vermin of the skies." Given their numbers, it's no surprise that the tiniest asteroids captured the lion's share of asteroid studies. However, scattered among these Lilliputian tribes lie a few giants, worlds that truly deserve the term minor planet. Vesta is one such, as are its cousins Ceres, Pallas, and Juno. And these larger bodies are now coming into their own as worlds to study.
Two streams of solar system research converge in the asteroid belt. The older arose from classical astronomy and seeks to understand the origin of the planets. The second, a child of the space age, is using the latest telescopes and techniques to delve into the individual histories of these bodies. Where the two streams intersect, scientists are at last unfolding a marvelously rich and detailed view of the solar system.
Asteroids were not always scorned as vermin. in fact, in 1801 when astronomer Giuseppe Piazzi discovered the first, which he named Ceres, it neatly filled an embarrassing gap. Astronomers had noticed the big jump between the orbits of Mars and Jupiter and had postulated a missing planet to fill the space.
Ceres' orbit fit well.
Then in 1802, Heinrich Olbers discovered a second, fainter object orbiting between Mars and Jupiter; he named it Pallas. Confronted with two objects to fill the gap, astronomers' thoughts shifted from missing planets to shattered ones. Could the original missing planet have somehow broken apart, they wondered? This seemed confirmed as intensified searches for additional bodies turned up Juno in 1804, Vesta in 1807, and Astraea in 1845. All lay within the gap between Mars and Jupiter.
But the pace of discovery was slow when astronomers worked at the telescope only by eye. With the advent of astronomical photography in the late 19th century, the discovery rate shot upward and hasn't slowed since. Planetary scientists have now cataloged more than 6,000 asteroids, and they estimate that between 100,000 and 1,000,000 additional bodies (with diameters between I km and about 40 km) remain to be found.
Such numbers can overwhelm. But from them a general picture of the asteroid belt has taken shape, and some curious features emerge. Interestingly, the belt isn't just a random mix of bodies; instead, it's stratified according to distance from the Sun. For example, the inner one-third of the belt has mainly asteroids with surfaces that are relatively reflective, somewhat reddish, and rich in iron-bearing minerals. The outer two-thirds of the belt is dominated by darker asteroids that resemble carbonaceous chondrite meteorites. Less populous classes also exist, including at least one believed to derive from Vesta itself. These objects appear to be covered in basaltic lava. This is the hallmark of planetary evolution and a telltale sign that whatever Vesta is, it is not geologically simple.
Asteroid statistics point out another anomaly. The total mass in the asteroid belt is surprisingly small - only about a tenth the mass of Earth's Moon. Moreover, of all the mass of asteroids a kilometer or larger in size, fully 40 percent reside in just the "Fab Four" - Ceres, Pallas, Juno, and Vesta - alone. Ceres, Pallas, and Vesta are also the only asteroids with diameters larger than 500 km. These, the very largest asteroids, form a clearly distinct class that seems to have "broken away" from the general asteroid population. How could this have occured?
To answer this question, planetary scientists first recognize that the present-day population of the asteroid belt is the result of both nature and nurture - that is, the population results from the belt's origin and its later evolution.
Three processes shaped the sizes of asteroids: growth through collisions among bodies that encounter one another at low enough speeds to combine; erosion (or catastrophic shattering) that occurs when bodies collide at high speeds; and orbital changes primarily forced by the gravity of massive Jupiter, lying just beyond the outer edge of the belt.
As with the formation of the planets themselves, asteroids grew initially in a bottom-up process: larger bodies came from smaller ones. The critical factor is that average collision speeds were low enough that collisions mainly resulted in growth, rather than destruction.
So far, so good. What complicates the process (and makes it more interesting) is Jupiter. This giant planet profoundly influenced the belt in two ways. First, Jupiter's gravitational stirring made asteroid orbits more eccentric. This sped up impact velocities so that most collisions produced fragmentation rather than growth. in fact, computer models of asteroid belt evolution indicate that essentially all the bodies smaller than about 350 km in diameter are the result of catastrophic, high-speed collisions, and that they are significantly younger than the age of the solar system.
Second, Jupiter's influence also created some special regions in the asteroid belt called resonances. These amplify the effects of gravitational perturbations: In some resonances, orbits become unstable; as a result, bodies can be ejected from the asteroid belt. Jack Wisdom of the Massachusetts institute of Technology showed in the early 1980s that this mechanism delivers many meteorites and some asteroids into Earth-crossing orbits.
More recently, George Wetherill of the Carnegie Institution in Washington, D.C., has uncovered a subtle interplay between collisions, gravitational perturbations, and Jovian orbit resonances that deeply affected the early evolution of the asteroid belt.
In his simulations, Wetherill found that collisions can nudge objects into resonances so efficiently that most of the bodies in the early asteroid belt were probably removed, primarily through ejection into Jupiter-crossing orbits. As the asteroid belt cleared out, collisions became infrequent, as they are today. With fewer and fewer objects nudged into unstable resonances, the rate of ejection decreased. Scientists find Wetherill's model attractive because it offers a natural explanation for the meager total mass of the asteroid belt.
Wetherill's results also imply that in its early days, the asteroid belt may have formed more objects in the Vesta to Ceres size-class. Most of these mega-bodies were probably ejected from the belt to suffer collisions with jupiter or were tossed out of the solar system by it. Thus it would seem that Ceres, Pallas, Juno, and Vesta grew large by accreting smaller objects in low-speed collisions, perhaps just as the belt was clearing out. Then, thanks to their size - and having escaped perturbation into unstable resonances - they survived to the present.
In the billions of years that have rolled by while these four objects orbited the Sun, little has changed on them. They have no detectable atmospheres. Few have satellites. And their surfaces remain today as they have been since they cooled after formation - cold! - with temperatures of -100[degrees] C (about 170 K) being typical.
Yes, some of their myriad brethren have bashed and battered them. Yes, solar radiation and cosmic rays have weathered their surfaces. And yes, their orbits have moved around a bit as collisions and perturbations worked their subtle synergy.
But with so little to change them, the Fab Four have remained almost-timeless relics of a distant past. This cannot be said for the terrestrial planets, which have evolved greatly from their formative state. Nor can it be said for most of the objects in today's asteroid belt, which are simple shards left from collisions that occurred only yesterday, considering the age of the solar system. Of all those. million-plus objects in the belt, just a handful - namely those larger than about 350 km in diameter - have survived largely intact since the early days of the solar system.
As four of the solar system's largest asteroids, Ceres, Pallas, Juno, and Vesta preserve a bit of the past that is otherwise mostly lost. Scientists look to these roundish little worlds on the frosty border between the inner and outer solar system in hopes that they will one day become scientific beachheads, perhaps even staging points, for the long jump humans will ultimately take across the cold, yawning vacuum to Jupiter.
Visualizing Vesta
Vesta's surface is so unlike that of any other asteroid, it's been placed into a class of its own. Not surprisingly, this uniqueness makes it a rather attractive target for many astronomers. Last fall, Ben Zellner of Georgia Southern University used the Hubble Space Telescope's Wide Field and Planetary Camera 2 to take pictures of Vesta as it completed one rotation. The images show much of Vesta's surface to a resolution of 70 km or better.) In them we see bright and dark features temptingly similar to regions on our Moon. This leads to the intriguing possibility that geologists may one day have a smaller cousin to our Moon to study and compare.
And that day may be not be far off. A proposal made by Cornell University's Joseph Veverka, NASA's Jet Propulsion Laboratory, and Martin Marietta Aerospace puts forth an unmanned mission to Vesta arriving in 2003. Imaging cameras would map areas of its surface to 10 meters, with some on-board instruments "tasting" the asteroid's chemical composition from a 250 km orbit.
What exactly makes Vesta so interesting?
For one thing, of all the large asteroids, Vesta has the most reflective surface. Its average visible-wavelength albedo, or efficiency at reflecting light, is 38 percent. Compared to the Moon's average albedo of 7 percent, Vesta is brilliant. In fact, even though at Vesta's distance sunlight is reduced to just 15 percent of the strength experienced on Earth, an astronaut on its surface would see a scene about as bright as on Earth's Moon. And the similarity wouldn't end there.
Like areas on the Moon humans have seen, Vesta's surface is covered in a fine, pulverized powder about 6 centimeters deep. Vesta is also composed of volcanic basalts created when its surface was melted, perhaps by some nearly catastrophic collision early in its history.
Vesta's intriguing characteristics led it to become one of the first asteroids compositionally fingerprinted. Over 20 years ago, Tom McCord (now at the University of Hawaii) developed a method called reflectance spectroscopy that uses light broken down into its component colors. With the technique, McCord accurately determined Vesta's mineral content.
Similar observations pioneered by Michael Gaffey of Rensselaer Polytechnic Institute, along with recent images taken by Hubble, reveal other features analogues to lunar maria. Some of these regions have minerals called pyroxenes and feldspars, typically found in the Earth's mantle. Perhaps craters punched through Vesta's basaltic crust to excavate material from its upper mantle.
Complementing this picture are actual pieces of Vesta believed to have have fallen to Earth as meteorites called basaltic achondrites. Careful work by astronomers like Richard Binzel of MIT reveal the way these chips off Vesta came to Earth.
A string of tiny meteoroids blasted off Vesta fell into orbits susceptible to the influences of Jupiter's gravity, which then perturbed them into Earth-crossing orbits. As a result, these basaltic achondrites, which have reflectance spectra, identical to the spectra of Vesta, fall to Earth on a regular basis. Whether this debris found its way to Earth via Jovian resonances or was blasted directly off Vesta's crust is uncertain. It's likely that both channels make the Vesta-to-Earth meteorite conveyor work.
Close-up on Ceres
The solar system's largest asteroid may be more like a planet than an asteroid: Its shape is spherical, and like Mars, this ruddy-colored minor planet might harbor water at its poles.
An astronaut orbiting Ceres would see a yellowish or perhaps slightly orange-tinged surface pockmarked and battered by craters. Studies made from Earth suggest the surface might be clay-like, consisting of soil that has water chemically locked into it and minerals similar to those found in meteorites called carbonaceous chondrites.
Groundbased evidence also indicates it's unlikely our astronaut would see dramatic differences in Ceres' surface color, save at the minor planet's poles. Several tantalizing lines of evidence point to the possibility that Ceres' poles may harbor a cache of frozen water, either mixed in the planet's clay-like soil or, perhaps, lying encrusted on the surface in frozen ponds as a patchy veneer.
While the evidence for water ice on Ceres is not definitive, it is highly suggestive. Larry Lebofsky of the University of Arizona and his colleagues have repeatedly identified the distinctive spectral fingerprint of water ice. Others, including Michael A'Hearn of the University of Maryland and Paul Feldman of Johns Hopkins University, have found evidence for hydroxide (OH), the most common product of water vapor ([H.sub.2]O), above Ceres' surface.
These and other findings have led to studies of the stability of water ice on Ceres. In the most complete study to date, modelers Fraser Fanale and James Salvail of the University of Hawaii believe that water frost would be stable just under, and perhaps even on, the cool surfaces of Ceres' poles. For no other asteroid is the possibility of water ice on its surface known to be so strong.
Moving in orbit around Ceres, an astronaut would notice its surface reflects less light that the lunar highlands - by almost 40 percent, and he might confirm groundbased radar and thermal measurements that indicate a thin, powdery surface overlies a much deeper, more compact layer.
Several orbits later, our astronaut would quickly come to the conclusion that change comes to Ceres only over a long time and through random meteorite impacts and incessant bombardment from cosmic rays, sunlight, and solar wind. We can only speculate what weathered vistas of mountains, rilles, valleys, or scraps might greet a visitor to Ceres. Our still-growing databank isn't very large.
Just because it's small doesn't mean that Ceres isn't interesting.
And, finally, a picture:
