I've
blogged in my past about my fascination with
Ceres, the first asteroid discovered by astronomers, for decades considered a planet, and most recently reclassified (along with Pluto) as a dwarf planet. Another asteroid is dear to my heart, a geologically interesting body that also has its own claim to dwarf planet status,
Vesta.
The discovery of Ceres in 1801 and Pallas in 1802 led German astronomer Heinrich Wilhelm Olbers to propose that the two objects were the remnants of a destroyed planet. In 1802 he sent a letter with his proposal to the English astronomer William Herschel, suggesting that a search near the locations where the orbits of Ceres and Pallas intersected might reveal more fragments. These orbital intersections were located in the constellations of Cetus and Virgo.[14]
Olbers commenced his search in 1802, and on March 29, 1807 he coincidentally discovered Vesta in the constellation Virgo. As the asteroid Juno had been discovered in 1804, this made Vesta the fourth object to be identified in the region that is now known as the main asteroid belt. This discovery was announced in a letter addressed to German astronomer Johann H. Schröter dated March 31. Olbers allowed the prominent mathematician Carl Friedrich Gauss to name the asteroid after the Roman virgin goddess of home and hearth, Vesta.
Vesta ended up losing its planetary status later in the 19th century, as the discovery of hundreds of other asteroids scattered in what came to be known as the asteroid belt led to the downgrading of these bodies. I think that this downgrading was a mistake in some respects, in that even the larger asteroids, like the larger moons of the outer Solar System (the four Galilean moons and Titan, called
"secondary planets" also in the 19th century), weren't considered to be worlds of the same import as the "actually existing" planets. We have sent space probes out to distant Neptune, but nothing to the Ceres and Vesta discovered more than a generation before Neptune.
This, thankfully, is changing. Bad Astronomy
linked to these
superb NASA images of the Vesta surface, part of an ongoing effort to map first Vesta then Ceres for the benefit of the navigators of the
Dawn space proibe, scheduled to visit Vesta in 2011 and Ceres in 2015.
Vesta is a very unusual asteroid, and the key to understanding its unusual nature lies in its density. This rocky world can claim a density of 3.42 grams per cubic centimetre. This is substantially above the estimated density of ~2.07 grams per cubic centimetre estimated for Ceres, almost exactly the same as the 3.34 grams per cubic centimetre of Moon, and not that far removed from Mars' 3.93 grams per cubic centimetre. Even though Vesta is a very small body, estimated to have a diameter of 550 kilometers and a polar axis 462 km, and with a surface gravity a bit more than two percent of Earth's, Vesta is a dense, rocky body. It's unique in having a rocky surface, perhaps similar in composition to the maria of the Moon, that's so reflective that Vesta is the only asteroid visible to the naked eye. The asteroid--the only surviving member of its class--formed as particles collided and condensed over several million years, internal heat produced by the decay of a radioactive isotope of aluminum common in the early Solar System, this melting resulting in a sorting out of Vesta's material by density before the too-small world began to cool.
Vesta's
complex surface is marked by one very, very big crater
discoveredd in 1997.
The most prominent surface feature is an enormous crater 460 kilometres in diameter centered near the south pole. Its width is 80% of the entire diameter of Vesta. The floor of this crater is about 13 kilometres below, and its rim rises 4–12 km above the surrounding terrain, with total surface relief of about 25 km. A central peak rises 18 kilometres above the crater floor. It is estimated that the impact responsible excavated about 1% of the entire volume of Vesta, and it is likely that the Vesta family and V-type asteroids are the products of this collision. [. . .] Spectroscopic analyses of the Hubble images have shown that this crater has penetrated deep through several distinct layers of the crust, and possibly into the mantle, as indicated by spectral signatures of olivine.
Several other large craters about 150 kilometres wide and 7 kilometres deep are also present. A dark albedo feature about 200 kilometres across has been named Olbers in honour of Vesta's discoverer, but it does not appear in elevation maps as a fresh crater would. Its nature is presently unknown; it may be an old basaltic surface. It serves as a reference point with the 0° longitude prime meridian defined to pass through its center.
The eastern and western hemispheres show markedly different terrains. From preliminary spectral analyses of the Hubble Space Telescope images, the eastern hemisphere appears to be some kind of high albedo, heavily cratered "highland" terrain with aged regolith, and craters probing into deeper plutonic layers of the crust. On the other hand, large regions of the western hemisphere are taken up by dark geologic units thought to be surface basalts, perhaps analogous to the lunar maria.
The large south-polar crater--visible in the two top images--may be the scar produced by a collision that ejected perhaps one percent of its mass via
at least one massive impact a billion years ago, creating a
signifcant family of asteroids was thrown off of the Vestan surface by this and other impacts.
Could Vesta be elevated to the status of a
dwarf planet? Maybe. The infamous
International Astronomical Union resolution defines planets as bodies massive enough to pull themselves into spherical shapes through their own gravity. Vesta isn't a sphere, perhaps a result of that collision, and it has only a quarter the mass of the already small Ceres. If
Dawn determines that Vesta is massive enough to mold itself into a sphere, and if it turns out that Vesta's oblateness is productive of the aforementioned massive asteroid collision, Vesta could join Ceres in that elite club.
Age of miracles and wonders.
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