[LINK] "Orcus and Vanth"

[rfmcdonald]

Astronomy has advanced amazingly, with probes resolving lakeshores on Titan and the atmospheres of gas giants orbiting other stars. It still has far to go, though, as the Planetary Society Blog's Emily Lakdawalla noted in a recent post about the plutoid Kuiper Belt Object 90842 Orcus and its very close-orbiting moon Vanth ("officially (90482) Orcus I Vanth"). Or are Orcus and Vanth co-planets?

An orbit of only 9000 kilometers is a really really close orbit. Still, Hubble is capable of resolving Orcus and Vanth as two separate (pointlike) objects. What they learned from looking at Orcus and Vanth with Hubble is that the two bodies have very different surfaces. Neither one shows the gray color and strong evidence of water ice that some other Kuiper belt bodies (like Haumea and Charon) do. There's some evidence for some water ice on Orcus, but Vanth looks like more like typical Kuiper belt bodies, reddish in color. It's not the color of Vanth that's odd; it's the fact that Vanth and Orcus have such different colors. In fact, Vanth and Orcus are less similar in color than any other binary pair yet observed in the Kuiper belt.

This dissimilarity in color has various implications for the origin of the Vanth-Orcus system but I'm more interested in what it means about the differences in physical properties between the two. It very likely means that the two have very different albedo, and albedo translates in a very direct way into size.

Size estimates are performed using thermal infrared data, which they got from Spitzer. Unfortunately, Spitzer has lower resolution than Hubble's visible cameras, so it can't resolve Orcus and Vanth separately; all it can see is a single point of light that's composed of light reflected from both Orcus and Vanth. So, for starters, Brown and his coauthors estimated the size that Orcus-Vanth would be if they were, in fact, one object. This comes out to be 940 ± 70 kilometers and 0.28 ± 0.04 for the albedo. That diameter, taken with the relatively well-known mass, produces a density estimate of 1.45 ± 0.3 grams per cubic centimeter, comfortably above the density of ice, meaning it's a mix of rock and ice.

So how big is Vanth relative to Orcus? The usual assumption is that Kuiper belt objects and their satellites have the same albedo, an assumption that's usually been borne out in observations of Kuiper belt binaries. If Vanth and Orcus do have the same albedo, then the fact that Vanth is 2.54 magnitudes fainter than Orcus would imply that Orcus' diameter would be about 900 kilometers, and Vanth would be 3.2 times smaller, 280 kilometers (which is just a bit bigger than Saturn's moon Phoebe).

But I just told you that the Hubble data suggests very different albedo for Vanth and Orcus. The paper says: what if Vanth's albedo is half that of Orcus? Then Orcus would be 820 kilometers in diameter, and Vanth 640 kilometers in diameter, and Vanth would be a "moon" with a whopping 50% the mass of its primary, and would be bigger than the very interesting-looking moons Mimas, Enceladus, and Miranda! This would be even more a binary system, a mutually orbiting dumbell planetoid, than Pluto and Charon are. And it makes me wonder if the story could be even crazier -- if there are reasonable choices for albedo for the two bodies that would actually result in the "moon," Vanth, being larger than its "primary," Orcus.

Go, read.