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The violent collisional history of aqueously evolved (2) Pallas
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Journal Article
The violent collisional history of aqueously evolved (2) Pallas
2020
Overview
Asteroid (2) Pallas is the largest main-belt object not yet visited by a spacecraft, making its surface geology largely unknown and limiting our understanding of its origin and collisional evolution. Previous ground-based observational campaigns returned different estimates of its bulk density that are inconsistent with one another, one measurement
1
being compatible within error bars with the icy Ceres (2.16 ± 0.01 g cm
−3
)
2
and the other
3
compatible within error bars with the rocky Vesta (3.46 ± 0.03 g cm
−3
)
4
. Here we report high-angular-resolution observations of Pallas performed with the extreme adaptive optics-fed SPHERE imager
5
on the Very Large Telescope. Pallas records a violent collisional history, with numerous craters larger than 30 km in diameter populating its surface and two large impact basins that could be related to a family-forming impact. Monte Carlo simulations of the collisional evolution of the main belt correlate this cratering record to the high average impact velocity of ~11.5 km s
−1
on Pallas—compared with an average of ~5.8 km s
−1
for the asteroid belt—induced by Pallas’s high orbital inclination (
i
= 34.8°) and orbital eccentricity (
e
= 0.23). Compositionally, Pallas’s derived bulk density of 2.89 ± 0.08 g cm
−3
(1
σ
uncertainty) is fully compatible with a CM chondrite-like body, as suggested by its spectral reflectance in the 3 μm wavelength region
6
. A bright spot observed on its surface may indicate an enrichment in salts during an early phase of aqueous alteration, compatible with Pallas’s relatively high albedo of 12–17% (refs.
7
,
8
), although alternative origins are conceivable.
High-resolution observations of the third largest asteroid, (2) Pallas, from SPHERE unveil a heavily cratered surface, probably due to Pallas’s inclined and eccentric orbit, a density almost equal to carbonaceous chondrites and hint at surficial salt-enriched spots.
Publisher
Nature Publishing Group UK,Nature Publishing Group,Springer Nature
