The Shape and Surface Variation of 2 Pallas from the Hubble Space Telescope
B. E. Schmidt,1,*
P. C. Thomas,2
J. M. Bauer,3
J.-Y. Li,4
L. A. McFadden,4
M. J. Mutchler,5
S. C. Radcliffe,6
A. S. Rivkin,7
C. T. Russell,1
J. Wm. Parker,8
S. A. Stern8
We obtained Hubble Space Telescope images of 2 Pallas in September
2007 that reveal distinct color and albedo variations across
the surface of this large asteroid. Pallas’s shape is
an ellipsoid with radii of 291 (±9), 278 (±9),
and 250 (±9) kilometers, implying a density of 2400 (±250)
kilograms per cubic meter—a value consistent with a body
that formed from water-rich material. Our observations are consistent
with the presence of an impact feature, 240 (±25) kilometers
in diameter, within Pallas’s ultraviolet-dark terrain.
Our observations imply that Pallas is an intact protoplanet
that has undergone impact excavation and probable internal alteration.
2 Department of Astronomy, Cornell University, Ithaca, NY 14853, USA.
3 Jet Propulsion Laboratory, Pasadena, CA 91109, USA.
4 Department of Astronomy, University of Maryland, College Park, MD 20742, USA.
5 Space Telescope Science Institute (STScI), Baltimore, MD 80302, USA.
6 Hydraulx, Santa Monica, CA 90401, USA.
7 Applied Physics Laboratory, Laurel, MD 20723, USA.
8 Southwest Research Institute, Boulder, CO 80302, USA.
* To whom correspondence should be addressed. E-mail: britneys{at}ucla.edu
