[Previous] | [Session 4] | [Next]
A. S. Rivkin (LPL/Univ. of Arizona), D. E. Trilling (Lick Observatory), J. H. Plassmann, R. H. Brown (LPL/Univ. of Arizona), J. F. Bell III (Cornell University)
We have obtained multispectral images of both the leading and trailing sides of Phobos from the IRTF at 10 wavelengths in the 1.65--3.5 \mum region. The observations were made on 26 April 1999 (UT), with the Cold Coronagraph (CoCo) on NSFCAM. The coronagraph effectively removes the scattered light of Mars from our observations.
We find no evidence for a 3-\mum absorption feature (indicative of water of hydration) to within 5-10% on either hemisphere. This finding increases the likelihood that the unexpectedly low density of Phobos found by Viking and the Phobos 2 spacecraft is due to macroporosity effects (as suggested by Avanesov et al. Plan. Space Sci. 1991), among others) rather than compositional effects, since the spectral signature of plausible lower-density materials such as hydrated minerals and water ice have not been found. This is similar to the situation for 253~Mathilde, where the NEAR spacecraft encounter found a low density (Yeomans et al. Science 1997) while Rivkin et al. (Icarus 1997) found an anhydrous surface mineralogy, leading to an interpretation that macroporosity effects were important.
The work of Murchie and Erard (Icarus 1996), using data from the Phobos 2 spacecraft, showed that Phobos could be separated into different geological units, based on color and morphology. The fresher areas associated with Stickney crater are bluer in color than the ``background'' areas. For comparison with this work, the ``blue'' unit dominates the leading hemisphere, the ``red'' unit the trailing hemisphere. In the 1.65--3.5 \mum region, we find the two hemispheres to have virtually identical spectra. When connected to visible spectra of Phobos (Murchie and Erard, among others), the leading hemisphere has strong similarities to T-class asteroid spectra. The spectrum of the trailing hemisphere resembles mature lunar soils more closely than any asteroid class.