Other
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p31e..03h&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P31E-03
Other
[5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
Motivated by the possibility that small amounts of adsorbed water may be stable on illuminated portions of the lunar surface, possibly as OH- , (e.g. Starukhina and Shkuratov, 2000; Hibbitts et al., 2009; Dyar et al., 2009; McCord et al., 2009) and to confirm not yet released observations by other spacecraft, we have reanalyzed the VIMS (Visual and Infrared Mapping Spectrometer) observations of the Moon for absorption features near 3 microns - a very strong absorption band due to the presence of OH- or H2O. The VIMS instrument onboard the Cassini spacecraft measures reflected solar and thermally emitted radiation from ~ 0.35 to 5.1 microns in 352 channels. On August 18, 1999, the Cassini spacecraft flew between the Earth and the Moon, within 380,000 km of the lunar surface at a phase angle of 90deg and a subsolar point of 0.33N, 257E (Brown et al., 2000), obtaining 12 observations fully within its field of view at a sub-spacecraft spatial resolution of about 190 km/pixel. Designed for operation at 10 a.u., the instrument offers the potential for very high SNR measurements of solar reflectance and thermal emission from of the lunar surface to investigate the presence and abundance of water and other trace materials, provided that significant calibration issues are appropriately addressed. Due to a combination of operating at higher temperatures than designed for and due to high thermal flux from the parts of the Moon’s surface that are warm, the VIMS instrument saturates in the infrared shortward of 1.65 microns and longward of ~ 3.5 - 4 microns, with longer wavelengths remaining useful for colder areas of the surface. In this preliminary effort, we have recalibrated each of the 12 observations relative to the equatorial limb by fitting the spectrum of the limb to a polynomial curve, resulting in an arbitrarily smooth spectrum, but deriving an empirical adjustment to the radiometric calibration that allows us to compare other areas of the Moon to the limb. This technique has the disadvantage of erasing any feature, real or artifact, in the spectrum of the limb, but will allow us to explore the possibility of stronger bands present elsewhere such as at the cooler higher latitudes or on the terminator. The mentioned disadvantage is mitigated by the fact that the spectrum of the equatorial limb is strongly affected by thermal emission near 3 microns and longer wavelengths, and any real spectral features will be muted, the absorption band in reflectance ‘filled in’ by the emitted energy. This subsequently-derived modification to the VIMS calibration is applied to three other selected regions: equatorial terminator, north polar, and south polar terrains. We currently find that the south polar terrain possesses a weak, ~ 3% feature beginning near 2.73-microns and not rising again, that may be consistent with a small amount of water or OH-. The northern terrain does not possess this feature. More detailed analyses will be presented, but additional, high spatial resolution spectra covering the 2.5-4 micron region are needed to better constrain the possibility of water and other trace materials on the lit portions of the Moon.
Hansen Gary B.
Hibbitts C.
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