DPS 2001 meeting, November 2001
Session 48. Mars Surface
Oral, Chairs: N. Barlow, J. Bell III, Friday, November 30, 2001, 4:30-6:40pm, Regency E

[48.04] H2O- and OH-bearing minerals in the martian regolith: Analysis of 1997 observations from HST/NICMOS

E.Z. Noe Dobrea, J.F. Bell (Cornell University), M.J. Wolff (Space Sciences Institute), P.B. James (University of Toledo)

In an effort to identify the as-yet unknown hydrated mineral phase" known to occur on Mars, we used the Hubble Space Telescope NICMOS instrument and 10 medium- and narrow-band filters in the range between 0.95 and 2.16 \mu m to image the Acidalia hemisphere of Mars (central longitude ~ 35\arcdeg W) in July of 1997 (Ls ~ 150\arcdeg). Our observations are at a spatial resolution of about 45 km/pixel, the best yet achieved at these wavelengths for most of the regions in this hemisphere. We present our calibrated and mapped observations and analyses, putting special emphasis on the search for evidence of H2O or OH absorption bands at 1.45 and 1.90 \mu m (the stretching and bending modes of water) within putative surface minerals. In preparation for the analysis, we have studied a wide variety of water- and OH-bearing mineral spectra convolved to the NICMOS bandpasses and have measured their band depths at 1.4 and 1.9 \mu m. On a 1.4- vs. 1.9-\mu m plot, there is a clustering of the minerals based on their OH and H2O content: generally speaking, OH-bearing minerals have a high 1.4-\mu m band depth and a low 1.9-\mu m band depth, and H2O-bearing minerals have both high 1.4- and 1.9-\mu m band depths.

In our NICMOS observations, the 1.4-\mu m band depth is greatest for the north polar cap and the classical “bright” regions (e.g., Arabia) and lowest for the classical “dark” regions (e.g., Acidalia). In contrast, 1.9-\mu m band depth is greatest for the north polar cap and the classical “intermediate” regions (e.g., the region between Arabia and Acidalia) and lowest for the classical “bright” regions. Atmospheric effects (water ice clouds) can also be discerned towards the limb in the 1.9-\mu m band depth map. Scatter plots of 1.4 vs. 1.9 \mu m band depth show that, although most of the observed surface plots in the same locations as minerals that do not contain OH or H2O, there are some regions (e.g., Arabia) that plot where hydroxylated minerals tend to lie (1.4-\mu m band depth of a few percent, no 1.9-\mu m band depth), and other regions that plot where hydroxylated and hydrated (having both OH and H2O) minerals tend to occur (e.g., Lunae Planum).