NASA Astrobiology Institute (NAI)

1. TES study of intracrater low albedo deposits, Amazonis Planitia, Mars

   Project Investigators: Karen Stockstill

   Other Project Members

   F. Scott Anderson (Collaborator)

   Victoria Hamilton (Collaborator)

   Summary

   We studied low albedo deposits in the floors of craters within the Amazonis Planitia region using data from orbiters of Mars to determine how these deposits were formed. These deposits are dominated by mafic minerals (olivine, pyroxene) with very low clay contents, which suggests that these deposits had limited contact with water.

   Astrobiology Roadmap Objectives:

   • Objective 2.1: Mars exploration

   Project Progress

   We studied low albedo deposits in the floors of craters within the Amazonis Planitia region using infrared data from Mars Global Surveyor Thermal Emission Spectrometer (TES) and Mars Odyssey Thermal Emission Imaging System (THEMIS), high resolution visible images from the Mars Orbiter Camera (MOC) and THEMIS, and topographic data from the Mars Orbiter Laser Altimeter (MOLA). Of the 23 Amazonis Planitia craters examined, we found satisfactory TES spectra for eleven of the low albedo intracrater features (Fig. 1). The average TES surface spectrum for each low albedo deposit was derived via linear deconvolution. Our end member set is a refinement of the set used by previous workers and includes spectra of eight atmospheric end members, a surface dust end member, and mineral end members; the minerals selected were important for determining whether these deposits were exposed to an aqueous environment and to what extent. Finally, we used the mineral abundances derived from linear deconvolution to calculate a bulk composition for each low albedo deposit. We then used these bulk compositions to assign the deposits to lithologic classes.

   TES spectral modeling of the low albedo deposits reveal a mineralogy that is dominated by mafic minerals (olivine, pyroxene) with a derived bulk chemistry that ranges from ultramafic to mafic (~40-52 wt. % SiO2) in composition. These bulk compositions are comparable to some Martian lithologies, but represent some of the lowest silica contents identified on Mars. To variable degrees, the spectra display an absorption centered at ~900 cm-1 that matches well to olivine lab spectra; this may reflect variation in the olivine abundance within the deposits. Indeed, three spectra with the strongest olivine absorptions (Fig. 2) contain among the highest olivine abundances (Table 1). The persistence of olivine and low clay content indicates limited contact with water. Furthermore, in some craters, possible local sources have been identified within the crater walls, floors and interiors; local sources may be present in additional craters, but obscured by dust. No regional sources were identified within a reasonable distance in terms of sand transport, but may also be obscured by dust.

   
   

   Eleven craters in Amazonis Planitia containing low-albedo deposits explored in this study. Craters labeled in red have a foidite composition, craters labeled in green have a picrobasalt composition and craters labeled in blue are basaltic.

   
   

   TES surface spectra of low-albedo deposits in Amazonis Planitia craters. The crater 17 spectrum is shown in brown.

   
   

   Table 1: Deconvolution results normalized to remove atmosphere and surface dust, rounded to nearest 5%.

   Mission Involvement

   Mars Global Surveyor

   Uses data from the TES (thermal infrared), MOLA (digital elevation models) and MOC (high resolution camera images).

   Odyssey

   Uses data from THEMIS (Visible and Thermal infrared).

Publications

Stockstill-Cahill, K.R., Anderson, F.S. & Hamilton, V.E.  (2008).  A study of low-albedo deposits within Amazonis Planitia craters: Evidence for locally derived ultramafic to mafic materials.  Journal of Geophysical Research, 113(E07008):doi:10.1029/2007JE003036.