MINERALOGY OF SAND- AND SILT-SIZE GRAINS ON MARS FROM PHOENIX OM IMAGES

The Microscopy, Electrochemistry and Conductivity Analyzer (MECA) on Phoenix Mars Lander (PHX) included an Optical Microscope (OM) that returned color images of soil material with a spatial resolution of 4 μm/pixel. The OM consisted of a high-resolution imaging system and an active visible-light sample illumination system composed of three LEDs: Blue (B, λ ~465 nm), Green (G, 524 nm), and Red (R, 636 nm). The upper limiting grain-size imaged (200 μm) was determined by the sieve through which sample was introduced by the Phoenix Robotic Arm (RA) into the MECA instrument; the lower limiting size was determined by the 4 μm/px limit of the optical system. Strong variation in the red reflectance (λ = 630-710 nm) from particle to particle led to the following classification; red and white fines, brown sand, and black sand.

Previous comparison of PHX OM three-channel RGB photometry of brown grains is consistent with library spectra for ferroan (Fo_<40) olivine prepared as <60 μm grains; nanohematite and some other varieties of hematite; and/or some library jarosites. It is more likely that the brown grains consist of olivine with nanohematite ferric coatings than that coarse silt- and fine-/very-fine-sand-size particles consist entirely of ferric minerals. Nanohematite and jarosite are among the candidate constituents of nanophase ferric oxides (npOx) that have been inferred from OMEGA orbital spectroscopy to occur in the area around the PHX landing site, and are both identified elsewhere on Mars by rover instruments and from orbit. Olivine was not detected from orbit in OMEGA spectra at latitudes as far south as the PHX landing site, probably due to its low abundance at the optically active surface relative to fine-grained ferric minerals.

Fayalitic olivine compositionally similar to that identified at the PHX landing site on the Martian arctic northern plains has been identified in Jezero Crater and elsewhere in and near Nili Fossae. Olivines compositionally consistent with PHX data (Fo_<40) occur in nakhlites (Fo_14-43). Fine-/very-fine-sand- and coarse-silt-size particles at the Phoenix landing site (both brown and black sand) are diverse and may justify further taxonomic subdivision based on other properties imaged by the OHX OM and comparisons with terrestrial analogs.