FERRIC OXIDE VARIABILITY IN THE MURRAY FORMATION FROM CHEMCAM PASSIVE REFLECTANCE OBSERVATIONS

Beginning on Sol 1157 with the target Augusta, relative reflectance spectra (400-840 nm) acquired with the Chemistry and Camera (ChemCam) instrument on the Mars Science Laboratory (MSL) rover Curiosity [1,2] began to document spectral features indicative of hematite on surface rocks [3-5]. As of this writing (Sol 1605), spectra of nearly all rocks and drill fines associated with Murray Formation mudstones have shown these features, although variable spectral contrast and peak reflectance wavelengths suggest differences in iron-bearing phases, crystallinity, and/or oxidation states [5-6]. CheMin X-ray diffraction measurements of drill fines show hematite to be the only crystalline ferric oxide. As such, observed spectral variations must arise from differences in hematite crystallinity or grain size, mixing with other phyllosilicates, or amorphous components detected within the mudstones. The 535 nm band depths have systematically increased since Sol 1300, suggesting increasingly greater hematite crystallinity. The 750 nm-840 nm spectral slope has decreased since about Sol 1400, consistent with increasing absorption near 860 nm, again related to greater hematite crystallinity. As the rover approaches a ridge known to contain hematite from orbital spectral observations, the MSL team will continue monitoring variations in hematite and other ferric oxides that may reflect important fluctuations in the availability and timing of oxidizing fluids within the Murray mudstones [5].

[1] Johnson, J.R., et al., Icarus, 249, 74–92, 2014; Johnson, J.R., et al, Amer. Mineral., 101, 1501–1514, 2016;[2] Wiens, R., et al., Space Sci Rev., 170, 167-227, 2012; Maurice, S., et al., Space Sci Rev., 170, 95-106, 2012;[3] Rampe, E., et al,. this conference;[4] Wellington, D., et al., AGU meeting abstract P23B-2175, 2016; this meeting abs. #292521;[5] Fraeman, A. et al., AGU meeting abstract P23B-2173, 2016; LPSC abstract #2185, 2017;[6] Johnson, J.R., et al., LPSC abs. #1310, 2017.