Synergistic Ground and Orbital Observations of Iron Oxides on Mt. Sharp and Vera Rubin Ridge

Abstract

Visible/short‐wave infrared spectral data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) show absorptions attributed to hematite at Vera Rubin ridge (VRR), a topographic feature on northwest Mt. Sharp. The goals of this study are to determine why absorptions caused by ferric iron are strongly visible from orbit at VRR and to improve interpretation of CRISM data throughout lower Mt. Sharp. These goals are achieved by analyzing coordinated CRISM and in situ spectral data along the Curiosity Mars rover's traverse. VRR bedrock within areas that have the deepest ferric absorptions in CRISM data also has the deepest ferric absorptions measured in situ. This suggests strong ferric absorptions are visible from orbit at VRR because of the unique spectral properties of VRR bedrock. Dust and mixing with basaltic sand additionally inhibit the ability to measure ferric absorptions in bedrock stratigraphically below VRR from orbit. There are two implications of these findings: (1) Ferric absorptions in CRISM data initially dismissed as noise could be real, and ferric phases are more widespread in lower Mt. Sharp than previously reported. (2) Patches with the deepest ferric absorptions in CRISM data are, like VRR, reflective of deeper absorptions in the bedrock. One model to explain this spectral variability is late‐stage diagenetic fluids that changed the grain size of ferric phases, deepening absorptions. Curiosity's experience highlights the strengths of using CRISM data for spectral absorptions and associated mineral detections and the caveats in using these data for geologic interpretations and strategic path planning tools.

Analyses of High‐Iron Sedimentary Bedrock and Diagenetic Features Observed With ChemCam at Vera Rubin Ridge, Gale Crater, Mars: Calibration and Characterization

“…If the iron oxide grain size increases (to nearly the size of the laser beam diameter), it would also increase the scatter of the FeO T abundances measured with ChemCam in a raster, as coarse grains cannot be statistically hit by the laser with the same proportion of the matrix for each observation point of the raster. This could be a partial explanation for the gray Jura part, as coarse-grained gray hematite grains are thought to be present in the bedrock according to MastCam and passive ChemCam observations (Jacob et al, 2020;L'Haridon et al, 2020) as mentioned in section 2, although these grains are probably small compared to the 350to 550-μm diameter of the laser beam. However, ChemCam rock analyses reveal that variability in iron content (when discarding diagenetic features) is not observed within the same ChemCam raster but is mainly observed between different targets, sometimes within a narrow range of distance (meter to 10-m scale), even though no change in rock texture is observed among them.…”

“…Abundant amorphous component (AmC) is also present (49 wt.%). The lack of ferric absorption features in ChemCam passive reflectance observations, made without utilization of the laser (Johnson et al, 2015(Johnson et al, , 2016, and Mastcam multispectral observations suggest that specular gray hematite could be located in the gray Jura samples (Horgan et al, 2020;Jacob et al, 2020;this issue;L'Haridon et al, 2020). The color of hematite turns gray instead of red when particle sizes are greater than~5 μm (e.g., Catling & Moore, 2003;Sherman & Waite, 1985).…”

APXS‐Derived Compositional Characteristics of Vera Rubin Ridge and Murray Formation, Gale Crater, Mars: Geochemical Implications for the Origin of the Ridge

“…They do not follow stratigraphy but instead cut across primary bedding features. Further details of the stratigraphy, sedimentology, textures, and spectral properties of the ridge strata are provided by Bennett et al (2018), Edgar et al (2020), Fraeman, Edgar, et al (2020), Fraeman, Johnson, et al (2020), Horgan et al (2020), and Jacob et al (2020).…”

Hydrothermal Precipitation of Sanidine (Adularia) Having Full Al,Si Structural Disorder and Specular Hematite at Maunakea Volcano (Hawai'i) and at Gale Crater (Mars)

“…We plot reflectance spectra for red and specular hematite in Figure 11. Two parameters used for spectral identification of red hematite, S750_500 (¼ slope between 500 and 750 nm) and BD535 (¼ band depth at 535 nm relative to a continuum defined by values of reflectance at 500 and 600 nm), are defined in Figure 11a (after Fraeman et al, 2020). Red hematite pigment HMS3 has a mean particle size of 0.14 μm (Morris et al, 1985), and specular hematite is the 500-1000 μm sieve fraction of a powder derived by grinding and washing hematite single crystal HMMG1 (Lane et al, 2002).…”

“…Journal of Geophysical Research: Planets scattered gray patches~10 m in diameter (interpreted as specular hematite) (Fraeman et al, 2020). Following the color, hydrothermal precipitation of specular hematite in indicated for the gray patches, but hydrothermal conditions are neither implied nor excluded for precipitation of red hematite in VRR relatively red areas.…”