A previously unidentified artifact has been found in Compact Reconnaissance Imaging Spectrometer for Mars targeted I/F data. It exists in a small fraction (<0.05%) of pixels within 90% of images investigated and occurs in regions of high spectral/spatial variance. This artifact mimics real mineral absorptions in width and depth and occurs most often at 1.9 and 2.1 μm, thus interfering in the search for some mineral phases, including alunite, kieserite, serpentine, and perchlorate. A filtering step in the data processing pipeline, between radiance and I/F versions of the data, convolves narrow artifacts (“spikes”) with real atmospheric absorptions in these wavelength regions to create spurious absorption‐like features. The majority of previous orbital detections of alunite, kieserite, and serpentine we investigated can be confirmed using radiance and raw data, but few to none of the perchlorate detections reported in published literature remain robust over the 1.0‐ to 2.65‐μm wavelength range.

Chloride deposits on Mars, first identified by Osterloo et al. (2008), are of particular interest because they record the last surface water present at a given location on the surface, as subsequent water events would dissolve them. These light-toned, indurated deposits (<1 km 2 -∼1,000 km 2 ) are found throughout the southern highlands, have been previously described as ancient (Noachian to early Hesperian in age; ∼4.0-3.4 Ga), and are often found in local topographic depressions, sometimes with channel-like features (

Visible-shortwave infrared microimaging reflectance spectroscopy is a new technique to identify minerals, quantify abundances, and assess textural relationships at sub-millimetre scale without destructive sample preparation. Here we used a prototype instrument to image serpentinized igneous rocks and carbonate-rich travertine deposits to evaluate performance, relative to traditional techniques: XRD (mineralogical analysis of bulk powders with no texture preservation) and SEM/EDS (analysis of phases and textures using chemical data from polished thin sections). VSWIR microimaging spectroscopy is ideal for identifying spatially coherent rare phases, below XRD detection limits. The progress of alteration can also be inferred from spectral parameters and may correspond to phases that are amorphous in XRD. However, VSWIR microimaging spectroscopy can sometimes be challenging with analyses of very dark materials (reflectance <0.05) and mineral mixtures occurring at a spatial scales multiple factors below the pixel size. Abundances derived from linear unmixing typically agree with those from XRD and EDS within ~10%.

Key Points:• Machine learning can be highly effective in exposing tiny outcrops of rare phases in CRISM • A new hydrated iron oxide phase, elsewhere on Mars attributed to akageneite, is detected in NE Syrtis and Jezero • Al clays, jarosite, chlorite-smectite, and hydrated silica are reported in Jezero Abstract A hierarchical Bayesian classifier is trained at pixel scale with spectral data from the CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) imagery. Its utility in detecting rare phases is demonstrated with new geologic discoveries near the Mars-2020 rover landing site. Akaganeite is found in sediments on the Jezero crater floor and in fluvial deposits at NE Syrtis. Jarosite and silica are found on the Jezero crater floor while chlorite-smectite and Al phyllosilicates are found in the Jezero crater walls. These detections point to a multi-stage, multi-chemistry history of water in Jezero crater and the surrounding region and provide new information for guiding the Mars-2020 rover's landed exploration. In particular, the akaganeite, silica, and jarosite in the floor deposits suggest either a later episode of salty, Fe-rich waters that post-date Jezero delta or groundwater alteration of portions of the Jezero sedimentary sequence.