Did the Curiosity rover just encounter an ancient soil profile on Mars?

The Mars Science Laboratory onboard Curiosity rover has been traversing up the slopes of Mt. Sharp since landing on Mars in 2012. In late 2021, the rover entered a new region of Mt. Sharp known as the “Sulfate-bearing unit” because of the widespread orbital detections of sulfate minerals from orbit (Figure 1).

The sulfate-bearing unit has spectral signatures of hydrated sulfates and is estimated to be ~700 meters thick in the northwest portion of Mt. Sharp (Rapin et al., 2021). Large-scale (5-10 meter) trough cross beds in the lowermost unit are consistent with an eolian sequence. Stratigraphically above the basal unit, a possible deflationary surface (the “Marker bed”) is topped by what appears to be a fluvial depositional system, which is consistent with an alternating wet-dry climate regime for this unit, rather than a monotonic shift to arid conditions. Since alternating wet-dry conditions in terrestrial environments can lead to regional-scale subaerial weathering of sediments, it is possible that individual weathering profiles could have formed in the sulfate-bearing unit during periods of subaerial exposure.

Figure 1. Putative paleosol within the basal sulfate-bearing unit (LSu) at Gale crater (a-b) inferred from decimeter-sized concretions (c), dark-toned nodular beds (d) and sulfate-enriched polygonal mudcracks (e-f), adapted from Rapin et al., 2022

The identification of what appears to be the first paleosol at Gale Crater (Rapin et al., 2022) (Figure 1) is consistent with the alternating wet-dry-hypothesis for the origin of the sulfate-bearing unit. Polygonal mudcracks and sulfate nodules are common features of sulfate-rich lake margin soils and are also common in more developed smectitic (montmorillonite/nontronite-rich), shrink-swell soils, classified as Vertisols in US taxonomy. These observations now present a unique opportunity for comparisons with terrestrial weathering profiles containing similar features such as mudcracks and nodular sulfate minerals which together can provide a reference frame for evaluating a subaerial weathering hypothesis for the origin of altered sediments at this location.

We have submitted a proposal the the NASA SSW program to fund a terrestrial analog study to evaluate a pedogenic origin for features observed in the sulfate-bearing unit by examining sulfate-rich paleosols with rover-like instruments (Figure 2). Wish us luck!

Figure 2. Desert roses (A-D) and crystals (E-F) in hand specimens of terrestrial paleosols, and G-H, comparisons with concretions at Gale crater: A, clay pseudomorph of gypsum desert rose from By horizon of Thamberalg pedotype in Ediacaran (599 Ma) Ranford Formation, Donkey Creek, Western Australia; B, silica pseudomorphs of gypsum desert roses in By horizon of Muru pedotype from Ediacaran (547 Ma) Ediacara Member in Brachina Gorge, South Australia; C, barite desert roses from Oligocene (20 Ma) Rockenberg Formation near Rockenberg, Germany; D, barite desert roses from By horizon of pedotype from Early Permian (270 Ma) Garber Sandstone near Cimarron City, Oklahoma; E, silica pseudomorphs of mirabilite in A horizon of Viku pedotype from Cryogenian (640 Ma) Reynella Siltstone Member Hallett Cove, South Australia; F, ripidolite pseudomorphs of kieserite in By horizon of Isi pedotype from Archean (3700 Ma) Isua Greenstone near Isukasia, Greenland; G, Mastcam image (Sol 1277) showing concretions from the Murray Formation; H, Mastcam image (Sol 3396) showing concretions and evaporite pseudomorphs from the clay-sulfate transition. Terrestrial specimens in Condon Collection of Museum of Natural and Cultural History, University of Oregon are (a), R4185 (b), F6345 (d), field photographs (c, e-f).

Rapin, W., Dromart, G., Rubin, D., Deit, L. Le, Mangold, N., Edgar, L. A., et al. (2021). Alternating wet and dry depositional environments recorded in the stratigraphy of Mount Sharp at Gale crater , Mars. Geology, 49(7), 842–846.

Rapin, W., Sheppard, R., Dromart, G., Schieber, J., Kah, L., Rubin, D., et al. (2022). The Curiosity rover is exploring a key sulfate-bearing orbital facies. Lunar and Planetary Science Conference, 2473.

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