A Geologic Study of Three Types of Sulfates within Aram and Aureum Chaos, Mars

We are conducting a coordinated effort to investigate the sulfate-bearing deposits within Aram and Aureum Chaos where three types of sulfates have previously been identified. Using improved Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) image processing to produce Map-Projected Targeted Reduced Data Record (MTRDR) images, we have mapped out the occurrences of polyhydrated sulfates (PHS), monohydrated sulfates (MHS), and ferric hydroxysulfate outcrops (FHS; Fe³⁺SO₄OH) beyond what was mapped previously. High Resolution Imaging Science Experiment (HiRISE), Context Camera (CTX), High Resolution Science Camera (HRSC), and Colour and Stereo Surface Imaging System (CaSSIS) images were used to map and characterize the morphologies and locations of sulfates within these two chaos regions. We then used these morphologic characteristics for each type of sulfate where CRISM targeted coverage existed to extrapolate the sulfate occurrences within both chaos regions outside of the CRISM images. Digital Terrain Models (DTMs) made from HRSC, HiRISE, and CTX stereo pairs were used to establish stratigraphic relationships between the different sulfates and determine thicknesses of deposits. We also mapped out the distribution of a spectrally bland, crenulated, layered, light-toned caprock unit within both chaos regions because it is found in association with the sulfates. 

Our results for both chaos regions reveal that the PHS-rich units are always stratigraphically above the MHS-rich units where they occur together. We found that the PHS unit is typically smooth, darker-toned, and contains several layers of meter-scale thickness forming broad plateaus that are commonly covered by ripples and debris. Steeper cliffs along specific layers within the PHS unit correlate to brighter, jagged, and heavily fractured outcrops. Spectra of the PHS unit have bands near 1.44 and 1.93-1.95 µm and a drop in reflectance near 2.42 µm, similar to spectra of rozenite (Fe²⁺SO₄•4H₂O) and starkeyite (MgSO₄•4H₂O). In contrast, the MHS is brighter and rougher with scalloped fracturing, and can be either massive or very finely layered, with the finely layered material typically exposed beneath the overlying crenulated caprock. The MHS spectra are consistent with szomolnokite (FeSO₄•H₂O) and kieserite (MgSO₄•H₂O). We also detected kieserite in dark-toned talus and eolian ripples adjacent to or mantling MHS outcrops, indicating erosion of the outcrops produces finer-grained kieserite-bearing debris, but why the debris is darker than the outcrops from which it is derived is unknown. Interestingly, some of the brightest and clearest outcrops within both chaos regions had some of the weakest sulfate signatures in CRISM images, suggesting other factors are influencing brightness besides the presence of sulfates. The FHS, which is spectrally characterized by a strong absorption at 2.236 µm, occurs in smaller, patchy outcrops along the lower-lying floors of both chaos regions and it generally appears dark-toned and heavily fractured with ridges sometimes present. The FHS is always adjacent to or directly beneath the MHS, and is also associated with chaos terrain where it typically occurs along the chaos block edges.

Recent laboratory studies indicate that the FHS can only form by heating hydrated ferrous sulfates above 100°C [Bishop et al., 2025]. We find that the FHS is only associated with Mg-bearing MHS, perhaps because the Fe-bearing MHS was transformed to FHS due to heating, leaving behind the Mg-bearing MHS that is now seen from orbit. Moreover, the correlation between upturned and tilted chaos blocks and FHS is consistent with heating in the subsurface that caused conversion of the MHS to FHS, as well as the tectonic disruption that created and uplifted the chaos blocks. The geothermal heating could have arisen from intrusion of magma that never reached the surface but caused heating along the chaos floors due to their deeper depths. We did not find any FHS within Iani Chaos even though there are MHS in Iani that are much closer to Aram than those at Aureum. These observations suggest the subsurface heat source extended from Aram to the southwest where it reached northeastern Aureum rather than towards the southeast to reach Iani. Alternatively, heating could have reached portions of northwestern Iani Chaos but there were no hydrated ferrous sulfates present for conversion to FHS. Deposition of the sulfates within these chaos regions occurred in the Late Hesperian to Early Amazonian, implying the magmatic heating event that altered the MHS to FHS also took place relatively recently in martian history (<3 billion years ago). 

 

References: 

Bishop, J.L., Meusburger, J.M., Weitz, C.M. et al. Characterization of ferric hydroxysulfate on Mars and implications of the geochemical environment supporting its formation. Nat Commun 16, 7020 (2025). https://doi.org/10.1038/s41467-025-61801-2