Discovery of alunite in Cross crater, Terra Sirenum, Mars: Evidence for acidic, sulfurous waters

Abstract

Cross crater is a 65 km impact crater, located in the Noachian highlands of the Terra Sirenum region of Mars (30°S, 158°W), which hosts aluminum phyllosilicate deposits first detected by the Observatoire pour la Minéralogie, L'Eau, les Glaces et l'Activitié (OMEGA) imaging spectrometer on Mars Express. Using high-resolution data from the Mars Reconnaissance Orbiter, we examine Cross crater's basin-filling sedimentary deposits. Visible/shortwave infrared (VSWIR) spectra from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) show absorptions diagnostic of alunite. Combining spectral data with high-resolution images, we map a large (10 km × 5 km) alunite-bearing deposit in southwest Cross crater, widespread kaolin-bearing sediments with variable amounts of alunite that are layered in <10 m scale beds, and silica-and/or montmorillonitebearing deposits that occupy topographically lower, heavily fractured units. The secondary minerals are found at elevations ranging from 700 to 1550 m, forming a discontinuous ring along the crater wall beneath darker capping materials. The mineralogy inside Cross crater is different from that of the surrounding terrains and other martian basins, where Fe/Mg-phyllosilicates and Ca/Mg-sulfates are commonly found. Alunite in Cross crater indicates acidic, sulfurous waters at the time of its formation. Waters in Cross crater were likely supplied by regionally upwelling groundwaters as well as through an inlet valley from a small adjacent depression to the east, perhaps occasionally forming a lake or series of shallow playa lakes in the closed basin. Like nearby Columbus crater, Cross crater exhibits evidence for acid sulfate alteration, but the alteration in Cross is more extensive/complete. The large but localized occurrence of alunite suggests a localized, high-volume source of acidic waters or vapors, possibly supplied by sulfurous (H 2 S-and/or SO 2-bearing) waters in contact with a magmatic source, upwelling steam or fluids through fracture zones. The unique, highly aluminous nature of the Cross crater deposits relative to other martian acid sulfate deposits indicates acid waters, high water throughput during alteration, atypically glassy and/or felsic materials, or a combination of these conditions.

How to Search for Life in Martian Chemical Sediments and Their Fluid and Solid Inclusions Using Petrographic and Spectroscopic Methods

“…Although no evidence of life has been found on Mars to date, several observations have suggested that past life may have been possible. Minerals that form on Earth from water, such as chlorides, iron oxides, and sulfates, including hydrated forms of such minerals, such as gypsum, have been found on Mars (e.g., Clark and Van Hart, 1981;Burns, 1987;Klingelhöfer et al, 2004;Clark et al, 2005;Osterloo et al, 2008;Farrand et al, 2009;Glotch et al, 2010;Ehlmann et al, 2016). Terrestrial life is found in the same places as terrestrial water; even the most extreme aqueous environments on Earth, such as acid brine lakes, hot springs, subglacial lakes, and deep ocean trenches, host life (i.e., Mikucki et al, 2010;Zaikova et al, 2018).…”

“…Terrestrial acid brines, such as modern acid saline lakes on the Yilgarn Craton in Western Australia and volcaniclastic-hosted acid salars in northern Chile, precipitate an unusual suite of minerals, including halite, gypsum and other sulfate salts, iron oxides, jarosite, alunite, kaolinite and other clay minerals, and opalline silica (e.g., Risacher et al, 2002;Benison et al, 2007;Story et al, 2010;Karmanocky and Benison, 2016). These same minerals are found in several places on Mars, including Meridiani Planum, Gusev Crater, and Gale Crater (e.g., Burns, 1987;Klingelhöfer et al, 2004;Madden et al, 2004;Clark et al, 2005;Bibring et al, 2006;Osterloo et al, 2008;Farrand et al, 2009;Glotch et al, 2010;Ehlmann et al, 2016). For these reasons, as well as for their thermochemical stability in Martian conditions, acid brines have been proposed as a likely past liquid on Mars (Burns, 1994;Clark, 1994;Benison and LaClair, 2003;Squyres et al, 2004;Benison and Bowen, 2006;Benison et al, 2008b).…”

Challenges in the Search for Perchlorate and Other Hydrated Minerals With 2.1‐μm Absorptions on Mars

“…Alunite is a hydroxylated aluminum sulfate mineral (KAl 3 (SO 4 ) 2 (OH) 6 ) which forms in acidic conditions, either connected to magmatic fluids or as a product of acidic alteration (Ehlmann et al, , and references therein). The detection of alunite and its discrimination from other phases relies on detection of an absorption at 2.17 μm, a doublet at 1.43 and 1.48 μm, and weaker absorptions at 1.76, 2.32, and 2.52 μm (e.g., Swayze et al, ).…”

“…The detection of alunite and its discrimination from other phases relies on detection of an absorption at 2.17 μm, a doublet at 1.43 and 1.48 μm, and weaker absorptions at 1.76, 2.32, and 2.52 μm (e.g., Swayze et al, ). We examined all three alunite detections currently reported in the peer‐reviewed literature: the CRISM‐type spectral region (Viviano‐Beck et al, 2014; Ehlmann et al, ), a second region from Ehlmann et al ()/Carter et al (), and the mixed alunite deposit in Wray et al (). For all three exposures, we found that alunite's diagnostic absorptions persisted in both ratioed I/F and ratioed radiance data (Figures S8–S10).…”

Global seasonal variations of the near-surface relative humidity levels on present-day Mars

“…Mars has been of key importance in the search for extraterrestrial life for decades now (Gargaud et al, 2011). During its geological history there were periods when the planet likely had lakes (Cabrol and Grin, 1999, Ehlmann et al, 2016, Fassett and Head, 2008, rivers or maybe even larger bodies of liquid water (Tokano, 2005). A recent discovery of organic molecules found at Gale crater provided further arguments that Gale crater was potentially habitable about 3.5 billion years ago (Kate, 2018).…”