Are amorphous materials in Gale Crater “has beens”?

Session: Advancing Mineral Science and Exploring Planetary Surfaces: In Honor of MSA Dana Medalist, Elizabeth B. Rampe, Part I

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Abstract:

Perhaps one of the most important, but difficult to interpret, observations resulting from X-ray diffraction (XRD) analyses by the Chemistry and Mineralogy (CheMin) instrument on board the Curiosity rover has been the preponderance (~15-70%) of amorphous materials in all samples analysed at Gale Crater. Amorphous materials of similar age (i.e. billions of years old) have not been identified on Earth, implying that our continuously habitable surface environment has curtailed their longevity. Thus, the character of amorphous components on Mars may offer constraints on the duration of aqueous conditions in Gale Crater and on ancient Mars more generally.

Amorphous materials in Gale Crater could be either 1) formerly crystalline materials that became amorphous through subsequent alteration – in other words, “has beens”; or 2) materials that formed without long-range order and remained that way. We test the “has been” pathway by subjecting a Mars-relevant clay mineral (montmorillonite) to silica-poor solutions and analyzing changes to its crystallinity and chemistry. The alteration solutions ranged from an acidic sulfuric/hydrochloric acid solution (pH = 0) to a dilute, near-neutral (pH = 6.6) solution at high water-to-rock ratios. XRD analyses performed on samples exposed to the pH = 0 solutions for 67 days demonstrate a substantial loss of crystallinity, but those exposed to circumneutral pH solutions maintain crystallinity over these timescales. Analysis following subsequent, much longer-term exposure (ca. 1150 days) to these fluids (to be completed during summer 2025) will help to extrapolate to longer timescales. Considered alongside other experimental studies that demonstrate the conversion of other clays, basaltic minerals, and basaltic glass to amorphous silica, we conclude that alteration of Mars-relevant minerals – particularly by strong acids – can yield amorphous materials over geologically short timescales. However, if Martian amorphous materials are indeed “has beens”, their long-term preservation would require limited, subsequent interaction with circumneutral pH fluids, since subsequent water-rock interaction would promote crystallization. Ultimately, while amorphous materials in Gale Crater continue to resist simple explanations, their presence may be a key factor in improving our understanding of the arc of Martian habitability.