ROVERS AND ORBITERS REVEAL WIDE-RANGING AQUEOUS CONDITIONS ON MARS

Less than a decade ago, Mars was a world with much morphologic and little mineralogic evidence for liquid water. Gray hematite detected in a few regions might have formed in water (or not), and minor secondary minerals in Martian meteorites and soils lacked geologic context. A revolution began in 2004, with the landing of Spirit and Opportunity and the arrival of OMEGA on Mars Express. This presentation will cover a subset of the diverse aqueous settings inferred jointly by the Mars Exploration Rovers, OMEGA and other orbital instruments.

Recent alteration has been surficial and limited, but non-zero. Coated rock surfaces on the Gusev plains are enriched in sulfur relative to rock interiors, and the soil profile studied by Spirit at Troy suggests geologically recent mobilization of soluble sulfates. These data are consistent with orbital chemical and textural evidence for sulfate cements in Amazonian and older dust mantles across Mars.

Around the Home Plate volcaniclastic mound, Spirit found deposits enriched in ferric sulfates and/or opaline silica, consistent with hydrothermal alteration. Potentially analogous deposits identified from orbit in the Sisyphi Montes, Nili Patera caldera, and possibly Terra Sirenum suggest that volcanic hydrothermal environments may have been widespread during the Hesperian.

Sulfates constitute up to ~40 wt.% of the Late Noachian to Hesperian sedimentary rocks explored by Opportunity. Orbital results suggest that these hematite- and sulfate-rich evaporites formed commonly across Mars’s lower latitudes—e.g., in Gale crater. Columbus crater may show the result of similar chemistry in a deep lake: more crystalline secondary aluminosilicates are accompanied by a sequence of sulfates that evolves from less soluble phases (e.g., gypsum) to soluble salts as the brine concentrates.

Mg-rich carbonates were found by Spirit in the Columbia Hills and in soils, and by CRISM in Late Noachian outcrops near the Nili Fossae. Magnesite is the most soluble of the common carbonates, and one hypothesis for its origin in the Columbia Hills is via dissolution of older, buried carbonates. CRISM has indeed found Fe/Ca-carbonates that may be Early Noachian, although how much CO₂ was stored in such reservoirs remains unknown.

Collectively, these observations imply diverse aqueous conditions on Mars.