GUSEV CRATER POSTMORTEM: ONGOING EXPLORATION FIVE YEARS AFTER SPIRIT

The Spirit rover conducted operations from January 2004 until March 2010, extending its primary mission lifetime by a factor of ~25 and drive distance by >10 times its original specification. Gusev crater was chosen as a landing site because of evidence in orbital images for an ancient lake and the desire to investigate a potentially habitable environment. The crater floor was found to be covered by olivine-rich basalts that are now recognized and mapped as Early Hesperian volcanic (eHv; formerly Hr, Hesperian ridged plains), one of the more ubiquitous terrain types on Mars. This result was evident prior to Spirit’s arrival, but apparently not recognized. A kipuka rising from the plains flood basalts, dubbed Columbia Hills, was reached by Spirit and found to contain a remarkably diverse set of rocks representing a range of primary and secondary geologic processes. Among these, the plagioclase-rich clastic rocks known as Wishstone class now are recognized as the precursor to a style of alteration not apparent on Earth in which isochemical processing results in an amorphous mineral(s) overprinting.

More profound evidence for the role of water was found in the form of outcrops known as Comanche that are rich in Mg-Fe carbonates (16-34 wt%) and outcrops and soil composed of nearly pure opaline silica. The carbonates initially were suggested to be the result of hydrothermal dissolution, transport, and re-precipitation of pre-existing carbonates from subsurface carbonate-bearing rocks. A recent alternative hypothesis demonstrates that water-limited leaching of formerly widespread Algonquin-like tephra deposits by ephemeral lake water, followed by transport and evaporative precipitation of the fluids into the Comanche outcrops, can explain their chemical, mineralogical, and textural characteristics. The opaline silica occurrences adjacent to the “Home Plate” feature initially were thought to result from acid-sulfate leaching of basaltic precursor rocks by fumarolic steam condensates. But subsequent work showed the viability of a geyser/hot spring model in which the silica formed as a primary precipitate. New observations from a geyser field in the Atacama Desert now provide robust supporting evidence for this model and even reveal indications of potential biosignatures in the Martian silica.