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Opportunity, Meridiani, and the Search for Life on Mars


IPGP - Campus Jussieu


Séminaires généraux de l’IPGP

Salle Bleue

Andrew KNOLL


Abstract: On January 24, 2004, the Mars rover Opportunity landed on the Meridiani plain of equatorial Mars, drawn by the high hematite content of the plains surface, as observed from orbit. Early in its mission, Opportunity established that the Meridiani region is underlain by sedimentary rocks, best exposed in crater walls. These rocks contain several lines of evidence for liquid water at the time the regional succession accumulated. First, sand grains in outcrop rocks contain a mixture of silicate and sulfate minerals formed during aqueous alteration of parent basalts. The physical stratigraphy of a 7+ m stratigraphic section in Endurance crater is dominated by meter-scale eolian cross-beds and eolian sheets sands, but cm-scale festoon cross beds at the top of the column record local reworking by water. And hematite concretions, sulfate cements, and secondary porosity formed by the dissolution of diagenetically emplaced euhedra all record groundwater activity in Meridiani sands. Despite this evidence for water, the Meridiani region was arid, acidic and oxidizing at the time its exposed stratigraphy developed. Recent analyses of Meridiani data illuminate properties of its ancient water that bear on questions of habitability. Strong acid, recorded by the hydrated iron sulfate mineral jarosite, is compatible with life, as shown by the relatively diverse microbiotas of acid mine drainage sites of Earth. Water activity, however, is another story. As brines percolated through Meridiani sediments, they became highly saline. Only a handful of terrestrial organisms are known to grow at water activity values comparable to those calculated for Meridiani brines at the point where Mg-sulfate minerals precipitated. Sodium chloride in Meridiani rocks would have precipitated at a water activity well below anything known to support life on Earth. Studies of late stage alteration, developed on rock exteriors since the current plain surface developed, indicate that water has been uncommon in the Meridiani region during the past several hundred million years. Overall, Opportunitys discoveries suggest that by the time Meridiani beds formed, ca. 3.5-4.0 billion years ago, Earth and Mars had gone down different environmental paths. On Mars, the window for life on the planetary surface was probably closing. If it was ever open, the habitable window extended through the first 500 million years or so of martian history. A spectroscopic signature of clay minerals, identified by the OMEGA instrument on the Mars Express orbiter, suggests that the oldest martian terranes differ from Meridiani in ways that are consistent with a more hospitable surface environment. Examples of these oldest terranes are accessible to the Mars Science Laboratory, scheduled for 2009 launch. -------------------------------------------------------------------------------- Séminaires généraux de l'Institut de Physique du Globe de Paris --------------------------------------------------------------------------------