Introduction to special issue

Opportunity Runneth Over

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Science  03 Dec 2004:
Vol. 306, Issue 5702, pp. 1697
DOI: 10.1126/science.306.5702.1697

Actually, the Opportunity rover never did run across the martian surface; it was more like a stop-and-go boogie at an average crawl of 3.6 meters per hour. Nor did the mobile rover go very far in its first 56 sols and 56 nights, spending most of its sunlit working hours in the 20-meter-diameter Eagle crater that it serendipitously bounced into. Still, it was the hottest rover on six wheels, on this cold and desolate planet, from the moment that it put up its mast and scanned the broken but distinct outcrop that rimmed part of its landing site. Never before had a slab of martian bedrock been seen or touched by robotic inquisitiveness, including NASA's successful predecessors: Spirit from the Mars Exploration Rovers (MER) mission, Sojourner rover from the Pathfinder mission, and the landers from the Viking 1 and 2 missions. Here within Meridiani Planum, just meters from the landing pod, was a layered rock, bent and fractured by impact events, but still partially intact and providing a history of environmental changes.

Opportunity turned a trickle of in situ evidence for liquid water on Mars into a flood. After the overview of the first 90 sols of the mission by Squyres et al. (p. 1698), the next eight papers describe in some detail the fine-scale layering, the hydrated minerals, the enrichment of sulfur, and the salty surfaces that suggest that the layered rock formed by repeated cycles of flooding, evaporation, and desiccation or erosion. Opportunity also found the source of the hematite (Fe2O3) that was remotely sensed over hundreds of kilometers by the Thermal Emission Spectrometer (TES) on the Mars Global Surveyor orbiter. The hematite is mostly concentrated in millimeter-sized spherules, initially called blueberries by the MER team. The spherules are embedded in the layered rocks and distributed as loose particles on the plains. These plains probably represent a lag deposit: a thin layer of larger particles left over after disaggregation of the rocks and removal of smaller particles by the wind. The embedded spherules are randomly oriented relative to the layering, which suggests that they formed later by secondary alteration. The last two papers describe the composition, opacity, and dynamics of the martian atmosphere measured with the instruments on Spirit and Opportunity.

In his Perspective (p. 1689), Kargel offers one scenario to explain the bedrock. There are other possible scenarios; however, they all require liquid water and acidic conditions. Both MER rovers are still collecting data almost a year later, and Spirit has recently found some bedrock too. The flood of new in situ evidence for liquid water allows for the possibility of microbial organisms living, perhaps transiently, when the acidic brines were active, like terrestrial acidophiles. Opportunity's run is definitely not over yet.

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