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Spirit and Opportunity find the chemistry of water

Built for 90 sols apiece, Spirit lasted six years and Opportunity fourteen. Within eight weeks Opportunity had jarosite and hematite spherules: rock soaked in acidic water.

A microscopic image of small grey spheres scattered across fine-grained Martian soil.
Hematite-rich spherules, nicknamed blueberries, in the soil beside Stone Mountain, part of the rock outcrop at Meridiani Planum. Opportunity's microscopic imager made this mosaic; the spherules are concretions that grew inside the rock as groundwater moved through it. · Image: NASA/JPL/Cornell/USGS

Spirit landed in Gusev Crater on 4 January 2004 UTC, Opportunity in Meridiani Planum on 25 January. Its airbags dropped it into a twenty-two-metre crater named Eagle Crater, a few metres from exposed bedrock — the first outcrop a Mars mission could drive to and touch. Steve Squyres called it a hole in one.

At that outcrop, El Capitan, the Mössbauer spectrometer identified jarosite, a hydrated iron sulfate that forms only in acidic, sulfur-rich water; the microscopic imager showed millimetre-scale hematite spheres — "blueberries" — weathering out of it. The rock was cross-bedded in curving festoon shapes, which form under flowing water, and riddled with voids left by dissolved crystals. NASA announced on 2 March 2004; the papers ran in Science that December. Meridiani had been flooded, evaporated and dried repeatedly: habitable in principle, though strongly acidic.

Gusev was chosen because a vast channel drains into it: the floor ought to have been an old lake bed. It was basalt. Spirit crossed volcanic plains for five months to the Columbia Hills, where it found altered, hydrated rock. Its right front wheel seized in 2006, and it drove backwards afterwards, dragging the wheel like a plough; in 2007 the furrow turned up almost pure silica, the signature of a hot spring or fumarole, which on Earth swarms with life.

Both were built for 90 sols and about 600 metres of driving. Spirit managed 7.73 kilometres in six years, until on 1 May 2009 it broke through a crust into soft sulfate-rich sand at Troy, could not tilt its panels toward the winter sun, and stopped transmitting on 22 March 2010; NASA gave up listening on 25 May 2011. Opportunity drove 45.16 kilometres over fourteen years and four months, finding clays and gypsum veins laid down by neutral water at Endeavour Crater's rim, until a planet-wide dust storm ended it in June 2018. Gusts and dust devils kept sweeping the panels clean, stretching a 90-sol power budget into years.