oining the Hot Papers in Physics at #3 is a spectacular contribution describing the first results from the Mars Exploration Rover mission. As I write, the duration of the rover Spirit’s mission is 780 sols (martian days), while Opportunity, the subject of #3, is on sol 759. Both rovers are almost 700 sols beyond their warranty periods. On December 12, 2005, Opportunity had its first birthday, on Mars, having spent a full martian year (670 sols) on the surface. Opportunity is part of NASA’s Mars Exploration Program, which seeks to use robotic and remote sensing technology to answer the question: was there ever life on Mars?

Of the many investigations for the Mars Exploration Program, the top priority is the possible presence of liquid water on Mars, either in its ancient past or preserved in the subsurface today. Water is key because almost everywhere we find water on Earth, we find life. Martian astrobiology asks: Did Mars once hold a vast ocean in the northern hemisphere, as some scientists believe? If so, how did the ancient water world transform to the dusty deserts of today?

Mars Odyssey, a remote sensing mission lofted in 2001, immediately detected enormous quantities of subsurface water in the polar regions by detecting the gamma rays emitted when cosmic rays collide with H atoms in the soil. Reinforcing this result, subsequent neutron spectroscopy showed that the buried water follows geographical features.

The surface and orbital missions currently "following the water" are striving to understand the present martian environment. Signs of subsurface water could come from hot springs or hydrothermal vents. Mission scientists want to explore features like dry riverbeds, ice in the polar caps, and rock types that only form when water is present. The later objective, interpreting a stratigraphic record to find conditions suitable for life, is what #3 is all about, and explains the stir it is causing.

Mission scientists, guided by thermal emission data from Mars Global Surveyor (MGS), had selected Meridiani Planum as a suitable landing site because it is rich in hematite, a mineral needing water for formation. The MGS result for Meridiani was a beacon: no other site came close as a former watery environment at the surface. Opportunity landed in tiny Eagle crater, a great stroke of luck. Its sister craft, Spirit, had landed on featureless lava and had to trundle over to Columbia Hills to start serious geology. Opportunity, by contrast, struck the mother lode in its very first image, which showed an outcrop of bedrock only yards away.

At the microscale level the outcrop has four constituents: coarse sand-sized grains, gray spherules embedded in rock, cement, and prismatic vugs. The elemental composition is extraordinary because of the high concentration of sulfur: nearly 25% by mass of the composition is SO₃, 20 times higher than in the basaltic rocks examined by Spirit. Opportunity’s Mössbauer spectrometer detected the ferric sulfate mineral jarosite throughout the outcrop. The concentration of Br in the rocks is highly variable. The spherules are rich in hematite, and differ in composition from the rocks in which they are embedded. Spherules are also strewn across the soil throughout the landing site, and they appear to be responsible for the strong hematite presence picked up by MGS.

Eagle crater is rich in stratification styles, with planar lamination, cross-bedding, cross-stratification, and undulatory lamination. Some geometries are signs of deposition of dunes; others point to sediment transport in aqueous ripples. Overall, the bedforms are consistent with a subaqueous origin. Team leader Steve Squyres (Cornell University) and his 18 associates note the evidence for diagenesis, involving fluid. The observed diagenetic features are consistent with formation in groundwater. Most conspicuously, the hematite spherules are interpreted as concretions formed within sandy sediments.

The inferred history of this outcrop is extraordinary for what it tells us about ancient Mars. At this location we have episodic shallow inundation by surface water, followed by evaporation and desiccation. The rock lamination is evidence for inundation by water, while the mineralogy points to dissolved salts. In other words, liquid water once flowed through the rocks, changing their composition and chemistry.