Because of the rover's unique mobility, scientists wanted interesting landing sites with less than ten to twenty percent rock coverage. Rubble was trouble. But few anticipated what all that flatness would finally deliver - not only for easy movement of their mobile laboratory, but also for its remarkable science returns.

The rover's story, as detailed in the eleven Science papers this month, is mainly one about water and salt.

But a thread of this adventure includes the physical landscape itself. It is thus surprising when the flattest spot - Mars without the rubble - served up pictures of the first bedrock found on another world.

On earth, bedrock is common in northern New England, particularly Maine and New Hampshire, the Granite state. But the wind blows around enough dry dust on Mars to cover what might be exposed bedrock. This debris layer blankets most of the rest of the planet.

Additionally, meteors have pulverized the martian surface leaving a thick crushed layer. The thing about bedrock is it is stationery: always has been, always will be, at least as far as knowing its original location.

Why this is important follows from the preserved geochemistry that such bedrock shows. Bedrock marks the spot, just because its native location is known. Bedrock at Meridiani thus becomes the important control experiment that was missing from previous expeditions.

When geologists see bedrock, then they know they are not studying rocks carried from another place on Mars. So the first surprise for Opportunity was finding bedrock.

Along with bedrock came sediments, the delicate layering of time-tagged deposits. Repeated cycles of wet and dry periods can shape these layers into an intricate record of the planet's history. Such fine laminations also showed rippling effects at the Opportunity site.

Although not exactly like a shoreline, these thick and thin beds could span up to a meter and may have arisen from wind-driven water flow in the distant martian past.

One of the great challenges of doing this kind of remote geology is the lack of an obvious time scale. The marker between one rhythm to another is unknown.

It may not be annual. But if an inferred timeline presents itself, just as yearly tree rings are used to date Earth events, then the layers of sediment tell an important story.

So far, the timeline at Meridiani goes something like this: 600 meters of sediments may have formed in about 250,000 Mars years (or sols, each about 600 days or two earth-years).

This doesn't mean that water flowed on Mars a half-million earth-years ago, because there is no obvious way to know when any rhythmic cycle might have started. But simply guessing at the persistence of water and sedimentary rocks hints at a much warmer and wetter climate than today's.

It would be remarkably useful to know the start and end dates. When did water stop flowing on Mars? No one knows at the moment.

After finding bedrock and sediments, Opportunity allowed scientists to start telling the martian story using its many instruments. One chapter that continues to reveal surprises is the red planet's geochemistry. What are the rocks and sediment made of?

One thing the team was looking for was minerals that could dissolve in water. On a dry Mars, the salts will be all that remains after the water leaves.

Examples of soluble salts include traces of chlorine and bromine. Both were found at Meridiani Planum, along with very high concentrations of sulfates. So soon after the instruments looked closely at the Opportunity landing site, the surrounding soil, rocks, and outcrops were found to be salty.

The story was different on the other side of the planet. The Spirit rover had found something more shaped by lava than brines. A rubble field of volcanic rocks and inorganic soils was found there, at Gusev crater.

The Spirit rocks were harder to drill and grind than those at Opportunity, since the rubble was strewn across Gusev crater from past lava flows and meteor strikes. If Opportunity's site was like a salt-flat, then Spirit's site was like an old lava field.

One can ask the question: what would a Mars rock feel like? Would it be heavy or light? Would it crumble when squeezed?

In imagining the texture of the rocks found by the Opportunity rover, the mission team compared them to a spongy sandstone.

They were pockmarked, porous, dried and cracked. The voids and holes in these spongy rocks may have arisen from repeated cycles of evaporation to harden the surfaces followed by a washing away to dissolve the more soluble interior portions.

So the story Opportunity told was figuratively a 'wash-out' - however, the role of evaporation had not yet been clarified and another piece of puzzle was falling into place.

Where evaporative layers might deposit chlorine or bromine, the heavier, less soluble element (chlorine) was found on the bottom layers.

Such dried beds are typically separated like this, since more concentrated brines come later in the drying and by then, only the most soluble salts remain as the solution evaporates, thickens and concentrates.

A final clue was detection of the mineral jarosite, which requires highly acidic brine. Taken together, the chemistry of Meridiani was rich in sulfur, iron and magnesium, while also stratifying deposits in a way consistent with evaporation and acidity.

If a sample of this composition was sought on Earth, the most likely candidate would be water draining from acid mines. These mines would be full of polluted areas, turned orange and red from rusting washouts of iron sulfide ores.

Mars, was it the rusting world? Prior to this mission, the Meridiani plains were compared to the Rust Belt states, those in the middle north of America (Michigan, Ohio, Pennsylvania). The other comparison was to the red dirt found in Oklahoma and northern Texas - the so-called Red River region.

"It's an incredibly rich data set," principal investigator for both science missions, Steve Squyres told Astrobiology Magazine.

"The data from the first 90 sols of both landing sites is now out there in the Planetary Data System, and anybody can access it and do science with it. To a certain extent, it's going to be very satisfying to just sit back and watch people do science with this data. We worked very hard to build these rovers, and we've worked very hard to collect the data."

What does this synopsis hint at for habitability? One thing the Viking probes found in the 1970's was Mars is rusting. Indeed the soil was considered highly reactive and oxidizing with the corrosive strength of hydrogen peroxides.

The challenge for life at Meridiani is daunting. To survive requires tolerance for extreme conditions: supercold, salty and acidic. While individually not outside the bounds of Earth organisms, the biological hurdle is a challenging one. A supercold world of acidic brines may have once been Mars.

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