NASA's intrepid rover Opportunity spent two years exploring the geology of Mars' Victoria Crater, often perched perilously on the crater's edge.
The results of that effort are now detailed in Friday's issue of the journal Science. They've given scientists a clear view of some of the processes that have sculpted the Martian surface, including evidence that water shaped much of the entire region where the crater is found.
Another separate study, detailed in Thursday's issue of the journal Nature, looked at the question of how liquid water might have formed on early Mars. The research shows that even though the planet's surface temperature could have been well below freezing, water might still have flowed there if enough minerals were dissolved in it.
Opportunity, trundling across the Martian surface for more than five years now, arrived at Victoria crater 952 Martian days into its journey on the planet.
The rover had previously explored the Eagle and Endurance craters, about 3.7 miles (6 kilometers) away from Victoria. Mission scientists chose Victoria as the next crater to explore because "it was the biggest crater we could possibly find," said Steve Squyres of Cornell University, and the lead scientist for the Mars Exploration Rover Project.
The science team hoped that Victoria's depth — of about 400 feet (120 meters) — might shed more light on the geology of the Meridiani Planum region, an area about the size of Oklahoma.
Scientists had already seen distinctive sediment layers in Endurance, as well as evidence that water had been an active ingredient in shaping those layers. But Victoria offered a different perspective of the region because it is deeper than Endurance and sits slightly higher, offering a chance to peek at different layers of the ground.
During its two-year expedition there, Opportunity drove to the edges of promontories along Victoria's serrated edge, allowing it to look across to the next ridge and image the layers of sediment. The rover also drove down into the crater to get a look at what was inside.
What it found were layers of the same sulfate-rich sandstone that Opportunity had found at Endurance, with evidence that water had weathered away minerals in the rocks, then evaporated, leaving behind salts that eventually solidified into rock once again.
The findings show that water acted at both the Endurance and Victoria sites, suggesting that it did so across the whole Meridiani region. Squyres told Space.com that this was probably "the most significant finding" of the Victoria expedition.
Opportunity also found "gorgeous, striking evidence" of dune structures preserved in the rocks, Squyres said. There had been some signs of dunes at Endurance, but "Victoria really enabled us to nail that problem," he said.
While driving up to the crater and around it, the rover also saw clusters of meteorites that Squyres and the other mission scientists think could be fragments of the impactor that created the crater.
'A whole new mission'
With its exploration of Victoria completed, Opportunity is already on its way to its next target, Endeavour Crater, 7 miles (11 kilometers) away.
"We're hoping to find some very different things there," Squyres said.
Endeavour is a much older crater than Victoria and Endurance. The later craters were blasted into the Martian surface after the sediment that Opportunity examined were put in place, "so what lies beneath [those layers] is still a mystery to us," Squyres explained.
The impactor that created Endeavour crashed into Mars before any of those sediments were laid down, so scientists are hoping it will give Opportunity a look at the bedrock that lies underneath the Meridiani Planum region. Essentially the trip will be "like a whole new mission," Squyres said.
Cold and wet
While Opportunity and other Martian probes have found ample evidence that liquid water once flowed on the Red Planet, explaining how the water became liquid is a trickier prospect.
Mars is a very frigid place, with surface temperatures well below the freezing point of water. Geologists suggest that the features they see were created by flowing water at a time when Mars was warmer than it is now. But climate modelers can't add enough carbon dioxide (a planet-warming greenhouse gas) to their Mars atmosphere models to get the temperature high enough to keep water from freezing.
The solution laid out in the new study, which also used computer models, suggests that salts dissolved in the water could have kept the water liquid by dropping its freezing point (much like road salt melts ice on roads and sidewalks).
"People have been speculating about that for years," said Giles Marion, a geochemical modeler with the Desert Research Institute in Reno, Nev. Marion has looked at this issue himself, but was not involved with the new study.
The study used chemical analyses done by Pathfinder, Viking, Spirit and Opportunity to see if the salts they observed in the Martian dirt could sufficiently lower the melting point of water. It worked — the salts could explain how water remains liquid on a freezing cold planet.
The new study adds to the support for this idea, said both Marion and Jim Head of Brown University, who was also not involved in the study. But whether ancient early Mars was warm and wet or cold and wet is "still an open-ended question," Marion told Space.com.