Gusev Crater, one of the most explored sites on Mars, is back to being thought of as the site of an approximately 4 billion year old lake. The question of whether Gusev once held a large body of water has bedeviled scientists since Spirit landed there, and thinking on the topic has swung backwards and forwards several times over the last decade. Consequently, the latest paper in Geology may not be the final word, but for the moment it is what most of us want to hear.
Even before Spirit landed there Gusev was thought of as a likely place to have once held water. What looks like a meandering river breaches its southern rim, and this was one of the main reasons the site was chosen to host NASA's precious explorer.
However, as Spirit began to move it found it was rolling over volcanic rocks, rather than the sedimentary ones it was seeking. But despair turned to hope again as Spirit slowly rolled into the Columbia Hills five months into its exploration. There Spirit found hematite, a rock that is only formed in water, but still no lake sediments. This suggested that maybe Mars had once had liquid water, but not nearly enough of it to fill the crater. An alternative theory, that the hematite formed in hot volcanic springs, started to win favor.
The issue is not just that scientists are really keen to get as close as possible to Martian sedimentary rocks, important as that is. The question of whether Mars' “Noachian Period” is aptly named lies at the heart of our efforts to understand the red planet. Water erosion is the most popular explanation for the fact that craters 3.7 billion years old on Mars are much more worn down than those even a few hundred million years younger. Nevertheless, it is not the only one, and even if water bodies existed we don't know how large or frequent they were. Resolving this is central to subsequent Martian geology,
In 2010, while studying the Columbia Hills outcrop Comanche, Spirit found a rich load of magnesium-iron carbonate minerals. While also indicative of water, these too were ascribed to hydrothermal activity.
However, just as Spirit kept on coming back from every obstacle that seemed set to end its mission, the Gusev as lake theory will not lie down. Professor Steve Ruff of the University of Arizona, one of those who identified the minerals at Comanche has reconsidered.
"We looked more closely at the composition and geologic setting of Comanche and nearby outcrops. There's good evidence that low temperature surface waters introduced the carbonates into Comanche rather than hot water rising from deep down," says Ruff.
Ruff's explanation for Comanche goes like this: The Columbia Hills and surrounding areas were once covered in volcanic ash to form what is known as tephra. Then the crater flooded, presumably through the southern valley. The lake evaporated in the manner of desert lakes on Earth, and carbonate residues were left behind. This probably occurred multiple times, laying down multiple carbonate layers, like the salt pans that are common in dry terrestrial locations.
"The lake didn't have to be big," Ruff explains. "The Columbia Hills stand 300 feet high, but they're in the lowest part of Gusev. So a deep, crater-spanning lake wasn't needed." Meanwhile, “The wind has eroded most of that deposit, also carrying away much of the evidence for an ancient lake." In this view Comanche is one of the last remnants of both the tephra and the carbination.
Spirit lived up to its name, battling on for more than 20 times its original planned mission, but it fell silent shortly after examining Comanche. Future missions to Mars will be highly tempted to boldly go where no droid has gone before, but Ruff thinks these are the rocks we're looking for.
"Going back to Gusev would give us an opportunity for a second field season there, which any terrestrial geologist would understand," Ruff argues. "After the first field season with Spirit, we now have a bunch more questions and new hypotheses that can be addressed by going back."
It is hoped that future missions will collect samples for return to Earth, where they can be studied in a manner no rover can match. "Scientifically and operationally it makes sense to go to a place which we know has geologically diverse -- and astrobiologically interesting -- materials to sample," Ruff argues. "And we know exactly where to find them."