BioEd Online

Was there once a sea in Mars' Meridiani Planum? Probably not, according to a new model of water flow on the red planet.

The Martian rover Opportunity found lots of mineral salt deposits on the bottom of the Meridiani Planum, just south of the planet's equator. The favoured explanation for this was that a broad sea or series of ponds had once covered the entire region, leaving deposits behind as the water slowly evaporated. But because there isn't a large basin in that area in which so much water could have collected, researchers have been left perplexed.

Nown Jeffrey Andrews-Hanna and colleagues at Washington University, St Louis, Missouri, propose an alternative theory: the water swelled up from underground, pushed up by the formation of a gigantic volcano, and evaporated bit by bit as it reached the surface. Their work is published today in Nature¹.

Forced up

The team used a whole-planet hydrological model to see what would have happened to the planet's groundwater as the giant volcanic Tharsis region formed, which is thought to have happened some 3.8 to 3.5 billion years ago and deformed an area of 30 million square kilometres, dominating Mars's western hemisphere. Their conclusion is that the volcanic eruption would have forced up groundwater, and the most likely place for it to have burst forth is smack where the purported 'sea' once was. "Meridiani Planum just jumped out," says Andrews-Hanna.

The result provides a much-needed mechanism for how those salts appeared, says John Murray of the Open University in Milton Keynes, UK, who spotted a frozen lake on a different region of Mars in 2004. "The sea would have had to have been 8 km or so in depth. That's a huge amount of water," he says, and there is no obvious evidence of a deep seabed in this area.

Andrews-Hanna says that his models add weight to theories that early in its history Mars' water cycle was dominated by rain and erosion, but then it dried out and groundwater dominated the hydrology of the planet.

The hypothesis can now be tested with data from current and future missions, says Alfred McEwen, principal investigator on the High Resolution Imaging Science Experiment (HiRISE).

These underground processes could still be happening today, suggests Andrews-Hanna — although the harsh cold climate makes it less likely that water would break through now. And if water is still flowing underground today, it might just be possible that it could support life, Andrews-Hanna suggests. The quest continues.