— Much of Mars’s water may still be present in the form of buried ice - this region near the equator that may consist of jumbled blocks of ice beneath a shroud of dust (Image: ESA/DLR/F U Berlin/G Neukum)
A winding canyon called Nanedi Vallis seen here in an image from NASA's Mars Odyssey spacecraft is one of many features on the planet’s surface that suggest past water flow (Image: Malin Space Science Systems/NASA)
An ocean of water once wrapped around Mars, suggests the discovery of soil chemicals by NASAs rovers. But the same chemicals also indicate that life was not widespread on the planet at the time the ocean was present.
Sulphates, which form most readily in liquid water, had already been detected by the Spirit and Opportunity rovers. The minerals have been interpreted as evidence for past bodies of water on the surface. But it has not been clear how large these bodies of water might have been.
Now, a new analysis of rover data suggests that the sulphates were once dissolved in a planet-wide ocean. The study was carried out by James Greenwood of Wesleyan University and Ruth Blake of Yale University, both in Connecticut, US.
The researchers point out that phosphates, which are also linked to water, are also present at both sites. More importantly, the ratio of phosphates to sulphates is about the same at both locations. They say the most likely explanation for this is that any local variations were smoothed out by mixing in a planet-wide ocean.
Some researchers argue that a broad, flat area in Mars's northern hemisphere is the relic of an ancient ocean, and point to rock weathering that could have been caused by seawater. But the uniform phosphorus-to-sulphur ratio is the first chemical evidence that such a large body of water might have once existed.
The phosphorus was probably leached from rocks in the form of calcium phosphate, the researchers say. The fact that it appears to have been dissolved and mixed with sulphates in large amounts suggests that the hypothesised ocean must have been very acidic, because calcium phosphate only dissolves well in acidic water.
A phosphorus-rich ocean is a bad sign for past Mars life. Phosphorus is an important element for life on Earth, and is quickly extracted from the environment by organisms. If life were extensive on Mars, it would not have left so much phosphorus dissolved in the water, the researchers say.
"To a first order approximation, you couldn't have had a biosphere that was anything like the one on Earth," Greenwood says.
Lots of lakes?
Michael Wyatt of Brown University in Rhode Island, US, says the idea of a past ocean on Mars fits well with the phosphorus and sulphur data, but adds that several smaller bodies of water might also explain it.
"It's kind of hard to pin down smaller bodies of water versus one large ocean," he told New Scientist. Another possible explanation for the data would be many bodies of water that have similar chemistry, he says.
The researchers admit that the similar phosphate-to-sulphate ratio seen on opposite sides of the planet could also arise if wind mixed these materials together after the bodies of water disappeared.
Journal reference: Geology (vol 34, p 953)