Expert: Mars Mineral Not Proof of Water

Nov. 9, 2006 — A Martian mineral touted as proof of past lakes on the Red Planet may be the result of a rather dry acidic fog instead, say geologists.

When the sulfur-rich mineral jarosite was found in layered rocks of an area known as Meridiani Planum by a Mars Exploration Rover, NASA experts were quick to say it was evidence there was once a shallow lake there, which left the mineral as a salt crust when it evaporated.

But new research into how jarosite coatings form on Earth rocks at the summit of the Kilauea volcano in Hawaii points to a much drier origin.

Noxious sulfur dioxide gas is constantly spewing from Kilauea's Halemaumau crater, report University of California at Davis geologist Peter Schiffman and his colleagues in the November issue of the journal Geology. The gas sticks to the ground downwind of the crater in the Kau Desert as a sulfuric acid fog.

There the volcanic rocks are coated with a thin mineral layer that includes jarosite.

Schiffman and his team simulated the unusual chemical circumstances of the Kau Desert rocks and found that the jarosite probably forms when the volcanic acid fog soaks the surfaces of the rocks, then evaporates away, leaving behind volcanic minerals that react with the sulfuric acid and create the jarosite coating.

The jarosite is just one piece of evidence among many that scientists use to build the case that Mars was once a wet planet, but it was held up by many as the most conclusive to date. Schiffman's work calls that conclusion into question.

"While there is an increasing amount of evidence that suggests the existence of liquid water on the surface of Mars at some time in the past," reported Schiffman, "there is no a priori reason that standing bodies of liquid water must have existed at the time of jarosite formation."

Some researchers even believe that jarosite is evidence of a drying environment, rather than standing water. In other words, the Meridiani Planum jarosite could have been created in the hundreds of million of years after a very early wet period — billions of years ago — which might have been the only time Mars had any standing water.

"(The geochemical simulation) implies that the precipitation and persistence of jarosite is a better indicator of arid than wet environments," Schiffman reported.

It's even possible that the jarosite found on Martian rocks is also a Kau Desert-like coating, through the rovers can't magnify the rocks enough to say for certain, said James Greenwood, a Martian rock researcher at Wesleyan University.

The rovers identified Martian jarosite by its optical signature, rather than visually spotting the mineral crystals.

"I think it's a reasonable hypothesis," said Greenwood of the reasoning that jarosite came from a dry fog. "The only problem is at Meridiani there is a lot of sulfur" and at Kilauea almost all of the sulfur is trapped in the jarosite.

That might be a crucial difference between the two environments, and needs to be studied, he said.

The discovery of jarosite was touted as the evidence of past standing water on Mars because it's a mineral that contains oxygen-hydrogen pairs in its structure — which on Earth means water was somehow involved in the mineral's birth, Greenwood explained.

The good news is that the drier way to form jarosite doesn't preclude the possibility of life having once lived on the Red Planet.

"There are plenty of acidic organisms that live under acidic conditions on Earth," said Greenwood.