When it comes to testing a controversial hypothesis, Paul Knauth opts for the “lions' den” approach, presenting his ideas at meetings and conferences. But by any standards, the talk he gave at a meeting about Mars in October 2004 was more than a little provocative. Knauth, a geologist at Arizona State University in Tempe, presented evidence suggesting that the large depression seen by the Mars Rover Opportunity was not evidence for a now-vanished body of water on the red planet.

Together with fellow Arizona geologist Donald Burt, Knauth thought that rather than the remains of an evaporated lake, the depression might be the result of a ‘base surge’ — a ground-hugging cloud of gas and ash — caused by either a volcanic explosion or an impact from a meteor.

So when the question-and-answer session at the Mars meeting began, Knauth took the plunge. “I stood up and gave an alternative view — it was one of the most stressful talks I ever did,” he says. “Of course a lot of people jumped up, very unhappy about it.”

But afterwards, several people not associated with the Rover project came up to him and said “right on”, he remembers. That motivated Knauth and Burt to call in Ken Wohletz, an expert in base surges based at Los Alamos National Laboratory in New Mexico. Together the three worked out the hypothesis that appears on page 1123 of this issue.

The trio concentrated on minerals such as the sulphate deposits detected in the depression on Mars. The Rover team explains these deposits in terms of an acidic lake evaporating to leave the mineral. But the three geoscientists felt that other minerals present in the Mars basin did not square with this explanation.

They also noted that the Rover team talked about acidic weather on Mars — another factor that seemed wrong to Knauth. “If it were true, there would be clay minerals all over Mars,” he says. Acid rain occurs when sulphur dioxide is emitted into the atmosphere, perhaps from a volcano, and mixes with water vapour. When the rain falls on basalt it creates sulphate deposits. But clay is a by-product of the process, and Opportunity has so far produced no evidence of this.

Instead of a dried lake, Knauth and his colleagues think that the depression is more like a desert — although that doesn't mean that they say there is no sign of water on Mars. Small amounts of water would have been needed to redistribute the chemistry after the base surge and to help create the small iron-rich spherules that litter the site.

Field trips to the sites of base surges on Earth convinced Knauth and his colleagues that their hypothesis had merit. They were amazed by the geological similarities between the sites and the martian landscape. “We felt like we were walking around on Mars,” Knauth says of one place in New Mexico.

Knauth knows that he, Burt and Wohletz are “outsiders” in the Mars field, but he feels that they can play an important role in the scientific dialogue. Several Mars researchers have welcomed their input, even though they disagree with the hypothesis. Part of the reason the Rover team put its data and images online is so that other scientists could weigh in and discuss the findings — whether to agree or disagree. “And here we come,” says Knauth.