Did water leave calling cards in a Martian crater?

For nine years, exquisite images of gullies on Mars have triggered a debate over the geophysical hand(s) that carves them.

Now, planetary geologists from Brown University have taken an up-close look at one of these gullies and the debris fan forming at its base. The verdict: liquid water did the deed. And it appears to have flowed episodically at the site within the past 1.25 million years.

You can find the formal report on their work here in the March issue of the journal Geology. A plain-English version appears here.

Liquid water, of course, is one of the necessary ingredients for organic life. That's why NASA's mantra for the past decade's worth of missions to Mars has been: Follow the water. On Earth, gullies form as water races down slopes, especially in deserts -- and Mars these days is nothing if not a planet-wide desert. So it's tempting to point to gullies on Mars and exclaim: Water did it!

Not necessarily, caution some scientists.

Some scientists point out that some of these fans may result from slumping.

Five years ago, for instance, Rutgers University's Tony Shinbrot and come colleagues calculated that at least some of the gully-and-fan formations could be the result of what, in effect, is too much debris gathering on a slope, then gently sliding down in a slow-mo landslide.

Others suggest that some gully fans form after a sudden outburst of groundwater that punches through a weakened spot in a crater or valley wall.

And it's been hard to figure out how old these gullies are. They clearly are young compared with the rest of the marscape because they aren't covered by any other features. But how young?

The hunt for gullies

Enter Samuel Schon, a grad student at Brown working on his PhD. He's focusing his efforts on trying to figure out how gullies form on Mars. He explains that he was looking through some new images from the Mars Reconnaissance Orbiter's HiRISE camera hunting for gullies.

He came across one in particular on the inside slope of a three-mile-wide crater in the Promethei Terra region of Mars, about 35 degrees south of the martian equator. Think the equivalent of Santiago, Chile, for a rough idea of just how far south.

During a visit with him at Brown, Mr. Schon recalled that he took a look at the image and thought: "Wow! Here's this gully that looks typical in most respects" to others he'd seen. But the crater had tiny craters of its own -- the result of an impact some 60 miles away.

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