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.
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.
And some of the debris flowing down the gully was crater covered an older fan that did have some of these "secondary" craters on it. And this newer material was itself crater free.
When he looked closely at the image, he found evidence for three distinct episodes of material moving downslope to cover much of the oldest, crater-scarred debris fan.
The closer Schon looked, the less plausible slumping or a landslide became as the explanation for the gully and its fan. The edginess of the formations and other features were all wrong for a dry explanation.
So what would the source of the water be? Enter James Head III, Mr. Schon's thesis adviser.
Dr. Head and colleagues have been looking at the landscape, analyzing a unique layer of material that covers much of the marscape in a latitude band roughly ranging between 30 and 60 degrees north and south of the equator. The layer has been interpreted as a relatively smooth layer of soil and ice several meters thick -- a byproduct of snowfall during the comings and goings of ice ages on Mars.
The crater in question falls into this latitude zone, where this cloak of icy dust or (dusty ice) would be most vulnerable to episodic melting during the periods between the Mars's glacial maximums.
He and others have dubbed this aspect of the martian surface "pasted-on terrain." It can cling to slopes -- in this case a section of a crater slope that receives the most sunlight. At some point since the last ice age, he and Schon suggest, ices in this pasted-on material probably melted, briefly sending mud flowing down the side of the crater.
And as for timing, Schon turned to the craters. He found a crater some 60 miles away from the gully that bore the hallmarks of an impact crater. In fact it was a two-fer: a four-mile-wide dent in the crust nested inside a 11-mile-wide crater -- both very fresh. Material blasted from these arced over the marscape. Some landed in the gully's crater to form the secondary craters.
How old did you say it was?
Using craters on the surface as a kind of clock, Schon estimates that the gully's initial, cratered fan formed 1.25 million years ago.
For Head's money, this would be an interesting place to look for evidence of past or present microbial life.
Scientists have long realized that liquid water was present on Mars during its earliest times. "All of a sudden, we've got liquid water, and we can't even date how young it is because it's so young," he says. "Maybe these are the kind of places where life might be – a kind of refuge."
He points to radiation resistant algae that survive frigid Antarctic winters.
"They form the algal mats and wait around until one day it melts," he says. They thrive for a brief instant in time, "then they dry up and curl up, and then the wind blows them away."
"A million of them won't do any thing because the water won't be there" when they land. "But one of them will" if it lands in the right spot. It wakes up, thrives briefly in the water -- and the cycle repeats. As for the other million? Wind can continue to whisk them to new, possibly more-hospitable locations. Freeze-dried life: Just add water!