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NASA will soon launch two missions to Mars. Known, temporarily, as Mars Exploration Rover (MER) A and B, the two rovers will land in different regions of Mars to search for evidence of water in Mars’s past.

Scientists and engineers have whittled down an initial list of dozens of candidate landing sites to four finalists. Of these four, two are clear favorites among the scientists: Meridiani Planum and Gusev Crater. A previous article discussed Meridiani Planum; this article will focus on Gusev.

Gusev Crater is almost exactly halfway around the red planet from Meridiani Planum. While the latter site is distinguished by the presence of hematite, a mineral typically formed by the long-term interaction of water with iron-bearing rock, Gusev is notable for its geologic setting.

Gusev is a large crater basin, some 170 kilometers (105 miles) across. Most scientists believe it once was fed by water flowing through an enormous valley channel, Ma’adim Vallis. Snaking its way northward along the Martian landscape for more than 900 kilometers (560 miles), Ma’adim Vallis is 1.5 times as long as the Grand Canyon.

In images of Gusev taken by the Mars Orbital Camera (MOC) aboard the orbiting Mars Global Surveyor (MGS) spacecraft, some exposed outcrops appear to show faint layering. The prevailing scientific theory is that Gusev Crater contains sediment washed down Ma’adim Vallis from the highlands to the south nearly 4 billion years ago.

Some researchers also believe that landforms visible in MOC images of the mouth of Ma’adim Vallis, where it enters Gusev Crater, resemble landforms seen in some terrestrial river deltas.

“Deltas of this nature take tens of thousands of years, hundreds of thousands of years on Earth to be formed,” says Nathalie Cabrol of the SETI Institute and NASA Ames Research Center. “So here you have the place where water has been acting for a long time. And depositing and eroding sediments and shaping the landscape for – possibly that long.”

MOC images indicate that there may once have been a very large lake upstream of Gusev, near the source of Ma’adim Vallis. “This large lake could have provided water over a long period of time to Ma’adim, channeling the water into Gusev,” says Cabrol.

No one knows how much water flowed through Ma’adim Vallis, or for how long. Cabrol says there are several possibilities. It may have been a slow, continuous flow that lasted for tens or hundreds of thousands of years. It may have been a punctuated series of massive outbursts. Or it may have been a combination of these two processes.

“When you look at the dynamic of a channel on Earth, usually because of climate change, because of changes in the source region, you will have a series of very different episodes, and all these dynamics will be recorded in the sediment. So our hope is that if this is fluvial, if this is related to water, then by looking at the sediment, the shape of the grain, the thickness of the deposits, we might be able to tell what really happened,” says Cabrol.

Steve Squyres of Cornell University, who is the principal investigator for the MER missions, agrees with Cabrol that Gusev would make an excellent landing site.

“What you’d be looking for in Gusev is deep, possibly long-lived standing water bodies,” he says. “I mean, a big lake. If Ma’adim Vallis was carved by running water – and I’ve heard no credible alternative to that hypothesis and there’s an enormous amount of evidence to support it – there’s just no way around the idea that Gusev had a great big lake in it. I mean, lots of water, and lots of sediments.

“We know from years and years of work in sedimentary and aqueous environments on Earth,” Squyres adds, “that those kinds of materials can be very, very good at preserving evidence of climatic conditions, what kinds of aqueous processes were going on, and whether there was any kind of prebiotic chemistry.”

There are those who see a different history written in Gusev’s layered sediments. One alternative hypothesis for their origin is that they are “airfall deposits,” dust and ash that has settled over time from the atmosphere. Some researchers believe the layering in Gusev is the result of a series of volcanic lava flows.

Cabrol thinks these are unlikely explanations for the landforms that comprise Ma’adim Vallis and Gusev Crater. But, she concedes, if MER were to confirm one of these hypotheses, the discovery would be of great scientific value – and spark an intense round of new discussion.

“If something like Ma’adim Vallis is actually a lava flow and looks so much like a fluvial channel, well, we better reassess what we think about the channels we’re seeing on Mars,” she says. “And if all the material we’re seeing in Gusev is airfall deposits we’re going to learn about the climate history of Mars.”

One potential problem with Gusev is that the sedimentary materials of interest, scientists believe, were mainly deposited 3.5 billion years ago, although there have been subsequent active episodes more recently. They may be buried too deeply to be accessible by MER’s instruments. “There is the risk, frankly at any of these sites, that the stuff that you’re interested in has been buried by some other stuff,” says Squyres.

Fortunately, he adds, nature has provided a possible solution to this dilemma: craters. “The good thing about the Gusev site,” he says, “is that there are all these craters. Mother Nature has dug a bunch of holes for us in the form of impact craters, and Gusev has a pretty high density of craters.

“The rover’s mobility is key to taking advantage of craters. You’re not necessarily going to land right next to a crater, but you can drive to one. Because of the fact that the crater density in Gusev is pretty high, the chances that these impacts have brought up from below the materials underneath are good enough that it has caused a strong community consensus behind this site as a high potential site to go to.”

Although Meridiani Planum and Gusev Crater are the clear landing-site favorites within the scientific community, the final decision on where the MER missions will land has not yet been made.

Over the course of the next month, computer-simulated landing scenarios will be run repeatedly. If these simulations turn up an unexpectedly high risk of failure – for example, if the terrain at one of the sites is deemed too rough to land safely, or if models of Mars’s air circulation indicate that high winds might cause the spacecraft to crash – Meridiani or Gusev could be replaced by one of the two alternate sites, most likely the site at Elysium Planitia.

The final decision, which will be made by Ed Weiler, NASA’s associate administrator for space science, is expected in April, just weeks before the first of the two MER spacecraft is scheduled for launch.