Washington, D.C.-Steep-sided gullies, sinuous channels, and deltas of debris captured by the high-resolution "eye" of the Mars Global Surveyor suggest that liquid water may be lurking at shallow depths beneath the martian surface, according to a report in the 30 June issue of Science.
The presence of liquid water in the upper crust of Mars could be a boon for future human expeditions to the planet that could use the water for everything from basic survival to fuel cell energy and rocket propellant, the Science authors say. Some scientists believe that if life exists on Mars, it will be found in or near a water source.
The martian landforms, which bear a striking resemblance to water-carved gullies on Earth, appear to be relatively recent, say authors Michael C. Malin and Kenneth S. Edgett of Malin Space Science Systems, Inc. The formations are unblemished by impact craters, freeze cracks, or windblown deposits.
"These gullies could be on the order of a million years old, or they could have formed yesterday," says Malin.
Unlike images captured by past missions, which indicate that large amounts of water may have flowed on the martian surface during the planet's ancient history, the images of these new and rare geologic features suggest a smaller-scale scenario of pent-up groundwater seepage bursting forth in short-lived torrents.
Only about 150 images of the more than 25,000 high-resolution views collected by the Mars Orbiter Camera onboard the Mars Global Surveyor show landforms that the Science researchers interpreted as signs of water seepage and runoff. Appearing on the interior cliff walls of impact craters, pits, and two major martian valley systems, these distinctive formations start high on slopes as triangle-shaped alcoves, created by the undermining and collapse of the cliff below a rock layer from which a liquid appears to have been seeping. V-shaped channels emerge from the apex of these alcoves and run downslope, splaying out in an apron of rock debris or a tangle of smaller channels at their ends.
Details of these landforms-their origin in distinct rock layers, their banked, winding, and often branching channel paths, and their final fans of debris embroidered with a pattern of smaller channels-are all characteristic of gully formations on Earth, say the researchers. After testing the possibility that the martian landforms could have been created by "dry" flows, like landslides or avalanches, Malin and Edgett conclude that the martian gullies may have been shaped by the same water-driven processes that create gullies on Earth.
"There's still a finite chance that they were formed some other way," says Malin, "but there's a high probability that they were formed by water."
The researchers calculate that at least 2500 cubic meters of water-enough to fill seven community swimming pools-were necessary in some cases to transport the amount of material seen in the debris fans. Scientists believe, however, that any water on the martian surface would evaporate quickly in a boiling froth. Malin and Edgett's challenge was to explain how large amounts of water could accumulate and survive to create these gullies.
They suggest that the water exists in a porous layer of rock buried a few hundred meters below the martian surface, kept liquid by the pressure exerted by overlying rock. Water seeps through the porous layer until it is exposed at the surface inside a crater or other depression. These first exposed trickles may freeze into a small ice "dam" at the surface. Meanwhile, groundwater continues to percolate through the rock and build up pressure behind this ice dam, creating a reservoir of fluid that eventually bursts through the dam and runs downslope, like the surge of flash flood waters through a desert arroyo.
If this is the correct scenario, the researchers suggest that the formation of the martian gullies may be linked to their location. More than 90 percent of the gullies occur in the planet's southern hemisphere, almost all of them are found on the pole side of 30 degrees latitude, and they are almost three times more likely to face the hemisphere's pole rather than the equator.
"These are cooler areas, areas further away from the sunlight and higher temperatures that you get on slopes that are near to the equator or face the equator," says Edgett. The lower temperatures in these areas, he explains, may slow down evaporation and help form the necessary ice barriers.
At several of the sites, the gullies group together and form clusters. According to the authors, this intriguing detail may point to a more defined martian groundwater system, similar to natural rock-bound aquifers on Earth.
"If these are aquifers," says Edgett, "one of the things we would like to do as part of the ongoing research effort is to try and map their location and extent."
The research team plans to continue its surveillance of the landforms to gather more evidence to support their water hypothesis and to see if they can catch the gullies in action.
"This is part of the puzzle that we've been putting together in the last two years of a new Mars," says Malin. "The possibility of liquid water, accessible over a substantially larger part of the planet than we thought, is quite exciting."