Enigmatic Martian Gullies
Evidence of permafrost and ground ice, surface water and glacial ice seen first in early 1970s images from Mariner 9 flybys and in later 1970s images from the Viking missions. Results from Mars Orbital Camera (MOC) and Mars Orbiter Laser Altimeter (MOLA) not only are consistent with the view that water shaped the geology of Mars - they say it happened very recently. The occurrence of gullies is quite rare: only a few hundred locations have been seen in the many tens of thousands of places surveyed by the orbiter's camera. Most are in the martian southern hemisphere, but a few are in the north.
Nearly all occur between latitudes 30 degrees and 70 degrees, and usually on slopes that get the least amount of sunlight during each martian day. If these gullies were on Earth they would be at latitudes roughly between New Orleans, Louisiana, and Point Barrow, Alaska, in the northern hemisphere; and Sydney, Australia, to much of the Antarctic coast in the south.
Eroded gullies on crater walls and cliff sides were first observed in images taken by the Mars Global Surveyor in 2000. Most of the models to describe these formations had water coming out of the ground in aquifers and springs, or had ice-impregnated soils melting during warmer climatic periods and causing landslides.
In the banner image (top), a section of what was released by the THEMIS team on September 2, is shown the northern rim of Hale Crater, located in Noachis Terra, a region which is heavily dissected by these enigmatic gullies.
The gullies observed in the images are on cliffs -- usually in crater or valley walls -- and are made up of a deep channel with a collapsed region at its upper end (an "alcove") and at the other end an area of accumulated debris (an "apron") that appears to have been transported down the slope. Relative to the rest of the martian surface, the gullies appear to be extremely young, meaning they may have formed in the recent past.
Because the atmospheric pressure at the surface of Mars is about 100 times less than it is at sea level on Earth, liquid water would immediately begin to boil when exposed at the martian surface. Investigators believe that this boiling would be violent and explosive. So it remains a subject of intense research to find out how can these gullies form.
The water supply is believed to be about 100 to 400 meters (300 to 1300 feet) below the surface, and limited to specific regions across the planet. Each flow that came down each gully may have had a volume of water of, roughly, 2500 cubic meters (about 90,000 cubic feet) -- about enough water to sustain 100 average households for a month or fill seven community-sized swimming pools.
Spacecraft images have revealed a number of other local effects attributable to water or ice on Mars:
Extensive, uncratered (and therefore recently resurfaced) areas of the northern plains and southern highlands covered by cracked polygonal terrain that closely mimics terrestrial permafrost areas
Lakes that held water for a thousand to ten thousand years, when climate must have been drastically different than it is now.
Gullies that have drained water and debris on Mars' surface within the past several million years
A whole assembly of features that is evidence for very recent glaciers, including crevasse-like fractures, moraines drained by converging tributaries, debris aprons deposited within the past several million years.
Megafloods that have possibly triggered climate change within the past 10 million years
A second question to answer revolves around trying to determine when a particular hydrogeological feature might first have been carved into the martian surface. The surrounding craters offer one of the best guesses for timing such events. Because it is so difficult to obtain soil samples from other planets, scientists often use such impact craters to date geological surface features. Knowing when and how often asteroids and comets bombard planets can help place surface features within a certain time frame. If a crater is superimposed on top of a feature, scientists know that the feature was there before the impact occurred. Likewise, if, for instance, a gully runs through the middle of a crater, they know that the gully came sometime after the impact.
Some scientists have said that the location of water-related features in the mid-latitudes must mean that they are very old. Many of these features are oriented on a south-facing slope, and at present, that region is much too cold to melt ice. This argument has been countered by the possibility of a lag reaction time to an earlier climate change. Some have considered these sites in the mid-latitudes would be a good place to search for either living or fossilized martian life. Even if life does not exist on Mars today, perhaps these gullies are candidates for where surface fossils might be brought up as clues to any past life on Mars.
As telescopes around the world focus on Mars because of its historic nearness to Earth, two NASA spacecraft are hurtling toward the Red Planet to look for evidence that it might once have been wet enough to sustain life. The landing locations under scrutiny now were chosen because of strong signs that they may have held bodies of water at one time.
One site, known as Gusev Crater, looks like a big lake bed with a winding riverbed feeding into it. The other site, Meridiani Planum, has the spectrographic signature of a mineral called gray hematite, which usually forms in the presence of liquid water. The two sites, both slightly south of the equator, are on opposite sides of the planet. By late October, officials at NASA Headquarters will have to commit to the rovers' landing sites.
Orbital projections of where Europe's first mission to the red planet, the Mars Express , along with where the two NASA Mars Exploration Rovers are right now, can be continuously monitored over their half-year journeys.