"We see features that look like gullies formed by flowing water and the deposits of soil and rocks transported by these flows. The features appear to be so young that they might be forming today. We think we are seeing evidence of a ground water supply, similar to an aquifer," said Dr. Michael Malin, principal investigator for the Mars Orbiter Camera on the Mars Global Surveyor spacecraft at Malin Space Science Systems (MSSS), San Diego, CA. "These are new landforms that have never been seen before on Mars."
The findings will be published in the June 30 issue of Science magazine.
"Twenty-eight years ago the Mariner 9 spacecraft found evidence -- in the form of channels and valleys -- that billions of years ago the planet had water flowing across its surface," said Dr. Ken Edgett, staff scientist at MSSS and co-author of the paper in Science. "Ever since that time, Mars science has focused on the question, 'Where did the water go?' The new pictures from Global Surveyor tell us part of the answer -- some of that water went under ground, and quite possibly it's still there."
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.
Left: This picture, which was taken in September 1999 by the Mars Global Surveyor spacecraft, shows a deep, prominent martian gully in a south-facing wall in Nirgal Vallis near 29.4°S, 39.1°W. Sunlight illuminates the scene from the upper left. At the bottom of the picture is a series of evenly-spaced, almost parallel ridges. These ridges are dunes created by windblown sand. The apron--the fanlike deposit at the lower end of the deep channel--at this location is seen covering some of the dunes. The sand dunes are thus older than the apron of debris that came from the channel. The dune field has no small meteor impact craters on it, so it, like the gully landforms, is geologically young--yet older than the apron. If the dunes are active in the modern environment--which is uncertain despite the apparent youth of the dunes--then the apron would have had to form within the past few centuries or less. [more information about this picture] Credit: NASA/JPL/Malin Space Science Systems
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 how can these gullies form? Malin explained that the process must involve repeated outbursts of water and debris, similar to flash floods on Earth.
"We've come up with a model to explain these features and why the water would flow down the gullies instead of just boiling off the surface. When water evaporates it cools the ground -- that would cause the water behind the initial seepage site to freeze. This would result in pressure building up behind an 'ice dam.' Ultimately, the dam would break and send a flood down the gully," said Edgett.
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 camera. Most are in the martian southern hemisphere, but a few are in the north.
Right: Evidence for Recent Liquid Water on Mars: Gullies at 70°S in Polar Pit Walls. Gully landforms proposed to have been caused by geologically-recent seepage and runoff of liquid water on Mars are found in the most unlikely places. They typically occur in areas that are quite cold--well below freezing--all year round. Like the old adage about moss on trees, nearly all of them form on slopes that face away from sunlight. Most of the gullies occur at latitudes between 30° and 70°. [more information about this picture] Credit: NASA/JPL/Malin Space Science Systems
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.
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. The process that starts the water flowing remains a mystery, but the team believes it is not the result of volcanic heating.
"I think one of the most interesting and significant aspects of this discovery is what it could mean if human explorers ever go to Mars," said Malin. "If water is available in substantial volumes in areas other than the poles, it would make it easier for human crews to access and use it -- for drinking, to create breathable air, and to extract oxygen and hydrogen for rocket fuel or to be stored for use in portable energy sources."
Above: This 3D view of v-shaped "seepage" scars on the western wall of a 50 km-diameter impact crater in southern Noachis Terra at 65°S, 15°W requires red-blue 3D glasses for proper viewing. [more information about this image] Credit: NASA/JPL/Malin Space Science Systems
"This latest discovery by the Mars Global Surveyor is a true 'watershed'--that is, a revolution that pushes the history of water on Mars into the present," said Dr. Jim Garvin, Mars Program Scientist, NASA Headquarters. "To follow up on this discovery we will continue the search with Mars Global Surveyor and its rich array of remote sensing instruments, and in 2001, NASA will launch a scientific orbiter with a high spatial resolution middle- infrared imaging system that will examine the seepage sites in search of evidence of water-related minerals.
"Furthermore, NASA is in the process of evaluating two options for a 2003 mission to Mars, both of which could provide independent information concerning the remarkable sites identified by Malin and Edgett."
See more pictures at the Malin Space Science Systems web site.
JPL manages the Mars Global Surveyor Mission for NASA's Office of Space Science, Washington, DC. Malin Space Science Systems built and operates the camera system. JPL is a division of the California Institute of Technology, Pasadena, CA.
JPL's industrial partner is Lockheed Martin Astronautics, Denver, CO, which developed and operates the spacecraft.
Mars Global Surveyor Home Page - from NASA/JPL
Malin Space Science Systems -- MSSS operates and processes data from instruments on planetary missions under contract to the National Aeronautics and Space Administration (NASA).
Science@NASA Stories about Mars:
Martian Swiss Cheese -- March 9, 2000. New pictures from NASA's Mars Global Surveyor spacecraft show exotic terrain made of dry ice near the Red Planet's south pole.
Unearthing Clues to Martian Fossils -- June 11, 1999. The hunt for signs of ancient life on Mars is leading scientists to an other-worldly lake on Earth.
The Red Planet in 3D -- May 27, 1999. New data from Mars Global Surveyor reveal the topography of Mars better than many continental regions on Earth.
Search for Life on Mars will Start in Siberia -- May 27, 1999. NASA funds permafrost study to support astrobiology research.