|ESA Science & Technology||30-Jun-2005 14:15:23|
Last Update: 08 Jun 2004Until very recently, planetary scientists had thought that Mars is a cold, dry planet today, although it hasn't always been so. Then in the summer of 2000, NASA released MGS images showing evidence of very recent seepage of ground water from crater and valley slopes in the southern hemisphere. It seems that substantial reservoirs of the water that once ran so copiously on the surface may still exist.
The evidence of freely flowing water early in Mars' history is dramatic. Not only does the planet boast the largest volcanoes and deepest canyons in the Solar System, it also shows evidence for the most catastrophic floods.
Large outflow channels, each of which could have been formed only by the massive release of water over a short period of time, scar four regions: Chryse-Acidalia, Elysium Planitia, the eastern Hellas Basin and the Amazonis Planitia. Several of these channels drain into the northern plains, lending support to the existence of an ancient ocean over most of the northern hemisphere. However, there is other evidence for flowing water in earlier times.
The southern highlands are criss-crossed by valley networks that must also have been formed by water. And many craters, especially at high latitudes, are surrounded by fluidised ejecta that resemble the ring of splattered debris around a stone dropped in soft mud. Also, the surface appears scoured by a fluid, probably water, on the northwest of Elysium Mons.
The history of water on Mars as revealed in these three different types of evidence, suggests a dramatic change in the climate about 3.8 billion years ago. The atmosphere today is too cold to support liquid water on the surface for long and too thin to support ice - any ice that does form will quickly sublimate into water vapour. But before 3.8 billion years ago many scientists think that liquid water must have existed on the surface for quite some time.
"The valley networks formed more than 3.8 billion years ago. They must have been carved by rivers fed by rainfall or groundwater sapping," says Francois Costard from the Laboratoire de Giologie Dynamique de la Terre et des Planhtes, Orsay, France. "Liquid water must have been on the surface for a long time to form them, so the temperature and pressure must have been higher then than now."
From a distance, the valley networks resemble river valleys on Earth. But the camera on NASA's Mars Global Surveyor, which is taking closer shots than ever before, is revealing some notable differences. The valleys tend to start and end up with the same width and shape and they have few small tributaries up-stream. This argues for an underground source of water rather than run-off after rainfall. Once on the surface, though, the water must have remained for a long time to carve out the valleys some of which are more than 200 km long. That could only have happened if Mars was warm and wet when they formed more than 3.8 billion years ago.
Craters formed in soft, probably water-logged ground. Note the splatter marks (lobate flows) around it. Many craters on Mars, especially at latitudes greater than 45o, are surrounded by lobed patterns suggesting that the impacting object struck wet or icy ground.
"Above 45o the temperature is always below freezing, so there is stable ground ice under the surface. In these regions even the smaller craters exhibit lobate flows, which suggests that the ground ice is near the surface," says Costard.
By looking at the size of craters surrounded by lobate flows it's possible to estimate the depth of the ice table beneath the ground.
"At the equator, the top of the ground ice table is 300 m to 1 km down. At mid and high latitudes, it's 150-300 m. Modelling suggests an ice thickness of 1-3 km at the equator and 3-7 km at mid and high latitudes," says Costard.
These depths are well within the expected range of the MARSIS experiment which will sound for underground water from the Mars Express orbiter.
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