All known forms of life on Earth require the presence of liquid water. Mars is an attractive planet to search for extraterrestrial life because its surface contains clear evidence that water flowed across it. There are large channels and valley networks, both of which seem to require large amounts of flowing water. The meteorites from Mars contain hydrated minerals, indicative that water was present in their magmas, hence available to be transferred to the atomsphere to produce a far wetter climate than possessed by present-day Mars. How much warmer and wetter the atomsphere was is not known with certainty, but there certainly was abundant flowing water, especially early in Martian history.
|Large channels like this one in Kasei Vallis indicate that water once flowed in prodigious amounts on Mars. However, this does not imply that it had to be incredibly rainy on Mars. In fact, it may not be possible to form such huge floods by rainfall alone. The water more likely emerged from the ground when ice melted rapidly, perhaps because of magmas moving through the crust. The water would end up spurting from the ground, sweeping downhill and eroding the landscape. (23oN, 65oW, NASA photo.)|
|This photograph of an area near the mouth of Ares Vallis in Chryse Planitia shows the power of the surging water. Flood waters flowing from the bottom to the top of the image were diverted by two craters 8-10 kilometers in diameter. Two streamlined islands were formed. (20oN, 31oW, NASA photo.)|
|Valley networks also indicate the presence of liquid water on the surface of Mars. Some may have been formed by groundwater flowing onto the surface, but others resemble typical branching drainage networks on Earth. However, this and other networks on Mars lack the small-scale streams feeding into larger ones. This may indicate that rainfall was not the only process at work to provide the water to carve the valleys. (42oS, 92oW, NASA photo.)|
This branching, or dendritic, drainage network in South Yemen was photographed by the Space Shuttle. Note that it is more intricate than the network on Mars, with many smaller streams flowing into larger ones.
Calculations suggest that the amount of water required to form channels and valley networks on Mars could have been a few percent of the volume of Earth's oceans, although some estimates place the amount at much less than one percent. On Mars as on Earth, there would have been seas and land masses, not a global ocean.The presence of water on Mars, at times flowing in great rivers and standing in lakes (which were probably frozen on top), makes it promising to search for life on this desert-like, reddish planet. (NASA photo.)
|Most of the prominent valley networks occur in the ancient highlands of Mars. This region is characterized by numerous large craters that have been strongly eroded. Since most large craters formed before about 3.8 billion years ago (an age inferred from studies of large craters on the Moon and from lunar samples), erosion rates must have been quite high, certainly much higher than they have been since that time. ALH 84001 is an old rock, formed in the ancient highlands and was involved in a large cratering event 4.0 billion years ago. Conditions in the ancient highlands would have made it likely that the rock was exposed to water, either on the surface or flowing through cracks beneath the ground. (NASA photo.)|
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