|ESA Science & Technology||29-Jun-2005 10:07:35|
Morphology & Erosion
Liquid surface water isn't essential to form most of the younger water features. Outflow channels, for example, with an average age of 3.5 billion years and some as young as 1 billion years, require ground ice and extensive underground aquifers for their formation, but not necessarily a continually replenished source of surface water. The outflow channels are truly enormous. Typically, they are a few tens of kilometres across at their source, expanding to hundreds of kilometres across downstream.
Discharge rates have been estimated at 10 000 times the average discharge of the Earth's largest rivers, such as the Mississippi, and 100 times the peak discharge of the largest known flood on Earth. This occurred in the Scablands region of the north-west US about 10 000 years ago when a dam made from ice melted suddenly to release a vast amount of water from Lake Missoula. "The resemblance between the Lake Missoula floods and the Martian outflow channels is quite striking," according to Michael Carr from the US Geological Survey in his book 'Water on Mars'*.
Some outflow channels, such as those to the east of the Valles Marineris, seem to have occurred, like the flood in the Scablands, when the containment around a lake was breached suddenly. But others emerge from regions where the ground seems to have collapsed downwards leaving a jumble of large blocks of rock. "These areas of chaotic terrain are the source of many of the outflow channels. They are huge collapsed fissures in the ground, which are much bigger than comparable features on Earth" says Costard. Vast volumes of fast flowing fluid created streamlined islands like these in the Chryse Basin.
Comparison with similar features on Earth, is revealing. Areas of chaotic terrain in Siberia formed when ice filling the spaces in the upper layer of rock and soil melted suddenly, causing the rock to fracture and collapse. On Mars, the ground ice must have trapped a large quantity of water in an underground aquifer at very high pressure. The water would have been released when the ground ice melted suddenly, perhaps when the temperature of the atmosphere increased. But a warm climate isn't essential. Other, more likely mechanisms are the heat generated by an impact, volcanic eruption or sudden underground heating of the aquifer. Several of these mechanisms may account for different floods, which seem to have occurred episodically over a span of more than 2 billion years. "Events occurring before 3.8 billion years ago didn't involve ground ice. But after that time, ground ice became important" says Costard.
*'Water on Mars', Michael H. Carr, 1996, Oxford University
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