Identification of Former Lakes on Mars
||Facing south, a perspective view looking up Ma'adim
Vallis.. A view across part of the large former lake shows its deep
Dr. Ross Irwin
Dr. Ted Maxwell
We are examining basins in the Martian highlands that may have contained lakes in the past. The best means for identifying paleolakes are deltas and continuous terraces at a constant elevation around a basin, or basins that have a contributing valley and an outlet but no incision of the lower-lying basin floor. Shore landforms of terrestrial lakes are typically small and fine-grained, so they might be difficult to observe or preserve on Mars, particularly as the available topographic data has coarse resolution. Many basins on Mars have one or more of these features, but other basins have a contributing valley but no delta or outlet, and we are looking for reasons why some craters and not others might have filled with water.
Our discovery of a lake that overflowed to carve Ma'adim Vallis is the first good indication that a large expanse of the highlands was formerly flooded with water. In addition to the elevation of the outflow spillway, the lake level is indicated by a nearly constant elevation of the present boundary between basin plains and highland terrain that is cut by river valleys, like one would expect for a former lake level. The lake interior contains several deep, eroded impact craters that are presently bowl-shaped because of sediment fill along their margins (unlike the usually flat-floored eroded craters on Mars). Other evidence is outlined in the June 21, 2002 Science paper. The shoreline indicated in this manner also gives us the right amount of water needed to carve Ma'adim Vallis. It is hard to reconcile a large, long-lived lake at this high elevation without some means of recharging highland aquifers. The fact that the valley networks that fed the lake (in addition to input from a regional groundwater table) originate near the crests of sharp ridges suggests that precipitation was the ultimate water source.
The best example of a large valley created by long-lived erosion is the Grand Canyon, which is somewhat smaller than the lake's outflow valley, Ma'adim Vallis. The Grand Canyon has a small interior valley containing the Colorado River, which transported all of that sediment out of there over a span of 5 million years. Ma'adim Vallis, for comparison, has an interior channel 3 to 5 km wide. Ma'adim Vallis also begins full-born at a drainage divide, like the spillway of a dam, and there is no other way to get large amounts of water up and over a divide than by having it pond behind the divide. Ma'adim Vallis is also much larger than the typical Martian valley networks that formed by slow erosion, and Ma'adim had adequate flow to overcome the many obstructions in the landscape related to impact cratering over its 900 km length. All of these indicators, as well as the means by which water was released to carve this large valley, suggest that a catastrophic flood was the eroding agent.
Images of the Ma'adim Vallis region on Mars and renderings of the former
for this paper
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