analogs of Martian
Geologic Features Relating to Water
|Martian gullies on a crater wall. Notice the white frost on and above the rim and the eolian sand dunes at the base|
|If you look at any
map or picture of Mars, you can probably find tons of features that look
very similar to ones here on Earth. Volcanoes, canyons, valleys, craters
and plains are thrown all over the planet and they all mean something. By
understanding how large scale and small scale features had formed, one can
begin to understand the history that Mars went through to get to its present
state. There are certain features here on Earth that are dispersed
everywhere throughout Mars which are understood to be formed by fluids.
Such features include ones formed by flowing fluids such as large outflow
channels and valleys and ones formed by standing bodies of fluid such as
shoreline features, deltaic sediments, intracrater sediments, and finely
layered bedding. On Earth it is understood that these features are
created by water and on Mars this is the most likely culprit although CO2
has been proposed for some features as well.
Giant Outflow Channels
Outflow channels are large channels that are formed by huge catastrophic floods. On Mars, these channels are tens of km across and hundreds to thousands of km long. The floods may have started from ice being melted due to igneous activity and then being violently erupted from the bedrock. The flood would destroy the bedrock causing the ground to collapse to form what is called chaotic terrain. Chaotic terrain are areas of angular, jostled blocks and jumbled depressions. These areas are found at the head of outflow channels. However, not all outflow channels need to start at chaotic terrain. Some can start in deep cracks know as grabens which are downthrown linear blocks between faults. From there, massive amounts of erosion would occur as the water drains to the low points on the planet. On Mars, these low points are the northern plains where shoreline sediments have been found to be deposited. This is consistent with the assumption that the northern plains of Mars may have been a gigantic ocean. These channels are most likely Hesperian in age but some may be as young as Amazonian in age. (Check out the Martian Geologic Time Scale) The Martian floods were of enormous size, sometimes twice as much as the largest floods known on Earth. An example of a Martian outflow channel on Earth would be the Channeled Scablands of Washington State that where formed by catastrophic floods from Lake Missoula during the last ice age.
Branching patterns of valley networks resemble fluvial systems here on Earth. The interpretation is that these valleys were a result of surface runoff. Under the current surface conditions on Mars, liquid water cannot exist in equilibrium. Because of this and the fact that the valleys date from the ancient Noachian period, it has been thought that early Martian history was warm and relatively wet. However, the Martian valleys seem to be less developed than ones here on Earth with a lack of small scale tributaries feeding into larges streams. Therefore it has also been thought that the valleys may have been formed by groundwater current. There are a few networks that seem to be Amazonian in age on several Martian volcanoes. This raises important questions regarding the stability of liquid water on the surface in what was presumed to be a very cold, dry climate throughout most of Marsí history.
Deposited sediments are of high interest due to the fact that they may preserve fossils. Features in the northern plains and in craters are indicative of sedimentary deposits possibly from outflow channels and valleys. Sequences of layered sediments have been observed in canyons which are believed to once contain standing bodies of water.