Different relief forms originating probably from liquid water activity have been observed on the Martian surface: treelike systems of fluvial valleys morphologically similar to terrestrial river systems, great mega outflow channels without inflows, which have no terrestrial analogues, and thin gullies on crater walls. Dating of these forms based on crater density shows that valley systems cut through only the oldest, most cratered surfaces of the Noachian epoch, 4.6-3.9 billion years old. It was first proposed by Sagan et al. (1973) that treelike valleys were generated by erosion of superficial run-off, and hence indicate the existence of dense atmosphere and precipitation. Thus, in the Noachian epoch Martian conditions were most similar to the Earth: precipitation fall-out and superficial run-off were formed, and constituted numerous river systems.
The mega outflow channels cut through the less cratered, i.e., younger rocks of Hesperian age, 3.9-1.5 billion years. From the beginning of the Hesperian epoch, the heating of Mars by meteorite bombing began to reduce. As a result, ground freezing started and permafrost formation took place. From this time on, Martian conditions favored a permanently frozen envelope. Geological premises for the appearance of liquid water on the surface arise only in places where permafrost was melted through by magma as a result of tectonic and volcanic activity, in places with high concentrations of water-soluble salts in rocks, or due to hydro-explosions; in this way, huge canyons were formed (Carr 1979).
In the Amazonian epoch (1.5 billion years ago) Mars lost tectonic activity, and liquid water hypothetically was only able to get to the surface in places of ground-water (thermal or overcooled brines) seepage, and formed thin gullies on the crater walls (Malin and Edgett 2003). According to other hypotheses, these gullies were formed without water. It is important to note that some of them are probably present-day geological formations; the multiple image of one of the crater slopes in 2005 detected a new scour, filled up with light material, which was not visible on the image from the year 1999 (http://www.nasa.gov/mission_pages/mars/images/).
The recent terrestrial cryosphere is a result of the last (Cenozoic) era history. But there is evidence that this sphere periodically occupied the Earth's surface for tens and hundreds of millions of years during its early history: for example, in the early and late Proterozoic (2.4-2.1 and 1.0-0.6 billion years ago) and in the early and late Paleozoic (460-420 and 330-230 million years ago). This fact indicates that Earth and Mars underwent similar stages of development in the earliest parts of their history, which is important for the life searching theory.
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