"Ice covered lakes that form in polar regions after large impacts

This is a possibility that was highlighted recently with the close flyby of Mars by the comet Sliding Spring in 2014 C/2013 A1 Sliding Spring. Before its trajectory was known in detail, there remained a small chance that it could hit Mars. Calculations showed it could create a crater of many km in diameter and perhaps a couple of km deep. If a comet like that was to hit polar regions or higher attitudes of Mars, away from the equator, it would create a temporary lake, which life could survive in.

Models suggest that a crater 30 - 50 km in diameter formed by a comet of a few kilometers in diameter would result in an underground hydrothermal system that remains liquid for thousands of years. This happens even in cold conditions so is not limited to early Mars, so a similar impact based temporary underground hydrothermal system could be created today if there was a large enough impact like Sliding Spring. The lake is kept heated by the melted rock from the initial impact in hydrothermal systems fed by underground aquifers.

Temporary lakes resulting from volcanic activity

There is evidence that volcanism formed lakes 210 million years ago on one of the flanks of Arsia Mons, relatively recent in geological terms. This may have consisted of two lakes of around 40 cubic kilometers of water, and a third one of 20 cubic kilometers of water, which probably remained liquid for hundreds, or even of the order of thousands of years.

Two views of Arsia Mons, based on Viking orbiter imagery and Mars Global Surveyor elevation data, from the south (top) and north (bottom).

Arsia Mons is the southernmost of the volcanoes of Tharsis Montes. It is depicted using a Viking image mosaic draped over MOLA topography. The topography shows the caldera structure and the massive flank breakouts that produced two major side lobes on opposite sides of the volcano. The vertical exaggeration is 10:1.

There is evidence of lakes that formed 210 million years ago on the flanks of Arsia Mons. Compared with the 4.5 billion year history of Mars, this is relatively recent. It may have had two lakes of around 40 cubic kilometers of water, and a third one of 20 cubic kilometers of water, which stayed liquid for centuries, possibly for millennia."

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