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Cool image time! The photo to the right, rotated, cropped, and reduced to post here, was taken on September 30, 2020 by the high resolution camera on Mars Reconnaissance Orbiter (MRO).
It shows one half of what scientists have dubbed a pollywog crater, in which there is a single breach in the crater wall, aligned with the low point in the crater’s floor. Such craters suggest that they were once water- or ice-filled, and that they drained out through the breach either quickly in a single event or slowly over multiple events.
The second image below was taken by the wide angle context camera on MRO, and not only shows this entire crater, but several other adjacent craters, all of which show evidence of glacial fill in their interiors. The latitude here is 34 degrees south, placing these craters within the mid-latitude bands where such glacial features have been found by scientists in great numbers.
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The crater in the first image above is in the center right.
To get a deeper understanding of what we see here, I contacted Edwin Kite of the University of Chicago, who requested this specific image. He provided me a copy of a paper just published in the journal Earth and Planetary Science Letters that tried to model the drainage events that formed these craters and their exit breaches.
The paper’s model predicted that drainage through the breach would have occurred in a single event for at least 2 of 21 studied Martian pollywog craters. Actual observations of those craters on Mars however does not match this model, and instead suggests the drainage occurred over a longer time span across multiple smaller events for all the observed pollywogs.
These results suggest that the craters were once ice-filled, ice brought there during the periods when the planet’s rotational tilt, or obliquity, was high, as high as 60 degrees. At that time, the mid-latitudes were colder than the poles, so that water would sublimate away from the poles and fall as snow in the mid-latitudes.
In the case of this crater the exit breach was formed when that ice began to sublimate away as the obliquity began to lower to the present tilt of 25 degrees. That sublimation took place across many small seasonal and cyclical events, both by sublimation and short flows through the breach. While the sublimation would not have caused the erosion that formed the breach, the small flows would have. To quote their conclusions:
Pollywog craters were predominantly filled with ice that underwent a small amount of melting many times (e.g. seasonally). This formed small melt ponds that drained over the crater rim, contributing a small amount of valley floor erosion each time the climate enabled meltwater to form.
The cartoon to the right, taken from figure 2 of the paper, illustrates the process.
Since all the craters in the context image appear to have glacial features in their interior, this conclusion appears sensible. And since three of four of the large craters appear to have one or more exit breaches, this conclusion also suggests that this process was not necessarily unusual when Mars’ mid-latitude glaciers had just passed their peak and were beginning the process of disappearing.