Click for full image.
Cool image time! The photo to the right, rotated, cropped, and annotated to post here, was taken on September 17, 2021 by the high resolution camera on Mars Reconnaissance Orbiter (MRO). It shows us a very small section of Mars’ north pole icecap.
What are we looking at? The picture was taken in summer, so by this point the thin mantle of dry ice that falls as snow in the winter and covers the north pole down to about 60 degrees latitude has sublimated away. This surface thus is water ice interspersed with Martian dust.
Yet, unlike the Antarctic icecap on Earth, the ice surface is not smooth and flat. Instead, this Martian ice has a surface that is a complex arrangement of hollows and ridges, all about the same size. Why?
And what are the two larger white spots? What caused them and why are they the only differently-sized objects in the picture?
The full resolution close-up, found at the image website, provides some answers to these questions.
Click for full image.
Both white spots are small craters that are apparently filled with water ice. Though these craters are very likely relatively recent when compared with the ice cap below, they still could be quite old. As I noted in June 2019 article,
The polar cap … is 600 miles across and a little less than 7,000 feet deep. It is made up of many seasonal layers, like the icecaps on Earth, with the bulk a mixture of water ice and cemented dust and sand. The very top layers, dubbed the residual icecap, is about three to six feet thick made up of frozen water having a volume about half of Greenland’s icecap. While this water could sublimate away, data suggests it is, like the icecaps on Earth, in a steady state, neither gaining or losing volume with each Martian year.
The map to the right of the north pole icecap shows the location of this image by the yellow dot. As it is deep inside that cap, there is a lot of ice below it, going down many thousand feet.
When the impacts hit, each melted those icy top layers which then quickly refroze, leaving the interiors of both craters bright with ice. The surrounding curlicue terrain is probably the result of eons of ice sublimation followed by later ice deposition, causing the surface to roughen with time.
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