A Martian starburst spider

A Martian starburst spider
Click for full image.

Cool image time! The photo to the right, cropped to post here, illustrates an example of a wholly unique Martian phenomenon, that is not only unique to Mars but is also found only in its south polar regions. The image was taken on July 17, 2020 by the high resolution camera on Mars Reconnaissance Orbiter (MRO).

What we are looking at is a permanent spider formation etched into the layered deposits of ice and dirt that cover the widest area surrounding Mars’ south pole. The blue dot just north of Chasma Australe on the overview map below shows the location of these starbursts, on those layered deposits.

Each winter the poles of Mars are blanketed with a thin mantle of dry ice, generally less than six feet thick. When spring arrives and sunlight hits this mantle, it heats the ice and sand on which the mantle lies, and that warmth causes the mantle’s base to sublimate back into gas. Eventually gas pressure causes the mantle to crack at its weak points so the gas can escape. By the time summer arrives that mantle is entirely gone, all of it returning to the atmosphere as CO2 gas.

This sublimation process differs between the north and south pole, due to the different terrain found at each. In the north the mantle mostly lies on ice or sand dunes, neither of which is stable over repeated years. Thus, the mantle weak points do not occur at the exact same place each year, even though they occur at the same type of locations, such as the base and crests of dunes.
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Land of swiss cheese and spiders

Swiss cheese on Martian south polar cap

Time for some cool images! In one of their periodic captioned releases of an interesting high resolution image, the Mars Reconnaissance Orbiter (MRO) science team this week released a picture of the strange “swiss cheese” terrain found throughout the Martian southern polar cap. (I have already highlighted in an early post the spiders that form in the south pole as the carbon dioxide evaporates.) The image to the right is a cropped section of that image, which you can see in its entirety if you click on it.

The South Polar residual cap is composed of carbon dioxide ice that persists through each Martian summer. However, it is constantly changing shape.

The slopes get more direct illumination at this polar location, so they warm up and sublimate, going directly from a solid state to a gaseous state. The gas then re-condenses as frost over flat areas, building new layers as the older layers are destroyed.

The captioned link above also included a link to a gif animation showing how this terrain has changed since 2009. The holes have become bigger, their cliffs retreating with time.

The section I highlight above not only shows the retreating swiss cheese dry ice, you can also see ghosts of several buried craters slowly becoming visible as the dry ice evaporates away.

This is only one of many images taken of the south pole by MRO. In the October archive release, I found almost two dozen, and that’s only the images taken during August of this summer. MRO takes images of the south pole regularly to track its changes, though I suspect it took more this summer because the global dust storm blocked imagery in the middle latitudes. Below and to the right is just one of these images, a particularly good illustration of the swiss cheese formation.
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