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.
In the south however the ground is more stable, so that each year the trapped gas at the base of the mantle travels upward along the same exact paths, repeatedly etching into that ground the meandering spider patterns we see in the image above.
Think of these lines as river tributaries, except that the flow here is gas and it is upward to the surface, not downward following gravity. Moreover, in the case of these starbursts, the flow is going uphill on all sides of a small peak, until the separate flows meet at the top, where they escape.
Essentially the starbursts are vaguely reflecting the surface topography of the layered ice/dust deposit on which they sit. At this spot there several high points, and surrounding each is a starburst.
I’ve said it before, but it bears repeating: Mars is strange. Mars is alien. Mars epitomizes the universe in all its glory.
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