Behind The Black – Robert Zimmerman

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Cool image time! The picture to the right, cropped to post here, was taken on December 29, 2023 by the high resolution camera on Mars Reconnaissance Orbiter (MRO). It shows strange spidery formations on the rim of a 17-mile-wide crater about 500 miles from the south pole of Mars.

Scientists think these spider features are formed due to the seasonal cycle on Mars. In the winter at the poles the carbon dioxide in the atmosphere falls as snow in the polar regions, creating a thin dry ice mantle that covers everything. When spring arrives, sunlight goes through the clear mantle to heat its base, causing that dry ice to sublimate into gas that is trapped below the mantle. Eventually that mantle cracks at a weak point and the gas escapes, spewing dark dust on its top. By summer the mantle is entirely gone, and the black splotches disappear as they blend back into the same colored ground.

At the south pole the ground appears to be firmer and more structurally sound than at the north pole. The trapped gas appears to travel upward along the same tributary paths to the same escape points each year, thus carving these spidery features that are permanent features.
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Cool image time! The picture to the right, cropped to post here, was taken on December 20, 2023 by the high resolution camera on Mars Reconnaissance Orbiter (MRO). It shows what the science team labels as “Dunes with Blotches.”

The blotches, or as I call them splotches, are the round dark patches on dunes themselves. Though their darkness is reminiscent of the dark patches that appear as spider features in the south polar regions of Mars, there are problems linking the two. The spiders form when the winter mantle of dry ice that falls as snow begins to weaken when the Sun reappears in the spring. Sunlight travels through the clear dry ice to warm the base of the mantle, causing it to sublimate into carbon dioxide gas. That gas however is trapped at the base, and only escapes when the thin mantle cracks at weak points. As the gas puffs out it carries with it dust, which leaves dark patches on the surface that disappear when the mantle disappears entirely by summer.

In the southern hemisphere at the poles the ground is somewhat stable, so the trapped gas appears to travel along the same paths each year to the same weak spots. This in turn causes it to carve spidery patterns in the ground, like river tributaries, except here the tributaries of gas flow uphill to their escape point. At the north pole the ground is not as stable. Instead we have many dunes, so that the dry ice mantle sublimates away at different places each year. There is no chance to form such spider patterns over time.

Making these splotches more puzzling is the season. This picture was taken in the winter, at a time one would think no dry ice is sublimating away.
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