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.

The white dot on the overview map to the right marks the location, inside a half-mile-deep and 12-mile-wide unnamed crater in the high southern latitude of Mars. At 63 degrees south latitude this location is just outside the arctic circle of Mars, which means there will be a day/night cycle year round, even if the days are extremely short in the winter.

The splotches therefore could be caused by sunlight during the short winter days, shining through the dry ice to create CO2 gas which then bursts through the mantle at dune weak points. In the full image it appears these weak points are more often found at the low points between dunes. In the cropped section above however the weak points are found both high and low on dunes.

This variety of weak point locations, combined with the dunes here, suggest that at this southern location the formation process is more like that seen in the north. The dunes are less stable, so weak points appear at different spots each season in a more random manner, and no spiders form.

One last thought: Note the colors. The orange near the bottom of the picture, on a part of the crater floor where there are no dunes, suggests Martian dust and debris. The blue color on the dunes suggest that water ice is being ejected from below by the CO2 gas when it escapes. These dunes therefore might also be impregnated with glacial material.