Click for full image.
Cool image time! The photo to the right, rotated, cropped, reduced, and enhanced to post here, was taken on June 7, 2021 by the high resolution camera on Mars Reconnaissance Orbiter (MRO).
The left image shows what the scientists have dubbed a “lozenge-shaped depression” in the middle of an unnamed 60-mile-wide crater in the southern cratered highlands of Mars. The right image shows the same exact depression, but I have brightened the photo in order to see the details in the shadowed depression.
Though the image is inconclusive, the bottom of the darkest spot in that depression cannot be seen, suggesting it could be an entrance into a larger void below.
Even if there is no voids below, why is this depression here? What caused it? The wider view of MRO’s context camera below might give us a hint.
Click for full image.
The white box indicates the area covered by the photo above. The crater’s north and south rims can be seen at the top and bottom of the photo.
At about 28 degrees south latitude this crater is not expected to show much evidence of ice in its interior, and that generally appears to be the case, based on the visual look of the crater floor. If the crater floor had buried glacial fill, you would expect at this latitude to see more erosion features and some bedrock, as seen by a similar crater at a slightly higher latitude highlighted as a cool image in October 2020. The floor’s smoothness suggests instead that we are looking at bedrock, similar to a different crater floor featured in a cool image in August, 2021.
However, if you look closely at the two interior crater in the crater’s northern quadrant you can see what looks like a splash aprons surrounding each, the kind of apron you see often surrounding craters in the northern mid-latitudes, where there is much evidence of near surface ice.
Such aprons however could instead be impact melt and thus volcanic in nature, not the result of melting ice.
If there is buried ice here at this latitude, it would have to be underground to prevent it from sublimating in the warmer equatorial temperatures. The depression suggests that there might an ice layer below ground, and that it might even be sublimating away to leave cave voids behind. That lozenge-shaped depression on the surface could thus be a sinkhole entrance as well as the outlet in which that gas is escaping.
All guesses on my part. What reinforces my hypothesis to my eye is the look of the other small interior craters close to the depression. Though they do not appear to have aprons, they also appear to have impacted into something somewhat soft, like ice.
If this crater has a subsurface ice layer, it would be the lowest latitude such a thing has been identified, and would strengthen the possibility that future colonists will be able to find mineable underground ice practically anywhere on the Martian surface.
Any papers relating to this depression will likely be published in a year or so. Stay tuned. The data from the Martian orbiters continues to make Mars more and more enticing.