Click for full image.
Cool image time! The photo to the right, cropped to post here, was taken on October 31, 2020 by the high resolution camera on Mars Reconnaissance Orbiter (MRO). Labelled simply as a “Terrain Sample”, it was not taken as part of any specific science research but because the MRO science team need to regularly take pictures to maintain the camera’s temperature. When such engineering images are required they try to pick spots of some interest, but sometimes the resulting picture is somewhat bland.
If you look at the full image, you will see that blandness generally describes it. However, in the upper left corner was a most intriguing-looking crater, which I have focused on above. From all appearances, when this impact happened the ground was quite soft, almost like mud, and thus the ejecta splattered away not as individual rocks and debris but as a flow.
The map below gives a little context, but really doesn’t explain this crater fully.
The white box indicates the location of this crater. It is right on the edge of the Medusae Fossae Formation, a gigantic field of volcanic ash about the size of India from which most of the dust on Mars is thought to come.
However, the number of craters in the full image suggests we are not looking at that ash field. Other images of the Medusae formation (see this one for example) generally show a craterless terrain, the craters buried by the more recent ash fall.
The number of craters also suggests that the surface here is not a lava flood plain. The nearby recent lava flows, such as the Athabasca Valles flood lava, wipe away the craters to leave a smooth plain (see this image for example).
It is possible the craters were caused by the spray of secondary ejecta from a more recent nearby large impact. To the southwest of this image is such a larger crater, but I do not know when that impact occurred.
The latitude is 2 degrees south, so this is right on the equator. We would not expect to see buried ice here, or at least, if there is, it will be deep underground and would likely not have caused this feature.
The only explanation that comes to my mind is that this impact is old, and that it happened when the planet’s equatorial tilt was much greater, and this location was at a higher latitude, when ice might have been more present.
To put it mildly, I do not take that explanation very seriously. We have here a mystery, something that is not unusual when you look closely at the surface of Mars. This mystery however is not readily explained by what I so far know of Mars.
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