Lopsided ejecta from Martian crater

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Crater with unequal ejecta

Cool image time! The image on the right, reduced and cropped to post here, comes from the December image release from the high resolution camera of Mars Reconnaissance Orbiter (MRO). (If you click on the image you can see the full resolution uncropped photograph.) Released without a caption, the release itself is intriguingly entitled, “Crater with Preferential Ejecta Distribution on Possible Glacial Unit.

The uneven distribution of ejecta material around the crater is obvious. For some reason, the ground was preferentially disturbed to the north by the impact. Moreover, the entire crater and its surrounding terrain look like the impact occurred in a place that was saturated somewhat with liquid, making the ground soft like mud.

That there might have been liquid or damp material here when this impact occurred is reinforced by the fact that this crater is located in the middle of Amazonis Planitia, one of the larger regions of Mars’ vast northern lowland plains, where there is evidence of the past existence of an intermittent ocean.

This however really does not answer the question of why most of the impact’s ejecta fell to the north of the crater. From the release title is appears the planetary geologists think that this uneven distribution occurred because the impact occurred on a glacier. As the ground has a lighter appearance just to the south of the crater, I suspect their reasoning is that this light ground was hard bedrock while the darker material to the north was that glacial unit where the ground was more easily disturbed.

This is a guess however (a common requirement by anyone trying to explain the strange features so often found on the Martian surface). Other theories are welcome of course, and could easily be correct as well.



  • wodun

    In the full picture, there is an interesting crater (?) down from the one in the cropped picture. It is round like a crater but looks like something whelmed up from underneath.

  • wodun

    Welled up*

  • wodun: Y’know, I never even noticed that, being entirely focused on the main subject of the image. Very intriguing.

  • Dick Eagleson

    Also agree that the big round feature wodun refers to is quite interesting. Maybe it’s a very old crater that got ground down uniformly by subsequent water ice glacial activity with its modest remaining ring wall now filled uniformly by a combination of material deposited when the glacier subsequently melted and supplemented and smoothed by eons of dust accumulation from Martian windstorms.

    But anent the crater with the lopsided ejecta blanket, I note on the full-resolution image that there is also a lopsided pattern within said crater as well. It sort of resembles an inverted sunburst or an inverted bivalve shell in terms of its striations. The “focal point” of these striations is notably nearer the “North” wall of the crater (12 o’clock position) which also happens to be the side of the crater where the ejecta outside the crater was seemingly flung the furthest.

    To me, this indicates the crater was probably made by a bolide coming in at a fairly shallow angle relative to the Martian surface with whatever was left of it burrowing in and stopping much closer to that 12 o’clock focal point and most of the ejecta being “splashed” along that 12 o’clock vector. I am given to understand that even impacts at fairly shallow angles of incidence will still produce round craters when they occur at the velocities typical of meteoric, asteroidal and cometary collisions in the inner Solar System. Only impacts occurring nearly tangentially produce elongated impact craters.

  • Dick Eagleson: Interesting analysis. Note that you are correct: Round craters result even when the meteorite comes in at a shallow angle.

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