New craters and wind
A Piece of Mars: The two small dark craters (2.25-2.4 m, or 7.4-7.8 ft across) are brand new, having appeared in CTX images sometime between May 2007 and April 2008. They punched through a layer of bright dust and threw up some darker material, which the wind carried downwind (near-surface winds blowing from the southwest, and higher winds blowing from the southeast). Application of an atmospheric model could further constrain the season and time of day when the impact happened, based on the prevailing wind direction. This picture from May 2016 shows the wind streaks are still there, having faded only a little in the 8-9 years since they formed. (HiRISE ESP_045798_1965, NASA/JPL/Univ. of Arizona)
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I’ve had a look at the earlier image and there seems to be no change at all to the distribution of dark stuff in the intervening years. To me this argues for the asymmetry being created right at the time of the impact, but doesn’t prove this. If there were significant change after the event, why did it stop altogether within a few months?
Impact craters are usually circular even when the impactor has arrived at an angle well off vertical. Only when the angle is very shallow does a fast moving primary impactor produce an elongated crater and an asymmetric spread of ejecta. These caters look quite circular but there’s not a lot of pixels to go on. I wonder how sure we can be that the impacts themselves were completely symmetric, that without wind they would have produced a radially even spread of ejecta. There are structures reminiscent of crater rays in the dark deposit that may have been created by the impact itself. I can’t explain these unusual craters with any confidence, but for now I’m guessing that the impacts themselves may not have been symmetric. That the wind blowing at the time of the event could accomplish this isn’t as plausible, because it would require that Mars’ thin wind could reverse the direction of flying sand in the air. Or should I imagine the sand sized ejecta blasted mostly straight up so that the wind would have the greatest effect on it’s movement during it’s brief flight? I’ve not seen a suggestion that sand is ever suspended in the air on Mars.
Sand probably isn’t suspended often on Mars, but an impact might be able to briefly kick up a small amount.
My inclination would be to do an aeolian study of the region and see 1) if other aeolian features might give clues to the prevailing wind and activity level (e.g., barchan migration rates and directions) and 2) use an atmospheric model to determine what typical wind patterns are in this area. That may or may not determine whether the streaks formed during or after the impact, and even if it did support one or the other, it wouldn’t be conclusive.
It would be really hard to determine whether the impact event came from a shallow angle. Typically these small impacts occur as clusters, because the smaller impactors often break up in the atmospshere. There was something about the size pattern of impacts that I read in a paper once… hmm. I didn’t show it, but there is a smaller impact associated with this just to the south of these two. If I get a spare moment I’ll look up that paper and see if it provides any wisdom (it’s the Golombek et al. paper on small impacts visited by Opportunity).
Got it. This is from Golombek et al. (2010), 10.1029/2010JE003628, where they cite a couple of paper by Ivanov et al.:
“[15] Modeling by Ivanov et al. [2008, 2009] shows that
these clusters form by fragmentation of weak impactors in the atmosphere and that the dispersion of the cluster is related to the density and strength of the impactor. Some of these clusters have one relatively large crater and a few smaller craters, whereas others consist of a number of small craters. Many consist of dispersed large and small craters similar to the Resolution cluster explored by Opportunity. Modeling shows that the largest craters are typically farther downrange as the largest fragment with the least atmo- spheric deceleration would be expected to travel the farthest.”
So although it’s only a data sample of 3, the two larger impacts are to the north and a smaller impact is to the SSE, suggesting the impactor came from the SSE. This would be consistent with the more prominent, more proximal dark ejecta being blown downwind by the impact itself. The fainter, more distant could be either by the prevailing wind or the impact.
Yes, I’ve seen that smaller third impact in the Hirise image now. It has a very similar asymmetry to the other two. At least we can rule out an asymmetric target. All three impacts can’t by chance be straddling the same discontinuity. A low angle (asymmetric) collision is supported, if not very conclusively, by the positions and sizes of the three craters we can see. So in the end this isn’t telling us much we can be sure of about Mars’ wind or it’s capacity to carry solid pacticles.
I would like answers to two inter-related questions: What is the grain size of the dark material that has dispersed from the craters? and: How much of the dispersion of the dark stuff took place during the impact event vs subsequently? If we are seeing a distribution pattern that was created during the event, and the asymmetry results from the wind during the event, then I’d have to guess that all the dark material is made up of dust sized particles. If the impact had produced ejecta in pieces granule sized or larger, this would be thrown away from the crater in trajectories largely unaffected by wind. The total absence of dark deposits south of the craters is striking, implying that nothing, not even coarse material, was moved south. Perhaps the target was all fine grained soil. Sand is maybe another case, but I’m doubtful that blast impelled sand grains in trajectories near the ground could be stopped at the crater rim by wind. They would create a temporary wind of their own moving away from the crater. I’ve read quite a lot about Mars impacts and I’ve not come across the idea that ejecta might sometimes be emplaced asymmetrically because of wind. This makes sense maybe if the ejecta is dust.
I can’t make sense of what I see without more information. Perhaps most of the movement of the dark stuff took place in winds after the impact? Its odd that this feature hasn’t faded over years? Maybe this is just a part of Mars where there is no net dust deposition in the current era. I’d like to look at the earlier image to see if there’s been any change at all.
I suspect the dark material is sand-sized and finer. I agree that coarser grains are less likely to be significantly influenced by the wind.
My reasoning that the wind tail occurred during (or shortly after) the impact are that:
1. There are a number of images of this area in the ensuing years, none of which show that the wind streak lengthens. So unless the impact occurred during or just before the strongest wind event(s) in the past 10 years (possible but perhaps unlikely), the material must have been kicked up into the prevailing wind during the impact.
2. The streak ends in a slightly hummocky area, so if material had slowly migrated downwind, it would have accumulated (gotten stuck) in the hollows in this terrain, and yet there are no apparent accumulations.
Those reasons are circumstantial, but make me lean toward the entire wind streak being emplaced in a single event, most likely related to the impact.
On asymmetric emplacement: the martian atmosphere is so thin that the vast majority of ejected blocks are unaffected by the wind. Most impact geologists are not aeolian folks – they’re interested in impacts that are quite often billions of years old, and it’s unlikely that wind streaks like these last that long. There are some on Venus, though – some large impacts have enormous asymmetric wind streaks that suggest material during the impact was kicked up high enough in the atmosphere to be caught up in strong winds aloft. On Mars, I’ve only seen wind streaks like these on very recent impacts (such as those that have been observed to form in the last couple of decades).