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The recent news story NASA’s InSight Lander Detects Stunning Meteoroid Impact on Mars, about an impact that occurred on Mars on December 24, 2021 (event S1094b), shows a picture of the impact crater, with white features that are interpreted as blocks of water ice:

What’s more, the meteoroid excavated boulder-size chunks of ice buried closer to the Martian equator than ever found before – a discovery with implications for NASA’s future plans to send astronauts to the Red Planet.

PIA25583: HiRISE Views a Mars Impact Crater Surrounded by Water Ice, Credit: NASA/JPL-Caltech/University of Arizona PIA25583 Credit: NASA/JPL-Caltech/University of Arizona

How do we know that this ice is from Mars and not from the impactor, which would then be a comet and not an asteroid?

I guess it all comes down to inferring the density of the impactor, but its dimensions and mass seems to be poorly constrained. The NASA press release says "5 to 12 meters", without precising the shape. This IPGP press release (in French) says the object was between "250 and 650 tonnes". Assuming a spherical shape, combining these numbers can yield any density between 0.28 g cm$^{-3}$ (12 m sphere weighting 250 tonnes) and 10 g cm$^{-3}$ (5 m sphere weighting 650 tonnes). (I won't consider this Imperial College press release, as it is full of "units" even worse than the imperial system: "one and a half times the size of Trafalgar Square", "van-sized", "the area inside London’s M25 motorway" [sic]...)

Comets have a density of about 0.6 g cm$^{-3}$, while iron meteorites can be as dense as 8 g cm$^{-3}$, with rocky bodies in between, so they all seem to fall in the possible range.

I've glanced over the two Science papers associated with the story (#1, #2), but the ice is barely mentioned, and I could not find any mass or dimension of the impactor.

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    $\begingroup$ One) An object that small would be a comet fragment, not a comet. Two) The article said the meteroid would have burned up in Earth's atmopshere before hitting the ground. It is possible that a comet, much less dense and more fragile than a meteoroid, would have broken up even in Mars's less dense atmosphere. Three) possibly the pattern of ice resembles mattr ejected from impact craters. Those are two possible reasons they might have for considering the ice to be from Mars instead of from a comet. $\endgroup$ Oct 28, 2022 at 15:42
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    $\begingroup$ Good question! Potentially related in Earth Science SE: Have "frosty meteorites" ever been observed soon after landing? Are there photos? $\endgroup$
    – uhoh
    Oct 29, 2022 at 21:59

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From Richardson et. al, all the small, high-speed debris gets ejected from an impact first, and ends up furthest from the impact point. This would include any intermixing of the impactor with Martian material at the center of the crater, supposing the impactor wasn't entirely vaporized on impact.

The last of the ejecta to exit the crater are the largest pieces of existing planetary material, which come from the very outside edges of the crater at the lowest speed. The paper has a nice figure that I've referenced in a number of other answers:

enter image description here

Most of the reflective ejecta we see is within two crater radii from the impact point. So, according to impact models, it was ejected last, and it originated from the area just inside the crater, rather than from the impactor itself.

While we may not know if the white ejecta is water ice, we can be pretty sure it isn't part of the impactor.

Here is the same picture with a scale:

enter image description here

And here is a zoomed-out picture of the before and after the impact on the surface.

enter image description here

If any chunks of the impactor were turned into ejecta, they lie furthest from the impact crater.

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  • $\begingroup$ The distance of ejected material from crater is indeed a very convincing evidence, thanks! $\endgroup$ Nov 2, 2022 at 9:14
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This is an application of Occams razor.

If you look at the paper on Science, you'll see it doesn't talk much about the ice at all! Just a couple of asides, in a caption to the picture:

The light-toned material [...] is inferred to be water ice ejected during the impact.

We know from other impacts that ice is often exposed (eg Widespread Exposures of Extensive Clean Shallow Ice in the Midlatitudes of Mars). Those impacts can't all be by comets. And we would expect the impactor to be completely destroyed on impact, and any water vaporised (the impact had the energy of a small atomic bomb). So that would not be consistent with "boulders of ice". So the simplest explanation is that the water ice seen here is from the subsurface of Mars. This is what the paper means by "is inferred to be".

The application of Occams razor: accept the simplest explanation that fits the observations leads to the given conclusion. But as with all applications of the razor, further observations may change that conclusion.

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    $\begingroup$ Thanks for the link, it was an interesting read! However, the study is about midlatitudes, and says that "the lowest-latitude ice detection in the south is now at 46.2°S", while "in the northern hemisphere, the lowest-latitude detection remains at 39.1°N". The new impact is located at 35°N. I'm not saying it's impossible that there is ice at this low a latitude, but we can't say it's common. It's the first time that we find ice this low. That's why I'm wondering if there could be another explanation, or at least if the impactor origin had been considered, and on what basis we can rule it out. $\endgroup$ Oct 31, 2022 at 10:25
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    $\begingroup$ Well, there are lots of difficulties with supposing the ice was in the impactor. Perhaps these could be overcome, perhaps they would the hypothesis out. The main on is that the impactor would not leave "boulders". However, if it looks like a duck, and quacks like a duck... the chances are... $\endgroup$
    – James K
    Oct 31, 2022 at 17:46
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It is not necessarily a comet if the ice/snow was from the impactor, because asteroids called chondrites have water and chondrites have high mass (approx 1000 kg chondrite found at earth because chondrites have iron in the Jilin meteorite shower) but there is no proof that it was a chondrite.

So the Martian atmosphere (100 times thinner than Earth's) still could cause a comet to evaporate (because the comet would start regelation and vaporize (due to the collision of air molecules in front of the comet creating pressure and heat. ) as it is more volatile than iron (2,862 °C), nickel (2,730 °C), copper (2,562 °C) and will condense and freeze (which will not make the bolders of ice ), which would also not cause the impact and seismic activity and If it was a comet the ice would not cling to the boundary of the crater but would rain from the cloud and be a bit larger in radius.

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