How do we know that ice from the S1094b event was from Mars surface and not from the impactor?

Question

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 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.

Answer 1

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:

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:

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

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

Answer 2

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.

Answer 3

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.