Actually, yes it can. It can thicken the atmosphere enough and kick up a lot of vapor to change the climate on Mars forever. It could very well do that and there is evidence that suggests that it can thicken Martian atmosphere and after the dust settles the planet may warm itself up.

There is always a chance for anything. have you witnessed a comet that size smashing into Mars? No? Okay then. So an appropriate answer would be to reserve possibility it could happen.

If you want to talk about stripping away an atmosphere then you could explain to me why Earth still has an atmosphere after the large meteor impact occurred that wiped out the dinosaurs?

If you want to get technical, Mars acquired its thin atmosphere because of the very weak Magnetism it has. Unlike Earth, Mars is not safe from the Sun's radiation and the components in the atmosphere are hurled into space.

Lastly, another theory is that the Universe is Expanding... true? Yes, very true. So is it the continents spreading apart or is it actually the Earth expanding and separating the land masses? Like I said, anything is possible.

"a comet strike on Mars could jumpstart terraforming efforts by delivering a big load of volatiles (water, methane, CO2 and so on), plus a lot of heat energy that could release water and other volatiles from the martian soil."

An asteriod/meteoriod of that size would very likely produce a large amount of meteorites, but they will be fragments.

Yes. I didn't mean that none of it would penetrate, just that it would not penetrate intact; i.e. that it would disintegrate above the surface.

I think it's safe to say that nothing will penetrate intact, no matter the size, composition, speed, etc.

Of course it will... there are plenty of size/composition/speed combinations that can allow for intact atmospheric passage. If that wasn't true then none of our astronauts would have ever made it back to Earth alive.

A sufficiently large or dense or slow object could survive, and this has happened many times in the history of our planet.

And when I say intact, I don't mean 0 mass loss during atmospheric passage, I mean insignificant mass loss.

If I lend you my car and you return it like this then I would hardly call it intact

Sufficiently slow, yes, but the object has to obey the laws of gravity, so it can't go below 11 km/t

And I car wouldn't look as good as this if it entered the atmosphere at 11 km/s...

Can you give any examples from the history of our planet?

If you look at meteorites, even dense ones, you can clearly tell from their surfaces that they have had a brutal encounter with our atmosphere. Even if the mass loss was insignificant (which I still believe unlikely, but I don't deny the possibility that more than half of the object can survive is a single piece), I would hardly call it intact.

Yeah, you'll be totally intact if you fall off a cliff, but not after the landing...

That is exactly what I have been trying to say this whole time!

Indeed. I was just trying to reinforce it.

We're obviously talking a bit past each other here.

Even meteors too big to disintegrate before impact, will be explosions in progress, they just haven't had the time to finish at impact. If the Earth somehow got replaced with vacuum and the atmosphere stayed as a shell, I think you would need a very big object not to break up at all having gone through the atmosphere. But this is getting very hypothetical.

I wouldn't say it's all that hypothetical

Hypthetical in the sense that it's an even more stretch of the term "intact" if the object, though still in one piece technically, already before impact has seismic waves inside which will surely rip it apart if it wasn't for the impact, but it hasn't been time for it simply because such waves travel slower than the impact speed, and surely there will be "superseismic" effects as well, such as from the (vaporised) front of the impactor travelling slower than the rear part.

[etc]

The impact program seems to agree on that, but it seems to miss one possible outcome totally, that the object simply bounces off Earth's atmosphere if the angle is very low. The program will have any object reach the ground intact no matter the size if the angle is small enough. In reality the object would go into space again.

Your understanding of the physics of meteor fragmentation is incorrect.

Meteors do not breakup because of seismic waves traveling through them.

They break up if the differential pressure over their surface is greater than their material strength, which leads to fracturing.