1) The impactor was a large icy body flung into the inner solar system by Jupiter. It would have the KE to smash the planet, but most of its remains would vaporize and be carried away by the solar wind. This would explain how Mars would briefly have enough liquid water to form the surface features we see today.
That brings up something else that puzzles me. If comets were swarming thick enough back in the day to end up burying 3/4 of Earth's surface area under several miles of liquid impact debris, why don't we see evidence for at least the same amount, if not more, flooding on Mars? Instead, AFAIK, something less than 1/2 of Mars was ever underwater, and none of that was very deep. That would make it seem like Mars took a disproportionately smaller number of comet hits.
Sure, Mars is a smaller target. However, it's got a bigger orbit so has potentially more crossing paths, and marginal comets might hit it that would vaporize it before coming this far in. Also, Mars formed slightly earlier so has had more time to be hit. And if you assume an average size for all these water-carrying comets, each hit on Mars would result in a greater proportion of its smaller surface area being covered than the same hit would cause here on Earth.
Thus, I'd expect to see evidence that Mars at one time had at least as much surface water as Earth, if not more. But that's not the case. Why not? If the water was blowing away as fast or faster than it was arriving, that would make me think it wasn't arriving very fast. But then how so did our oceans come into being so rapidly?
2) The impactor was actually Mars' twin in the same orbit ... The collision would be low velocity by astronomical standards, it would still have made a mess but without flinging chunks of planet all over the place.
3)It did make a mess, but between the tug of Jupiter's gravity and just the passage of time, all of the debris either fell back to Mars (most of its craters are "self inflicted"), escaped, with the remainder consolidated into dusty little Demos and Phobos.
Working on the assumption that the Borealis Basin hit was fairly late in the process, #3 would lead me to expect a much higher number of craters in the southern hemisphere than you'd normally expect. There'd be the expected number from the regular bombardment, plus a bunch more from debris from the big hit later. Meanwhile, the northern hemisphere would have a lot fewer craters than you'd expect, because the big hit would have wiped out all the older ones; most of the craters there would be from its own debris. Also, I'd think most of the craters from the debris would show signs of fairly low-angle impact, instead of nice round things from straight-in hits. Is that the case?
I read that since Phobos's orbital period is less than Mars's rotational period, tidal forces act to slow Phobos down and drop it into an even lower orbit (as opposed to Earth's moon where tidal forces act to push the moon into a higher orbit). Eventually, Phobos will fall out of the sky.
I've heard that, too. That would be cool to see. I'd expect this to be a very low-angle hit, which would make an interesting impact structure
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