Situation: A planet similar in size to the Earth has a Eurasia+ sized continent at its South Pole. It is hit by a huge asteroid near its South Pole. This would produce a very large crater/basin. Things would be very bad for a long time, but after a long while equilibrium of sorts would be reached. Assuming that the basin was completely landlocked, I assume that snow and ice would slowly start to collect in it and be unable to escape.

Could such an impact basin be large and deep enough to absorb all of a planets oceans as precipitated ice and snow and leave the rest of the world a desiccated desert?

I assume that this would not be possible with Earth’s oceans as their volume is simply too large, but perhaps a planet with much less water - one tenth-one to one hundredth of Earths water?

If not what would stop it?

Assume no magic and all physical laws are as we would expect.

  • $\begingroup$ If snow and ice are unable to leave the basin, how do they get inside? $\endgroup$
    – Cadence
    Jun 18, 2020 at 19:53
  • $\begingroup$ @Cadence by snow fall from clouds. So water in the form of ice can get in, but glaciers have nowhere to flow to so can't get out, at least initially if my assumptions are correct... $\endgroup$
    – Slarty
    Jun 18, 2020 at 20:02

1 Answer 1

  1. We have a perfect example on our own Earth.

    Sea levels at present are very much lower than what is usual in the last few hundred million years. This is due to the Antarctic continent trapping a significant part of the planet's liquid water, so that global sea levels fell by about 60 meters (180 feet) when the Antarctic ice sheet formed about 40 million years ago.

    (Actually, it was not at all sudden; it took some ten or fifteen million years for the Antarctic ice sheet to reach its current thickness.)
    (Fun factoid: when the Antarctic ice sheet formed, the CO2 level in the atmosphere was between 20% and 60% higher than what we have now.)

    Luckily, the average depth of Earth's planetary ocean is about 3700 meters (12,000 feet), so that our cold Antarctic continent traps only about 1.6% of the water in the ocean.

  2. I have no good idea of how you could do much better than our own Antarctic.

    You can imagine some contrived geographical arrangement which could trap maybe twice the amount of water trapped by the real-life Antarctic, but not really much more.

    • You cannot really have a very large very deep basin isolated from the ocean.

      The largest deep dry basin which ever existed on Earth is the Mediterranean sea, which, at a certain point of its history, became isolated from the ocean and dried out; it has an area of about 2.5 million square kilometers (about one million square miles) and an average depth of 1500 meters (4900 feet), giving a volume of 3,750,000 cubic kilometers (900,000 cubic miles). But that's only 0.7% of the volume of the planetary ocean, less than one half of the volume of water trapped in the Antarctic.

    • The thing with large impact craters is that they cannot really be all that large. If Earth were hit by an asteroid or something capable of creating a crater with a volume similar to the Mediterranean, the energy released by the impact would be so high that the crust will melt, thus immediately filling the hole. Liquids don't really suffer holes in their surfaces.

  3. Water is a weird substance. In particular, water ice sublimates, that is, solid water ice transitions (slowly) directly into gaseous water vapor without passing through an intermediate liquid phase. (That's why washed clothes will dry if hung outside during dry cold days in winter; housewives knew it since times immemorial.) This means that a small amount of water will always escape the cold reservoir.

  • $\begingroup$ Probably fair to say that the greater the size of the basin the less likely it is to be isolated from the sea, but I suspect in the unusually circumstance given (a very large polar continent with a polar impact) it is at least possible. You make a good point concerning melting, but very large impact basins have formed on other planets, such as the Hellas basin on Mars which has a volume of 20 - 30 million cubic km en.wikipedia.org/wiki/Hellas_Planitia $\endgroup$
    – Slarty
    Jun 19, 2020 at 6:31

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