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The setting: In trying to terraform Mars quickly, I am imagining that Phobos, (the bigger of its two moons) holds ten-quadrillion tonnes of water ice beneath its stony regolith. I am further supposing that Phobos is shattered by controlled impacts/explosions, sending this ice raining down on Mars as comets to fill up its empty seas. (This is not important to the question)

The question: I’ve learnt that ten quadrillion tonnes of water is significantly less than the 1.5 quintillion tonnes of water found on Earth. However, given that Mars’ surface is only equal to that of all the earth’s land, how much of Mars would be covered by this amount of water and would it be enough?

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  • $\begingroup$ Now suppose this would work, how to get all the ice melt... oceans consist of fluid water. $\endgroup$
    – Goodies
    Commented Jan 14, 2023 at 11:29
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    $\begingroup$ Plausibly relevant XKCD. $\endgroup$
    – Cadence
    Commented Jan 14, 2023 at 12:29
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    $\begingroup$ A gentle reminder that you've made more than 70 questions and received nearly 260 answers and yet you've only managed to accept a single one. Consider going back to your previous questions and either accepting some answers, or clarifying why the current answers are inadequate. $\endgroup$
    – Starfish Prime
    Commented Jan 14, 2023 at 12:42
  • $\begingroup$ Enough for what? $\endgroup$
    – Mary
    Commented Jan 14, 2023 at 15:56
  • $\begingroup$ "ten quadrillion tonnes" is a measure of weight, not mass nor volume. It is unclear if this is earth-weight or mars-weight or phobos-weight. $\endgroup$
    – user535733
    Commented Jan 16, 2023 at 0:16

3 Answers 3

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All of physics is revised.

Phobos is only 10 trillion tons. If it somehow has magically generated a million/ thousand times more water than its mass that water is clearly supernatural and unusual, and we can't really predict the properties of that water

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Probably a few small seas

Assuming an ice content of 50% (by mass, of course), Phobos has approximately 5.0916208 × 1015 litres of water, or about 5 quadrillion. Phobos is not 100% water, its surface is surprisingly mars-like. I'm not sure how to calculate the depth of water, but I think this should fill a few small seas. Earth's oceans are 1.355 sextillion litres, but mars is much smaller than Earth. Assuming the water is spread evenly across the planet, the hellas planitia should fill up first, as it's the lowest.

And as for if it's "enough", unfortunately, not likely. Of course, the surrounding areas could be habitable, but then the rest of the planet would be barren. I don't know enough about climate stuff to be sure, but I think the surrounding land would likely be habitable.

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Ignoring the questionable numbers for the amount of water you're hoping to get: Phobos is an irregular body about 27x22x18 km in size, which should directly give you a rough idea how big a body of water you could possibly fill with it. More precisely, it has has about 5800 km^3 of volume, a little more than Lake Michigan. With that volume of water, you could fill a ~300 km diameter crater to a similar depth.

Additionally, remember that the ground is somewhat porous and permeable. How much water soaks into the ground will be both very dependent on location and very difficult to estimate based on current data.

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