I know the surface gravity of Mars is 38% of ours and the liquid core has already shut down a long time ago, so its water cycle is virtually non existent. Weather forecasts are also bad: a seasonal G5 solar storm is just too frequent.

Nonetheless I have great faith in Mars's gravity and magnetic field to protect a large body of water similar to our own ocean. In that case I wonder would the Martian ocean last or this is an incorrect assumption? This is a thought experiment, just handwave how the water ends up there.

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    $\begingroup$ Wikipedia: "Mars does not have a global magnetic field." It does mention a "remanent" field, but that only exists in the southern hemisphere. $\endgroup$
    – Tom
    Dec 10, 2023 at 7:30
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    $\begingroup$ what-if.xkcd.com/54 has all the answers you could wish for. $\endgroup$
    Dec 10, 2023 at 9:04
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    $\begingroup$ Downvote for clear lack of research. $\endgroup$
    – jdunlop
    Dec 10, 2023 at 10:08
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    $\begingroup$ Mars is at an average of -60C. The ocean would freeze solid, and then stay that way forever. Water vapor is not a potent enough greenhouse gas to raise the temperature above freezing. $\endgroup$
    – causative
    Dec 10, 2023 at 19:02
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    $\begingroup$ @causative Some parts of Mars get above freezing temperature during the day, so some water would turn to vapour (no liquid phase given the ambient pressure), and either be stripped off the planet like most of the atmosphere, or refreeze (possibly in a different location). So the ocean would very (very very) slowly reduce and change shape. $\endgroup$
    – jcaron
    Dec 11, 2023 at 13:33

3 Answers 3


The average atmospheric pressure on Mars is 610 Pa.

Water at 0 C has a vapor pressure of about 610 Pa, too, but at that pressure is still a liquid.

Immediately after being magically poured on Mars surface, some of the water of the ocean would start evaporating, cooling the remaining part until the liquid reaches about 0 C. This evaporation would look like a briskly boiling almost everywhere. Meanwhile the water vapor would saturate the atmosphere, reaching an equilibrium.

Solar activity would slowly break down water molecule in the atmosphere and carry them away, and this would slowly deplete Mars from the newly acquired oceans. But don't despair, it will take some million years to fully dry Mars up, so you have time to build that beach resort funded by those venture capitals.

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    $\begingroup$ next question would be, how long would it take for humans to move all the water on Earth to Mars? $\endgroup$ Dec 11, 2023 at 3:18
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    $\begingroup$ Are you saying most of the water (ie the not evaporating part) would quickly freeze? $\endgroup$
    – quarague
    Dec 11, 2023 at 8:13
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    $\begingroup$ @quarague Prima facie yes, in that scenario. After all, even CO2 freezes on the poles. But then we should ask an atmospheric scientist what the greenhouse effect of so much water vapor is (it is surely strong). I suppose enriching Mars's atmosphere with water vapor is at the core of any terraforming plan. $\endgroup$ Dec 11, 2023 at 17:08
  • $\begingroup$ Where would the beach resort be? i.e., how much dry land would remain when it stabilizes? $\endgroup$ Dec 11, 2023 at 19:53

The entire ocean has a mass of about $1.4 × 10^{21}$ kilograms. Mars, meanwhile, has a mass estimated at $6.39 × 10^{23}$ kilograms, which means that the addition of the water would increase Mars's mass by about $0.22\%$.

This would result in an increase in gravity. From $3.71\ \frac{m}{s^2}$ (about 38% of Earth's gravity) it would go up to about $3.72\ \frac{m}{s^2}$, which would need to be accounted for when calculating spacecraft trajectories and orbits, but not much more.

Due to the low temperature and low pressure, water that is exposed to air would do weird things, but due to the high thermal capacity of water, any water that's surrounded by water would stay liquid and transform Mars from a desert planet to an archipelago with only a few islands planet-wide.

Eventually all the liquid water would freeze and the water vapor in the atmosphere would get eroded away by solar radiation, but this is something that happens on cosmic timescales (many millions of years).

  • $\begingroup$ the mass is consequent enough that its velocity must match Mars', otherwise Mars' orbit will be screwed, and possibly the rest of the solar system with it $\endgroup$
    – njzk2
    Dec 10, 2023 at 15:29
  • $\begingroup$ "all the liquid water would freeze" would it? (would the evaporated water create enough of an atmosphere to have enough pressure for solid water?) $\endgroup$
    – njzk2
    Dec 10, 2023 at 15:30
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    $\begingroup$ @njzk2 This is a very complicated physics problem, but my intuition says no. Because of the low temperature of the atmosphere, the saturation vapor pressure is very low, meaning that even a RH of 100% wouldn't result in the atmosphere being that much thicker. Since the average temp of the oceans is like 3.5 degrees, this means that very close to the surface you'd probably more wet atmosphere, but without something to kick off a spiraling greenhouse effect, I don't think it would be significant enough to stop the surface from boiling everywhere. $\endgroup$
    – Dragongeek
    Dec 10, 2023 at 20:31

LDutch has it right for a magical water exist-ifier magically exist-ifying water at Mars's surface.

The answer with actual pouring, in case that's of interest, is roughly 0 incredibly violent seconds.

Mars surface escape velocity: about $5000 m/s$

$T = 0.5mv^2$ gets us about $10^7 J / kg$ of gravitational binding energy at the surface.

Specific heat of water is about $4\times 10^3 J/kg$ per degree

Latent heat of vaporization of water is about $2\times 10^6 J/kg$ per degree

Specific heat of steam is about $2\times 10^3 J/kg$ per degree (before we let it expand)

Mars is much more massive than the ocean so the ocean will get most of the kinetic energy in the collision.

Start the water at 0 degrees (273 K) and consider a kilogram:

$4\times 10^5 J$ to heat to 100 degrees (373 K).

$2 \times 10^6 J$ to vaporize it.

$2\times 10^3 J/K$ to superheat the steam

$\text{(most of)}10^7 J - 2 \times 10^6 J - 4\times 10^5 J)/(2 \times 10^3 J/K) \approx 5 \times 10^3 K$ of temperature difference.

So a right-after-impact temperature of around 5400K.

We pour the oceans on Mars. The ocean briefly flashes (almost exactly) as bright as the surface of the sun, before a fireball of water vapor and vaporized rock explodes off of the surface. Roughly half of it is moving faster than escape velocity, so it immediately leaves forever. Most of is moving close to escape velocity, so it ends up far out in space, to be gently stripped away by solar wind or accreted by Mars' moons over the course of millions of years.

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    $\begingroup$ Does this assume a particular speed of the pouring? If so, why? $\endgroup$
    – sehe
    Dec 10, 2023 at 23:03
  • $\begingroup$ Only that it's fast enough that most of the water gets to the surface. Otherwise you get the same end result, but with less dramatic fireworks. $\endgroup$
    – g s
    Dec 11, 2023 at 0:14
  • $\begingroup$ Given the very low atmospheric pressure on Mars, the boiling temperature is a lot lower, there's actually no liquid phase. $\endgroup$
    – jcaron
    Dec 11, 2023 at 13:22

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