[Space] Getting Mars to 1% hydrographic coverage

[Agemegos]

Originally Posted by AlexanderHowl

You need enough atmosphere to prevent water from boiling off.

No you don't. The vapour pressure of water is determined by temperature alone, and the partial pressures of gasses are separable. Which means that if you add enough other gasses to the atmosphere to raise the boiling point of water to above the ambient temperature the water will evaporate anyway — it will just do so without boiling. You need enough water to make up the water-vapour component of the atmosphere at the prevailing temperature, plus some over for the liquid cover. The amount required depends on temperature but not the presence of other gasses. The water vapour will provide enough pressure to prevent boiling, since boiling occurs only where saturated vapour pressure exceeds ambient pressure.

In other words, if you put a lot of water on the surface of Mars it would boil owing to the low pressure, and go on boiling until either it established an atmosphere of water vapour with enough pressure to halt the boiling, or boiled away short of that point. If you added enough other gasses to suppress the boiling then the same amount of water vapour would evaporate off as boiled off in the other case, and the water vapour component in the total atmosphere would be as massive as the atmosphere of water vapour in that other case.

EXCEPT THAT the other gas you added would exert a greenhouse effect, which would warm the surface, raise the saturated vapour pressure, increase the amount of water vapour in the atmosphere, and cause more water to evaporate away from the liquid bodies.

And if you are thinking about the thermal escape of water vapour to space (Jeans escape), that's controlled by temperature, molecular mass, and the escape velocity. Through it may be termed "boiling off to space" it's more like an evaporation; pressure doesn't come into it.

[Agemegos]

It might be attractive to start making your first lake on Mars at Hellas Planitia, which is a vast impact basin seven kilometres deep. It is deeper and smaller than the northern polar basin, so it would be easier to fill with a pool of vapour, and it already retains a barometric pressure twice that at the datum elevation.

To a certain extent, Hellas Planitia might be where your water ends up whether you want it to or not.

[DanHoward]

It isn't possible to terraform Mars without a thicker atmosphere and the atmosphere isn't possible without restoring the planet's magnetic field. Without a magnetic field, any atmosphere you try to build will be stripped away by the solar wind.

[Agemegos]

Originally Posted by DanHoward

It isn't possible to terraform Mars without a thicker atmosphere and the atmosphere isn't possible without restoring the planet's magnetic field. Without a magnetic field, any atmosphere you try to build will be stripped away by the solar wind.

How quickly, though? If we had the absurd shipping capacity to supply Mars with an atmosphere to start with we might be able to keep it topped up.

[dcarson]

Originally Posted by Agemegos

How quickly, though? If we had the absurd shipping capacity to supply Mars with an atmosphere to start with we might be able to keep it topped up.

Can't find tha article I read years ago but it had the Moon retaining a breathable atmosphere for at least 10s of thousands of years if you created one on it. So at least that long I'd say, maybe 100s of thousands.

[Michael Thayne]

Originally Posted by AlexanderHowl

You need enough atmosphere to prevent water from boiling off, which would realistically be around 5% Earth at the equatorial region. Given the area and gravity of Mars roughly balances out for this purpose, that would require around 250 trillion metric tons of gases (preferably carbon dioxide because of its greenhouse properties). The ice caps of Mars possess insufficient carbon dioxide, as their dry ice reserves are just frozen portions of the atmosphere that melt every summer. At the very least, you would likely need to transport ~68 trillion tons of carbon from the Main Belt (you can liberate oxygen for cheaper from the rocks of Mars).

Gah, how did I manage to forget that the boiling point of a liquid is affected by temperature?

Originally Posted by Agemegos

No you don't. The vapour pressure of water is determined by temperature alone, and the partial pressures of gasses are separable. Which means that if you add enough other gasses to the atmosphere to raise the boiling point of water to above the ambient temperature the water will evaporate anyway — it will just do so without boiling.

AlexanderHowl's point—which I'm now slapping my forehead for not thinking of myself—is that keeping water liquid will require both raising the overall pressure to about 0.05 atm and raising the partial pressure of water vapor to about 0.006 atm. Either alone won't cut it.

[Agemegos]

Originally Posted by Michael Thayne

Gah, how did I manage to forget that the boiling point of a liquid is affected by temperature?

What? The boiling point is a temperature.

AlexanderHowl's point—which I'm now slapping my forehead for not thinking of myself—is that keeping water liquid will require both raising the overall pressure to about 0.05 atm and raising the partial pressure of water vapor to about 0.006 atm. Either alone won't cut it.

I don't think that is correct.

A liquid boils at the temperature where its vapour pressure is sufficient to open a bubble against the ambient pressure. So high pressure will suppress boiling, i.e. raise the boiling point. Whenever you see a liquid boiling it must be the case that its saturated vapour pressure is higher than total ambient pressure, and that means that its saturated vapour pressure must be higher than the partial pressure of its vapour above it. And that is the definition of the air above it not being saturated with vapour. A saturated vapour always exerts enough pressure to prevent boiling at the ambient temperature¹; an unsaturated vapour admits evaporation even if the liquid is not boiling.

So it doesn't matter whether the liquid is boiling. What matters is whether the vapour above it is saturated. If not, your lake or sea will evaporate and blow away even without boiling.

_______
¹That's how pressure-cookers work.

[Anthony]

Originally Posted by DanHoward

It isn't possible to terraform Mars without a thicker atmosphere and the atmosphere isn't possible without restoring the planet's magnetic field.

To be fair, on the scale of terraforming, making a superconducting loop magnet capable of shielding a planet isn't all that big.

[Fred Brackin]

Originally Posted by Anthony

To be fair, on the scale of terraforming, making a superconducting loop magnet capable of shielding a planet isn't all that big.

You could dig a tunnel from pole to pole through Mars' dead core and set up a rather large bar magnet as an alternative.

[Anthony]

Originally Posted by Fred Brackin

You could dig a tunnel from pole to pole through Mars' dead core and set up a rather large bar magnet as an alternative.

Dunno how hard that would be, but in general the handy thing is that the energy content of a magnetic field is proportional to volume times intensity^2, and the intensity required is inversely proportional to the radius of the screened volume, so the energy requirement is linear in radius. It's generally going to beat mass shielding for more than a few hundred meters or maybe a kilometer.