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Old 02-21-2020, 01:45 PM   #1
Michael Thayne
 
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Default [Space] Getting Mars to 1% hydrographic coverage

Terraforming Mars is unlikely to be easy. This got me thinking: what's the minimal amount of Mars terraforming that would be interesting in GURPS terms? Looking at the habitability rules in GURPS Space, I notice that any hydrographic coverage, even 1%, is good for +1 to habitability. So what would it take to get Mars there?

My understanding of the physics is that in order to have liquid water of the long haul, you first need to add lots of water vapor so that the water vapor and liquid water can be in equilibrium. Average surface temperatures of a newly-terraformed mars are likely to be just above freezing, so we can probably use the vapor pressure of water at 0 C for these estimates, which happens to be just over 0.006 atm, coincidentally roughly equal to Mars' current atmospheric pressure. So we're talking about getting Mars' atmospheric pressure to at least 0.012 atm, which is technically no longer "trace" atmosphere according to GURPS but instead "very thin", not that that will do much for Mars' habitability by itself.

At first I thought this would also mean doubling the mass of Mars' atmosphere, but given that Mars' atmosphere is mostly CO2 and IIUC gas pressure is a function of number of molecules rather than mass, we might only be talking about a 40% increase in the mass of Mars' atmosphere. Since Mars' atmosphere is about 25 trillion tons, that means we need about 10 trillion tons of water. My big question is: how much of that could you get by melting Mars' polar ice caps with big solar mirrors? That, I think would represent less than 1% of the ice found in Mars' polar caps, though I really don't know how much of those you're going to melt even with solar mirrors that get the average surface temperature above freezing.

So, people who have more expertise on this sort of thing than I do, what say ye?
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Old 02-21-2020, 03:43 PM   #2
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Default Re: [Space] Getting Mars to 1% hydrographic coverage

You only actually need equilibrium where the bodies of open water are; most parts of earth's surface have less than 100% relative humidity. Unfortunately, you also need to account for all the water which condenses out of the atmosphere in places other than your bodies of open water and does not immediately return to them (i.e. it freezes or seeps into the ground and stays there).

Also, atmospheric pressure at the surface is equal to the weight of the atmosphere above that same area of surface, though the lower molar mass does mean the scale height of the atmosphere will increase.
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Old 02-21-2020, 05:34 PM   #3
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Default Re: [Space] Getting Mars to 1% hydrographic coverage

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Also, atmospheric pressure at the surface is equal to the weight of the atmosphere above that same area of surface, though the lower molar mass does mean the scale height of the atmosphere will increase.
D'oh. I was thinking of the ideal gas law, but that's for pressure due to heat (or at a more micro level, the momentum of individual gas molecules), not pressure due to gravity.
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Old 02-21-2020, 05:47 PM   #4
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Default Re: [Space] Getting Mars to 1% hydrographic coverage

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Originally Posted by Michael Thayne View Post
D'oh. I was thinking of the ideal gas law, but that's for pressure due to heat (or at a more micro level, the momentum of individual gas molecules), not pressure due to gravity.
You're thinking of Gay-Lussac's law. The Universal Gas Law
PV = nRT
combines Boyle's, Charles', and Gay-Lussac's laws into a single relationship among four variables. Note that only R is a constant in that equation. The atmospheric gasses conform to that law (and to the hydrostatic condition giving pressure as a d.e. of altitude (give or take local weather), so therefore to the barometric formula), but they accommodate changes in temperature and molar quantity by changes in volume (a.k.a. by expanding upwards, a.k.a. altering the atmospheric scale height).
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Old 02-22-2020, 07:48 AM   #5
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Default Re: [Space] Getting Mars to 1% hydrographic coverage

Given that the temperature of Mars at the equator can actually get fairly high - up to 70 degrees F (20 degrees C) on a summer day, it would probably be easier to create hydrographic coverage there. The main difficulty would be getting enough liquid water that it doesn't just freeze over the night (temperatures even in summer on Mars can get down to -100 F/-73 C) and stay frozen; also needs to not freeze completely during the Martian winter, or only so much that it can still melt in the spring.

Of course, adding water and water vapor could prevent temperatures from plummeting as far as they do, now.
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Old 02-22-2020, 08:14 AM   #6
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Default Re: [Space] Getting Mars to 1% hydrographic coverage

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).
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Old 02-22-2020, 08:30 AM   #7
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Default Re: [Space] Getting Mars to 1% hydrographic coverage

Wouldn't a nice dusting of black carbon also help with warming the local region a bit? Or would that cause a bigger hassle than solve?
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Old 02-22-2020, 08:52 AM   #8
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Default Re: [Space] Getting Mars to 1% hydrographic coverage

One of the advantages of adding water vapor to the atmosphere is that it's heavy (relatively), so even when it boils off, it should stay on Mars for far longer than lighter gases.

Adding more CO2 would certainly help, though. If you have sufficiently convenient wormhole technology, you could transfer hot CO2 from Venus to Mars, though I'm not sure how much H2SO4 would also be swept up. Probably depends on how high up the Venus end of the wormhole is.
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Old 02-22-2020, 09:17 AM   #9
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Default Re: [Space] Getting Mars to 1% hydrographic coverage

That would not be a good idea, for a couple of reasons. First, the relative orbital velocity of Venus around the Sun is around 10 km/s, meaning that the velocity of the atmosphere would exceed the escape velocity of Mars. Second, the hot CO2 would be hot enough to escape Martian gravity even without the extra velocity. Even with superscience, it would make more sense to mine it in the Main Belt and just ship it (portals would still be a bad idea because of the relative velocity difference).

Water vapor breaks up into hydrogen and oxygen in the upper atmosphere relatively quickly without a thick atmosphere, so the loss of hydrogen would be substantial without a decent atmosphere. At 5% Earth atm, the rate would probably be around 1% every 2500 years, so it would be manageable for a short time. At 100% Earth atm, the rate would be 1% every million years, which is quite good.

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Old 02-22-2020, 12:40 PM   #10
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Default Re: [Space] Getting Mars to 1% hydrographic coverage

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Originally Posted by Prince Charon View Post
One of the advantages of adding water vapor to the atmosphere is that it's heavy (relatively)
It's much lighter (molar mass 18) than nitrogen (28), oxygen (32), or carbon dioxide (44) and not appreciably heavier than methane (16) or ammonia (17).
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