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Theoretically if we could figure out some way to get Mars' air pressure above the triple point of water, it would become completely habitable for most earthly plants, because (theoretically) if the air pressure were higher, the greenhouse effect would be stronger and the temperature during nights would stay high enough to keep the plants alive. The moon, on the other hand, regardless of how much frozen water it may have, will NEVER become habitable by any kind of known creature. And the moon's element composition is not such that it would be economical to mine it without some sort of ecosystem going. Everything it has we have on earth-- and more. (Some astronomers think the moon was created from the earth. I forget what that strange theory is called.) All the moon is good for is fancy-- there is no reason to set up a permanant colony there, unless it makes travel to Mars easier or faster. And I'm not swayed very much by that argument.
I'm not sure this is really the place, but let me explain to you why Mars currently (and maybe never) has no life. If Mars was the same size as earth or Venus, it probably WOULD have life. Venus has no life because (and I'm sure everyone knows this) it's too close to the sun and a runaway greenhouse effect overwhelmed it, pushing its temperatures to around 650K (unbearable by any known life) and its atmospheric pressure to around 90X the earth's (such that every probe we sent there basically imploded within minutes). The sulfuric acid clouds don't help either, but that's a result of the greenhouse effect.
Mars is such a distance from the sun that if it had an atmosphere the same strength as earth's or Titan's current ones or Venus' former one, its greenhouse effect would give it an average climate only a few degrees less than earth's. Secondary planetary atmospheres (initially all planets have hydrogen/helium atmospheres, like the 4 major outer planets still do) are assumed to have been created by outgassing. Even today the earth is still "outgassing"-- that is, releasing gasses from its crust which rise and become part of the atmosphere until the lighter gasses (hydrogen and helium mainly) escape because earth's gravitational pull isn't strong enough to hold them. The rest (C02, oxygen, nitrogen, etc.) usually combine with other elements and the molecules become too heavy and fall back to earth.
Some outgassing is just natural (sort of like fumes) but during the teritary planets' early stages the primary mode was volcanic activity. And here's where Mars went wrong. Ignoring Io for a second, the bigger a body is, the longer it will remain geologically unstable. Venus and Earth are basically the same size, and today both of them still have volcanic eruptions and earthquakes, although FAR less than they did say, 3, 4 billion years ago, when their cores were still very hot (from the creation) and therefore still highly unstable. Mars is roughly half the size of either of them. Its core cooled down much more quickly than either of them (can't remember exactly when, but about within a billion years of its creation because the formula is exponential to size). Therefore Mars hasn't had any significant volcanic activity-- or outgassing-- in billions of years.
(As I already mentioned, Io is different, because its so close to massive Jupiter that the gravitational pull makes its core unstable-- that's why it has volcanoes. There are closer satellites, but none big enough to be geologically active to begin with.)
Now here's where your mind will be blown, because I'm well rehearsed in exobiology, meteorology and geology and I still don't quite understand it.
At some point a few hundred million years after the creation of the solar systems, Venus, earth and Mars were EXACTLY alike, atmospherically speaking. The original helium and hydrogen atmospheres had escaped into space and outgassing had produced a mostly carbon dioxide atmosphere of about 1000 mb (760 is what the earth is today). Water oceans existed on all 3 planets because the pressure was easily within the triple point of water. (For those non-physics majors, liquid water can only exist at a certain range of pressure-- the triple point is more or less this range. Below the minimum ice doesn't really melt-- it just sublimes into water pressure and skips the liquid state altogether.)
Venus, as I said, was too close to the sun-- even though it had the same size atmosphere as earth, it was heated too much and its oceans gradually turned to vapor-- which in turn made the atmosphere heavier-- which made the greenhouse effect stronger, which heated the planet up even more, which caused more water to evaporate-- RUNAWAY GREENHOUSE EFFECT. All of its water ended up in the sky before life had a chance to form.
If the earth had been a few million miles closer to the sun, the same thing would have happened. Instead an uneasy temperate balance eventually ensued-- for a while. During this period of time this extra distance bought, lightning, sunlight and volcanic activity sparked the primordial soup of organic carbon materials in its waters and through complicated processes that biblical scholars say are hogwash, single cellular life arose. (I feel no need to get in an evolution/spontaneous life debate here.)
Now what's really important is that this original life was plantlife-- it made its own food through sunlight and the plentiful supply of CARBON DIOXIDE and released OXYGEN "waste" back into the atmosphere. What the early plants did for life is take the carbon dioxide out of the atmosphere to slow down the greenhouse effect AND replace it with a friendlier gas. The CO2 stayed in their bodies until they died, when the CO2, like them, became part of the earth. Had these plants not done that, eventually the earth, too, would have had some sort of runaway greenhouse effect, ultimately destroying whatever life had been created. The reason the earth would have had a runaway greenhouse effect is because the stupid planet kept right on outgassing carbon dioxide and eventually the atmosphere would have gotten so thick with it if not for the plants that it would have gotten bigger than Venus' and therefore just as strong, despite its farther distance from the sun.
Mars, however, basically stopped outgassing during this time because it was so small. Well, as it turns out, plants aren't the only things that get rid of CO2. So do OCEANS-- eventually CO2 falls into the ocean and combines with water to form carbonates. Which is what happened to Mars. With the outgassing/carbonate cycle broken, most of its CO2 fell into the ocean and became carbonates. And remember, that's what most of its atmosphere was to begin with-- CO2. At this point Mars was doomed, because without the outgassing its atmospheric pressure fell and fell and fell until it was down to a few milibars (just as today). As it turns out, the minimum triple point of water is about 6 or 7 milibars-- OOOOPS. Without liquid water, life as we know it cannot exist. Furthermore, 6 or 7 milibars creates a pretty weak greenhouse effect, and Mars also turned too cold for liquid water to exist. Well, on average-- sometimes the equator might rise above freezing-- but the water still doesn't melt. The pressure is too low. It just sublimes from ice into water vapor.
If any life arose during Mars' oceanic stage, it was probably quickly eradicated once the outgassing stopped and the atmospheric pressure fell.
As far as colonies are concerned, the temperature of Mars is not important-- people live at least temporarily in Anarctica, don't they? And Mars' equator usually is warmer than Anarctica. Furthermore, completely sun-powered greenhouses could easily raise the temperature inside to the point that plants could grow in Martian soil, using Mars' own CO2 and releasing oxygen into the atmosphere, making it non-lethal to animals after a few... well... million years, probably!
But that's not the big problem. The pressure is the big problem. No matter how hot the greenhouses make it, liquid water still can't exist without enough pressure. And creating that type of pressure requires substantial energy, even for a little greenhouse. The question is, is it worth it? Getting to Mars is a much simpler issue than living there, which is why we haven't gone there yet.
As for Titan, considering we can't see its surface, it has to be a dark, creepy world. And definitely cold. I'm not sure anything can be done for it. Attempting to make the atmosphere stronger is pointless-- at that distance from the sun, instead of increasing the greenhouse effect you're just making the planet darker. And I don't think we want to know about any kind of life that can live off methane!
Life on Europa would have to be extraordinarily weird. Basically we'd have to cut a hole through the ice (which, if I'm not mistaken, is about 100 km thick), stick a tube down to the water and either live on the surface ice in what are basically grounded spaceships or live in the icy water in submarine-type things. Seeding Europa... the problem here is that probably no sunlight reaches the ocean, and most earthly life requires sunlight at some stage of the foodchain. Not to mention the fact that although Europa has liquid water beneath the ice, it has no atmosphere at all! We'd have to bring on or make one-- which leads us right back to the Martian problem.
Despite its drawbacks, Mars is definitely the way to go. All we need is a good solution to the pressure problem and we'll be breathing nice fresh Martian air in no time!
wasting server space, your neighborhood astronomer,
The Creep, The Freak & The Loser
PS I don't think I made one part of this clear (dealing with carbonates) but I'll worry about that if somebody actually reads this and asks.
By the way, I'm sure someone will ask: I know all this because of A) several astronomy classes B) lots of unnecessary geeky reading C) a few years ago, back when I thought I would become a computer programmer, I was working on a video game that dealt with planetary exploration and exploitation (kind of like "Master of Orion" without the war), and in order to make it realistic as possible I did a LOT of research about planetary evolution. Found out WAY more than I ever wanted to know. The game itself was never completed, although I still have a partial copy that works up to a point. Later I decided to turn the game into a screenplay. Trust me-- it's better than "Wing Commander"! Mmmm... (Bruckheimer and Spielberg are trying to beat me to the punch with "Martian Chronicle" type things in the works.)
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