Canid117 said:
The moon has water. As does Mars, and many other bodies in our solar system. Hell, comets are mostly ice. Water is not as rare as you seem to think. Our proximity to our sun is also not incredibly rare. Scientists have discovered several planets of similar size to earth which are a similar distance from their own suns. All of this is assuming that the aliens eat drink and breathe the same things we do. Do you have any idea how huge the galaxy is? Millions upon millions of solar systems each one of them with the potential to support life and with all the different ways that life can exist you are still assuming that we are going to run into something like Klingons? Humans with slightly different facial structure and green blood? We don't have shit that aliens want so why worry?
Water, but not LIQUID water, not GIGANTIC OCEANS, lakes and swamps of water, not a Water cycle of evaporation, rain and ice and so on. Those planets, moons and comets cannot in and of themselves easily support life of a wayward alien race.
I really can't even begin to fully explain how vital water is for life, all life, even alien life. Scientist have gone as far as to say complex life cannot develop on a planet WITHOUT large quantities of liquid water. Aliens may not breathe oxygen, they may not be able to eat the hydrocarbon chains we consider food, but almost certainly they will be hugely dependant on not just water to drink occasionally, but as a key part of their environment.
Water is a substance that is perfect for maintaining equilibrium, it is a massive heat store, it soaks up the suns energy preventing overheating on the sunny side, and on the dark side it acts like a giant hot-water bottle, preventing everything freezing. That's why deserts get so incredibly hot during the day and frostbite inducing cold at night, no water to act as a heat trap.
You know water has a heat capacity 8 times as high as steel? That means 1 kilo of water at 100-deg C has Eight times the energy of a 1 kilo block of steel at the same temperature.
The fact that most living creatures have a large proportion of their body mass as water is to aid in thermo-regulation, enzymes for maximum efficiency are only stable within a very narrow temperature range. Then of course sweating can quickly cool off.
Evaporation and precipitation acts as effectively a chemical scrubber for the atmosphere, it is the reason why Venus is choked in clouds of sulphuric acid, it had no water and rain cycle to scrub the volcanic gases that built up over billions of years. Rainfall keeps our atmosphere stable and regular, and to spite our best efforts also regulates the concentration of CO2 in the atmosphere.
The ice water on the Moon, Mars and in other parts of the solar system will be vital for any space operations either by us in the future or any "visiting" aliens but it is not sustainable, it can only be used when processed by large machinery for artificial environments.
This is why scientist are so keen to find anywhere else in the Solar system or known universe that has liquid water. It really is a totally unique compound, as it makes a planet either a dead-bare planet or hellish acidic planet. In fact I have heard that it is so important it is not only considered practically a requirement for complex life but even a GUARANTEE, that if liquid water has existed on a planet for long enough (few billion years) then spontaneous life of some sort is inevitable.
there is also a pretty good chance any alien species will breathe oxygen at least in part, what is the worrying thing is what they exhale, they could easily breath in nitrogen gas and exhale hydrogen cyanide (which is not inherently dangerous to all life). There is also a chance a gas we consider inert and harmless to us and breath in all the time may be toxic to them, like Argon which makes up almost 1% of our atmosphere (doesn't have to be reactive, just block some of their biological chemical-receptors).
Most likely any difference will be in the detail, such as they are comfortable with a very different partial pressure of oxygen, and a different secondary gas, perhaps even helium.
The idea is Alien spaceships could simply enter our earth's atmosphere, land and get about colonising, mining and stripping resources not to leave but to stay. They wouldn't need space suits to work in the open, likely only a gas mask, though they may even resort to genetic modification of their own bodies or some other adaptation to breathe our unique atmosphere and likely the UV radiation from the sun will be either too high or too low for their liking. But genetic modification for dealing with a dry, or frozen planet, nuh uh, ain't gonna happen, if it can't be done in 3 billions yeas it likely can't be synthesised by science in any decent time.
One distinct possibility is they will attempt geo-modding, or should I say "Xeno-forming", transforming our planet to more suit their own adaptations. That's the way we humans do it, we change our environment to suit ourselves.
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