ivier wrote:
i think you do not fully appreciate how thick the martian atmosphere really is, NASA is designing planes to fly high above it's surface.
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the creatures would not have to have much of a pressure increase to keep thier homes always above the water tripoint, there are places on mars where this already occurs, the deepest/warmest regions, like canyons, near the equator! Life on mars would have to adapt to the conditions there in order to survive, the pressurized colony is the only thing no earth life form does, and thats because no earth life form needs to.
I do appreciate the density of the martian atmosphere. It's low. Flying an aircraft is one thing - a lightweight design is feasable, especially given the lower gravity needed for the lift to overcome. Absolute density of the air is the key factor for aeronautical considerations, the absolute pressure is of lower concern. Although the two are directly related, they are not the same.
To clarify that remark, even if mars had an atmosphere at the same density as earth, the pressure would still be much less as the force exerted depends on the gravitational pull.
Going back to living systems, there are a great many things beyond the triple point of water that would affect chemical/biological properties. For one thing, the solubility of gasses decreases sharply with pressure.
When you consider how insoluable molecular oxygen is at STP here on earth, you can see that even at an optimistic 10mb pressure at the bottom of say Valles Marineris, a small puddle of water (likely to already be highly saturated with minerals) would mean that no appreciable trace of oxygen could stay solvated. Only extremophile, probably anerobic, microscopic life forms are likely to find this surface conditions endurable.
For anything more advanced, like insect life, solvating gasses in its bodily fluids (or whatever rudimentary circulatory system it may have) is likely to be inordinately difficult even at several times the mean surface pressure of the outside. Just look at the binding efficiency curve of myoglobin / haemoglobin and other oxygen binding systems here (all very efficient at their job) versus the partial pressure of O2. It falls off very sharply as the pressure falls.
This is one reason why higher animal life on earth struggles to survive at altitudes where the overall pressure, proportion of oxygen, and temperature ranges are still much more tolerable than mars.
Given the idea life conquers all extremes, the lack of any higher organisms at these altitude on earth suggests that mars, where the pressure is already far lower, atmospheric oxygen is about 0.13%, harsh infiltered solar radiation and extreme temperature shifts, the likelyhood existance of anything beyond extremophile life becomes even more remote.
if it was a slow leaking of the atmosphere into space as some believe then they would most definately had plenty of time to adapt. if it was a sudden event, as i suggested, with a massive impact destroying the atmosphere, then there will be a much smaller chance of such an adaptation is far less likely, but still possible. the earliest forms of life adapt the quickest.
It's not impossible both are responsible. An earlier cataclysmic loss of atmosphere is possible, given some of the massive impact sites, but whatever atmosphere remained is being scavenged by the solar wind, if the measurments made by some of the first probes to orbit the planet are to be accepted.
