Every time I read scientific articles about space exploration and the search for life on other planets, there is a lot of emphasis on planets like Mars that may have once had water on the surface, or once had an oxygen rich atmosphere, etc.
But, : why do water and oxygen need to exist on a planet for us humans to think the planet may be hospitable to life? Sure, they’re crucial elements to life on Earth and carbon based organisms like ourselves. But are scientists so sure that water and oxygren needing life is the only kind we’re going to be able to find? Couldn’t there be creatures elsewhere that evolved on planets that have no water or oxygen?
I’m not saying we go turn over every rock on the moon to see what’s crawling underneath, but it seems that we’re limiting our search to what we know, versus keeping our minds open to the possibilities that the vastness of the universe offers.
I don’t think scientists discount the possibility of other types of life, it’s just that life is more likely (at least, as best we can figure) with the presence of liquid water. It’s a great solvent that can allow all kinds of chemical reactions to take place. In some cases, you’re right, there’s probably a life-as-we-know-it bais.
An oxygen atmosphere may be a sign of life because it would be unusual for oxygen to exist in such a free state in large quantities. (I think) Earth’s early atmosphere did not have lots of oxygen until photosynthetic life evolved (oxygen was a waste product of that life).
Methane in an oxygen atmosphere would be another good sign of life because oxygen readily breaks down methane…Earth has methane replenished in the atmosphere mainly due to biological sources.
There are some basic themes that underlie a search for worlds that have/had surface water and free atmospheric oxygen:
[/list]
[li]We want to understand how life arose on this world, and looking for Earth-like examples may help us achieve that goal (the more examples among which we can compare and contrast, the better).[/li][li]In our search for [intelligent] life elsewhere in the universe, it would be easiest for us to identify life surviving in conditions that are somewhat Earth-like, and that have similar environmental requirements as Earth organisms.[/li][li]If we take the long-term view and wonder where humanity might migrate to some day, a planet that’s as close as possible to our own would be easiest to colonize first (little/no significant terraforming issues to confront).[/li][/list]
It’s easy to think of organisms having a life cycle that don’t require oxygen; after all, plenty of microbes here on Earth (the anaerobic bacteria and many archaea, for example) survive just dandy without it. Life without water poses a bit more of a problem, simply because water is such a handy solvent in which chemical reactions can take place.
Basing life on an element other than carbon has also been the subject of debate in the past. I’m not a biologist, but I seem to recall silicon-based life (as an example) being considered unlikely owing to the energetically unfavorable chemical reactions being required to sustain such an organism.
Scientists have been keeping their minds wide open about life elsewhere for a long time, but you have to cut us a little slack - we have to start somewhere, rather than blindly poking around. Currently there is a lot of funding interest in life in extreme environments here on earth as an analog for Earth-like life elsewhere; it’s a good starting point. How do you develop a plan to look for life, the nature of which is completely speculative?
Besides - and importantly - there’s the completely political aspect of making one’s proposed research palatable to the public by emphasizing the human impact of such research, which is a little hard to do if you build a proposal around the search for organisms that MIGHT be based on silicon or some other element. Do you remember Senator William Proxmire’s “Golden Fleece Awards”? Every year he singled out federally-funded research grants that sounded “ridiculous” as examples of how your tax dollars were “going to waste.” Sometimes it WAS true, but the rare stinkers were unfairly portrayed as being representative of federally funded research projects in general. So over the past decade, federal funding has been geared more toward applied research, or at least toward projects that had a demonstrable human impact factor. NASA is not immune to such pressures, and so its research objectives are defined accordingly.
If you are unhappy about the “lack of imagination” illustrated by currently funded space exploration and astrobiology projects, you have your elected officials (and many of their constituents) to blame… get out there and pester them until they start changing their minds!
There aren’t a heck of a lot of good candidates besides water for a medium, but oxygen isn’t necessarily a must. After all, we have anaerobic life all over the earth.
There may be some practical limits to the size and/or complexity of anaerobic life forms, however. AFAIK, the only types we know to exist are all bacteria or single-celled organisms.
The problem with not having water is, for most of the chemistry we understand to be associated with life, some medium is required which readily allows chemical reactions. Solid-state chemical reactions don’t seem to cut it.
My $.02 perspective is that recent knowledge (for instance, the microbes that live in the hellish Pacific vents) leads one to the conclusion that life is not, in fact, fragile and unlikely. Further, planetary formation would appear (at this point) to fairly common. Conclusion: there’s just gotta be little green men somewhere!
So howcome we don’t know? (Who was the physicist who answered Enrico Fermi’s question by saying “We ARE here, Enrico, we’re disguised as Hungarian physicists”?)
Might we steal a trick from Larry Niven? If we look for vehicles, we might assume that some of them must be traveling “away” from our position. (OR decelerating “towards” our position). Might we look for very localized energy sources, shedding spectra that would suggest artificial sources?
We have a certain experience with life that uses water and oxygen that makes us think (not necessarily correctly) that we’ll recognize it when we see it.
We don’t have any experiences with any other kinds, nor do we have a “quantum biochemistry”, that would allow us to say, “Under these conditions, such-and-such chemicals would generate the phenomenon that we call ‘life’”.
Therefore, it behooves us (and even more so, those who depend on government research grants) not to spend a few terabucks on the matter and, when asked, “Did you find life on Altair VIII?”, respond with: “Ummm, we haven’t the foggiest notion”.
Actually, the evidence that we have suggests that all life on Earth has a common origin. If life were arbitrary and highly likely, we would expect there to be dozens, if not hundreds, if not millions of different ‘original’ lifeforms, all unique and unrelated, and many of their ancestors surviving to this day through the same process of natural selection.
Alternate conclusion: Life on Earth is the result of a (nearly) impossible fluke and it’s not probable that it could occur elsewhere or elsewhen…
…either that, or some non living entity manufactured life, as we know it, as a quirky little lab experiment just to see if it could be done…
I recall reading somewhere that oxygen was necessary in the atmosphere of a life supporting planet, not because all life needs to breath free oxygen, but because the ozone layer blocks ultraviolet radiation that can break down the bonds that make up the cells of all lifeforms.
Of course, life on earth started out without the ozone layer, it just probably wouldn’t have gotten anywhere if it didn’t release free oxygen which in turn developed into the ozone layer.
