It was always my understanding in physics and biology that life as we know it[sup]TM[/sup] would need water to exist. That does leave a possibility (we don’t know how small or large) that life as we don’t know it could exist without water. But if life as we don’t know it is out there, we might not even recognize it as life when we see it. Who’s to say that methane-based life has locomotion or reflects light within the human-visible spectrum? etc…
Probably a dumb question, but could mercury be used as a solvent instead of water?
Almost certainly not. The issue is not just one of providing a solvent (or a liquid). The issue is that anything with the properties of life (any physical systems capable of evolution by natural selection, which requires reproduction with with variation) will need a complex macromolecular chemistry, and the only sorts of chemical systems we know capable of exhibit the right sort of complex chemistry are organic molecules in an aqueous environment. Water is not just acting as a solvent, in this situation, its unique ionic properties are essential to the functioning of biochemical macromolecules. Most types of solvent do not have anything approaching these properties. They are certainly not shared by mercury or liquid methane.
Also, the only other element apart from carbon capable of forming macromolecular structures of any size and complexity is silicon, and it is not at all certain that even silicon compounds can attain the anything like the range of molecular size and complexity that carbon does. We do not have a lot of options for types of life supporting chemistry. Indeed, there may well only be the one.
So the argument is not just “All life we know of depends on water, so therefore all possible life must depend on water,” (that, indeed, would be a very weak argument), the argument is that everything we know about chemistry suggests that macromolecular functioning of a sort capable of sustaining life process can only arise when carbon based macromolecules interact with water. Chemistry is a finite system. There are only a limited number of chemical elements in the universe, and we know the properties of all of them pretty well. We are not going to discover any other relatively simple compounds that are common in the universe (as water is) that have anything like its properties. Neither are we going to discover any other elements, apart from carbon (and just possibly silicon) capable of forming large, complex and stable macromolecules.
And that’s a really important point. There may be other biochemistries possible, but how are you going to concentrate the elements needed to support them? Back in hte old days of science fiction there used to be a lot of aliens who breathed chlorine instead of oxygen. And that would certainly be possible, chlorine is almost as electronegative as oxygen, and it would be chemically possible to use chlorine for many of the same functions as oxygen.
The problem is that chlorine is orders of magnitude less common than oxygen. So how does a planet with a chlorine atmosphere form? Where does all the chlorine come from? A large fraction of the Earth’s crust is oxides–various elements combined with oxygen. Oxygen is everywhere. Chlorine isn’t. So a chlorine biosphere is biochemically plausible, it isn’t geologically plausible.
So, to take the example of mercury, where are the mercury oceans or lakes coming from? How is it that this particular planet got stuffed full of mercury? On Earth mercury is an extremely rare element, and readily forms compounds with other elements, so you don’t find lumps of metallic mercury but rather cinnabar or other ores. So while we can imagine a planet with oceans of mercury, it’s very hard to imagine how such oceans could form geologically.
Interesting question. When I was a kid, scientists thought water hardly existed anywhere off the earth, and that it was necessary for life. Now they think water is all over the place, and not necessarily necessary. According ot all these documentaries I’ve been seeing, now they think that liquid may be necessary for life to form, basically to be kind of a soup within which chemicals can mix and react with each other.
I’m not sure why a gaseous atmosphere wouldn’t work for the same reasons.
I do not know when you were a kid, but it is several decades since I was a kid, and scientists were definitely prepared to believe that there was water elsewhere in the universe then. I can remember hearing quite serious speculations about the possibility that Venus might be covered with oceans of carbonated water, and that certain seasonal changes in markings observed on Mars might be a sign of spreading and dying back as ice in the Martian polar regions melted and re-froze. They may not have known as much as we do now about where water was actually to be found, but they were certainly not ruling out the possibility that it was out there, in quantity.
I can also remembering hearing speculation, when I was kid, about the possibility of silicon based life. My impression is that this is now considered a lot less plausible than it was once thought to be.
Do you actually know of any contemporary scientists (with the relevant expertise - as opposed to people from unrelated fields bullshitting), who take seriously the possibility that water may not necessarily be necessary for life?
Is it possible for complex enough self replicating organic molecules to exist in gaseous form? Not necessarily carbon based ones, have we observed any other element that can form long chain complex molecules as a gas?
I recall first hearing about the “liquid but not necessarily water” idea on Brian Cox’s Wonders of the Universe, and it was based at least in part on the discovery of liquid methane lakes on… Titan? … eh, one the moons of a gas giant. Obviously it is just speculation because none has been found. But here is a story with NASA’s Chris McKay doing some of the speculating
I dunno.
Unlikely. In general, the more massive a molecule, the higher its boiling point, since a big heavy molecule is harder to break away from the surface of a liquid than a small lightweight molecule. At the same time, in general, the more complicated a molecule, the higher its boiling point, since complicate structure provides more “hooks” for it to hang on to other molecules. So a large, complex molecule would have a really hard time being a gas. Now, you could just suppose that this gaseous life is living in a much hotter environment, but then you’ve got to worry about that heat breaking internal bonds in the large molecule, and turning it into a bunch of smaller molecules.
Life on our planet exists in some terrible conditions that we used to think was impossible live in.
Some live in high PH
others Low PH
Some without oxygen
some live inside rocks
some high salt concentrations
some in nuclear plants
Some in almost no water
high temperatures
low temperatures
Life will grow where it can and it can thrive in difficult conditions.
These little devils are pretty hardcore.
Yes it’s Titan, these articles imply that data from Cassini has found that hydrogen is being consumed at the planets surface more than expected, and one explanation is methane based life consuming it as a food source.
No, we can make some guesses based just on chemistry and other physical sciences. They may be poor guesses (I don’t really think so), but they will still be informed ones.
I disagree. Especially given the conditions under which it started. One point - the extreme locations we find some life in today - that’s not an environment that existing life moved into - that’s more like the environment life started in. And the conditions that exist there are more like that elsewhere than current Earth. Pretty hellish, that’s where it gets its name from
Is it not amazing that water is both an extremely common molecule, and has unique properties useful for many things, including life?
Are there other molecules, whether common or not, which could serve as a good matrix for life?
The wiki section on “non-water solvents” in “hypothetical types of biochemistry” is interesting.
Methane, Ammonia and Hydrogen Fluoride have been proposed, among others.
Thanks for the link! Very interesting.
From there, Googling and clicking took me to a website based on Cairns-Smith’s proposal that the first living organisms to exist on Earth were clay minerals. Scientists seem to thing the hypothesis is false, but it still may be fun to read about.
This isn’t quite right. Yes, mercury being rare is a good point. But mercury readily forming compounds is not. Oxygen also readily forms compounds, yet we’re all breathing in an ocean of it.
How is that possible? Because oxygen is a metabolic waste product of the majority of the living biomass on this planet. If we discovered a planet with oceans of mercury, it would be a good sign of some sort of life that is producing elemental mercury. Of course, since mercury is much more rare than oxygen, this is unlikely.
Supercritical Carbon Dioxide is (as best I can tell) a solvent, and its what the atmosphere of Venus is constituted of, at a density like that of the ocean. And of course Carbon Dioxide includes Carbon.