An intriguing new study by Rao, Garrett-Roe & Hamm in the Journal of Physical Chemistry (B), appears to offer a clue or two. By applying modeling techniques usually reserved for the study of complex and dynamic systems, they investigated what might be going on in liquid water on a moment-by-moment basis. The dipolar nature of water molecules means that weak electrostatic bonds (hydrogen bonds) are readily made and broken between structures. So in a crowd of water molecules all manner of temporary arrangements can be made as they jostle around. This new study indicates that there may be two main flavors of these arrangements - one a rather 'blobby' mass of several molecules, and the other a more regular, crystalline arrangement. These structures are exceedingly fleeting - breaking up and re-assembling many times a second at room temperature. The result is, and this is a very crude phrasing, a bit like a 3D piece of velcro that is constantly morphing into different shapes.
So, all jolly nice, but why do we care? Imagine throwing some other molecules into this sticky nest. Perhaps some carbonates, some amino acids, even some proteins. The shape-shifting water structures can provide the perfect chemical incubator - from catalyzing reactions to determining structural forms of complex organic molecules. This cuts to the heart of one of the long running discussions that crops up in the search for life. Why couldn't you have biology that uses something other than water? Why not methane, or hydrogen peroxide, or ammonia?
The answer may now be clearer - as far as anyone knows these other solvent-like molecules do not exhibit this same type of behavior - so it may be that they offer far fewer, if any, pathways for complex chemistry. Water may be far more intertwined in the processes of life than we had suspected.
Water is polar. It does this transient hydrogen bonding between the molecules. It also is the only solvent where the solid form is less dense than the liquid form. As far as I know, the other solvents, Methane, Ammonia, and Hydrogen peroxide do not have the range chemical reactions that occur in them than water does.
ReplyDeleteThe other issue is Carbon. Carbon forms a larger variety of macromolecules than any other element. As in the case of the solvent, the greater the range of chemistry, the more likely that is the basis of biology, since biology is the most complex form of chemistry.
Biology most likely can only be Carbon-based in aqueous solution.
The other mentioned solvents occur at much lower temperature, which would make the corresponding chemical reactions much slower as well. If there's life on Titan, it would have to be very slow and very simple life.
kurt9: With regards to chemistry, there is no such thing as simple life. The whole notion of distinction between simple vs. complex life is flawed. There is nothing simple about a bacterium, and while it is ok to say a mammal is more complex than a bacterium, this is a (almost subtle) matter of degree, and involves at most one or two orders of magnitude, depending on how you define complexity. Furthermore, the biochemistry of mammals is of exactly the same complexity as that of the most humble microbe, and the notion that there is some chemistry that allows life, but not "complex" life is not tenable.
ReplyDeleteAs to solvents other than water and structural components other than carbon, we have to be very careful when we make statements such as "water is the only ...". There is a very strong myopia effect, because we know carbon and water biochemistry. It is not easy to rule out the others, as it involves proof of a negative.
To address some of your claims directly, acetic acid is known to expand upon freezing, so there goes one uniqueness assumption. Also, I do not think a good case has been made that this particular property is required for life.
The assertion that carbon is the only possible basis for biochemistry is similarly shaky, and far from established. I am sure you know all of the candidates listed, e.g. here:
http://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry
To prove none of them could be the basis of life is, in my opinion, nearly impossible and certainly has not been done.