Now that we have found so many exoplanets, we might start to wonder whether they have life. It so happens I am going to give a presentation on this at a conference in about three weeks time, hence the temptation to focus attention on the topic. My argument is that whether a place could support life is irrelevant; the question is, could it get started? For the present, I am not considering panspermia, i.e. it came from somewhere else on the grounds that if it did, the necessities to reproduce still had to be present and if they were, life would probably evolve anyway.
I consider the ability to reproduce to be critical because, from the chemistry point of view, it is the hardest to get right. One critical problem is reproduction itself is not enough; it is no use using all resources to make something that reproduces a brown sludge. It has to guess right, and the only way to do that is to make lots of guesses. The only way to do that is to tear to bits that which is a wrong guess and try again and re-use the bits. But then, when you get something useful that might eventually work, you have to keep the good bits. So reproduction and evolution have opposite requirements, but they have to go through the same entity. Reproduction requires the faithful transmission of information; evolution requires the information to change on transmission, but eventually not by much. Keep what is necessary, reject that which is bad. But how?
Information transfer requires a choice of entities to be attached to some polymer, and which can form specific links with either the same entity only (positive reproduction) or through a specific complementary entity (to make a negative copy). To be specific they have to have a strongly preferred attachment, but to separate them later, the attachment has to be able to be converted to near zero energy. This can be done with hydrogen bonds, because solvent water can make up the energy during separation. One hydrogen bond is insufficient; there are too many other things that could get in the road. Adenine forms two hydrogen bonds with uracil, guanine three with cytosine, and most importantly, guanine and uracil both have N-H bonds while adenine and cytosine have none; the wrong pairing either leads to a steric clash that pushes them apart or ends up with only one hydrogen bond that is not strong enough. Accordingly we have the condition for reliable information transfer. Further good news is these bases form themselves from ammonium cyanide, urea and cyanoacetylene, all of which are expected on an earth-like planet from my concept of planetary formation.
The next problem is to form two polymer strands that can separate in water. First, to link them something must have two links. For evolution to work, these have to be strong, but breakable under the right conditions. To separate, they need to have a solubilizing agent, which means an ionic bond. In turn, this means three functional valence electrons. Phosphate alone can do this. The next task is to link the phosphate to the bases that carries the information code. That something must also determine a fixed shape for the strands, and for this nature chose ribose. If we link adenine, ribose and phosphate at the 5 position of ribose we get adenosine monophosphate (AMP); if we do he same for uracil we get uridine monophosphate (UMP). If we put dilute solutions of AMP and UMP into vesicles (made by a long chain hydrocarbon-based surfactant) and let them lie on a flat rock in the sun and splash them from time to time with water, we end with what is effectively random “RNA” strands with over eighty units in a few hours. At this point, useful information is unlikely, but we are on the way.
Why ribose? Because the only laboratory synthesis of AMP from only the three constituents involves shining ultraviolet light on the mixture, and to me, this shows why ribose was chosen, even though ribose is one of the least likely sugars to be formed. As I see it, the reason is we have to form a phosphate ester specifically on the 5-hydroxyl. That means there has to be something unique about the 5-hydroxyl of ribose compared with all other sugar hydroxyl groups. To form such an ester, a hydroxyl has to hit the phosphate with an energy equivalent to the vibrations it would have at about 200 degrees C. Also, if any water is around at that temperature, it would immediately destroy the ester, so black smokers are out. The point about a furanose is it is a flexible molecule and when it receives energy (indirectly) from the UV light it will vibrate vigorously, and UV light has energy to spare for this task. Those vibrations will, from geometry, focus on the 5-hydroxyl. Ribose is the only sugar that has a reasonable amount of furanose; the rest are all in the rigid pyranose form. Now, an interesting point about ribose is that while it is usually only present in microscopic amounts in a non-specific sugar synthesis, it is much more common if the sugar synthesis occurs in the presence of soluble silica/silicic acid. That suggests life actually started at geothermal vents.
Now, back to evolution. RNA has a rather unique property amongst polymers in that the strands, when they get to a certain length and can be bent into a certain configuration and presumably held there with magnesium ions, they can catalyse the hydrolysis of other strands. It does that seemingly by first attacking the O2 of ribose, which breaks the polymer by hydrolysing the adjacent phosphate ester. The next interesting point is that if the RNA can form a double helix, the O2 is more protected. DNA is, of course, much better protected because it has no O2. So the RNA can build itself, and it can reorganise itself.
If the above is correct, then it places strong restrictions on where life can form. There will be no life in under-ice oceans on Europa (if they exist) for several reasons. First, Europa seemingly has no (or extremely small amounts of) nitrogen or carbon. In the very thin atmosphere of Europa (lower pressures than most vacuum pumps can get on Earth) the major gas is the hydroxyl radical, which is made by sunlight acting on ice. It is extremely reactive, which is why there is not much of it. There is 100,000 times less sodium it the atmosphere. Nitrogen was undetected. The next reason is the formation of the nucleic acid appears to require sunlight, and the ice will stop that. The next reason is that there is no geothermal activity that will make the surfactants, and no agitation to convert them to the vesicles needed to contain the condensation products, the ice effectively preventing that. There is no sign of hydrocarbon residues on the surface. Next, phosphates are essentially insoluble in water and would sink to the bottom of an ocean. (The phosphate for life in oceans on Earth tends to come from water washed down from erosion.) Finally, there is no obvious way to make ribose if there is no silicic acid to orient the formation of the sugar.
All of which suggests that life essentially requires an earth-like planet. To get the silicic acid you need geothermal activity, and that may mean you need felsic continents. Can you get silica deposits from volcanism/geothermal activity when the land is solely basalt? I don’t know, but if you cannot, this proposed mechanism makes it somewhat unlikely there was ever life on Mars because there would be no way to form nucleic acids.