If the surface of Mars is such a hostile place, is the deep subsurface any better for life? This is the question that we have recently started to answer with our new publication in Communications Biology from myself, Michel Jaworek, Roland Winter and Charles Cockell.
So it’s fairly uncontroversial to say that the surface of Mars isn’t a nice place to be. The temperature fluctuates from ok to extremely cold, there’s constant incoming radiation, it’s salty, dusty and maybe most importantly there is essentially no liquid water. All in all, not the first place you would expect to find life on Mars. However, deep beneath the Martian surface there may be environments that are less hostile to life. Examples of such environments would be the subglacial lakes reported by Orosei et. al. in 2018 and Lauro et. al in 2020 or the deep ground water as suggested in Clifford et. al. 2010.
These deep environments are however not without their own challenges. Firstly, they would be extremely cold, and I mean cold. We are talking about environmental temperatures lower than 210 K (-63 °C), which is pretty far below the freezing point of water. But wait, lakes have been found beneath Mars, so how can they contain liquid water if it’s so cold? Well that leads us to our second problem, the salts. For liquid water to exist at such low temperatures we need salts in the solution to depress the freezing point of water low enough so that it doesn’t freeze. So it’s cold salty water, how bad can it be? Well the reigning champion of freezing point depression is our nasty friend the perchlorates, and for water to be liquid at such temperatures it has to be packed with them. Perchlorate salts aren’t something you come across much on Earth unless you’re from the Atacama or work at a munitions factory. On Mars however, they’re everywhere, and they’re bad news for life. We also have a third contestant in the extreme environmental parameter contest, and that is the high pressures which you would experience in the deep subsurface, and importantly, we just don’t know how life responds to both pressure and perchlorate salts.
So what did we do? Well, being biochemists we immediately ignored life itself and decided to look at a model enzyme and examined how its stability and activity changed with increasing concentrations of magnesium perchlorate and increasing pressures. Unsurprisingly we observed that magnesium perchlorate reduced the activity of our enzyme at ambient pressures, and we can use this as our starting point to compare our further observations to. So what happened when we increased the pressure? We found that the activity of our enzyme increased with pressure, even in the presence of perchlorate salts, and it was interesting to note that the greatest proportional increase happened with 0.25 M magnesium perchlorate. I’ll not go into the detail, but the reason why this happened is due to the fact that our enzyme has something called a “negative activation volume”. This essentially means that throughout the course of the enzymatic reaction, the volume of the enzyme decreases, and pressure loves it when you decrease the volume of something, and so it favours the enzyme reaction, thus increasing the activity. So if life on Mars was trying to avoid the hostility of the surface, it could gradually work its way into the subsurface, and if its vital biochemistry had a negative activation volume, it could expect to see an increase in its biochemical activity, effectively undoing the negative effects of the perchlorate salts.
It wasn’t a complete victory over the perchlorate salts though. We found that the salts reduced the temperature at which our enzyme melted and also lowered the pressure at which it unfolded. So while you can recover enzymatic activity that was lost due to the perchlorate salts, the folded phase space of our enzyme was ultimately constricted in the presence of perchlorates. This may have important implications. When you look at the phase diagram for life, it looks surprisingly similar to the phase diagram of proteins, as opposed to lipids or DNA. This means that protein stability is a good proxy for the feasibility of life, and our results suggest that perchlorate salts will constrict the possible environmental conditions in which life can survive. So it’s not all good news.
As with any study there are caveats. The concentrations of perchlorates that we used were far from the saturation point. This is simply because saturated magnesium perchlorate obliterates life and it will be a monumental amount of work (or a stroke of luck) to find life or biochemistry that can function at such high concentrations of perchlorates. Secondly, we had to ignore the low environmental temperatures and this was largely a technical consideration. Most lab equipment is designed to work really well at room temperature and ambient pressure. So making a spectrophotometer that works at 2 kbar is hard enough without also asking it to cool down to 200 K. However, watch this space, as the low temperature work is underway and will hopefully be coming to a publication near you soon….or however long peer review takes.
