Evidence of Martian Life Found in a Holland and Barretts?

With my looming redundancy date and seemingly receiving even more rejections than jobs I’ve applied for it was refreshing to get some good news this morning. We’ve had another paper accepted on our work on why answering the question “Is there life on Mars?” is actually really difficult (with current techniques).

This one has the typically short and punchy title of:'Transformation of cyanobacterial biomolecules by iron oxides during flash pyrolysis: Implications for Mars life detection missions'. Sadly I couldn't get my preferred title (see above) passed my co-authors...

It'll probably be a few weeks until it's officially published online but there's an open access preprint here.

Around 4.5-4 billion years ago Mars was less of a Red Planet and much more like the early Earth. At this point in time Mars still had a CO₂ rich atmosphere, replenished by volcanoes and protected by an active magnetosphere. This provided both a greenhouse effects and increased surface pressure which meant liquid water was stable at the surface. Rivers flowed and filled asteroid impact craters to form crater lakes which may have provided an environment quite hospitable to life.

Early Mars was probably a much nicer place than it is today

For this project I needed a life form that represented the sort of organism that may have evolved and colonised these environments, the remains of which could be preserved in the sort of lake sediments that NASA’s Curiosity has been exploring at Gale Crater, Perseverance is exploring at Jezero Crater, and ESA’s Rosalind Franklin may find at Oxia Planum in 2022.

What I needed was a simple, single-celled, prokaryotic photosynthesiser, capable of adapting to extreme environments. I needed a cyanobacteria, one form of which is the blue-green algal ‘scum’ which can cause poisonous blooms in ponds, while another can be bought from your local health food shop as a supplement to put in nutritious smoothies…

That’s right, we’re using Spirulina as a stand in for alien life.

Tasty smoothie or alien life?

In our last paper we looked at how, when Mars rovers heat up sediment samples any fatty acids present, which could be evidence of biological processes, will react with iron oxides and be transformed into highly ambiguous aromatic organic molecules which won’t look much like evidence of life at all. There's loads of background detail about the search for life on Mars in that post and others (including here, here and here) so I'll keep this brief and won't repeat it all now (I'm quickly writing this while procrastinating rather than doing interview prep).

This time round we wanted to make the system a little more complex by using whole bacteria (the Spirulina) to see if the same thing happened with intact, pristine biological matter. Unsurprisingly we found out that, yes, on heating the organic molecules released from the bacterial cells will still react with the iron oxides, transform into more ambiguous phases, and make it very difficult to be 100 % sure whether we’ve found life or not! The detected organic products look pretty much identical to those which could just have been delivered to the surface of Mars by meteorite impacts.

We did find some hope in that haematite, which is a common iron oxide on Mars, is nowhere near as problematic as the other iron oxides we studied, and that some biomolecules (isoprenoid hydrocarbons) had better survivability than others, handily these are also the molecules most likely to survive over the billions of years since Mars  was widely habitable. We also established that suites of molecules, which would be pretty much useless on their own, could in combination be relatively strong evidence of life – and at least be a good sign we should take a second look at the sample using a wider variety of techniques.

Graph of detected products from the Spirulina pyrolysed on the various iron oxide minerals, quartz used as a less reactive comparison. The original biomolecules (the blue, purple, green) which would be solid evidence of life are lost and transformed into more ambiguous species represented by the yellow, red and black. AS can be seen survivability of 'useful' biomolecules is higher at lower Spirulina concentrations in the haematite than other iron oxides (quartz as expected only has minor adsorption effects)

As with the last paper, we came to the conclusion that despite the problems, we shouldn’t try to avoid iron oxides altogether (this would be pretty much impossible on Mars anyway, iron oxides being everywhere are the reason it’s red). This is because iron oxides are both good for long term preservation of organic matter (it sticks to iron which offers some protection, known as the ‘rusty sink’ effect) and may indicate habitability as they are often formed in the presence of water and could provide an energy source for specialist microorganisms.

Instead, we should look for areas where any of the more problematic iron-bearing species will have been transformed to more amenable haematite. These are areas with evidence of long-lived (or shorter but hotter) oxidising fluid flow in the subsurface. These areas, such as Vera Rubin Ridge at Gale Crater and the clay units at Oxia Planum, have the added bonus of potentially acted as a refugia for life as conditions at the martian surface became more inhospitable. Bacteria may have retreated to them and eeked out an existence in the subsurface, ‘feeding’ on energy produced in chemical reactions for millions/billions of years after the water on the surface dried out.

Vera Rubin Ridge, as viewed by Curiosity, may be a good place to search for evidence of ancient life (credit NASA)

The next step of this research, already submitted, is to look at how ancient organic matter preserved over millions of years (in the form of kerogen) reacts with these problematic iron oxides.  It could be expected that this would to be more resistant to transformation as it has already lost much of its reactant groups but, spoiler alert, it’s not. Hopefully more details on that soon!

I should add that the publication of this paper also involved one of the best experiences of peer-review that I have had. Both reviewers clearly knew the topics and methods involved well and provided highly insightful comments picking up on mistakes and pointing out gaps in the manuscript and encouraging me to make it into a better, more impactful piece of work. I wish more reviewers were like this!