Mars has long captured researchers' imaginations as a planet that could have supported alien life in the past. That being said, it's completely lifeless now, as far as we can tell. New research proposes an interesting explanation for why this is: Martian life could have driven itself extinct.
We've been hearing a lot in these past few years about man-made climate change, the threat it poses to our way of life and to life on Earth as a whole. It's all for good reason: climate change can easily run rampant and lead to some very dramatic consequences. Venus, Earth's sister planet, is an excellent example of where such changes could lead.
Mars is a much less violent place, but new research suggests that we stand to benefit from learning the lessons that the Red Planet can teach us. At one point, according to the authors, Mars likely had similar environmental conditions to our own planet, and its soils harbored similar life -- methane-producing microbes like those that thrived in Earth's primordial oceans. But it may have been that the emergence of life itself was what, ultimately, caused Mars to become a lifeless rock.
Out with a bang
Some of the oldest life forms on Earth were microbes known as hydrogenotrophic methanogens, beings which 'ate' carbon dioxide (CO2) and hydrogen (H2) from the atmosphere, releasing methane gas (CH4) in return. They were quite successful, colonizing all the oceans on the planet and draining almost all the hydrogen from the air. For comparison, during their day, hydrogen made up around 0.01-0.1% of the atmosphere. Today it is barely present, at around 0.00005%.
Over time, they released huge amounts of methane, a powerful greenhouse gas, into the air. This was actually beneficial; at the time, the Sun was less bright than today, and the methane helped keep temperatures on Earth stable and comfortable despite our star's lower output of energy. As such, these early methanogens set up the stage for life to further develop and evolve on the planet.
This view, that life creates and perpetuates the conditions for a self-regulating system that fosters the development of more life, is encapsulated today in the Gaia hypothesis.
But on Mars, it is possible that life had the opposite effect. Instead of creating the conditions it needed to further develop, the emergence of life here actually made itself go extinct.
The team investigated the earliest days of the Martian biosphere using a series of computer models, each aimed at a different aspect of their life.
The first one looked at the planet's atmosphere based on factors such as volcanic emissions and gas venting into space. The second model investigated the planet's crust to determine factors such as porosity, chemical makeup, water content, and average temperatures. Based on the results from these two steps the team ran their third model, which sought to estimate how methanogenic microbes would have fared on early Mars, with special emphasis put on studying their energy needs.
Out of all three, the last model allowed the team to assess the habitability conditions in Mars' ancient underground, and compare these to the conditions on Earth (and how microbes evolved there).
We have evidence of liquid water on Mars' surface in the past, meaning that, at one point or another, its climate was much milder than what we see today. As such, the team assumed that Mars had a much denser atmosphere than today (similar to that of modern Earth), which was rich in CO2 and H2, similar to Earth at the same time. This combination makes H2 a very potent greenhouse gas, even more potent than methane is in Earth's modern atmosphere -- helping to explain Mars' warmer climate in the past.
According to the first two models, however, the team is confident that even this atmosphere could not make Mars hot enough for liquid water to cover its surface. Much of the planet was covered in ice, and methanogenic microbes could not survive in these areas, as it was too cold for them and the ice caps physically blocked gases from reaching the underground (where the microbes lived).
That being said, the warmer regions around Mars' equator could sustain liquid water, and methanogenic microbes could survive underground in these areas. The first few hundred meters of crust in these equatorial areas were in the right temperature range and had access to enough gas for methanogenic microbes similar to those on modern Earth to be able to thrive, the team explains.
We don't yet have evidence that this happened such as fossils from Mars. But the research shows that it was theoretically possible for such microbes to live in the Martian crust around four billion years ago.
Now, the interesting part. While on Earth methanogenic organisms led to an increase in the greenhouse effect, on Mars they would do the opposite. As we've seen before, in an atmosphere that starts rich in CH4, hydrogen is a very powerful greenhouse gas. By consuming CO2 and H2 and releasing CH4, Martian microbes would have actually worked to make the planet colder by lowering its atmosphere's ability to retain heat.
The team estimates that such microbes could have lowered the planet's surface temperatures by several tens of degrees. On the one hand, this increased ice cover, making less and less of the planet viable for microbes to inhabit. On the other, it would push these organisms deeper and deeper into the crust, running away from the cold surface; over time, this would cut them off from their supply of gas, making them starve out.
Humanity today seems to be behaving contrary to the Gaia Hypothesis. Our activity is actually degrading the fine-tuned set of conditions that have kept life on Earth going up to now. Viewed in that light, the results of this study should help as a sobering wake-up message. If it is possible for even single-cell organisms to destroy the environments that sustain them, it would definitely be possible for a complex, intelligent, and highly advanced species to do the same.
The paper "Early Mars habitability and global cooling by H2-based methanogens" has been published in the journal Nature Astronomy.