Short answer: We don’t know. Longer answer: read on!
BINTEL thanks Dr. Jon Clarke, President of the Mars Society Australia and Adjunct Professor at the Centre of Excellence in Astrobiology at Amity University, for this article that discusses just how hard it is to answer the question of “is there life on Mars?”
Determining whether life ever arose on Mars is NASA’s top science priority for Mars. That this questions remains unresolved despite nine successful Mars surface missions by NASA and a further then flyby and orbiter missions is testimony to the complexities and ambiguities of working remotely on Mars with the very limited capabilities of remote explorers. A very different situation from much of the 19th and early 10th century when the likelihood of life of some sort on Mars was almost taken for granted. As the prospect of life on Mars receded with discoveries of the true hostility of the martian environment – low pressure and temperatures, scarcity of liquid water, high levels of hard ultraviolet light reaching the surface, elevated cosmic radiation, and frequent meteorite impacts – so the import of the question has been raised, especially in the media, to near-existential levels. “finding life on Mars will prove we are not alone!” is a common theme.
An unlikely but intriguing discovery – an astronaut finds a fossil on Mars
If this is the case, why look for life on Mars? One reason of course is that the historic interest in Mars and its link to the possibility of extra-terrestrial life is still a sufficient driver. Another reason is that, even though we know that the surface of Mars is far more hostile than people in the 19th century could possibly imagine, surface still contains all the chemical elements necessary for life, liquid water is present in some places for short periods, and there are multiple energy sources for life to exploit – light, warm, chemical energy gradients to name some – making it possible that some kind of life might survive in sheltered oases. We also know that early Mars, before 3.5 billion years ago, was very similar to early Earth , with a thick atmosphere, abundant liquid water, and active hydrologic cycle, active geothermal systems, and organic matter was being deposited in the lakes and deltas of that time. Life may well have been present at that time, as it was on Earth, as we know from rock found in the Pilbara of Western Australia. Mars is also a lot closer and easier to access than other astrobiologically interesting bodies in the solar system such as Europa, Titan, or Enceladus. Lastly, the presence or absence of life will affect how we conduct future human missions, how stringent precautions need to be against forward and backward contamination for example. Although of course large meteorite impacts have already been exchanging material between the two planets for millions of years.
Short of something climbing over a lander and sniffing the camera, look for life on Mars is a challenging prospect. The Viking landers were sent in 1976 with high expectations of getting definitive evidence, based on near 20 years of consideration of how to look for life. Nobody expected the ambiguous results that were obtained. Looking for past life will be even more difficult. Good evidence for life on Earth has been known from the Pilbara for about 40 years, but despite thousands of studies there remain some scientists who are unconvinced. Finding similar evidence on Mars – stromatolites, microfossils, isotopic anomalies – would not be convincing to some, given the very high burden of proof that would be expected.
Trenches excavated by the Viking 1 lander to look for signs of life (NASA).
The Viking landers looked for present life through searching for organic molecules and chemical activity, especially the exchange of gasses and the uptake and the release of labelled markers. In the event, the detection of organic molecules failed, and the results of the other experiments were equivocal. Subsequent missions were less ambitious, seeking to answer the basic questions to habitability instead. First there was the “follow the water” missions, orbiter, lander and rover missions to confirm the presence of last and, if possible, liquid water. The discovery of active flows in martian gullies, shallow martian ice, fossil hot springs, and dried up salt lakes, showed that that liquid water was a past and present reality of Mars, although now only ephemerally and in trace amounts. This led to the “follow the carbon” strategy, testing for the building blocks of life. This too is now answered in the affirmative, with the Curiosity and Perseverance rovers, which have found organic molecules and confirmed the presence of all the key elements of life. Actual life detection (past or present) most likely remains the province of future missions, although Perseverance may yet discover stromatolites in the lacustrine carbonate sediments along the margins of Jezero crater.
What are the implications of finding evidence of past or present life on Mars? Much depends on whether we can determine the nature of that life. What is its makeup, is it chemically similar to Earth life, or markedly different? Did it originate independently from that on Earth, a second genesis? If similar to Earth life, is it related, in which case did Earth seed Mars or did Mars seed Earth? If martian life represents a second genesis, it would suggest that, since life began on two planets in the solar system, it might be common in the universe. Especially if it represents a fundamentally different chemistry, such as not being nucleic acid and protein based. Conversely, if it is not a second genesis, this tell us little about whether life is common in the universe but does show that if it appears on one planet in a system, it can be transferred to others. Finding extant life could well lead to a backlash against Mars surface exploration – we might see “protect the pristine martian environment” and “protect Earth from martian lifeforms” campaigns by some environmentalists.
The Perseverance rover detected organic compounds in the sediments of Jezero crater on Mars (NASA image).
But what if we don’t find evidence for life on Mars? Carl Sagan once wrote that this would as important a discovery as finding life, as it would enable us to understand physical process in an Earth-like but lifeless planet. On Earth biology has affected the composition of the crust, atmosphere, and oceans, and the way many elements, especially carbon, hydrogen, oxygen, iron, phosphorous , nitrogen, sulphur, and manganese are cycled. Processes of erosion, deposition, precipitation are at least partly mediated by biology, which has been for the last 3.5 billion years and dominant for the last 2.5 billion. A Mars that has always been devoid of life may also point to a lesser prevalence of life elsewhere in the universe, indicating tighter constrains on its appearance. Lastly a currently lifeless Mars will greatly simply human activities on the surface
Settling the question of whether there is or was life on Mars is likely to be some distance in the future, deciding that there is no life there now will take longer, and that there was never life will take longer still. The type of detailed field investigations of biology and geology needed will most likely require teams of scientists working on the surface. By the time that happens life – human and terrestrial life – will be definitely established on Mars.
We don’t know if there is or has been life on Mars but hope that one day there will be (NASA image).