Latest evidence about life on Mars

Existence of life on Mars, More evidence of life but still no life  

Recently, NASA revealed exciting details of new findings from Mars. Scientists have discovered tiny patterns of unusual minerals in the clay-rich rocks on the edge of Jezero Crater, an ancient lake once fed by Martian river systems, and the exploration site of the NASA Perseverance Rover. The “leopard spot” patterns have been hailed as a potential sign of past microbial life due to their similarity with traces left behind by microorganisms on Earth. Researchers many times earlier thought they'd found life on the Red Planet. NASA's Perserverance captured a view of the 'Bright Angel' formation where the rover collected a sample which scientists say could be evidence of 3.5-billion year old life. Scientists are excited over the latest geological evidence for the existence of life on Mars. But as we have seen many times in the past, the Red Planet guards its secrets closely with the possibility of non-life explanations for the formations.

The jury is still out on whether these are actually signs of life, but this discovery has reignited the discussion about the previous existence of life on Mars, and the possibility that it could still survive there today. We’ll need many different lines of evidence to answer this question, but there is precedence for considering certain Martian environments as currently habitable. Early Earth and early Mars were relatively similar, but this similarity didn’t last long. Both had atmospheres and magnetic fields which offered some protection from harmful radiation originating from the Sun, along with bodies of liquid water on their surface. We know that these conditions led to the origin of life on Earth, so it is possible that the same could have happened on Mars. As geologist Joel Hurowitz said that the rock in question was found by the Perseverance rover, which has been exploring what was once a river delta and drained into an ancient Martian lake. Two chemicals found in the delta, vivianite and greigite, are most often formed on Earth by microbial activity in the soil, hence the excitement. But under certain conditions, they can also be formed by chemical reactions. In other words, we have potential evidence for life on Mars, but no conclusive proof. NASA's Perseverance rover discovered spots on reddish rock which scientists are considering as a potential biosignature for life. 

While life on Earth was beginning to thrive, Mars lost its magnetic field as its core cooled. This exposed the planet to harmful solar rays which began to erode the atmosphere. As the atmosphere disappeared, the Martian surface became colder and drier, eventually becoming the freezing desert we know today. This is why many scientists don’t expect to find living organisms on the surface of Mars, it is simply too inhospitable for life as we know it. Instead, the hope lies in uncovering microbial life hidden in protected underground or icy regions. Mars has been tantalizing us with the possibility of life on its surface since American astronomer Percival Lowell looked through his telescope in 1894 and thought he saw straight lines crisscrossing the planet. He called them canals, which are artificial, not naturally formed channels. Lowell believed these enormous features were engineered like the Panama or Suez Canals, but on a much larger scale. To him it suggested there was a civilization on Mars with amazing technical skills. It was not until the late 1960s, when robots were finally sent to Mars that we got the first close-up images of the surface and saw that the there was evidence of channels but not canals. Lowell's claims were fuel for science fiction writers and movie makers. It was not until the 1970s that robotic spacecraft sent to Mars debunked Lowell's theory, revealing a landscape pockmarked with craters which looked more like the moon than a habitable planet. But further robotic exploration did discover dried up river beds, or channels, proving that water once flowed on the surface billions of years ago.

On Earth where there is water, there is life, so perhaps there is or was life on Mars. It is just not in the form of cities and networks of canals. Possible locations for Martian microbial life include caves, inside or underneath ice sheets at the poles, or deep underground. All of these environments have analogues (environments with certain similarities) on Earth which host microorganisms. So it is not much of a stretch to consider that if life began on Mars, it could still be holding on in these extreme niches. Further compelling evidence came in 1976, when two Viking landers carried out experiments to look for life in the Martian soil. When the soil was warmed up and fed a drop of dilute nutrient solution, it gave off a gas as though microbes were present. But another experiment searching for organics found none, so it was assumed the reaction was chemical, not biological. Then there was great fanfare in 1996 about a Martian meteorite found in Antarctica which contained what appeared to be a tiny fossilized worm. Finally, real proof of Martian life. But again, the geochemists stepped in and declared the shape could also be formed by non-biological processes. Martian caves might be locations where life could survive today. 

Perhaps the most plausible of these is underground, the Martian subsurface. Extending from a few metres to several kilometres deep, it is thought to be the planet’s most stable and long-lived potential habitat. While the surface has been cold, dry and generally inhospitable for much of Martian history, the deep subsurface may have offered more favourable conditions. On Earth, the deep biosphere, the life which survives beneath the surface, provides a useful comparison. A substantial amount of Earth’s microbial life exists underground, surviving in cracks within rocks. These ecosystems are dominated by lithoautotrophs, microbes which get energy by feeding on those rocks. Methane, a potential byproduct of some lithoautotroph feeding habits, has even been detected on Mars. But there are many ways to generate methane underground without life, so right now this doesn’t tell us much. The debate continues, especially since NASA's Curiosity rover recently detected the largest organic molecules found on Mars to date, which might be fragments of a common component of life seen here on Earth. Despite more than a century of exploration, no definitive life has been found on the surface of Mars, at least not yet. But what about the possibility of life within the Martian crust? 

The potential for a deep biosphere hinges on factors including the availability of liquid water, a source of energy, space to live in and tolerable temperatures. There is possible evidence for the existence of liquid water below the surface of Mars, but this is still under debate. This would facilitate chemical reactions known as water-rock reactions which generate energy for microbes to live on. Because of its weaker gravity, rocks on Mars may be less compressed than those on Earth and remain more porous at depth, providing space for microbes to live in. At the same time, Mars produces less heat from its interior, which means temperatures suitable for life could extend nearly twice as deep underground as they do on Earth. As microbial biogeochemist Karen Lloyd says, Interterrestrials, there is a multitude of life forms that thrive deep underground on Earth. Kilometres beneath the surface, microbial life survives independent of sunlight, living in water that seeps through cracks in the rock. 

Mars too has a permafrost layer which raises the possibility of liquid water deeper down where microbial life could thrive. This could be a promising place to look. Scientists spend a lot of time analysing places on Earth, Mars analogues, to try to understand the possibilities for past and present life on Mars. These environments are not identical to Mars, but they share at least one important feature such as extreme dryness, high salt levels or high UV exposure. Earth’s deep subsurface is one example, and others include the Atacama Desert in South America, sediments at Lake Salda in Turkey, and salts found in Utah’s Pilot Valley. Researchers around the world are investigating these sites on Earth to better understand how Martian conditions might affect life and its preservation. As no one location on Earth could possibly match all Martian conditions, scientists also run controlled laboratory experiments. An example of this is the use of specialised “Mars chambers” to reproduce Martian environmental conditions such as its atmosphere, radiation exposure and temperature. All of these investigations combined help us to better understand the potential for life to exist on Mars.

Looking beyond Mars to the more than 6,000 verified planets orbiting other stars, and thousands more identified as candidates, many appear to have hostile surfaces. But that doesn't mean there couldn't be life. The universe might be teaming with life mostly hidden from view underground. Right now there is no conclusive evidence of life on Mars past or present. NASA’s “leopard spots” are the most promising signs we have, but these are still inconclusive. If life exists on Mars today, it is almost certainly not widespread like on Earth, our probes and rovers would have seen it. However, important opportunities lie ahead. The upcoming European Space Agency (Esa) ExoMars Rosalind Franklin rover will be able to drill up to two metres below the Martian surface. This will give us a chance to study the shallow subsurface of Mars which may contain living microorganisms. But this is only the start, most scientists agree that we will need to go deeper. NASA recently introduced a new class of astronaut candidates who will be among those to return to the moon and possibly to venture on to Mars. As they learn to fly spaceships and survive on other worlds, perhaps they should also get lessons on how to dig deep holes, so when they land on Mars they will have not just a flag to plant in the ground, but a drill. Drilling deep on Earth is still a huge challenge and there is so much we don’t know about our own subsurface life. Probing the deep subsurface of Mars will be a major scientific and engineering challenge, but one that may hold the key to finding existing Martian life.