“That Mars is inhabited by beings of one sort or other is as certain as it is uncertain what those beings may be.” With these dramatic words, the American astronomer Percival Lowell informed the world about a network of canals he thought existed on the Red Planet. Lowell conjectured that Mars was a dying, drying planet, whose inhabitants built straight channels to bring melt water from the polar caps to the arid equatorial regions. He produced elaborate maps of a canal network to support his theory.
That was in 1906, when the idea of life on Mars seemed entirely plausible. Then in the 1960s space probes sent to Mars failed to reveal any sign of the much-discussed canals. In 1977 two spacecraft called Viking landed on Mars and found a freeze-dried desert bathed in deadly ultra-violet rays. The craft were equipped with robot arms that scooped up dirt and then analysed it for microbes. Nothing was found. Mars looked completely dead.
Recently, however, opinion has begun to shift. Photographs of the surface show what look like dried-up river channels and lake beds. There are hints of an ancient ocean. Evidently Mars was once warm and wet and maybe not unlike our own planet. Could life have flourished there in the remote past? Might it still be clinging on today in some obscure niche?
There is a good chance we shall learn the answers to these questions in the coming decades. Mars is the one planet in our solar system that is just about accessible to human exploration, and the motivation to go there is strong. It could be our only chance of finding a second genesis – another location in the universe where life has emerged from nonlife. The consequences for science and philosophy are incalculable. The discovery of a second tree of life would transform our view, not only of the nature of life, but of our own place in the universe.
Long ago, Mars must have had a much thicker atmosphere laden with greenhouse gases such as carbon dioxide. This would have elevated the temperature and provided sufficient pressure for liquid water to reside on the surface. Estimates suggest this “Garden of Eden” phase ended as long ago as 3.5 billion years, although sporadic warming may have occurred since.
The best hope for life on Mars today is the subsurface zone. Over the past twenty years scientists have been astonished to find microbes dwelling deep inside the Earth’s crust. The depth of this hidden biosphere extends in places to several kilometres. Because temperature rises with depth (the Earth has a hot interior) deep-living organisms are heat-loving too. In some cases they thrive at temperatures above the normal boiling point of water. The energy source for many species of subsurface life derives not from sunlight but from chemical energy. Some organisms can take gases and minerals percolating up from the Earth’s crust and turn them directly into biomass.
The discovery of subsurface life capable of sustaining itself without sunlight greatly boosted the hopes for life on Mars. The Red Planet has a hot interior too. This is evidenced by extensive volcanoes, some of which may still be active. There are undoubtedly hot spots underground on Mars, where volcanic heat has melted the permafrost, providing liquid reservoirs that could harbour primitive microbial life. Subsurface Martian life might reveal its presence through gases such as methane seeping to the surface.
To properly study any deep-living Martian microbes it will be necessary to probe beneath the surface. Planned missions to Mars, such as the European Space Agency’s Beagle 2, will incorporate penetrators and drills, but they are unlikely to burrow deep enough to reach any living organisms.
Past life on Mars could have left traces in surface rocks. In 1996 NASA scientists announced that a rock blasted off Mars by an asteroid impact had been found to contain tiny features reminiscent of fossilised microbes. The meteorite, known as ALH 84001, has been subjected to intense scrutiny. Although the jury is still out, the prevailing view is that ALH 84001 will not provide definitive evidence for life on Mars.
If there was ever life on Mars there is a serious chance it could have spread to Earth by hitching a ride in a rock like this one. An average of one Martian meteorite per month strikes our planet, and over geological time billions of tonnes of Mars rocks have rained down on us. Experiments with artillery shells and centrifuges have shown that microbes could withstand the shock of being kicked off Mars. Once in space, the cold vacuum conditions would act as a preservative.
Radiation presents the greatest hazard to life travelling through interplanetary space. But cocooned inside a rock a couple of metres across, a microbe would be shielded from the worst effects. Calculations suggest that hardy bacteria could endure millions of years in orbit round the sun if ensconced within a suitable rock. The final hazard – high-speed entry into Earth’s atmosphere – need not present a problem because the frictional heat would not have time to penetrate to the interior of the rock.
This analysis carries a startling implication. Being a smaller planet than Earth, Mars cooled quicker, so it would have been suitable for life sooner. The deep subsurface, where early life could have sheltered from the ferocious barrage of impacting asteroids and comets, may have been habitable as early as 4.4 billion years ago. By contrast, Earth was probably a searing hell until 3.9 billion years ago. So perhaps life began on Mars and later spread to Earth inside ejected rocks. If this speculation is correct, then we will all be descended from Martians, and any organisms found there today would be our distant cousins.
Exciting though this prospect may be, the discovery that Mars and Earth life share a common origin would leave tantalisingly unsolved the biggest of the Big Questions of existence: are we alone in the universe? If life arose on Mars independently of life on Earth, then it would be clear that the laws of nature are intrinsically bio-friendly, able to coax life into being on any Earth-like planet. In that case, the universe should be teeming with life. But if Earth life is just a side-branch of Mars life – or vice versa – then it is still possible that life’s origin was a freak event, unique in the universe. Either way, the Red Planet is likely to contain vital clues to the answer.