One of the aims of the new Perseverance rover mission on Mars is to search for signs of early biological life. Is there any hope of success?
I’ll work through the reasoning that supports the investigation. The conclusion is: although there is no biological life on the surface of Mars now, we have no reason to think life didn’t arise there early on, and good reason to think that it may have arisen. Moreover, there is reason to think that signs of life might be relatively easy to find on Mars.
Earth and Mars were surely formed around the same time, around the same time the whole Solar system formed, which is usually quoted as 4.6 billion years ago. For a long time, stuff continued to rain down on all the bodies of the solar system, bombarding and churning up their surfaces.
Eventually, that rain subsided, leaving bodies like the Moon “maximally cratered” — that is, as new craters occasionally form on the Moon, they obliterate existing ones, leaving the total distribution of craters statistically constant. Nonetheless, some of the biggest, oldest craters on the Moon date from the last phase of that bombardment. The Moon has very little geological activity however, and no atmospheric erosion at all, whereas the surfaces of Mars and the Earth are greatly modified by both of these processes.
On the Earth, there are few meteor impact craters to be found, because the Earth’s crust is continually being deformed and re-molded by geological processes, and eroded by water and wind. Nonetheless, the oldest crater might be as old as 3 billion years: it is under the ice in Greenland, on a very old chunk of continental rock.
Mars is in between the Earth and the Moon in this sense. It had a lot of geological activity at some point, forming titanic volcanoes, something like continents and lowlands, and perhaps the 3000 km long rift valley.
But these features are some 3.5 billion years old. Little geological activity persists on Mars, and the surface of some regions is dominated by impact craters. Although there are clear signs that water once flowed on Mars, all those signs are likewise billions of years old. Although Mars has a thin atmosphere, and ices (both water and carbon dioxide ice) form at the poles, those erode the major geological features only slowly.
Life on Earth is also very old — the oldest direct micro-fossil evidence is some 3.5 billion years old, and there is indirect evidence of life much older than that. Directly detecting signs of early life on Earth is difficult, because the early life was microbial — and rarely left anything by way of fossils, and because so little of early rocks remain. But indirect evidence of early life is more abundant, such as mineral formations that are today produced by bacteria — and that is found to be more than 4 billion years old.
The point is, life arose on Earth very shortly (in geological terms) after the surface cooled enough to support the chemical molecules on which life is based, after liquid water could persist.
It is clear that, at the present time, nothing like Earth life could persist on the surface of Mars. It’s just too dry and cold and exposed to radiation. Was Mars always so different from Earth? Early on, it surely was more similar to Earth — that can be seen a couple of different ways.
The loss of Martian atmosphere can be understood. Due to the Sun’s radiation and the lower Martian gravity, Mars would have lost almost all of its atmosphere, even if had started with one similar to Earth’s. The speed of this process can be calculated; given the thin Martian atmosphere now, it surely did have an atmosphere similar in density to Earth’s a few billion years ago. Moreover, there remain clear signs of water flow, fossilized into the Martian rocks, some 3.5 billion years ago.
So around the same time as life was first getting started on Earth, it seems that Mars also had conditions amenable to life. Given that life arose straight away on Earth, we are left with no reason to think that it didn’t also arise on Mars.
Earth rocks that show the earliest life are of a similar age as the last formation of water flows on Mars. This strongly points to a good place to look for life on Mars: microbial deposits, and perhaps even micro-fossils. If you look in a place where there was likely a lot of water for a long time, you just might find 3.5 billion-year-old microbe fossils.
For the very reason that life can’t exist now on Mars — since Mars has experienced much less surface-reforming than Earth since fossil-containing rocks formed — fossil evidence of life might be easier to find on Mars, than on Earth.
There has also been some speculation that liquid water might still exist on Mars, deep underground, and that microbial life might persist in it even now. Maybe future explorations could look there, too.
Finally, I should point out that Mars is not the last hope of non-Earthling life in the Solar system. Several moons of Jupiter and Saturn appear to have deep liquid oceans beneath their ice surfaces. And they are just as old as the Earth and Mars…