Life on Mars is hard to find
The discovery of organic molecules on Mars is tantalizing — but not proof
Scientists are being very careful to point out that the recent discovery of organic molecules in Martian bedrock and methane in the Martian atmosphere, does not prove they have found life. True, it increases the odds that life may have existed — and could still — on the red planet. But actually finding that life, if it's there, will be extremely difficult.
Mars has tantalized us with the possibility that it could host alien life ever since astronomers first looked at it with telescopes. They saw white ice caps at the north and south poles, and markings on the surface that looked like lines, which they imagined could be natural channels or more speculatively, artificial canals built by an advanced civilization.
Unfortunately, when our robots visited the planet in the late 1960s and '70s, no canals or cities appeared in their cameras. Weirdly, some thought they saw a face the size of a mountain, which turned out to be nothing more than a natural hill that plays with shadows.
Still, there are features that look like dried up riverbeds and lakes suggesting that billions of years ago Mars was warm and wet like Earth. Liquid water is considered a precondition for life, so the question of whether life has ever existed on Mars is a valid one, but despite our best efforts, evidence for that life itself remains elusive.
In 1976, two Viking landers carried laboratories specifically designed to look for signs of life. Soil samples were placed in ovens, where water and nutrients were added, and instruments looked for gases such as oxygen, methane and CO2 that could be given off by microbial activity in the soil. Some gases were indeed detected, but Viking could not find any organics, chemicals based on carbon, that form life on Earth.
Either Martian life was based on something else or the chemical reaction that produced the gas was not biological. The debate among scientists about Viking's findings raged for years, but the results of the experiments are considered inconclusive.
Now, Curiosity has found organic molecules. But they don't necessarily provide evidence for life. They could have been produced through non-biological soil chemistry. They might also have fallen from the sky in carbonaceous chondrites, a type of meteorite that contains organics. The same is true of the methane in the atmosphere, which can be produced by chemical reactions involving water in the soil without biological activity.
So Mars continues to tantalize us with evidence that conditions were once right for life, and evidence that the ingredients for life are there, but still no absolute proof that life exists. So what will it take to actually find it?
On Earth, life is everywhere. It pokes out of the ground, crawls, walks or runs across the surface, swims in the water and flies through the air. But Mars appears totally barren. Nothing like a plant or animal has shown up in front of the cameras of Mars landers. It's too bad, really. Can you imagine the reaction if we saw a crowd of Martians gathered around one of our landers taking pictures of it wondering where the UFO came from?
We've found no footprints in the sand, no burrows between the rocks, and nothing flying overhead. So simply looking around has turned up nothing.
Another approach is to look directly for microbes in the soil, but that requires a powerful microscope, and so far, no lander has been equipped with one. There are plans to bring samples of Mars to Earth so they can be studied using laboratory equipment, but that is far in the future.
Finally, discovering fossils would be absolute proof that life existed there in the past, but fossil hunting is hard to do by robot. It takes the trained eyes of a paleontologist to scan a cliff face and pick out the rocks that used to be bones, which means we will likely have to wait until humans land on Mars before we find real evidence of life.
Ironically, the moment humans set foot on our neighbouring world, we will have contaminated it with our own organisms from Earth. Martian scientists will have to be extremely careful to ensure anything they do find is actually from Mars and not something they brought along themselves.
The quest for life beyond Earth is the pursuit of one of our most fundamental questions: Are we alone in the universe?
The possibilities seem endless, but alien life is not making it easy for us to find it.