It's easy to see geological features in our wonderful photographs of the solar system. Life though is more elusive. We could look straight at one of its habitats and see nothing.
This is a photograph of a place which scientists think may be one of the most hospitable places in our solar system outside of Earth.
It doesn't look like much, a few dark streaks that gradually spread over steep sun facing slopes on Mars in spring and summer. But these particular streaks only appear when temperatures rise above 0C. That rules out effects of dry ice, and wind, leaving liquid water as the only remaining hypothesis we have so far.
It's hard to explain those streaks as water either, but they are rare, and may depend on geothermal heating, or deliquescing salts, and unusual chemistry and geology. There are a few published ideas about how they might form.
So, if there is life on Mars, it might be in that photograph. But most of it would probably be hidden from view, a cm or so below the surface, so we wouldn't expect to see much.
The Mars surface is almost unimaginably harsh with strong Ultra Violet light, cosmic radiation, and near vacuum for the atmosphere. Almost all Earth life would die there. But there are microbes that could survive there.
UV light is easily blocked by shadow, like ordinary light. A few mms of soil can block it. Some cyanobacteria also have pigments in their cells that block out the worst of the UV while letting light through for photosynthesis, and may be able to survive in partial shadow on Mars.
As for cosmic radiation, the levels on the surface of Mars are roughly similar to the interior of the ISS. It can destroy even the most radiotolerant microbes such as radiodurans in its dormant sate after a hundred thousand years or so. But if the microbes are able to wake up more often than that, even briefly, they can repair their damage and keep growing.
Most life would probably be a cm or so below the surface, hidden from the UV, and the rapid drying effects of the daytime sunlight and the vacuum.
Similar situations apply on nearly all the solar system bodies. If there is life there, it is likely to be hiding. Not from us, but from the harsh conditions on the surfaces of the planets and moons of our solar system.
Here is an aerial view of the McMurdo valleys in Antarctica. It's cold and dry like Mars, though the thick Earth atmosphere makes it more habitable than anywhere on Mars. And life is there, though you can't see any signs of it in the photographs. The only way to find it is to look inside rocks or dig into the soil.
If there is life outside of Earth, most of it is almost certainly hidden from view like this. The life in the McMurdo valleys is also sparse, and it includes some of the longest lives microbes on earth. Some of them have lifetimes of millennia, for a single microbe, because it metabolizes so slowly. The populations are also scattered. Curiosity could look straight at them and see nothing.
Even a rover equipped with biosignature detectors might have to drill down, and search many times before it finds anything.
Then, on Earth every possible habitat, such as cooling lava, gets inhabited within a few months of formation. On Mars then it must be far harder for life to spread. For all we know, it might take thousands, or millions of years for a new patch of these warm seasonal flows to get colonized.
The search for ancient life is likely to be just as hard. Curiosity has found some organics at last, but it took it over a year to find them. Some processes seem to destroy all organics on the surface, probably a combination of cosmic radiation and chemical processes. The scientists have hypothesized that this patch must have got uncovered recently. Perhaps it was covered by other layers that got eroded away.
You might think the obvious thing to do is to return a sample to Earth to analyse. But, so far anyway, there's no reason to believe that this is ancient life, though it might be. It might just be organics from meteorites. Curiosity is not equipped to tell the difference.
Indeed, we have only sent two spacecraft anywhere with the ability to detect life directly, the two Viking landers, in the 1970s. One of their experiments, the labelled release, gave strange results that scientists argue about to this day. Most are sure that it only detected some unusual chemistry, but we haven't sent a follow up mission to check on this. So meanwhile we can only speculate about what it found.
We have plenty of new biodetection instruments we could send, far more sensitive than anything Viking had. In fact, we have sent one of those into space, but it hasn't yet reached its target. It's due to land on a comet later this year (autumn 2014). That's the Philae lander from the ESA comet encounter mission.
Philae will test for chirality. All DNA spirals in a clockwise direction, and this seems likely to be a characteristic of life, that it would create asymmetrical molecules, and that only one of the two possible forms would replicate. But Philae is not searching ffor life on the comet. Rather, it is looking to see if the chemicals delivered to Earth were already biased in one wy or another before life arose, perhaps helping life to get started, and it might explain why DNA spirals in a clockwise direction.
We could have sent similar instruments to Mars long ago, certainly it would have been possible to send an instrument like that on Curiosity. But most mission scientists didn't think that we were likely to find life on Mars, until quite recently - it was the Phoenix mission in 2008 that changed things along with new evidence from the orbiters of the ancient oceans and many new features such as the warm seasonal flows. Again all this happened in the last five years or so, and it takes some years to get the instruments space hardy and certified for launch.
ExoMars in 2018 or 2020 will be the first mission to Mars to search for life directly. It will have the ability to test for chirality and biosignatures and also to drill two meters below the surface; deep enough to find materials that might perhaps be less degraded by cosmic radiation.
There are many other instruments like that which we can send. That includes SETG able to do DNA sequencing, possibly a miniaturized scanning electron microscope, more sensitive biosignature detectors able to detect chirality, and experiments that look for life directly, to see if they can spot it reproducing. We could send good optical microscopes to Mars also.
In my next article, "Surprising places for life in the Solar System" I'll look at some of the potential habitats suggested by scientists, from the likely to the unlikely. This includes of course various habitats on Mars, also Europa, and Encladus. It also includes the clouds of Venus, and then with more exotic chemistry there is the possibility of life at ultra low temperatures on Titan, sulfur based life on Io, of course Carl Sagan's original gas giant biosphere idea (now not thought so probable as it was in the 1970s), life in liquid nitrogen on Triton, and other possibilities such as the poles of Mercury, some asteroids, and comets.
There are surprising places to look for past life also including Phobos, and our own Moon which just possibly may have meteorite debris from the early solar system with evidence of life.
There may be life in all these places, for all we know, or none of them. We wouldn't have seen it yet if it is there, not if it is cryptic, hidden below the surface, as it almost certainly would be.
The search has only just begun. We haven't sent anything to these places with a decent chance of finding cryptic life beneath the surface or sparse traces of ancient life. And life able to survive in such harsh conditions is likely to be so low in populations and slow growing that it would have a negligible effect on the planet atmospheres.
We have exciting times ahead of us. And if we don't find life in any of these places, then we will want to find out why it didn't arise. We may still learn much from looking at life's failures as well as its successes. How far did chemistry develop in the direction of life in these places? Did proto cells develop, able to reproduce after a fashion, but not exactly? Or "soups" creating conditions where chemicals such as RNA or PNA could replicate but without cell walls? Or what happened?
