You might be surprised to know that there are quite a few places we might be able to go to find present day life or evidence of the evolution of life. It would be easy to miss it, because we have sent no life detection missions outside of Earth since the two Viking missions to Mars in the 1970s.
For sure, there is no planet like Earth, covered in forests and grasslands, but one thing we have discovered since the 1970s is that life need not be as easy to spot as that. We have found life on Earth in caves deep underground, in caves sealed from the surface for millions of years. We have found life around the hydrothermal vents at the bottom of the oceans, able to survive in complete darkness and some of it not dependent even on the oxygen in the water. We have found microbes in deep mines, and at high temperatures and low temperatures tens of degrees below zero.
We have also found life in Antarctica in dry valleys apparently completely devoid of life to the naked eye. The individual microbes there slowly metabolize beneath the surface of rocks, with lifetimes of millennia. Some life on Earth doesn't need liquid water to survive; some lichens in arctic and high altitudes can survive in freezing cold conditions using just the water vapour in the cold night air.
In the past Earth has plunged into periods of global ice cover, snowball ice. At those times it would seem almost devoid of life as Mars seems today - yet there was enough life left for evolution to continue afterwards. Earth was able to recover because of the warming effect of the carbon dioxide given out by volcanoes, accumulating over millions of years. If it wasn't for the process of continental drift subducting limestone, and then eventually volcanoes replenishing its atmosphere, Earth might have lost most of its atmosphere, and still be in a snowball phase today.
Would a snowball Earth lose all of its life? It seems unlikely, as some life would surely remain deep below the surface, in caves, hydrothermal vents, and kilometers below the surface, totally unaffected by the changes on the surface of the planet.
It also seems quite quite possibly that life would persist on the surface too, in places similar to the McMurdo valleys in Antarctica.
So, where else might we find signs of present day life in our solar system? And what about past life?
First, in the early solar system, during the "late heavy bombardment" about 4 billion years ago, there was much exchange of material between the planets. We don't know if life was also transferred at this time, especially since this happened in the very early solar system. But it seems possible it did.
We don't know too much about what life was like then, but from the evidence of the archaea, the most primitive seeming lifeforms, it seems reasonably likely that some of it might have been able to survive transfer from one planet to another via meteorite impact.
It also seems likely that Venus, Mars and Earth all had oceans at that time. Oceans which in the case of Earth at least already had the beginnings of early life. So, wherever the life started, it seems at least possible that some of the most hardy forms of life were actually shared with the other planets (while they most probably also had independently evolved indigenous life of their own).
There is no way to know if this picture is correct. But if it is, it suggests a great deal of potential for search for life elsewhere in the solar system.
First of all, present day Mars is still surprisingly hospitable - not for trees, plants, animals etc - but for life "living on the edge" like the microbes in Antarctica. When the scientists saw the data from Mariner and Viking in the 1970s, most more or less gave up on the idea of life on Mars. It seemed unlikely that anything could survive there. But many things make it seem much more promising as a location for life now.
The lichens in the DLR experiments, with their ability to metabolize and grow with no liquid water at all, just using moisture in a simulated Mars atmosphere. Microbes able to use deliquescing salts as a source for liquid in otherwise totally dry conditions - and the evidence of possible drops of liquid on the legs of Phoenix - and the presence of many salts on the surface of Mars able to deliquesce in this way at temperatures suitable for life, if mixed in the right proportions. It's not going to be easy to spot, most likely, but it may be there.
Then, if there was life on Venus in the early solar system, it might well have colonized the upper atmosphere, as it does on Earth. The conditions there are reasonably hospitable for life, certainly nothing like surface conditions, and there is indirect evidence which, just possibly, might be evidence of live in the upper Venusian atmosphere.
Then there is the possibility of past life on Mars. Any past life on the surface would have deteriorated due to cosmic radiation long ago. But some of the past life would surely get buried quickly in the huge floods on early Mars. If we know exactly the right place to dig, or are lucky, find caves or excavated deposits, we might be able to find pristine deposits on Mars that have been protected from cosmic radiation by ten meters or more of debris for billions of years.
This may be hard to find, as hard to find as the rarest fossils on Earth. Mars is a big planet, same land area as the Earth. We have the advantage that much of the surface geology is explosed with no trees or other vegetation to hide it. In many ways we may feel that we know the geology of Mars very well. But the deposits of most interest are ones that are covered by at least ten meters of material; harder to spot.
Then, for past life, we can try Phobos, innermost moon of Mars. It might have evidence of ancient Mars in the form of meteorites. Is thought that a significant percentage of all the material on Phobos comes from Mars directly through debris of meteorite impacts.
There may also be evidence of past life on the Moon too. Similarly to Phobos, it's thought that the impacts on Earth sent debris to the Moon and if we know where to look, we might find evidence of stages of evolution on the Moon that are no longer preserved on Earth. Especially so if any of it was buried deep, deep enough to be protected from cosmic radiation.
Then finally there might also be life in the deep oceans of some of the moons in our solar system. Particularly, Europa, which may have an oxygen rich ocean. The oxygen derives not from photo-synthesis but from hard radiation from Jupiter splitting the ice and so creating free oxygen. Eventually over geological timescales enough of it would reach the subsurface ocean to create levels of oxygen suitable for animals like fish, octopi etc, able to lead a highly active lifestyle. If such life exists, we would be completely unaware of it as yet.
Then, there are the ethane lakes of Titan beneath its thick methane / ethane haze of an atmosphere - the only atmosphere in our solar system as dense as the Earth's. Sadly there is probably no liquid water there normally. So if there is life then it probably takes a very different form from Earth life. But, some thing, it may also have subsurface oceans like Europa. If so, then this water may sometimes erupt to the surface as a "water volcano". Or materials from the surface may get subducted through geological processes and enrich the subsurface oceans.
There is Encladus too. It doesn't have the same promise of oxygen rich oceans, as Europa, but what it does have is water actually continually released into space as geysers today. This makes it a place where it would be especially easy for a robotic spacecraft to gather and analyze a sample - without disturbing any life there may be in the subsurface lakes or ocean itself.
So, the solar system is rich with possibilities for ancient life and even present day life. There are other more remote possibilities too.
Ceres and Vesta are large enough so that COSPAR haven't yet totally ruled them out as possible Category IV locations for life. Perhaps there might be processes that sometimes melt the surfaces, even occasional impacts could do that. Life then could perhaps survive for long enough to reproduce. It seems not impossible though perhaps unlikely that these objects may also have life on them.
You might think that surely we would have discovered life by now if it existed (except possibly in the seas of Europa). But that depends on how obvious and easy to find it is.
We do have wonderful geological photographs of many of these bodies. Particularly we can explore Mars in exquisite detail from space. But this doesn't tell us much about microscopic life on the planets and Moons, unless it is abundant enough to make noticeable changes to the appearance of the planet. As we saw with the analogy of the McMurdo valleys in Antarctica, there is no reason why it needs to be as easy to spot as that.
Cryptic life, possibly living beneath the surface of rocks or just below the surface of the soil, and slowly metabolizing with microbial lifetimes of centuries or millennia, would surely be almost impossible to spot from orbit or through its effect on the atmosphere.
When it comes to surface missions, then we have hardly explored the solar system at all. Even the Moon is barely explored from the surface.
Yes we have lots of rocks from the Moon but most of those were collected by jet fighter pilots with limited help from Earth. They had a series of courses, and a few field trips with a geologist to help them to identify interesting rocks, which surely helped, but it is not the same as sending a geologist to the Moon.
Only one scientist was ever sent to the Moon, and that just for the last mission, Apollo 17. His time there was all too short. Mission control had to warn him that he was running out of time, and to call him back to the module as he rushed trying to gather his last few specimens, surprised at how fast the time had passed.
As for Mars, we have sent several wonderful geology missions to the planet, but they have explored only a tiny fraction of the surface. They can't be sent to the most interesting terrain because the "landing ellipse" is so large for Mars (due to the thin atmosphere, too thin for parachute landings, too thick for completely controlled rocket landings). They need to be landed in flat uninteresting terrain and then take months to get to places of more interest.
Imagine exploring the Earth using a collection of four rovers, which you can only send to the flattest places, places like the Sahara desert perhaps - and they can only move a maximum of 100 meters a day? It would take centuries to explore much of the surface of the Earth by this method.
Then, also, they have no life detection capabilities. It is not through lack of interesting instruments to send to Mars. We have many of those now, the Urey suite, Astrobionibbler, both exquisitely sensitive to the presence of even single molecules of biological interest. Also BIOG, a miniaturized DNA sequencer could be sent there in 2016, and there is the latest version of Levin's labelled release with ability to detect chirality. And we haven't yet sent a single decently powerful optical microscope to Mars.
There is much talk about returning a sample from Mars to Earth. But the exobiologists say this is premature, that we don't know what to return, not if it is of biological interest. What we need to do is to start sending life detection instruments to Mars and to Venus, and to Europa, and Titan and so on, even to the Moon too, looking for evidence of present day and past life there.
There are two places we can safely attempt a sample return. First Phobos, any material on the surface of Phobos has probably been exposed to cosmic radiation for millions of years and so should be totally sterile. If there is any doubt about this, it could be exposed to sterilizing levels of hard radiation deliberately during the journey back or after return to Earth before the capsule is opened.
If I was planning a sample return from Mars, that's what I'd do. Return it from Phobos instead, as the Russians plan to do. This serves two purposes. First it tells us about the composition of Phobos which might be of great interest for in site resource utilization in Mars orbit. Also it would return samples from the Mars surface, pre-sterilized. I would perhaps do two identical samples (or two copies of each sample), first would do in situ experiments on Phobos itself, and then return the samples to Earth - but in carefully sealed containers. And then seal again a second time before return to Earth surface, all this is to be doubly cautious.
Then on return to Earth, put one of the samples through a particle accelerator to simulate an extra few million years of cosmic radiation exposure. After this it would be surely sterile, but not significantly degraded as the chances are that most of the material has been subject to those levels of cosmic radiation anyway.
Another option I wondered about was to send it through the Van Allen belt multiple times on return from Mars to sterilize it.
Then you can study it as a sterile sample. The other non sterilized one would be available if you found something of extraordinary interest that needed to be studied without sterilization but would be opened only with great caution and care.
