One of the greatest scientific mysteries is the question about the frequency and nature of life beyond the Earth. This is not a mystery because of any theoretical problems – it has just been difficult so far, with the technology we have had available, to investigate this question.

We can look at planets around other suns, but so far not with the detail necessary to identify life. This technology is rapidly progressing, however, and so we will likely have tantalizing clues in the coming years, and maybe a definitive answer.

We have been listening for intelligent signals from space, but so far have only sampled a tiny fraction of what might be out there. This is also looking for intelligent life only – and perhaps most life is not technology in a way that would lend itself to sending out radio signals.

Perhaps the best chance we have for finding extraterrestrial life is in our own solar system. Mars is a possible candidate – peharps there are microbes still clinging to existence on that dry cold world. More probes may give us an answer, but some believe we need to put scientists on the surface of Mars to do the kind of sophisticated investigation we would need to really answer the question.

Another possibility is Europa, one of the large moons of Jupiter. Europa is covered with ice, and we know it gets hotter toward its core, so it stands to reason that at some depth the ice is liquid water – and where there is liquid water there is at least the potential for life.

In recent years another moon has been added to the list of candidates for life – Saturn’s moon Enceladus. It was first observed that plumes of some gas or liquid was being ejected geyser style from the surface of Enceladus, immediately leading to speculation that this was liquid water. Perhaps, then Enceladus is like Europa with a liquid water interior.

Recently astronomers have made this possibility more likely by discovering that the plumes of liquid being ejected from Enceladus contain crystals of salt.  Salt water is even more likely to harbor life. It has previously been discovered that the plumes contain organic compounds.

Therefore, astronomers say, Enceladus has the three ingredients we know are necessary for life – liquid water, organic compounds, and an energy source. The energy source is the tidal forces from Saturn itself – which stretches Enceladus as it orbits its parent planet, heating up its interior. The same is true for the large moons of Jupiter, the innermost of which, Io, is heated up so much by tidal forces that it is literally churning itself inside out with volcanic activity.

So now we have at least two candidates in our own solar system for liquid water that may contain life – Europa and Enceladus. This raises the interesting possibility that life in the universe may be more commonly found in such locations than on the surface of planets with atmospheres heated by their proximity to their parent star. Perhaps there are many more opportunities for life in moons orbiting gas giants. Perhaps there is more life living off of tidal friction than solar energy.

At this point we can only speculate. That is the frustrating thing about our current state of knowledge of life in the universe – we have an “N” of 1. Until we start discovering life elsewhere we have no way of knowing what is common and what is rare when it comes to life.

The question of life elsewhere in our solar system, to me, is no intriguing and potentially important that I think it deserves dedicated missions to investigate further. I would like to see probes to Europa and Enceladus designed specifically to look for life. Perhaps we don’t have the technology yet, but I think its’ something we should start seriously working on.

If we find life on one of these moons and it shares features with life on Earth, that would tell us something very profound about the origin of life. If the life we find is completely different, then we will have a completely separately evolved biochemistry to investigate, and we are also bound to learn a great deal about biology. Either way, the potential for new knowledge is astounding and deserves a high priority.