The ten most important questions for astrobiology: Number 9

Now we're down to the last two questions of the ten most important questions for astrobiology, a highly biased and personal overview of the primary motivations and challenges for this still emerging scientific 'interdiscipline'. Number 9 is big, not quite the biggest, but hugely important and rather remarkably getting ever closer to the realm of the answerable.

Is there microbial life elsewhere in the universe?

As has been discussed before in these pages, microbial life is the dominant form of life on Earth. It represents the majority of the total planetary biomass. It represents the greatest genetic diversity of living organisms. It represents the oldest continuous lineage of life on the planet. It represents the most flexible, adaptable, and metabolically versatile life on the planet - inhabiting the greatest range of niches. It represents the component of the biosphere that has, throughout Earth's history, been pivotal in establishing, and maintaining the bio-geochemical cycles and feedback systems that help govern atmospheric, marine, and surface chemistry and the global energy budget. It was here long before the likes of us, and will be here long after the likes of us.

So how do we find out whether this ancient and persistent mode of life exists beyond the confines of the Earth? Microbial life is one of the primary targets of solar system exploration and studies. When the Viking landers arrived on Mars in 1977 they performed carefully designed tests on the response of martian soil to being 'fed', as a means to see whether microbial life would perk up and show its presence. The results are still a matter of debate. Initially something indeed seem to be going on, even vigorously. Later analysis swayed opinion to consider this a result of the quirks of martian soil chemistry. Very recently then the possibility that the chemistry owes something to microbial life in the first place has been raised. With convincing geological evidence now in place that indicates Mars' on and off again acquaintance with surface aqueous environments, NASA's new rover Curiosity will carry with it a full instrument package dedicated to finding signs of extinct or even extant microbial life. European rover plans are also going to target such studies.

The likely subsurface oceans on Europa, as well as places like Ganymede, are an intriguing potential habitat for microbes like chemoautotrophs, among others. Similarly the liquid water zones that seem to be lurking inside Enceladus are prime locales. In fact, the more we know about terrestrial microbes, from bacteria to archaea, the more places begin to look attractive, even the mid-zones of the Venusian atmosphere and the porous interiors of comets and asteroids. Even if any organisms we find turn out to be relatives, transplanted around the solar system as lithopanspermia, it will be a stunning piece of evidence for the persistence and adaptability of microbial life.

Further afield, to small rocky exoplanets, the near-term goal of making rudimentary measurements of atmospheric chemical abundances is directly motivated by the relationship of out-of-equilibrium chemistry to life, and to the planetary engineering skills of microbes in particular. Indeed, making a planet long-term suitable for life in general may, in chicken-and-egg fashion, require the geo-engineering of microbial organisms. Life, much like its internal chemistry, can self-catalyze.

No, detection of atmospheric oxygen, carbon dioxide, even co-existing methane will not tell us precisely what is going on in these distant worlds, but the simplest answer would be that a microbial-type biosphere is present. Let's imagine that we find a nice rocky planet somewhere within 100 light years that is conveniently transiting its low-mass parent star so that we can see the atmospheric oxygen, water vapor, and a couple of other interesting molecular species. The trick will be to keep looking. Even a tidally-locked 'eyeball Earth' is likely to exhibit variations in environment with time. Populations of organisms are dynamic, stellar input is never perfectly constant and the responses of a biosphere to change are as much a fingerprint as is a single snapshot. Lurking in such data might be the clues to let us see whether a microbial world, like our own, exists or not.

From what we have learned about life on Earth then if we find no evidence for microbial organisms either in our solar system or beyond, it will dramatically alter not just the odds of finding life anywhere in the universe, but also our ideas and models for how life originates. For many of us this would be a disquieting prospect, and so we hold our breath to see whether the bacteria and archaea are indeed just local examples of a cosmic phenomenon.