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We live in a very big galaxy, which is itself only a speck in a vast universe. Still, this planet is our departure point for contemplating life besides our own in that universe. The question of whether we are one of many planets with biospheres, part of a very rare phenomenon called “life”, or whether we are all alone has haunted us down the corridor of time.
The notion appeared here and there in writings of both western and eastern antiquity, but Giordano Bruno’s musing in De l’infinito, Universo e Mondi (1584) explicitly states what has come to be a major part of the foundation of modern astrobiology: “Innumerable celestial bodies, stars, globes, suns and earths may be sensibly perceived therein by us and an infinite number of them may be inferred by our own reason.”
In Bruno’s time, such ideas were not welcomed by his brethren in the clergy but, in our own, the possibility that other worlds are out there and, most importantly, that we can understand them, fires our imaginations. The goal of finding other life has led us to monumental engineering feats, sending missions to the sibling planets and moons in our solar system. It has driven us to peer with amazingly sophisticated instruments into the realm of those other suns to search for the “other earths” that danced in Bruno’s mind.
What we now call astrobiology encompasses a huge field of study, ranging from the foundations of our biochemistry on Earth and how such materials were created, all the way to the consideration of how many “average biospheres” we might expect to be sharing our galaxy, and what proportion might develop complex multicellular life such as that on our home planet.
The active hunt for life through orbital and landed space missions extends from the rusty desert of Mars, where we can imagine someday setting our own footprints, to thinking about how to search in the mysterious fluid-filled icy moons of the gas giant planets. Two such moons are receiving special emphasis: Europa, orbiting Jupiter with its craggy, ice-cliff-covered surface, and Saturn’s Enceladus, which is beckoning us to sample her vapour plumes as clues to what might hide in her interior.
Even more exotic targets exist for thinking about hypothetical organisms with a significantly different chemistry to ours. These include places like Titan, the cold and hazy moon of Saturn, whose landscapes seem so similar to ours that I can imagine myself taking a nice spring stroll there. However, the rock I would step on is ice, the “soil” on my boots would be dark organic ooze, the rain falling on my umbrella would be liquid methane, and a balmy day could reach -179C. I would have to be a very different creature to the one I am to enjoy my stroll!
When it comes to searching for fellow creatures in the cosmos, we are armed with two primary tools. First, what we understand about the planet that gave us life, and second, one of our greatest inventions, the powerful tools of deductive and inductive science. Together these comprise our handbook for understanding nature.
The only tangible scientific model that we have to help us look for life on other planets is our biosphere here on Earth. Of course, there are tremendously useful computer models that can give us great insights, but as a hands-on exercise we have long turned to our planet’s remarkable environments. Each of these has lessons to teach us about how organisms cope with extreme conditions.
We could make a huge list of the variety of homes that our planet offers, including the deep sea hydrothermal vents, the ice caves of Antarctica, boiling volcanic pools, cold methane sea-floor seeps, sulphuric acid caves and mines, and many more. What we know about life in such extreme environments has greatly expanded since our first attempts at extraterrestrial life detection on the two Viking Landers that went to Mars in the mid-1970s.
On Earth, it is becoming ever clearer that life, the oceans, the atmosphere and the geology of our planet have profoundly affected each other over time and continue to act as a gigantic interconnected system. Will that be true on other planets as well? The rock record shows that for three-quarters of our planet’s tenure as a home for life forms, microorganisms made up the entire biosphere.
Only after several billion years did a great flowering of large forms appear, made out of many cells. Today, we know these as plants, fungi, and animals (including ourselves). Alas, our eyes are not microscopes, but if they were we would be able to directly see the microorganisms that still dominate our environment and the microbes that live on inside us, making each person an ecosystem in their own right.
In my own previous research at the New Mexico Institute of Mining and Technology, our team sought out spectacular underground landscapes as potential partial models for other planets and moons. Caves and fractures exist in almost every type of rocky material, from carbonates to water ice to rock salt to granite and more, and at temperatures from at least 60C to well below freezing. Certain of these conditions are relevant to other types of planets and moons that might harbour life.
Some of the most chemically and thermally extreme caves on Earth are inhabited by a stunning array of unusual microorganisms. Some eat their way through bedrock, some produce extreme acid conditions, some create elaborate biominerals and rare cave formations, and many produce compounds of potential pharmaceutical and industrial significance. We have studied them as living specimens, but also observed the physical and chemical biosignatures, or life traces, that they leave behind.
These can include whole fossils, microfossils and far more subtle textures in and on rocks that show that microorganisms or other creatures were once active there. Geochemical biosignatures can show where organism activities have affected what is preserved in the chemical record.
Sometimes these microorganisms have made complicated hieroglyphic-like patterns, easily visible to the naked eye.
The complex chemical composition of our planet’s atmosphere is also a type of biosignature: many of its gases like ammonia and methane are generated largely by life, and must constantly be replenished by biological activity. The free oxygen in our atmosphere is itself a major biosignature of photosynthesis.
I think that what we astrobiologists are really striving to develop is what I have long called a “Field Guide to Unknown Organisms” to help us decide where and how to look for life. I often think how challenging it would be if we were astrobiologists belonging to an intelligent species on a planet around one of our closest three neighbour stars, Alpha Centauri A, B, or Proxima Centauri.
There we would sit, trying to determine not only if there were signals of life on that distant, tiny, blue-coloured rocky planet circling around Sol, but also trying to figure out what kind of life, how it makes its living, and whether it is similar to us Alpha Centaurians, or very different? Would we be able to work it out? We hope the answer is yes for our hypothetical fellow astrobiologists, and that we can also do so for ourselves.
Truthfully, we do not yet know whether the answer is yes. So far, we appear to be the only planet in our own solar system with globally conspicuous surface life, at least at present. Perhaps Mars may have had such life in its past, but if it still resides there today, it must be in hidden niches on the surface or the subsurface. Of course, we are very interested in evidence of past life on the planet as well, and missions are being envisioned with both states in mind. If any life does reside in the liquid interior oceans of icy moons, it clearly is not visible on the surface and will require us to be clever and persistent if we are to find it.
I have come to think of Earth as an example of what I call a Type 1 Biosphere. The planet possesses clear indications of life on its surface, powered largely by ample sunlight that drives photosynthesis. In contrast, I have hypothesised a Type 2 Biosphere that would be cryptic, with no life detectable on the surface and where geological chemical sources from within the planet may provide the energy needed to sustain life. Planets such as current-day Mars and the icy moons could be examples of such Type 2 Biospheres if they are indeed life-bearing.
People often ask me why astrobiologists seem fixated on finding Earth-like planets around other stars. The answer is very simple: we are trying to find something that is very challenging to look for, namely signs of life on incredibly distant objects. As careful systematic scientists, we try to start with what we know, and what we know for sure is that an Earth-like planet (ours) actually did give rise to life (us). That doesn’t mean that astrobiologists have ruled out very different planets as possible homes for life, but it does mean that how we go about studying them is far more complex.
I believe we will need a variety of lines of evidence collected at exactly the same sampling spot if we are to provide a convincing enough demonstration of likely life when we eventually make such claims. What might those be? We certainly know that we are looking for the presence of chemical compounds that are related to those that make up Earth life, mostly organic carbon compounds and a few others. Hopefully, such materials will have sufficiently elaborate structures, for example DNA, that they are unlikely to be the product of non-biological processes. Other smaller and simpler organic compounds may be weaker evidence, since there are non-biological ways in which they can be made.
The ability to look at structures with different degrees of magnification and determine whether they are really biological or simply mimicking biology is another way to approach the problem of life detection. Here again, images alone are relatively weak evidence. What about properties of life that we observe here? If we saw movement that couldn’t be explained away by more ordinary non-biological means, we might conclude that whatever was moving might be alive. Perhaps there might be concrete traces of the byproduct of different types of metabolisms that involve geologically long-lasting compounds.
What about cases where life may be present but is so different from our own that we do not see the types of chemical signals that we expect? This idea has arisen in the minds of a number of astrobiologists, and we are contemplating possible life signals that do not directly depend upon the chemical details that guide many Earth biology investigations.
One possibility might be microscopic detection of shapes that appear too structurally elaborate to be a mineral form. Such a clue would be tantalising but would also need to have chemical and other data to strengthen any claims that life is involved. What about clues as to how organisms behave as they try to acquire the energy that they need to live?
Perhaps they may arrange themselves in space to maximise chances of gaining enough of the things they need to prosper. Many of these subtle aspects, if present, could be thought of as “universal biosignatures”, meaning that they could be a byproducts of life’s basic needs without specifying a particular chemistry.
However we ultimately choose to shape future astrobiology space missions, we know that the challenges are many. The data may be confusing and the properties of extraterrestrial microorganisms or other creatures may be very different and hard for us to initially comprehend. But the promise of discovering something as profoundly meaningful for our science and our lives as biological beings with a different history than ourselves will make the search worthwhile, no matter what we find as we go.
Penelope Boston is the director of the Nasa Astrobiology Institute
Photographs by Caleb Charland