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We live in a very large galaxy, which in itself is just a spot in a vast universe. Even so, this planet is our starting point to contemplate life besides ours in that universe. The question of whether we are one of the many planets with biospheres, part of a very rare phenomenon called "life", or if we are alone has chased us down the corridor of time.
The notion appeared here and there in writings of Western and Eastern antiquity, but the reflection of Giordano Bruno in De l'infito, Universo e Mondi (1584) explicitly states what has arrived to be an important part of the basis of modern astrobiology: "innumerable celestial bodies, stars, globes, suns and earths can be perceived sensibly by us and an infinite number of them can be inferred by our own reason".
In Bruno's time, such ideas were not welcomed by his brothers in the clergy, but, in our opinion, the possibility that other worlds are there and, most importantly, that we can understand them, triggers our imagination. The goal of finding another life has led us to monumental engineering feats, sending missions to the planets brothers and the moons of our solar system. It has led us to look for incredibly sophisticated instruments in the realm of those other suns to look for the "other lands" that danced in Bruno's mind.
What we now call astrobiology encompasses a vast field of study, ranging from the basics of our biochemistry on Earth and how these materials were created, to the consideration of how many "average biospheres" we might expect to share our galaxy, and what proportion could develop a complex multicellular life like that of our planet of origin.
The active search for life through orbital and terrestrial space missions extends from the rusty desert of Mars, where we can imagine someday establishing our own footprints, even thinking about looking at the mysterious icy moons full of fluid from the giant of gas planets. Two moons of this type are receiving a special emphasis: Europe, orbiting Jupiter with its rugged surface covered with ice and Saturn's Enceladus, which invites us to test its steam plumes as clues to what might be hidden inside.
There are still more exotic targets for thinking about hypothetical organisms with a chemistry significantly different from ours. These include places like Titan, the cold moon and Saturn's nebula, whose landscapes seem so similar to ours that I can imagine taking a nice spring walk there. However, the rock I would step on is ice, the "floor" of my boots would be a dark organic slime, the rain that would fall on my umbrella would be liquid methane, and a nice day could reach -179ºC. It would have to be a creature very different from the one I have to enjoy my walk!
When it comes to looking for creatures in the cosmos, we are armed with two main 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 manual to understand nature.
The only tangible scientific model 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 a great idea, but as a practical exercise we have used for a long time the remarkable environments of our planet. Each of these has lessons to teach us how organisms face extreme conditions.
We could make a huge list of the variety of houses that our planet offers, including deep-sea hydrothermal vents, Antarctic ice caves, boiling volcanic pools, cold methane seeps on the ocean floor, acid caves sulfuric and mines, and much more. What we know about life in such extreme environments has expanded greatly since our first attempts to detect extraterrestrial life in the two Viking Landers that arrived on Mars in the mid-seventies.
On Earth, it is becoming increasingly clear that the life, oceans, atmosphere and geology of our planet have deeply affected over time and continue to act as a gigantic interconnected system. Is that true on other planets too? The rock record shows that, for three-quarters of the possession of our planet as a home for life forms, microorganisms formed the entire biosphere.
Only after several billions of years did there appear a great flowering of large forms, made of many cells. Today, we know them as plants, mushrooms and animals (including ourselves). Unfortunately, our eyes are not microscopes, but if they were, we could see directly the microorganisms that still dominate our environment and the microbes that live inside us, turning each person into an ecosystem in their own right.
In my In his own previous research at the Institute of Mining and Technology of New Mexico, our team looked for spectacular underground landscapes as potential partial models for other planets and moons. There are caves and fractures in almost all types of rocky material, from carbonates to water ice and rock salt to granite and more, and at temperatures of at least 60ºC to well below the freezing point. Some of these conditions are relevant to other types of planets and moons that could harbor life.
Some of the most extreme chemical and thermal caves on Earth are inhabited by an amazing variety of unusual microorganisms. Some make their way through the bedrock, some produce extreme acid conditions, some create elaborate biominerals and rare cave formations, and many produce compounds of potential pharmaceutical and industrial importance. We have studied them as living specimens, but we have also observed the physical and chemical biofirms, or traces of life, that they leave behind.
These may include whole fossils, microfossils, and much subtler textures in and on rocks that show that microorganisms or other creatures were once active there. The geochemical biofirms can show where the activities of the organism have affected what is preserved in the chemical record.
Sometimes these microorganisms have made complicated hieroglyphic patterns, easily visible to the naked eye.
The complex chemical composition of our planet's atmosphere is also a type of biofirm: many of its gases, such as ammonia and methane, are generated in large part by life and must be constantly replenished by biological activity. The free oxygen in our atmosphere is in itself a great biofirm of photosynthesis.
I think that what astrobiologists really strive to develop is what I have always called a "Field Guide for 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 the three closest neighboring stars, Alpha Centauri A, B or Proxima Centauri.
We sat there, trying to determine not only if there were signs of life on that distant, tiny, blue colored rocky planet circling around the Sun, but also trying to discover what kind of life, how it makes a living , and if it is similar to us Alpha Centaurians, or very different? Could we solve it? We hope that the answer is affirmative for our hypothetical astrobiologists, and that we can also do it for ourselves.
To tell you the truth, we still do not know if the answer is yes. So far, it seems that we are the only planet in our own solar system with surface life visible worldwide, at least at present. Maybe Mars could have had such a life in the past, but if it still resides there today, it must be in hidden niches on the surface or in the subsoil. Of course, we are also very interested in the evidence of past lives on the planet, and missions are conceived with both states in mind. If any life resides in the liquid inner oceans of the icy moons, it is clearly not visible on the surface and will require us to be intelligent and persistent if we want to find it.
I have come to think of the Earth as an example of what I call a Type 1 Biosphere. The planet has clear indications of life on its surface, driven largely by the abundant sunlight that drives photosynthesis. In contrast, I hypothesized a Type 2 Biosphere that would be cryptic, with no detectable life on the surface and where geological chemical sources from within the planet can provide the energy needed to sustain life. Planets such as the current Mars and the icy moons could be examples of such Type 2 Biospheres if in fact they are carriers of life.
People often ask me why astrobiologists seem obsessed with 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, that is, signs of life in incredibly distant objects. As careful and systematic scientists, we try to start with what we know, and what we know with certainty is that a planet similar to the Earth (ours) actually gave birth to life (to us). That does not mean that astrobiologists have discarded very different planets as possible homes for life, but it does mean that the way to study them is much more complex.
I think we will need a variety of lines of evidence collected exactly at the same sampling site if we want to provide a convincing demonstration of probable life when we finally make such claims. What could they be? Certainly we know that we are looking for the presence of chemical compounds that are related to those that make up the life of the Earth, mainly composed of organic carbon and some others. Fortunately, such materials will have sufficiently elaborate structures, for example DNA, which 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 observe structures with different degrees of magnification and determine if they are truly biological or simply mimic biology is another way of approaching the problem of life detection. Here again, the images alone are relatively weak evidence. What about the properties of life that we observe here? If we saw a movement that could not be explained by more common non-biological means, we could conclude that whatever is in motion could be alive. There may be concrete traces of the byproduct of different types of metabolisms that involve geologically long-lasting compounds.
What about the cases where life may be present but is so different from ours that we do not see the types of chemical signals? What we expect This idea has come up in the minds of several astrobiologists, and we are contemplating possible signs of life that do not depend directly on the chemical details that guide much of the Earth's biology research.
One possibility could be the microscopic detection of shapes that seem too elaborate structurally to be a mineral form. Such a clue would be tempting, but it would also need to have chemical and other data to strengthen any claim that life is involved. What about the clues about how organisms behave when they try to acquire the energy they need to live?
Maybe they can organize themselves in space to maximize the chances of getting enough of the things they need to thrive. Many of these subtle aspects, if present, could be considered as "universal biofirms", which means that they could be a byproduct of the basic necessities of life without specifying a particular chemistry.
However, ultimately, we choose to shape the future astrobiology space missions, we know that the challenges are many. The data can be confusing and the properties of extraterrestrial microorganisms or other creatures can be very different and difficult to understand initially. But the promise of discovering something so deeply meaningful to our science and our lives as biological beings with a history different from ours will make the search worthwhile, no matter what we find on the fly.
Penelope Boston is the director of NASA's Astrobiology Institute
Photographs by Caleb Charland
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