Are there other forms of life beyond the land borders? Astrobiology is a scientific discipline that tries to solve this question that is almost as old as humanity itself. However, finding clues to other lives in the vastness of the cosmos is not an easy task. To learn more about this exciting search, we spoke with Carlos Briones, PhD in Chemical Sciences and CSIC researcher at the Center for Astrobiology (INTA-CSIC) . Briones combines his scientific activity with popularization and has written several books, including Are we alone? In search of other lives in the cosmos (Ed. Critic 2020) , which offers us a very complete and updated tour of space exploration and research in astrobiology.
When talking about the search for other lives in the cosmos, we should begin by asking ourselves what life consists of: what requirements does an organism that we find on another planet have to have to consider it alive?
The definition of life is a topic that has been approached by scientists and philosophers for centuries. Today, thanks to molecular techniques and everything we know about biochemistry and genetics, we consider that these three characteristics must be found in a living being: a genetic material that can be replicated and transferred to progeny, a metabolism, which is the network of reactions of exchange of matter and energy, and a compartment that distinguishes the living being as such, that individualizes it from the environment.
We consider living beings to be entities that combine these three characteristics, and based on this, different definitions of life have been enunciated. One of the ones we like the most is that of the biochemist Gerald F. Joyce, according to which a living being is a self-replicating chemical system that evolves as a result of its interaction with the environment. This is what allows us to distinguish the living from the non-living on Earth, and what will allow us to identify it outside the planet as well.
And, with that definition we can consider viruses alive?
Viruses do not have metabolism, but rather use the metabolic machinery of host cells, and since they do not have an autonomous life they cannot be considered living beings, they do not have the three legs of life that we have considered before . This does not mean that they are not important, because viruses are fundamental agents in evolution and allow the exchange of genetic material. The tree of life would not be the same without viruses, but they per se , as such, should not be considered living beings.
Now that we know what an organism needs to have to consider it alive, we are going to advance a little further. We are going to another planet: what are the basic requirements that we should meet to consider that life is possible there? Is the presence of liquid water essential?
From what we know so far, life does require the presence of carbon and liquid water molecules. Very briefly, the requirements that we consider necessary to think that there could be life on another planet or satellite are: liquid water, organic molecules and an energy source , which is what will allow the chemical reactions necessary for life to take place. That is what we are looking for inside and outside the solar system.
When we think of the search for life on other planets, the reference is Mars, why is it so promising? Is it likely that there is no life on Mars today but there was in the past?
Mars is very interesting because, to begin with, we have it very close. The robotic missions we’re sending out take only seven to eight months to arrive, and that makes exploration there a lot easier.
Furthermore, the key to Mars is that, when life originated on Earth, roughly 3.8 billion years ago, Mars was very much like our planet , both geologically and chemically, and had a large ocean that it covered practically its entire northern hemisphere. Under these conditions, if life originated here, nothing prevents it from having originated on Mars as well. After all, Earth is not such a special planet, and if Mars was so similar and relatively close, life could have arisen there as well.
However, we know that at a certain moment in the history of Mars its conditions changed radically, the atmosphere was lost, plate tectonics stopped and part of the liquid water on its surface evaporated, becoming a dry desert with temperatures surface temperatures of -55 ° C. Therefore, in this supposed scenario that life had arisen on Mars, with the new conditions, two possibilities open up: perhaps life continues to exist there, perhaps sheltered in the subsoil of the planet, where there is liquid water and protection from radiation. . The other option is that life has disappeared but some kind of trace has remained, for example molecules of biological origin that we call biomarkers.
Since you mention this about biomarkers … life that potentially can exist on other planets or moons may not be based on the same molecular components as that on Earth. If it is different, how can we detect it? What clues are we looking for?
As we have commented, life is probably always based on carbon and water , because at a chemical level they are better than other alternatives, but that does not mean that the biochemistry generated from chemistry is the same. Perhaps there are lives that do not have DNA or proteins, but in that case they will be difficult to detect.
At the moment, what we are looking for are chemical molecules originated by biology and that are really biomarkers. There are many that are not, because for example a single amino acid or a small molecule can be chemically formed without the mediation of a living organism. There are compounds that have even been detected in nebulae, and some are molecules that seem very ‘biological’ to us, such as urea, but they are not biomarkers because they have been detected in places where there is no life.
So what kind of molecules are we looking for? For example, lipids such as those that form our membranes, and that are sufficiently complex , as is the case with cholesterol derivatives. There is no known chemical route that produces them, and what we do know is that, if they exist, probably in the past there was a life that originated them.
So, the grace is in looking for molecules that cannot be formed by geological or other processes, right?
Exactly. For example, a debate open right now on Mars has to do with methane, which can be of biological and geological origin. Small methane fumes have occasionally been captured on the surface of Mars, and even orbiters have been able to detect it. One of the issues that future missions to this planet have planned is to find out if this methane is of geological or biological origin.
This is the search for life at the molecular level, but we could also search for it at the morphological level: for example, fossils of recognizable bacteria, or fossils of layers of microorganisms such as stromatolites, which on Earth have biological origin. If we saw a similar layered arrangement on another planet or satellite, and the composition was consistent with a biological origin, we might also suspect that there was life there.
In any case, if any evidence of this type is found, it will be necessary to make sure that it is so. As Carl Sagan said, “extraordinary claims require extraordinary evidence”, and the scientific community will be with sharp knives to see if it is true or if it is a false positive, that is, something that looks like life but does not really. it is.
Now that we are talking about molecules, organic, you recently participated in the discovery in space of ethanolamine, a key molecule at the origin of life, why is this finding so important?
We really liked detecting this molecule because it has eleven atoms, it is quite large compared to those found in nebulae, and it also has four types of elements: carbon, hydrogen, oxygen and nitrogen. On the other hand, it is a molecule from which the amino acid glycine can be formed and it is also the one that forms the head of the phospholipids of our cell membrane. This means that, perhaps, one of the components of the membranes of all living beings – ethanolamine – could have reached Earth from space aboard meteorites.
What other organic molecules basic to life on Earth have been detected outside our planet?
There are about 240. For example formaldehyde, formamide, urea or glycol aldehyde, which is the simplest sugar known. Another very interesting molecule is phosphorous oxide, since phosphorus is essential for the life we know on Earth.
They are small molecules, between four and twelve atoms, but they can serve as precursors of prebiotic chemistry, that’s the grace. These molecules were able to integrate into the pathways of formation of larger molecules that originated life and enrich the prebiotic soup, that metaphor of Oparin that said that the mixture of compounds that were on Earth formed a soup and from there life emerged. Well, maybe that soup had, among other ingredients, some that came from space, like these molecules that we have detected.
So, these kinds of findings can tell us that, just as these molecules reached Earth, they could also have reached other habitable planets and that, if there were life on those planets, they could also be part of their organic chemistry …
That is just the reasoning behind this. Just as they have arrived here, since the Earth is not a special planet, they could have arrived at any other place like Mars or the moons of Jupiter or Saturn.
Through the molecular mediators of the cosmos, which are mainly meteorites and comet nuclei, a lot of chemical molecules can be distributed to different places. When you detect them in space, it means that they have been able to reach many places and it increases the probability that life could have formed outside the Earth as well.
In recent years there have been many advances in the field of space exploration and astrobiology, which are, according to you, the most promising?
The exploration of Mars itself will remain critical, and the arrival of more science missions to the moons of Jupiter and Saturn is also very promising. In addition, we have the search for planets outside the solar system that may have habitable conditions, that is, where other lives may exist. I think that these are three more interesting scientific spearheads in astrobiology. They would allow us to approach the old question of whether life has arisen here by chance or by necessity : are we alone because it was very difficult for other lives to occur, or are there more lives in the cosmos? They are questions of great depth, also on a philosophical level.
