ASTROBIOLOGY: HISTORY, OBJECT OF STUDY AND IMPORTANCE - BIOLOGY

The astrobiology or exobiology is a branch of biology that deals with the origin, distribution and dynamics of life in the context of both our planet, as the entire universe. We could say then, that as a science astrobiology is to the universe, what biology is to planet Earth.

Due to the broad spectrum of action of astrobiology, other sciences converge in it such as: physics, chemistry, astronomy, molecular biology, biophysics, biochemistry, cosmology, geology, mathematics, computing, sociology, anthropology, archeology, among others.

Figure 1. Artistic interpretation of the connection between life and space exploration. Source: NASA / Cheryse Triano

Astrobiology conceives life as a phenomenon that could be "universal." It deals with their possible contexts or scenarios; its requirements and its minimum conditions; the processes involved; its expansive processes; among other topics. It is not limited to intelligent life, but explores every possible type of life.

History of astrobiology

The history of astrobiology perhaps dates back to the beginnings of humanity as a species and its ability to question itself about the cosmos and life on our planet. From there arise the first visions and explanations that are still present in the myths of many peoples today.

The Aristotelian vision

The Aristotelian vision considered the Sun, the Moon, the rest of the planets and stars, as perfect spheres that orbited us, making concentric circles around us.

This vision constituted the geocentric model of the universe and was the conception that marked humanity during the Middle Ages. Probably could not have made sense at that time, the question of the existence of "inhabitants" outside our planet.

The Copernican view

In the Middle Ages, Nicolás Copernicus proposed his heliocentric model, which placed the Earth as one more planet, revolving around the sun.

This approach profoundly impacted the way we look at the rest of the universe and even look at ourselves, since it put us in a place that perhaps was not as "special" as we had thought. Then the possibility of the existence of other planets similar to ours and, with it, of life different from the one we know.

Figure 2. The Heliocentric system of Copernicus. Source: Public domain, via Wikimedia Commons

First ideas of extraterrestrial life

The French writer and philosopher, Bernard le Bovier de Fontenelle, at the end of the 17th century already proposed that life could exist on other planets.

In the mid-18th century, many of the scholars associated with Enlightenment were writing about extraterrestrial life. Even leading astronomers of the time like Wright, Kant, Lambert, and Herschel, assumed that planets, moons, and even comets could be inhabited.

This is how the nineteenth century began with a majority of academic scientists, philosophers and theologians, sharing the belief in the existence of extraterrestrial life on almost all planets. This was considered a sound assumption at the time, based on a growing scientific understanding of the cosmos.

The overwhelming differences between the celestial bodies of the solar system (regarding their chemical composition, atmosphere, gravity, light and heat), were ignored.

However, as the power of telescopes increased and with the advent of spectroscopy, astronomers were able to begin to understand the chemistry of nearby planetary atmospheres. Thus, it could be ruled out that nearby planets were inhabited by organisms similar to terrestrial ones.

Object of study of astrobiology

Astrobiology focuses on the study of the following basic questions:

What is life?
How did life arise on Earth?
How does life evolve and develop?
Is there life elsewhere in the universe?
What is the future of life on Earth and elsewhere in the universe, if it exists?

Many other questions arise from these questions, all related to the object of study of astrobiology.

Mars as a model for study and space exploration

The red planet, Mars, has been the last bastion of hypotheses of extraterrestrial life within the solar system. The idea of existence of life on this planet initially came from observations made by astronomers in the late 19th and early 20th centuries.

They argued that the marks on the Martian surface were actually channels built by a population of intelligent organisms. These patterns are now considered to be the product of the wind.

The missions

The Mariner space probes exemplify the space age that began in the late 1950s. This era made it possible to directly visualize and examine the planetary and lunar surfaces within the solar system; thus ruling out the claims of multi-celled and easily recognizable extraterrestrial life forms in the solar system.

In 1964 NASA's Mariner 4 mission sent the first close-up photographs of the Martian surface, showing a basically desert planet.

However, subsequent missions to Mars and the outer planets allowed a detailed view of those bodies and their moons and, especially in the case of Mars, a partial understanding of their early history.

In various extraterrestrial settings, the scientists found environments not very different from inhabited environments on Earth.

The most important conclusion of these first space missions was the replacement of speculative assumptions with chemical and biological evidence, which allows it to be objectively studied and analyzed.

Is there life on Mars? The mission

In the first instance, the results of the Mariner missions support the hypothesis of the non-existence of life on Mars. However, we must consider that macroscopic life was being sought. Subsequent missions have cast doubt on the absence of microscopic life.

Figure 3. Orbital and terrestrial probe of the Viking mission. Source: Don Davis, via Wikimedia Commons

For example, of the three experiments designed to detect life, carried out by the Viking mission's ground probe, two were positive and one negative.

Despite this, most scientists involved in the Viking probe experiments agree that there is no evidence of bacterial life on Mars and the results are officially inconclusive.

Figure 4. Landing probe (Lander) of the Viking mission. Source: NASA / JPL-Caltech / University of Arizona, via Wikimedia Commons

Missions

Following the controversial results of the Viking missions, the European Space Agency (ESA) launched the Mars Express mission in 2003, specifically designed for exobiological and geochemical studies.

This mission included a probe called Beagle 2 (homonymous to the ship where Charles Darwin traveled), designed to search for signs of life on the shallow surface of Mars.

This probe unfortunately lost contact with the Earth and could not carry out its mission satisfactorily. Similar fate had the NASA probe "Mars Polar Lander" in 1999.

Mission

After these failed attempts, in May 2008, NASA's Phoenix mission reached Mars, obtaining extraordinary results in just 5 months. Her main research objectives were exobiological, climatic and geological.

This probe was able to demonstrate the existence of:

Snow in the atmosphere of Mars.
Water in the form of ice under the upper layers of this planet.
Basic soils with a pH between 8 and 9 (at least in the area close to the descent).
Liquid water on the surface of Mars in the past

The exploration of Mars continues

The exploration of Mars continues today, with high-tech robotic instruments. The Rovers missions (MER-A and MER-B) have provided impressive evidence that there was water activity on Mars.

For example, evidence of fresh water, boiling springs, a dense atmosphere and an active water cycle have been found.

Figure 5. Drawing of the Rover MER-B (Opportunity) on the surface of Mars. Source: NASA / JPL / Cornell University, Maas Digital LLC, via Wikimedia Commons

On Mars, evidence has been obtained that some rocks have been molded in the presence of liquid water, such as Jarosite, detected by the MER-B (Opportunity) Rover, which was active from 2004 to 2018.

The Rover MER-A (Curiosity) has measured seasonal fluctuations in methane, which has always been related to biological activity (data published in 2018 in the journal Science). He has also found organic molecules such as thiophene, benzene, toluene, propane, and butane.

Figure 6. Seasonal fluctuation of methane levels on Mars, measured by the Rover MER-A (Curiosity). Source: NASA / JPL-Caltech

There was water on Mars

Although the surface of Mars is currently inhospitable, there is clear evidence that in the distant past, the Martian climate allowed liquid water, an essential ingredient for life as we know it, to accumulate on the surface.

Rover MER-A (Curiosity) data reveals that billions of years ago, a lake within Gale crater contained all the ingredients necessary for life, including chemical components and energy sources.

Martian meteorites

Some researchers consider Martian meteorites as good sources of information about the planet, even suggesting that there are natural organic molecules and even microfossils of bacteria. These approaches are the subject of scientific debate.

Figure 7. Microscopic view of the internal structure of the ALH84001 meteorite, showing structures similar to bacilli. Source: NASA, via Wikimedia Commons

These meteorites from Mars are very rare and represent the only directly analyzable samples of the red planet.

Panspermia, meteorites and comets

One of the hypotheses that favors the study of meteorites (and also comets), has been called panspermia. This consists of the assumption that in the past the colonization of the Earth occurred, by microorganisms that came inside these meteorites.

Today there are also hypotheses that suggest that terrestrial water came from comets that bombarded our planet in the past. In addition, it is believed that these comets may have brought with them primal molecules, which allowed the development of life or even already developed life lodged inside them.

Recently, in September 2017, the European Space Agency (ESA) successfully completed the Rosseta mission, launched in 2004. This mission consisted of the exploration of comet 67P / Churyumov-Gerasimenko with the Philae probe that reached and orbited it, to then descend. The results of this mission are still under study.

Importance of astrobiology

Fermi's paradox

It can be said that the original question that motivates the study of Aastrobiology is: Are we alone in the universe?

In the Milky Way alone there are hundreds of billions of star systems. This fact, coupled with the age of the universe, suggests that life should be a common phenomenon in our galaxy.

On this subject, the question asked by the Nobel Prize-winning physicist Enrico Fermi is famous: "Where is everyone?", Which he asked in the context of a lunch, where the fact that the galaxy should be full was discussed of life.

The question ended up giving rise to the Paradox that bears his name and which is stated in the following way:

The SETI Program and the Search for Extraterrestrial Intelligence

One possible answer to the Fermi paradox could be that the civilizations we think about are actually there, but we haven't looked for them.

In 1960, Frank Drake along with other astronomers started a Search for Extraterrestrial Intelligence (SETI) program.

This program has made joint efforts with NASA, in the search for signs of extraterrestrial life, such as radio and microwave signals. The questions of how and where to look for these signals have led to great advances in many branches of science.

Figure 8. Radio telescope used by SETI in Arecibo, Puerto Rico. Source: JidoBG, from Wikimedia Commons

In 1993 the US Congress canceled funding to NASA for this purpose, as a result of misconceptions about the meaning of what the search implies. Today the SETI project is financed with private funds.

The SETI project has even spawned Hollywood movies, such as Contact, starring actress Jodie Foster and inspired by the novel of the same name written by world famous astronomer Carl Sagan.

Drake's equation

Frank Drake has estimated the number of civilizations with communication skills, using the expression that bears his name:

N = R * xf _p xn _e xf _l xf _i xf _c x L

Where N represents the number of civilizations with the ability to communicate with the Earth and is expressed as a function of other variables such as:

R *: the rate of formation of stars similar to our sun
f _p: the fraction of these star systems with planets
n _e: the number of Earth-like planets per planetary system
f _l: the fraction of these planets where life develops
f _i: the fraction in which intelligence arises
f _c: the fraction of communicationally fit planets
L: the “life” expectancy of these civilizations.

Drake formulated this equation as a tool to "size" the problem, rather than as an element to make concrete estimates, since many of its terms are extremely difficult to estimate. However, there is consensus that the number it tends to throw is large.

New scenarios

We should note that when the Drake equation was formulated, there was very little evidence of planets and moons outside our solar system (exoplanets). It was in the 1990s that the first evidence of exoplanets appeared.

Figure 9. Kepler telescope. Source: NASA, via Wikimedia Commons

For example, NASA's Kepler mission detected 3,538 exoplanet candidates, of which at least 1,000 are considered to be in the "habitable zone" of the system under consideration (distance that allows the existence of liquid water).

Astrobiology and the exploration of the ends of the Earth

One of the merits of astrobiology is that it has inspired, to a large extent, the desire to explore our own planet. This with the hope of understanding by analogy the operation of life in other settings.

For example, the study of hydrothermal vents on the ocean floor has allowed us to observe, for the first time, life not associated with photosynthesis. That is, these studies showed us that there can be systems in which life does not depend on sunlight, which had always been considered an indispensable requirement.

This allows us to suppose possible scenarios for life on planets where liquid water can be found, but under thick layers of ice, which would prevent the arrival of light to organisms.

Another example is the study of the dry valleys of Antarctica. There they have obtained photosynthetic bacteria that survive sheltered inside rocks (endolytic bacteria).

In this case, the rock serves both as a support and as a protection against the adverse conditions of the place. This strategy has also been detected in salt flats and hot springs.

Figure 10. McMurdo Dry Valleys in Antarctica, one of the places on Earth most similar to Mars. Source: US Department of State from United States, via Wikimedia Commons

Astrobiology perspectives

The scientific search for extraterrestrial life has so far been unsuccessful. But it is becoming more sophisticated as astrobiological research produces new insights. The next decade of astrobiological exploration will see:

Greater efforts to explore Mars and the icy moons of Jupiter and Saturn.
An unprecedented ability to observe and analyze extrasolar planets.
Greater potential to design and study simpler life forms in the laboratory.

All of these advances will undoubtedly increase our likelihood of finding life on Earth-like planets. But perhaps, extraterrestrial life does not exist or is so dispersed throughout the galaxy that we have almost no chance of finding it.

Even if the latter scenario is true, research in astrobiology increasingly broadens our perspective of life on Earth and its place in the universe.

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ASTROBIOLOGY: HISTORY, OBJECT OF STUDY AND IMPORTANCE - BIOLOGY - 2025