How scientists search for extraterrestrial life

2024 Author: Seth Attwood | [email protected]. Last modified: 2023-12-16 15:55

Perhaps there are other inhabited worlds somewhere in the universe. But, until we found them, the minimum program is to prove that life outside the Earth is at least in some form. How close are we to that?

Recently, we increasingly hear about discoveries that "could indicate" the existence of extraterrestrial life. Only in September 2020, it became known about the discovery of phosphine gas on Venus - a potential sign of microbial life - and salt lakes on Mars, where microbes could also exist.

But over the past 150 years, space explorers have more than once passed off wishful thinking. There is still no reliable answer to the main question. Or is there anyway, but scientists are cautious out of habit?

Telescope lines

In the 1870s, the Italian astronomer Giovanni Schiaparelli saw long, thin lines on the surface of Mars through a telescope and declared them "channels." He unambiguously titled the book about his discovery "Life on the planet Mars". "It is difficult not to see on Mars pictures similar to those that make up our terrestrial landscape," he wrote.

In Italian, the word canali meant both natural and artificial channels (the scientist himself was not sure of their nature), but when translated, it lost this ambiguity. Schiaparelli's followers have already clearly stated about the harsh Martian civilization, which, in an arid climate, created colossal irrigation facilities.

Lenin, who read the book by Percival Lowell "Mars and Its Canals" in 1908, wrote: "Scientific work. Proves that Mars is inhabited, that the canals are a miracle of technology, that people there should be 2/3 times larger than the local people, moreover with trunks, and covered with feathers or animal skins, with four or six legs.

N … yes, our author cheated us, describing the Martian beauties incompletely, should be according to the recipe: "The darkness of low truths is dearer to us than we are raising deceit". Lowell was a millionaire and former diplomat. He was fond of astronomy and used his own money to build one of the most advanced observatories in America. It was thanks to Lowell that the topic of Martian life hit the front pages of the largest newspapers in the world.

True, already at the end of the 19th century, many researchers were dubious about the opening of the "canals". Observations constantly gave different results - the cards diverged even for Schiaparelli and Loeull. In 1907, biologist Alfred Wallace proved that the temperature on the surface of Mars is much lower than Lowell assumed, and the atmospheric pressure is too low for water to exist in liquid form.

The interplanetary station "Mariner-9", which took photographs of the planet from space in the 1970s, put an end to the history of the canals: the "canals" turned out to be an optical illusion.

Since the second half of the 20th century, the hopes of finding a highly organized life have diminished. Studies using spacecraft have shown that the conditions on the nearby planets are not even close to those on Earth: too strong temperature drops, an atmosphere without signs of oxygen, strong winds, and tremendous pressure.

On the other hand, the study of the development of life on Earth has spurred interest in the search for similar processes in space. After all, we still do not know how and thanks to what, in principle, life arose.

Many events have taken place in this direction in recent years. The main interest is the search for water, organic compounds from which protein life forms could form, as well as biosignatures (substances that are produced by living things) and possible traces of bacteria in meteorites.

Liquid proof

The presence of water is a prerequisite for the existence of life as we know it. Water acts as a solvent and catalyst for certain types of proteins. It is also an ideal medium for chemical reactions and transport of nutrients. In addition, water absorbs infrared radiation, so it can retain heat - this is important for cold celestial bodies that are quite far from the luminary.

Observational data show that water in solid, liquid or gaseous state exists at the poles of Mercury, inside meteorites and comets, as well as on Jupiter, Saturn, Uranus and Neptune. Scientists have also suggested that Jupiter's moons Europa, Ganymede and Callisto have vast subsurface oceans of liquid water. They found it in one form or another in interstellar gas and even in incredible places like the photosphere of stars.

But the study of traces of water can be promising for astrobiologists (specialists in extraterrestrial biology) only when there are other suitable conditions. For example, temperatures, pressure and chemical composition on the same Saturn and Jupiter are too extreme and changeable for living organisms to adapt to them.

Another thing is the planets close to us. Even if today they look inhospitable, small oases with "remnants of former luxury" can remain on them.

In 2002, the Mars Odyssey orbiter discovered deposits of water ice beneath the surface of Mars. Six years later, the Phoenix probe confirmed the results of its predecessor, obtaining liquid water from an ice sample from the pole.

This was consistent with the theory that liquid water was present on Mars quite recently (by astronomical standards). According to some sources, it rained on the Red Planet "only" 3.5 billion years ago, according to others - even 1.25 million years ago.

However, an obstacle immediately arose: water on the surface of Mars cannot exist in a liquid state. At low atmospheric pressure, it immediately begins to boil and evaporate - or freezes. Therefore, most of the known water on the planet's surface is in the state of ice. There was hope that the most interesting was happening below the surface. This is how the hypothesis of salt lakes under Mars arose. And just the other day she received confirmation.

Scientists from the Italian Space Agency have discovered at one of the poles of Mars a system of four lakes with liquid water, which are located at a depth of more than 1.5 kilometers. The discovery was made using radio sounding data: the device directs radio waves into the interior of the planet, and scientists, by their reflection, determine its composition and structure.

The existence of a whole system of lakes, according to the authors of the work, suggests that this is an ordinary phenomenon for Mars.

The exact specific concentration of salts in the Martian lakes is still unknown, as well as their composition. According to the scientific director of the Mars program, Roberto Orosei, we are talking about very strong solutions with "tens of percent" of salt.

There are halophilic microbes on Earth that love high salinity, explains microbiologist Elizaveta Bonch-Osmolovskaya. They release substances that help maintain water-electrical balance and protect cell structures. But even in extremely salty underground lakes (brins) with a concentration of up to 30% there are few such microbes.

According to Orosei, traces of life forms that existed when there were warmer climates and water on the planet's surface, and conditions resembled early Earth, could remain in the Martian lakes.

But there is another obstacle: the very composition of the water. The Martian soil is rich in perchlorates - salts of perchloric acid. Perchlorate solutions freeze at significantly lower temperatures than ordinary or even sea water. But the problem is that perchlorates are active oxidants. They promote the decomposition of organic molecules, which means they are harmful to microbes.

Perhaps we underestimate the ability of life to adapt to the harshest conditions. But to prove this, you need to find at least one living cell.

"Bricks" without firing

The life forms that live on Earth cannot be imagined without complex organic molecules containing carbon. Each carbon atom can create up to four bonds with other atoms at the same time, resulting in a tremendous wealth of compounds. The carbon "skeleton" is present in the basis of all organic substances - including proteins, polysaccharides and nucleic acids, which are considered the most important "building blocks" of life.

The panspermia hypothesis just asserts that life in its simplest forms came to Earth from space. Somewhere in interstellar space, conditions developed that made it possible to assemble complex molecules.

Perhaps not in the form of a cell, but in the form of a kind of protogenome - nucleotides that can reproduce in the simplest way and encode the information necessary for the survival of a molecule.

For the first time, the grounds for such conclusions appeared 50 years ago. Molecules of uracil and xanthine were found inside the Marchison meteorite, which fell in Australia in 1969. These are nitrogenous bases capable of forming nucleotides, from which nucleic acid polymers - DNA and RNA - are already composed.

The task of scientists was to establish whether these findings are a consequence of pollution on Earth, after the fall, or have an extraterrestrial origin. And in 2008, using the radiocarbon method, it was possible to establish that uracil and xanthine were indeed formed before the meteorite fell to Earth.

Now in Marchison and similar meteorites (they are called carbonaceous chondrites), scientists have found all kinds of bases from which both DNA and RNA are built: complex sugars, including ribose and deoxyribose, various amino acids, including essential fatty acids. Moreover, there are indications that organics are formed directly in space.

In 2016, with the help of the Rosetta apparatus of the European Space Agency, traces of the simplest amino acid - glycine - as well as phosphorus, which is also an important component for the origin of life, were found in the tail of comet Gerasimenko-Churyumov.

But such discoveries rather suggest how life could have been brought to Earth. Whether it can survive and develop for a long time outside of terrestrial conditions is still unclear. “Large molecules, complex molecules, which we would classify as organic on Earth without any options, can be synthesized in space without the participation of living beings,” says astronomer Dmitry Vibe. “We know that interstellar organic matter got into the solar system and the Earth. But then something else was happening to her - the isotopic composition and symmetry were changing."

Traces in the atmosphere

Another promising way to search for life is associated with biosignatures, or biomarkers. These are substances, the presence of which in the atmosphere or soil of the planet definitely indicates the presence of life. For example, there is a lot of oxygen in the Earth's atmosphere, which is formed as a result of photosynthesis with the participation of plants and green algae. It also contains a lot of methane and carbon dioxide, which are produced by bacteria and other living organisms in the process of gas exchange during respiration.

But finding traces of methane or oxygen in the atmosphere (as well as water) is not yet a reason to open champagne. For example, methane can also be found in the atmosphere of star-like objects - brown dwarfs.

And oxygen can be formed as a result of the splitting of water vapor under the influence of strong ultraviolet radiation. Such conditions are observed on the exoplanet GJ 1132b, where the temperature reaches 230 degrees Celsius. Life under such conditions is impossible.

For a gas to be considered a biosignature, its biogenic origin must be proven, that is, it must be formed precisely as a result of the activity of living beings. Such an origin of gases is indicated, for example, by their variability in the atmosphere. Observations show that methane levels on Earth fluctuate with the season (and the activity of living things depends on the season).

If on another planet methane disappears from the atmosphere, then it appears (and this can be recorded during, for example, a year), it means that someone is emitting it.

Mars turned out to be one of the possible sources of "living" methane again. The first signs of it in the soil were revealed by the devices of the Viking program, which were sent to the planet back in the 1970s - just with the aim of searching for organic matter. The discovered molecules of methane in combination with chlorine were initially taken as evidence. But in 2010, a number of researchers revised this point of view.

They found that the perchlorates already known to us in Martian soil, when heated, destroy most of the organic matter. And the samples from the "Vikings" were heated.

In the atmosphere of Mars, traces of methane were first discovered in 2003. The find immediately revived conversations about the habitability of Mars. The fact is that any significant amounts of this gas in the atmosphere would not last long, but would be destroyed by ultraviolet radiation. And if methane does not break down, scientists have concluded that there is a permanent source of this gas on the Red Planet. And yet, the scientists did not have firm confidence: the data obtained did not exclude that the methane found was the same "pollution".

But observations from the Curiosity rover in 2019 recorded an abnormal rise in methane levels. Moreover, it turned out that now its concentration is three times higher than the level of gas recorded in 2013. And then an even more mysterious thing happened - the concentration of methane again dropped to background values.

The methane riddle still has no unambiguous answer. According to some versions, the rover may be located at the bottom of a crater, in which there is an underground source of methane, and its release is associated with the tectonic activity of the planet.

However, biosignatures can be rather non-obvious. For example, in September 2020, a team at Cardiff University detected traces of phosphine gas on Venus, a special phosphorus compound that is involved in the metabolism of anaerobic bacteria.

In 2019, computer simulations showed that phosphine cannot be formed on planets with a solid core except as a result of the activity of living organisms. And the amount of phosphine found on Venus spoke in favor of the fact that this was not an error or an accidental impurity.

But a number of scientists are skeptical about the discovery. Astrobiologist and expert on reduced states of phosphorus Matthew Pasek suggested that there is some exotic process that has not been taken into account by computer simulations. It was he who could take place on Venus. Pasek added that scientists are still not sure how life on Earth produces phosphine and whether it is produced by organisms at all.

Buried in stone

Another possible sign of life, again associated with Mars, is the presence in samples from the planet of strange structures similar to the remnants of living beings. These include the Martian meteorite ALH84001. It flew from Mars about 13,000 years ago and was found in Antarctica in 1984 by geologists snowmobiling around the Allan Hills (ALH stands for Allan Hills) in Antarctica.

This meteorite has two characteristics. First, it is a sample of rocks from the era of that same "wet Mars", that is, the time when there could be water on it. The second - strange structures were found in it, reminiscent of fossilized biological objects. Moreover, it turned out that they contain traces of organic matter! However, these "fossilized bacteria" have nothing to do with terrestrial microorganisms.

Their size is too small for any terrestrial cellular life forms. However, it is possible that such structures point to the predecessors of life. In 1996, David McKay of the Johnson Center for NASA and his colleagues found so-called pseudomorphs in a meteorite - unusual crystalline structures that mimic the shape of (in this case) a biological body.

Shortly after the 1996 announcement, Timothy Swindle, a planetary scientist at the University of Arizona, conducted an informal survey of over 100 scientists to find out how the scientific community felt about the claims.

Many scientists were skeptical about the McKay group's claims. In particular, a number of researchers have argued that these inclusions may arise as a result of volcanic processes. Another objection was related to the very small (nanometer) dimensions of the structures. However, supporters objected to this that nanobacteria were found on Earth. There is a work that shows the fundamental indistinguishability of modern nanobacteria from objects from ALH84001.

The debate is deadlocked for the same reason as in the case of Venusian phosphine: we still have little idea of how such structures are formed. No one can guarantee that the similarity is not a coincidence. Moreover, there are crystals on Earth, such as kerite, which are difficult to distinguish from the "fossilized" remains of even ordinary microbes (not to mention poorly studied nanobacteria).

The search for extraterrestrial life is like running after your own shadow. It seems that the answer is before us, we just have to get closer. But he is moving away, acquiring new complexities and reservations. This is how science works - by eliminating "false positives". What if the spectral analysis misfires? What if methane on Mars is just a local anomaly? What if the structures that look like bacteria are just a game of nature? All doubts cannot be completely ruled out.

It is quite possible that outbreaks of life are constantly appearing in the Universe - here and there. And we, with our telescopes and spectrometers, are always late for a date. Or, conversely, we arrive too early. But if you believe in the Copernican principle, which says that the Universe as a whole is homogeneous and earthly processes must take place somewhere else, sooner or later we will intersect. It's a matter of time and technology.