August 6, 2019 : Specialists Of Exoplanets Or Exobiologists Recently Advanced That We Should Envisage Biosignatures Related To Other Solvents Than Water Like Liquid Methane That Can Be Found On The Surface Of Titan

Some planetologists argue that we should envisage the possibility of biosignatures related to other solvents than water in our search for signs of life beyond our planet. The only lifeform we know today is the lifeform found on our own world. That lifeform is intimately linked to liquid water. Our body is mainly composed of liquid water which represents about 65 percent of our weight. The concentration of water is even higher for organisms like plants or jellyfish. Liquid water is widespread on the surface of the Earth since oceans cover about 70 percent of the surface of our planet. On the basis of those observations, one can easily deduce that life can only form in an environment that is rich in liquid water. Most biologists believe that water is the only solvent that can lead to the formation and the development of life. The Earth is the only planet of the Solar System where liquid water can be found on the surface. Therefore, we don't know whether liquid water automatically leads to the formation or the development of life over time.

We know, now, another world of the Solar System where stable pools of liquid can be found. That world is Titan, the largest moon of Saturn. Most moons in the Solar System are devoid of any atmosphere but Titan is an exception. Titan contains a remarkable atmosphere which looks like the atmosphere of the Early Earth. The atmosphere of Titan is dominated by molecular nitrogen like the atmosphere of our planet today. However, the harsh environment of Titan's atmosphere is devoid of any significant amount of oxygen and contains a relatively significant fraction of methane. A meteorological cycle of methane comparable to the meteorological cycle of water on the Earth can be observed on Titan. In an environment relatively rich in carbon like our own environment, can life emerge or develop if there are stable pools of liquid methane or liquid ethane ? That's clearly a crucial question in the prospect of our understanding of the chemistry of organics or life in the universe. Some researchers discussed about that topic at the first annual TESS Science Conference at the Massachusetts Institute of Technology (MIT) on August 2, 2019.

One of the major goals of astrobiologists or specialists of exoplanets is to identify potential biosignatures on worlds evolving around other stars. That's why we develop more and more ambitious missions to identify and characterize exoplanets. For instance, TESS represents NASA's Transiting Exoplanet Survey Satellite which has been operating for about one year. The space telescope deduces the presence of exoplanets around stars thanks to the effects of transits, in terms of brightness variation, performed by exoplanets in front of their star. TESS has recently allowed us to identify an exoplanet which may be habitable. That exoplanet known as GJ 357 d is located about 31 light years away. At the recent TESS Science Conference, planetologists discussed about the prospects in terms of search for biosignatures with the upcoming missions related to the study of exoplanets. They pointed out that we should envisage exotic types of biosignatures related to various types of solvents such as liquid methane. In a liquid, more complex chemical reactions involving various elements or molecules can take shape compared to a dry environment like the environment of Mars. That's why one has to envisage any type of solvent in the field of exobiology.

At the TESS Science Conference, the researchers discussed about the potential outcome of concepts of telescope like HabEx or Habitable Exoplanet Observatory and LUVOIR or Large UV/Optical/IR Surveyor proposed by NASA. Those potential telescopes would be in a position to identify and characterize exoplanets. We would be in a position to determine whether those exoplanets contain biosignatures or whether they are potentially habitable. Thanks to the new instruments, the planetologists hope to obtain a spectrum of at least one potentially habitable world beyond the Solar System. HabEx can allow us to gather the precious clues needed to infer the potential presence of a biosphere. LUVOIR may allow us to determine the fraction of rocky exoplanets located in the Habitable Zone that contain water or that unveil biosignatures. Another concept or project known as the Origins Space Telescope can also bring a lot regarding the composition of extrasolar planets which appear in the habitable zone of their star. That telescope could study the environment of cooler dwarf stars instead of stars which are comparable to our Sun in terms of energy generated.

Some planetologists insist on the fact that one has to imagine the type of life that can exist elsewhere or the potential diversity of lifeforms. Our biosphere based on water may represent a type of biosphere among other types of biosphere based on other solvents or on other types of environments. One has to imagine the potential chemistry of other worlds and the potential biosignatures that are supposed to be closely related to the ingredients found in the environment of the exoplanet. The chemistry of other worlds is a large field of study or exploration. Our experiments must go beyond the experiments on typical biopolymers that may be closely related to life. Every world has its particularities. Venus looks like the Earth but its atmosphere engenders a huge greenhouse effect that prevents the development of typical life on its surface. But are there extremophiles based on carbon dioxide or sulfuric acid in the opaque atmosphere of Venus ? Triton contains geysers or hot springs. Is there a biosphere beneath its icy surface ? Pluto unveils a field of nitrogen ice and may contain complex organics or tholins on its surface. Can Pluto host an exotic biosphere ?

The chemistry of Titan's environment is particularly interesting because the environment of Saturn's largest moon is rich in organics and there are pools of liquid hydrocarbons in its high latitudes or in its polar regions. Water can't appear in its liquid form on Titan's surface because the environmental temperature is extremely low compared to the environmental temperature of the Earth. Chemical reactions tend to be slower in colder environments. A complex chemistry involving organics or hydrocarbons may be found in the environment of the Opaque Moon thanks to the action of UV light from the Sun in particular. The haze of Titan is generated under the influence of UV radiations from our star. That exotic world can teach us a lot regarding the chemistry of organics or even biology. Are there prebiotic molecules in Titan's lakes, seas or rivers for instance ? We know that, on Earth, one can encounter creatures, bacteria or archaea that can adapt to extreme environments. Creatures or bacteria can be found in extremely warm water or at an extreme pressure in the abyss of the ocean. Will we ever find extremophiles on Titan ?

The concept of habitable zone appears quite limited since it represents the theoretical distance at which the planet, the moon, the exoplanet or the exomoon can contain pools of liquid water on its surface. However, pools of liquid water can be found if only there is the right combination of pressure and temperature at the level of the surface. We must keep in mind that liquid water can also be found beneath the external crust of the planet or moon. Worlds like Europa, Enceladus, Titan, Pluto or Charon may contain a subsurface ocean dominated by liquid water for instance. There may be more lifeforms than we think. That's why one has to envisage an habitable zone for each solvent. Several types of pools of liquid may be encountered beyond the Earth. We have identified pools of liquid methane or liquid ethane on Titan. Pools of liquid carbon dioxide, liquid ammonia or liquid nitrogen may also exist. Planetologists or researchers try to reproduce exotic environments such as the environment of Titan in the laboratory in order to anticipate or to understand what we could encounter on other worlds.

Will we ever be in a position to identify clear signs of extraterrestrial life ? We intend to develop more and more powerful instruments to characterize other exoplanets or to identify biosignatures on distant worlds in the coming decades. So, one can be optimistic ! Some planetologists advance that we may find evolved life in the next hundred years. However, we don't know the fraction of worlds of liquid water that host life. If life automatically develops in an environment of liquid water, one can be optimistic regarding our search for extraterrestrial life. Perhaps we will find an extraterrestrial intelligence within the next 30 or 40 years. We have already identified several worlds that may look like the Earth or that may contain seas or oceans of liquid water. We may be quite lucky to have identified a world in the habitable zone around Proxima Centauri, the nearest star to the Sun. We may be in a position, one day, to send a probe to that world known as Proxima b. At the present time, we have a very limited knowledge about Proxima b but we know that the exoplanet evolving around a Red Dwarf is located in the habitable zone of the star. One must be optimistic and the exploration of the Solar System is only beginning.

The image above reveals a portion of Saturn and the disk of Titan in natural color. The view was produced on the basis of images acquired with the ISS Wide-Angle Camera of the Cassini orbiter, using red, green and blue spectral filters, on January 29, 2008. At the time of the observation, Titan was located at a distance of about 2.3 million kilometers or 1.4 million miles from the probe whereas Saturn was evolving at a distance of about 1 million kilometers or 630,000 miles from the spacecraft. Titan unveils lakes, seas and rivers of methane and ethane that may favor the development of life. Saturn may contain an ocean of metallic hydrogen. Can that hypothetical ocean engender life ? Image credit: NASA/JPL/Space Science Institute.

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