February 19, 2020 : New Quantum Mechanical Calculations Upon Azotosomes Imply That Their Development Is Unlikely In The Hypothetical Biosphere Of Titan
A new study entitled « Can polarity-inverted membranes self-assemble on Titan ? », published in Science Advances on January 24, 2020 and proposed by H. Sandström and M. Rahm, reveals quantum mechanical calculations upon azotosomes which imply that those molecules which represent nitrogen-containing compounds may not develop in the hypothetical biosphere of Titan. In a relatively recent past, some researchers had advanced the possibility of a Titanian biosphere involving azotosomes. Azotosomes appeared to be good candidates for the potential development of cell membranes of any exotic lifeform on Saturn's largest moon. The new research work tends to show that azotosomes are unlikely for any autonomous organism on the Hazy Moon. The scientists advance that in an environment where temperatures are extremely low and where liquid water is absent, cell membranes can be ruled out for any lifeform. In terms of thermodynamics, the formation and the development of azotosomes on the Opaque Moon appear unlikely. Those theoretical membranes may not be in a position to self-assemble in the extreme environment of the giant moon.
On the basis of what we observe in our own biosphere, we have had the opportunity to determine that the membranes of the cells play a key role for the development of typical organisms. The membranes of the cells are composed of lipid bilayers which are hydrophobic or amphiphilic. Life on Earth is based on liquid water and all creatures are composed of cells in which various chemical reactions can take shape without the potential disruptions in the potential interactions between liquid water and other compounds located outside the cell and the « factory » of the cell. Numerous researchers believe that the lipid bilayer membrane is a key aspect of life in an environment dominated by liquid water or in an environment where water can be found in its liquid form. In the Solar System, the Earth is the only planetary body where water can be encountered in its liquid form on the surface. On Mars, there are not the right combinations of atmospheric pressure and environmental temperature for the stable presence of liquid water on the surface for instance. On Venus, the greenhouse effect is so high that the environmental temperature at the level of the surface is too high for the presence of liquid water on the surface.
The biosphere of the Earth, based on lipids, cell membranes, cells, proteins, amino acids, DNA, RNA and sugars, evolves in an environment where liquid water is widespread. Moreover, life as we know it is dominated by liquid water and contains a relatively high concentration of carbon. Therefore, can we conclude that life can only form in an environment dominated by liquid water and can we conclude that life can only be composed of the key atoms that form the creatures of the Earth ? We sometimes say that we are CHNOPS because we are mainly composed of carbon atoms, hydrogen atoms, nitrogen atoms, oxygen atoms, phosphorus atoms and sulfur atoms. Our quest to determine whether an environment of stable liquid water automatically leads to the development of life will not be easy simply because, at the present time, no other world in the Solar System contains any stable pool of liquid water. However, several worlds located in the Outer Solar System may contain subsurface oceans rich in liquid water. That's the case, for instance, for Europa, an icy moon of Jupiter, Enceladus, the famous tiny moon of Saturn, Titan, the largest moon of the Ringed Planet or Pluto, the famous Dwarf Planet beyond Neptune.
We have realized that life has developed on the basis of a solvent since we are based on liquid water. Thanks to the Cassini/Huygens mission, we have discovered that there are lakes, seas and rivers on the Opaque Moon. Prior to the journey of the Cassini/Huygens spacecraft in the Saturn System, we had suspected that there may be lakes, rivers, seas or oceans of liquid methane or liquid ethane on the surface of the giant moon because there are the right combinations of atmospheric pressure and environmental temperature at the level of the surface of Titan for the potential presence of pools of liquid hydrocarbons on the surface of that enigmatic world. We also knew that the environment of Saturn's largest moon is relatively rich in methane. The Huygens probe did not land in the humid areas found in the high latitudes of each hemisphere. However, the aerial views obtained from the parachuted probe on January 14, 2005 have clearly shown the presence of a network of dark sinuous channels, on bright hills, probably related to meteorological phenomena. Can the stable pools of liquid engender exotic lifeforms ?
Titan appears to be the only extraterrestrial world in the Solar System containing stable pools of liquid. Methane, ethane and propane can appear in their liquid form on the surface in the harsh environment of the giant moon. Clouds of methane and ethane have been clearly identified from the Cassini orbiter during its long mission in the Saturn System. On that world, there is a meteorological cycle of methane comparable to the meteorological cycle of water on Earth. Researchers don't know the exact composition of the seas or lakes on Titan. They believe that they must contain mixtures of methane, ethane and nitrogen with concentrations of each compound which can vary geographically and seasonally. Curiously, the lakes, seas and rivers appear to be mostly found in the high latitudes of the northern hemisphere. Can methane or ethane represent efficient solvents for the development of a biosphere ? The best way to answer that question would be to send a new probe or drone to the lakes or seas of the north polar region. Kraken Mare, Ligeia Mare and Punga Mare are key targets. But one could also mention Ontario Lacus in the south polar region.
Due to the extremely harsh environment of Titan where ambient temperature can be around minus 179 degrees Celsius, minus 290 degrees Fahrenheit or 94 Kelvin, chemical reactions are extremely slow compared to the typical chemical reactions on Earth. However, the environment of Titan is rich in carbon, a key ingredient of our biosphere. Moreover, there are complex interactions between UV light from the Sun and organics, hydrocarbons or other compounds or particles taking shape in the upper atmosphere of the Opaque Moon. In other words, there is a soup of molecules in the upper atmosphere that can engender relatively complex or heavy molecules that can fall toward the surface where they can form a type of red or dark mud or sludge rich in organics or hydrocarbons. That's why the environment of Titan can tell us a lot regarding the secrets of organic chemistry or the secrets of life on Earth. Planetologists often say that Titan may represent a natural prebiotic laboratory. Titan looks like the Earth to a certain extent. The atmosphere of Titan is dominated by molecular nitrogen like the atmosphere of the Earth for instance.
Researchers try to imagine the potential lifeforms that are likely to develop in an environment comparable to the Titanian environment where molecular nitrogen can interact with molecules like methane or ethane to form various compounds. Some analyses based on molecular dynamics simulations had revealed that azotosomes or polarity-inverted membranes composed of nitrogen-containing molecules are kinetically stable and may develop in liquid methane, in liquid ethane or in the particular environment of Saturn's largest moon. A type of bubble structure, rich in nitrogen, is likely to take shape and develop in the potential solvent of Titan. Inside those bubbles, more complex chemical reactions could occur. But can biological cells develop in the pools of that enigmatic moon ? The new research work tends to show that azotosomes are too complex to develop in the harsh environment of the Orange Moon where the most complex molecules present in that environment may be less complex than the azotosomes theorized by some researchers.
The drone of the Dragonfly mission will study the linear and parallel dunes of Titan. Those dunes which look like the Seif dunes found in the Namib Desert on Earth from outer space may be rich in hydrocarbons and organics. Can they contain microorganisms or more complex creatures ? We'll see in the 2030's ! However, the drone may go to relatively dry areas or areas which are devoid of any pool of stable liquids. The best place to find complex molecules, prebiotic molecules or even lifeforms is probably the land of lakes, seas and rivers in the north polar region. The north polar region is currently experiencing the Summer season but the Cassini spacecraft is not in orbit in the Saturn System anymore to study the evolution of the north polar lakes or seas. Is their level or size diminishing due to evaporation processes related to seasonal factors ? Is the size or the level of Ontario Lacus found in the south polar region increasing due to relatively significant condensation processes related to seasonal factors ? Ontario Lacus was the first stable extraterrestrial lake or sea identified in history. The pool was spotted during the Summer season in the southern hemisphere. Ontario Lacus whose shape looks like the shape of a foot is now experiencing the Winter season. Those pools of hydrocarbons fuel our imagination regarding the strange molecules, organisms or phenomena we may encounter in those areas.
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The image in the upper part of this table reveals a raw view of Saturn's largest moon Titan obtained on September 12, 2017 from the Cassini spacecraft. The image whose file name is N00289163.jpg was acquired using the CL1 filter and the CB3 filter. The image had not been validated or calibrated at the time of the observation and was going to be archived with the Planetary Data System proposed by NASA. The image in the lower part of this table represents a colorized view of the raw image. One can clearly notice a portion of Kraken Mare. Credit for the raw image: NASA/JPL-Caltech/Space Science Institute. Credit for the colorization of the original image: Marc Lafferre, 2020. |
- To get further information on that news, go to: https://advances.sciencemag.org/content/6/4/eaax0272.