July 3, 2021: Can Liquid Methane Engender A Prebiotic Chemistry Or A Biological Chemistry On Titan ?
Most worlds in the Solar System are devoid of any significant atmosphere. Therefore, those worlds can't contain any stable pool of liquid on their surface because there is not the right combination of surface pressure and environmental temperature at the level of the surface. Mercury which is the nearest world to the Sun is devoid of any atmosphere so that the stable presence of liquid water can't be envisaged on its surface. Mercury is not massive enough and too warm at the level of the surface to retain any atmosphere. Thus, today, Mercury appears as a dry world that is heavily cratered. By contrast, Venus which is the second largest Terrestrial planet in the Solar System contains a thick atmosphere because that planet is massive enough to retain an atmosphere even if the energy received from the Sun at the level of Venus is higher than at the level of the Earth. The environmental temperature of Venus appears much too high to allow the presence of lakes, seas or rivers of liquid water on its surface. The atmospheric pressure on the surface of Mars is too low to allow the stable presence of liquid water and the environmental temperature at the level of the surface of the Red Planet is rarely above 0 degree Celsius, 32 degrees Fahrenheit or 273 Kelvin.
Several worlds in the Solar System are believed to contain lakes, seas, oceans or rivers of liquid water but the Earth appears to be the only world in the Solar System to contain lakes, seas, oceans or rivers of liquid water on its surface. Several icy moons found in the Outer Solar System are thought to contain pockets or a subsurface layer of liquid water beneath their external crust. That's the case for Europa, one of the main moons of Jupiter. That's also the case for Enceladus, a bright and icy moon of Saturn where geysers or cryovolcanoes have been identified in the fractures of its south polar region. In fact, several moons of Saturn may contain a subsurface ocean dominated by liquid water. One can mention Mimas, Tethys, Dione, Rhea as well as the Opaque Moon Titan. The biosphere we know is intimately linked to liquid water. A lifeform based on liquid water likely implies the stable presence of lakes, seas, oceans or pools of liquid water. The Earth appears to be the only world in the Solar System containing that type of hydrosphere between the external crust and the atmosphere. Can life emerge and develop from another solvent than liquid water ? That question appears fundamental and we may find major clues regarding that question very soon thanks to the exotic environment of Saturn's largest moon Titan.
Titan and the Earth appear to be the only worlds in the Solar System containing stable lakes, seas and rivers on their surface. However, the lakes, seas and rivers of Titan are not composed of liquid water due to the extremely low environmental temperatures at the level of the surface of the Orange Moon. The Huygens probe had recorded a surface temperature of about -179 degrees Celsius, -290 degrees Fahrenheit or 94 Kelvin. What type of stable liquid can be found on Titan's surface ? The atmosphere of Titan is relatively rich in methane and the haze and the surface of the giant moon are relatively rich in various hydrocarbons or organics. In fact, in the harsh environment of Titan, methane (CH4), ethane (C2H6) and propane (C3H8) can be found in their liquid form on the surface. The lakes, seas and rivers of the Opaque Moon may contain a mixture of methane, ethane and dissolved nitrogen. The composition of the liquid can vary depending on the latitude or depending on the season. The infrared or near-infrared views as well as the radar views acquired from the Cassini orbiter during its orbital dance in the Saturn System have clearly shown that the pools or rivers are mostly found in the high latitudes of the Opaque Moon.
The low or mid-latitudes of Titan appear to be relatively dry areas even if they can experience heavy rainfall events from time to time due to seasonal factors in particular. A remarkable contrast between relatively bright areas and relatively dark areas can be observed in the low or mid-latitudes of the Orange Moon. The radar views taken from the Cassini spacecraft clearly show that the relatively dark areas found in the low or mid-latitudes are dominated by Seif Dunes or linear and parallel dunes extending over long distances. Those exotic dunes may be relatively rich in hydrocarbons or organics. They may contain particles or molecules related to the haze that can produce heavy molecules. The heavier molecules will tend to fall toward the surface. One can imagine that the particles or molecules of the haze can engender a dark or red sludge or mud on the surface. That type of mud or sludge may be rich in hydrocarbons or organics. Planetologists have simulated the type of mud or sludge known as tholin that we can encounter on the surface of icy or Terrestrial worlds found in the Outer Solar System. The level of complexity of the molecules of that mud or sludge is a major topic of research.
Curiously, the infrared or near-infrared views as well as the radar views captured from the Cassini spacecraft reveal that the seas, lakes and rivers are mostly found in the polar regions or in the high latitudes of each hemisphere of Titan. The high latitudes of the northern hemisphere turn out to be the most humid areas on the Opaque Moon. How can we explain that surprising dichotomy in the distribution of lakes, seas and rivers ? Seasonal factors can play a major role in the level or in the size of the lakes, seas and rivers. Physical or orbital factors are likely to play a major role in the asymmetry between the south polar region and the north polar region of Saturn's largest moon. What type of chemistry takes shape in the exotic pools of liquid on Titan ? The potential interactions between the various elements or molecules found within the lakes, seas or rivers can engender relatively complex molecules. Can a prebiotic chemistry or a biological chemistry emerge and develop in the lakes, seas or rivers of Titan ? Biologists, exobiologists or planetologists tend to say that life on Earth has probably emerged or developed in an ocean, in a lake, in a sea or in a stable pond of liquid water.
Thus, a biosphere likely implies a solvent to emerge and develop. On Earth, the biosphere is based on liquid water and water represents the largest concentration of the composition of any species. On Titan, water appears in its solid form on the surface. Under the external crust of Titan, an ocean dominated by liquid water is likely present so that any lifeform based on liquid water can't be ruled out. The lakes, seas or oceans found in the high latitudes of Saturn's largest moon tend to be dominated by liquid methane which may represent a solvent for the development of a complex chemistry involving complex hydrocarbons or organics. In the harsh environment of Titan, chemical reactions will tend to be much slower than in the environment of the Earth. That's why the metabolism of any potential lifeform on Titan may be particularly slow. In our biosphere, the key molecules are amino acids, proteins, lipids, sugars, RNA or DNA. Can molecules like amino acids take shape in the exotic lakes, seas or rivers of the Opaque Moon ? Some future missions to Titan, involving probes, drones, boats or submarines, may tell us a lot regarding that fundamental question.
The humid area found in the high latitudes of the northern hemisphere of Saturn's largest moon is undoubtedly a major target for planetologists in terms of exobiology. The chemical properties of methane are very different from the chemical properties of water. Therefore, the chemical interactions within the lakes, seas or rivers of Titan can be very different from what we anticipate. Over millions or billions of years, interesting molecules can emerge within the stable pools of liquid methane on Titan. The lakes, seas and rivers of Titan will allow us to discover a new dimension of the Universe in terms of chemistry or phenomena. Soon, we will probably be in a position to decipher the mystery of the "Magic Island" phenomenon. Is there an ecosystem inside Ligeia Mare or Kraken Mare ? Is there a cryovolcanic activity beneath the pool of Ligeia Mare or Kraken Mare ? Are there pockets of liquid methane beneath the presumed icy shell in the north polar region ? The tiny moon Enceladus appears geologically active in its south polar region. Is Titan also geologically active in its south polar region or in its north polar region ? Researchers believe that there may be internal sources to the methane found in the atmosphere.
Planetologists are surprised to observe such a concentration of methane in Titan's atmosphere because methane tends to vanish in the long run under the action of ultraviolet light in the upper atmosphere of the Opaque Moon. The haze of Titan is closely related to the action of ultraviolet radiations from the Sun in the upper atmosphere. Elements, ions or molecules like nitrogen, molecular nitrogen, methane, radicals or nitriles tend to interact to form new molecules. Heavier molecules tend to fall to the surface. Any biosphere based on methane can mobilize that methane to form other hydrocarbons or new organics. Some potential species of Titan can mobilize hydrocarbons like methane, ethane or propane to generate new molecules. Any ecosystem can mobilize and recycle any key molecule like methane. The atmosphere of Titan looks like the atmosphere of the Early Earth as well as our current atmosphere to a certain extent. Titan's atmosphere is dominated by molecular nitrogen like the atmosphere of the Blue Planet. Both worlds contain a meteorological cycle involving the development of clouds and rainfall events. Is a biosphere based on liquid methane possible on Titan ? That's a major question that deserves our whole attention and that will be probably answered very soon.
The image above represents a portion of a radar swath of Titan acquired from the Cassini spacecraft during the T-98 Flyby of February 2, 2014. One can notice a portion of Ontario Lacus, the first pool of liquid clearly identified on Saturn's largest moon. Each side of the view represents about 100 kilometers. Ontario Lacus which is located in the high latitudes of the southern hemisphere may look like a typical pond on Earth. Can that pool engender complex organics ? Credit for the original view: NASA/JPL/Cassini Radar Team/PDS Image Atlas. Montage credit: Marc Lafferre, 2021.
- To get further information on that news, go to: https://www.nasa.gov/feature/nasa-s-search-for-life-astrobiology-in-the-solar-system-and-beyond.