October 28, 2023: A Biological Source To The Methane Of Titan ?
On Earth, hydrocarbons like methane, ethane or propane are key molecules driving our economies because they represent energy sources for our cars or for our industries for instance. Those hydrocarbons are closely related to our biosphere because they have been produced by life a long time ago. The exploration process of the various planetary bodies of the Solar System has allowed us to notice that many worlds contain a relatively high concentration of hydrocarbons like methane or ethane. That's the case for the Gas Giants Neptune, Uranus, Saturn and Jupiter and that's also the case for smaller worlds like Pluto or Titan. Most moons are devoid of any significant atmosphere but Titan represents an exception among the multitude of moons encountered in the Solar System. Titan contains a remarkable atmosphere which is completely opaque from outer space. That atmosphere is dominated by molecular nitrogen like the atmosphere of the Earth. However, oxygen is absent or almost absent in the atmosphere of that strange world. Planetologists have been surprised by the relatively high concentration of methane present in the atmosphere of Saturn's largest moon. Methane tends to disappear in the atmosphere in the long run.
How can we explain the relatively high concentration of methane in the atmosphere of Titan ? Is there a methane cycle between the interior of the moon and the atmosphere ? Is the methane of Titan's atmosphere closely related to cryovolcanoes ? Is the methane of the Opaque Moon closely related to a biological system evolving beneath the icy crust for instance ? Many questions can be put forward to account for the particular chemistry of Titan. In the oceans of our planet, one can encounter ecosystems that generate methane. Bacteria or archaea on Earth can produce methane in the absence of sunlight. Could we encounter that type of ecosystem on the giant moon of Saturn ? We can't rule out that hypothesis because Titan may contain a subsurface ocean dominated by liquid water. One can easily imagine extremophiles thriving in that type of subsurface ocean where pressures are high and where environmental temperatures can be relatively high despite the relatively low amount of solar energy received at the level of Saturn or Titan. The study of the abyss of our oceans where volcanoes can be found can allow us to better anticipate what one could encounter at the level of the floor of the presumed susbsurface ocean of Saturn's largest moon.
On Earth, methane can be found in the form of methane clathrates. Methane clathrates repesent icy cages trapping methane and preventing it from escaping to the surface. We have determined that those particular structures found on continental margins contain a significant amount of methane since they represent the largest sources of hydrocarbons on our planet. The origin, the formation and the development of methane clathrates represent a key subject for researchers. A recent research work entitled "Molecular basis for inhibition of methane clathrate growth by a deep subsurface bacterial protein", published in PNAS Nexus in Volume 2, Issue 8 in August 2023 and mobilizing a group of researchers involving Dustin J E Huard and Abigail M Johnson reveals that a particular bacterial protein (CbpAs) tends to prevent the growth of methane clathrates. Thus, those types of biomolecules may be fundamental for the stability and for the habitability of those icy structures. Are there comparable methane clathrates at the level of the floor of the hypothetical subsurface ocean dominated by liquid water on Titan ? An exotic form of methane clathrates can be imagined in the icy bodies of the Outer Solar System.
In the harsh environment of Titan, methane or ethane can be found in their liquid form on the surface. Like water on Earth, one can encounter methane in its liquid form and in its gas form on that world. The haze of Titan can engender methane under the action of ultraviolet light from the Sun in particular. Relatively complex hydrocarbons can take shape in the atmosphere or on the surface of the giant moon of Saturn. An exotic lifeform based on liquid methane or liquid ethane can't be ruled out at the level of the surface or in the air. The biosphere of the Earth is based on liquid water and on carbon in particular. Elements like nitrogen, oxygen, hydrogen, phosphorus and sulfur are also mobilized by the creatures of the Earth. On Titan, water can't appear in its liquid form on the surface because the environmental temperature is too low. Environmental temperatures on Titan evolve around -179 degrees Celsius, -290 degrees Fahrenheit or 94 Kelvin. If there is a lifeform at the level of the surface on Titan, it can't be based on liquid water. The solvent for any lifeform at the level of the surface on Titan would rather be liquid methane or liquid ethane because lakes, seas or rivers can be encountered on the surface of that intriguing world.
The Cassini-Huygens mission in the Saturn System has allowed us to discover the surprising landscape of Titan. Prior to the Cassini-Huygens mission in the Saturn System, researchers had suspected the presence of seas or oceans of methane or ethane on the surface of that moon. The Huygens probe has allowed us to discover a landscape made of networks of dark channels which are likely drainage channels related to methane rain. The Cassini orbiter has allowed us to identify lakes, seas and rivers of methane or ethane in the high latitudes of the giant moon. Some planetologists had thought that the dark areas which mark a sharp contrast with the bright areas in the low or middle latitudes probably represented oceans of methane or ethane but the radar views obtained from the Cassini spacecraft during its orbital dance in the Saturn System have shown that those dark areas are in fact dominated by linear and parallel dunes extending over long distances. The radar views acquired from the Cassini orbiter have revealed that the lakes, seas and rivers of Titan are mostly concentrated in the high latitudes of each hemisphere.
The north polar region of Saturn's largest moon appears to be the most humid area of Titan. How can we explain that dichotomy between the southern hemisphere and the northern hemisphere ? Seasonal factors must play a key role in the size and in the level of the lakes, seas or rivers but there must be other factors to account for the surprising asymmetry in the distribution of lakes or seas on Titan. Orbital factors must be taken into account in particular. The distance between Saturn and the Sun can significantly vary during the Saturnian year. Variations in the depth of the external crust can also play a significant role if there are internal sources to the lakes or seas we can see from the orbiter in the infrared or near-infrared images or in the radar views. The images obtained from the Cassini orbiter during its long mission in the Saturn System from 2004 to 2017 have allowed us to evaluate the influence of seasonal factors. Cloud systems can be found in the areas where the lakes, seas or rivers are located. Those clouds composed of methane or ethane can produce rain like the clouds of water we regularly encounter on Earth. A meteorological cycle involving methane can be imagined on Titan.
There may be several liquid layers beneath the external crust of Titan. One can imagine a liquid layer rich in liquid methane and liquid ethane beneath the external crust. At a higher depth, a layer of liquid water can also be imagined. If there are cryovolcanoes on the surface of Titan, materials, compounds or elements from the underground will be found on the surface. In a configuration of active cryovolcanoes present on the surface of that world, any probe would not have to dig deeply in order to analyze the potential composition of the presumed liquid layer. Frozen bacteria or archaea or fossils could potentially be found on the icy surface or on the brown or dark mud of Titan. Several types of lifeforms based on different solvents could be encountered by our probes in the future. Titan clearly appears to be a natural laboratory for the chemistry of organics and hydrocarbons. That natural laboratory can tell us a lot regarding the origin of life and regarding the mechanisms of formation and development of life. Any lifeform based on liquid methane or liquid ethane would probably have a very slow metabolism due to the low level of mobility of molecules and elements in the harsh environment at the level of the surface.
A new probe is expected to land on the surface of Titan in the 2030s. The rotorcraft of the Dragonfly mission will have the opportunity to study a land of dunes whose composition is probably relatively rich in hydrocarbons and organics. The rotorcraft will not evolve in the land of lakes, seas and rivers found in the high latitudes of the northern hemisphere but pockets of liquid methane or liquid ethane may be found during that mission. The Huygens probe had revealed a strange landscape composed of eroded stones or pebbles. The module had probably landed onto an ancient brook or stream. That can appear familiar at first sight but the composition of the pebbles of Titan must be radically different from the pebbles one can encounter on Earth. The pebbles of the Earth generally tend to be rich in silicon dioxide whereas the pebbles of Titan must be rich in water ice and must be composed of a mixture of hydrocarbons, organics and water. Titan is clearly less rich in heavy elements or richer in lighter elements than the Earth. That's why the composition of the ground on Titan must be quite original. Are there methanogens on Titan ? Is there a biological cycle based on methane on Titan ? We will probably have an answer to that question one day !
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The image in the upper part of the table represents a raw view of a portion of Saturn's largest moon Titan acquired on December 31, 2013 from the Cassini orbiter. The view whose file name is N00219137.jpg was captured on the basis of the CL1 filter and of the MT1 filter. The image had not been validated or calibrated at the time of the observation and a validated or calibrated version of the view had to be archived with the Planetary Data System proposed by NASA. The image in the lower part of the table represents a colorized version of the original view. Credit for the original view: NASA/JPL-Caltech/Space Science Institute. Credit for the colorization process of the original view: Marc Lafferre, 2023. |
- To get further information on that news, go to: https://astrobiology.nasa.gov/news/discovery-of-new-proteins-advance-understanding-of-climate-and-astrobiology/ and https://academic.oup.com/pnasnexus/article/2/8/pgad268/7242427?login=false.