February 15, 2024: Carbon Migration Into The Presumed Subsurface Ocean Of Titan Via Impact Cratering May Be Too Limited For The Development Of Life

A new study entitled "Organic Input to Titan's Subsurface Ocean Through Impact Cratering", proposed by a team led by Catherine Neish and published in the journal Astrobiology on February 2, 2024 suggests that the amount of organics and glycine in particular migrating from the surface to the hypothetical subsurface ocean of Titan under the action of impact cratering appears too limited for the emergence and the development of life. The planetologists assumed that impact craters engender melt deposits containing liquid water. The mean density of the material is higher than the mean density of the icy crust so that the material rich in organics and rich in glycine in particular will tend to migrate toward the presumed subsurface ocean rich in liquid water. The team was in a position to evaluate the amount of organic molecules that could be injected into the subsurface liquid layer. The specialists mobilized known yields for HCN (Hydrogen cyanide) and the haze hydrolysis of the giant moon to determine the amount of glycine generated in the melt lenses and obtained a range of potential flux rates of glycine from the ground to the presumed subsurface ocean.

The range of possible flux rates of glycine appears to be between 0 and 10^11 mol/Gyr for HCN hydrolysis and appears to be between 0 and 10^14 mol/Gyr for haze hydrolysis. These fluxes imply an upper limit for biomass productivity of about 10^3 kg C/year on the basis of a glycine fermentation metabolism. This upper limit appears clearly low for the potential emergence or development of life. This upper limit for biomass productivity is even significantly lower than recent evaluations of the hypothetical biomass production taking shape in the presumed subsurface ocean of the tiny moon Enceladus. Yet, Titan is regarded as a world rich in organics and hydrocarbons but the concentration of molecules rich in carbon elements inside the hypothetical subsurface ocean of the Opaque Moon may be too limited for the emergence or the development of any lifeform based on liquid water and carbon. However, the planetologists of the research work focused their attention on the influence of impact cratering upon the amount of organics transferred from the surface to the presumed subsurface ocean. They don't consider other potential sources of organics or hydrocarbons that can fuel that hypothetical ocean.

The volume of the internal ocean of Titan that may be mainly composed of liquid water may represent 12 times the volume of the oceans of the Blue Planet. If the concentration of organics evolving in that type of ocean is not high enough, the emergence and the development of complex molecules seem impossible or almost impossible. If the crust in the lower part of that ocean is rich in hydrocarbons, organics or molecules containing carbon elements, the hypothetical ocean can be fed by relatively high concentrations of organics or hydrocarbons so that complex molecules mobilizing liquid water and carbon elements can emerge and develop. Amino acids which represent the building blocks of life on Earth can be found on asteroids or comets. In the complex haze of Titan, amino acids can emerge and develop as well so that the soil or the crust of Titan can contain amino acids as well. The impact events involving meteorites or comets can engender subsurface pools of liquid water and migration phenomena of molecules rich in carbon toward the presumed internal ocean of the Orange Moon. The relatively high density of the molecules rich in carbon favors their infiltration inside the external crust whose mean density is relatively low.

Many worlds of the Solar System may contain a subsurface ocean rich in liquid water. In the Outer Solar System where the environmental temperature is too low for the presence of liquid water on the surface of any icy or Terrestrial moon, pockets or layers rich in liquid water can be found beneath the external crust of several moons thanks to tidal forces related to the gravitational action of the neighboring worlds and thanks to the right combination of environmental pressure and environmental temperature. During its long mission in the Saturn System from 2004 to 2017, the Cassini spacecraft has captured surprising views of the tiny moon Enceladus which appears to be an active world geologically speaking. Geysers were clearly identified in the Tiger Stripes of Enceladus located in its south polar region. The fractures of Enceladus reveal geysers or plumes rich in water ice and organics. Those phenomena demonstrate that there must be a layer rich in liquid water beneath its external crust which appears very bright. Tidal forces from the Gas Giant Saturn and from the other major moons of that planet clearly play a major role in the internal activity of Enceladus.

Enceladus and Titan are part of those worlds in the Solar System that may contain a subsurface ocean rich in liquid water. Another tiny moon of Saturn may also contain a liquid layer of liquid water. That's Mimas, a cratered moon which appears bright like Enceladus. Mimas evolves closer to Saturn than Enceladus so that the interior of the moon must be relatively dynamic due to tidal forces from Saturn and the other moons. Tethys, Dione and Rhea which are also rich in water ice may also contain a subsurface layer dominated by liquid water. Titan evolves much further from the Ringed Planet than Mimas and Enceladus but the tidal forces related to Rhea or Iapetus are likely to play a relatively significant role in the dynamics of the interior of Titan. The Huygens probe had recorded a surface temperature of around -179 degrees Celsius, -290 degrees Fahrenheit or 94 Kelvin on January 14, 2005 in the region of Adiri and Shangri-La. Eroded stones or pebbles probably rich in water ice had been observed from the module on the surface. Water can only appear in its solid form in that type of environment. But for the interior of the giant moon, that's a different story !

Any impact event on Titan will tend to bring a strong amount of energy to the external crust likely rich in water ice so that pockets of liquid water can temporarily develop. Migration processes of compounds rich in carbon or organics from the external crust to the internal ocean can be facilitated during those types of exogenic events. The team of Catherine Neish had calculated, on the basis of the presumed frequency of impacts involving meteorites or comets of different sizes, the weight of organics that can be transferred from the surface to the interior. The team had evaluated the weight of glycine transferred from the surface toward the potential internal ocean. That weight may be around 7,500 kg/year. That's very low for the simplest amino acid that appears in proteins in our biosphere. That's the equivalent of the weight of a male African elephant injected into the presumed subsurface ocean every year. That's not enough for a giant internal ocean whose volume may represent 12 times the volume of the ocean of our planet. Obviously, let's recall that our knowledge regarding the exact composition of that layer is based on assumptions, hypotheses or relatively weak clues. We may be wrong and there may be major surprises.

The analysis of the surface of Titan from outer space is quite difficult due to the presence of a relatively thick atmosphere which appears completely opaque in the visible spectrum from outer space. The Cassini orbiter was in a position to obtain images of the surface in the infrared or near-infrared spectrum as well as radar views of the surface or the topography. Thus, we know that the relatively dark areas which mark a sharp contrast with relatively bright areas in the low or middle latitudes are dominated by linear and parallel dunes extending over long distances. Those dunes may be rich in hydrocarbons and organics. The haze can engender relatively complex molecules containing carbon elements that can fall toward the surface to feed those dunes. Any impact event is likely to engender the transfer of a relatively significant concentration of organics or hydrocarbons toward the interior of the moon. The images acquired from the Cassini orbiter have clearly shown that the north polar region and the south polar region contain lakes, seas and rivers of hydrocarbons. The pools of Titan may be dominated by liquid methane or liquid ethane and may also contain dissolved nitrogen.

Those giant pools of liquid demonstrate that the interior of the moon may be relatively rich in hydrocarbons or organics, at least, in some areas like in the polar regions of that world. There is a methane cycle on the Opaque Moon comparable to the water cycle of the Earth. Methane on Titan can evaporate, form clouds and fall as rain feeding the pools or rivers. Can there be a methane-based life in the major pools of the giant moon of Saturn ? Can liquid methane or liquid ethane represent solvents for the development of an exotic lifeform ? The environment of Titan is likely to tell us a lot regarding the chemistry of organics and hydrocarbons. Chemical reactions in the harsh environment of the Opaque Moon must be particularly slow. However, in the interior of the giant moon, there may be the right combination of pressure and temperature for the presence of a relatively deep layer of liquid water where a water-based life could emerge and develop. The potential cryovolcanoes of Titan may contain clues upon the composition of the presumed internal ocean. The Dragonfly rotorcraft may find major clues regarding the characteristics of the hypothetical internal ocean.

The image above reveals a portion of a radar swath obtained with the Radar Mapper of the Cassini orbiter during the T17 flyby performed on September 7, 2006. Each side of the view represents approximately 100 kilometers. One can notice a circular feature that may represent an impact crater. That crater is known as the Ksa crater. Impact craters on Titan are likely to engender transfers of hydrocarbons or organics from the surface to the hypothetical subsurface ocean rich in liquid water. Credit for the original view: PDS Image Atlas. Montage credit: Marc Lafferre, 2024.

- To get further information on that news, go to: https://news.westernu.ca/2024/02/titan-non-habitable/ and https://www.liebertpub.com/doi/10.1089/ast.2023.0055.



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