March 10, 2018 : Does Titan Represent A Biosphere With Life Evolving In The Area Of The Ceiling Of The Icy Crust In The Presumed Subsurface Ocean Of Liquid Water ?

A recent study entitled  The Possible Emergence of Life and Differentiation of a Shallow Biosphere on Irradiated Icy Worlds : The Example of Europa , proposed by the astrobiologist Michael Russell of NASA's Jet Propulsion Laboratory in Pasadena, California and published in the journal Astrobiology, suggests that an alien life could evolve in the area of the ceiling of the icy crust of presumed ocean moons like Europa or Titan. Europa is thought to harbor a subsurface ocean of salty liquid water. Some moons like the moons of Jupiter, Ganymede and Callisto or the moons of Saturn, Enceladus and Titan, may also contain the same kind of subsurface ocean. Michael Russell and his collaborators advance that an exotic lifeform can potentially develop in the particular environment found at the boundary between the liquid and the icy layer of the crust thanks to the interactions between chemical energy going up from hydrothermal vents found on the ocean floor and oxidants going down from the surface and the icy crust.

The Earth appears to be the only world in the Solar System containing pools or oceans of liquid water on its surface because there is the right combination of environmental temperatures and atmospheric pressure on the surface at sea level for water to appear in its liquid form on the surface. The main source of energy of our biosphere is sunlight even if some ecosystems can benefit from the energy emanating from internal sources, volcanoes, geysers or hydrothermal springs and evolve in the absence of solar radiations. In the presumed subsurface ocean of liquid water of Europa or Titan, the creatures, from microorganisms to more complex organisms, can't rely on sunlight to evolve or metabolize. As a result, the hypothetical creatures may be mainly based on the heat or the energy rising from hydrothermal vents located in the seafloor. Researchers imagine a configuration involving chemical reactions between molecules or elements from hydrothermal vents or the seafloor and seawater. The heat generated by hydrothermal vents is likely to facilitate complex chemical reactions, to engender key molecules or key building blocks of life and to push microbes and nutrients upward.

Scientists imagine that, in the particular environment of Europa, Ganymede or Titan, high-energy electrons emanating from the Gas Giant around which the moons move (Jupiter for Europa and Ganymede and Saturn for Titan) play a key role for the hypothetical biosphere of the presumed ocean world. In fact, the high-energy electrons from the planetary environment would produce chemicals known as oxidants which are likely to be used by the organisms evolving beneath the icy crust. The oxidants may represent fuel for the hypothetical alien life. A parallel could be drawn between those oxidants and the oxygen that our biosphere uses to burn nutrients for energy. But the configuration imagined by researchers implies that the presumed icy crust of Europa or Titan allows the diffusion of the oxidants or organics from the surface of the moon to the presumed subsurface ocean of liquid water. One can easily imagine a blanket of microorganisms thriving beneath the ceiling of the external crust.

Michael Russell and his collaborators who performed this analysis which was supported by the NASA Astrobiology Institute clearly show the potential interactions between the presumed subsurface ocean dominated by liquid water and the external crust. Complex chemicals and even the building blocks of life can take shape and develop at the boundary between the presumed icy crust and the liquid. The group of scientists believes that, at the underside of Europa's icy blanket, a shallow biosphere representing a network of ecosystems can take shape and thrive. Michael Russell pointed out :  All the ingredients and free energy required for life are all focused in one place. Europa is devoid of any atmosphere which implies that sunlight is in direct contact with the soil. That is not the case for Titan which is covered by a significant atmosphere. However, complex hydrocarbons or organics can form and engender a haze in the atmosphere of Titan. Some complex organics or molecules can fall to the ground and generate a kind of sludge called tholin. Can the organics found on the soil go through the icy crust and reach the presumed subsurface ocean of liquid water ?

The researchers observed that the layers where ice and water are interconnected can host a significant variety of life on our planet. We know that dense communities of a multitude of species of algae, bacteria, protists and even multicellular invertebrates take shape annually beneath the sea ice near Antarctica when a large amount of solar radiations fuels or triggers the photosynthesis of the algae. One can also mention that streamers of bacteria can be found under the blanket of snow and ice in runoff from the sulfurous Gypsum Springs on Axel Heiberg Island in the Canadian High Arctic. The planetologists suggest that their finding may be useful in the prospect of a future in-situ mission to Europa. Engineers could develop a suited probe or robot to drill and to explore the underside of the presumed icy crust on Europa, Ganymede or Titan. The team of Michael Russell imagines that the hypothetical microbes of Europa could reach densities comparable to microbial mats located on our planet.

The origin of life on Earth remains a major mystery and several hypotheses have been advanced to unravel the secret of life on Earth. Some researchers argue that life may have taken shape first in a pond. Other researchers advance that the first spark of life may have taken shape in the sea or in the ocean. Some planetologists imagine that life may have come from outer space via comets or meteorites. Some specialists advance other hypotheses such as the configuration in which life may have first developed in our atmosphere under the action of ultraviolet light from the Sun which favors chemical reactions. Another hypothesis put forward by planetologists is the configuration in which life may have taken shape in the area of an hydrothermal vent on the floor of one of our oceans. Michael Russell argued :  If we were to find life on Europa, then that would strongly support the submarine alkaline vent theory,  which implies that life on our planet originated close to underwater alkaline hydrothermal vents.

Thanks to the Cassini mission, we know, now, that Enceladus, the small icy moon of the Ringed Planet Saturn, represents an active world with geysers or cryovolcanoes in the fractures of its south polar region. We may have identified geysers or sprays of water ice on Europa, as well. The images of Europa clearly show a fascinating world covered by a complex network of fractures or cracks of the bright surface. When we see that icy moon of Jupiter, one can easily imagine the presence of a subsurface ocean beneath the bright and relatively young surface of Europa. The presumed subsurface ocean of Europa is probably dominated by salty liquid water. The hypothetical creatures have to adapt to their environment where there is no solar light. Is the liquid rich in oxygen ? Is the liquid rich in CO2 ? Scientists have to imagine the potential ingredients and the potential chemical reactions beneath the icy crust of Europa. The tidal forces related to the gravity of Jupiter and the gravity of the other moons like Io, Ganymede or Callisto may bring a lot of energy to the presumed internal ocean of Europa.

The tidal forces undergone by Titan in the system of Saturn may be weaker than those undergone by Europa in the system of Jupiter. However, Titan is among those moons or planetary bodies which may contain a subsurface ocean of liquid water beneath their external crust. Some researchers had advanced that there may be a subsurface ocean of ammonia or a subsurface ocean of methane on Titan. We know, now, that there is a methane cycle on Titan with pools of liquid hydrocarbons on the surface in the high latitudes or the polar areas, evaporation processes, condensation processes with cloud formation and precipitation processes. The soil of Titan may be rich in water ice, hydrocarbons and organics. Interactions between water, hydrocarbons and organics become interesting if the environmental temperature allows water to appear in its liquid form. That configuration can be encountered at a certain depth beneath the surface of the giant moon. The content of the presumed subsurface ocean of the Opaque Moon may be studied by probes, rovers or drones in the area of cryovolcanoes on Titan. Infrared or near-infrared data acquired from the Cassini orbiter have allowed us to identify potential cryovolcanoes on Saturn's largest moon. The hypothetical subsurface ocean of Titan may be found at a particularly high depth beneath its soil. That's why we have to find geysers or cryovolcanoes in order to gather clues regarding the composition and the chemistry of the presumed subsurface ocean on the Orange Moon.

 

The image above corresponds to a composite view of Saturn's moon Titan and Jupiter's moon Europa represented at scale. The original view of Titan represents a natural-color image obtained on January 30, 2012 from the Cassini orbiter. The original view of Europa was obtained from the Galileo probe on September 7, 1996. Europa and Titan may harbor a subsurface ocean dominated by water beneath the external crust. Are there hydrothermal vents fueling hypothetical ecosystems inside those hypothetical oceans ?

Credit for the original image of Titan: NASA/JPL-Caltech/Space Science Institute.
Credit for the original image of Europa: NASA/JPL/DLR.
Montage credit: Marc Lafferre, 2018.


- To get further information on that news, go to: https://www.astrobio.net/alien-life/shallow-biospheres-exist-beneath-icy-ceilings-ocean-moons.

 

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