July 13, 2019 : A Team Of Researchers Led By Rosaly Lopes Tries To Understand
The Chemistry And The Dynamics Of Organics Or Hydrocarbons In The Titanian
Environment In Order To Evaluate Its Habitability
A group of scientists from NASA's Jet Propulsion Laboratory, led by Rosaly Lopes and funded by the NASA Astrobiology Institute, has undertaken major studies upon the chemistry and the dynamics of organics or hydrocarbons in the environment of Saturn's largest moon Titan. Multiple hydrocarbons or organics have already been clearly identified in the hazy atmosphere or on the surface of the giant moon of the Ringed Planet. The environmental temperature of Titan is particularly low with a mean temperature around minus 179 degrees Celsius, minus 290 degrees Fahrenheit or 94 Kelvin but complex chemical reactions involving hydrocarbons, organics or nitrogen can occur in that harsh environment. The atmosphere of the Opaque Moon may look like the atmosphere of the Early Earth which also represented a soup of complex hydrocarbons. Planetologists have been in a position to infer the presence of a subsurface ocean beneath the external crust of Titan thanks to gravity measurements performed during flybys carried out by the Cassini orbiter. Thus, some researchers believe that the presumed ocean of water may potentially host life.
The team of Rosaly Lopes tries to identify the potential chemical interactions between the presumed subsurface ocean, the presumed icy crust, the soil and the atmosphere. Is there a cycle of organics or hydrocarbons between the hypothetical ocean and the hazy atmosphere ? Scientists know that ultraviolet light from the Sun plays a key role in the chemistry of Titan's haze or in the chemistry of Titan's upper atmosphere. Hydrogen, methane and nitrogen can engender complex organics or hydrocarbons that can represent prebiotic molecules. Can any exotic lifeform use relatively simple organics or hydrocarbons produced in Titan's atmosphere ? The researchers have determined four major objectives. The first goal is to understand the potential mechanisms in the movement of organic molecules between the atmosphere, the surface and the presumed ocean. The second goal is to determine the types of process taking shape within the subsurface ocean which are likely to make it habitable. The third goal is to identify the potential biosignatures the hypothetical life can generate. The fourth goal is to understand the potential mechanisms in the upward movement of the potential biosignatures toward the surface where we could identify or detect them.
The project regarding the study of organics or hydrocarbons in the Titanian environment has been financed by the NASA Astrobiology Institute for five years until April 2023. At the present time, the group of scientists is composed of 30 members working in various institutions. Rosaly Lopes pointed out : « Under each objective we have several investigations, and each investigation has a lead investigator. » The researchers are organized in a way that allows them to work in synergy and to share their outcome in a fluid way for new studies and new goals. Mike Malaska from JPL, who is a geochemist and Deputy Principal Investigator on the new project, explained : « Our science is following the organic molecules on their path from the top of the atmosphere where they get constructed, down through the crust and into the ocean, and if there's biology happening down there, how those organics work their way back up to the surface and become visible. » The mission is ambitious but the researchers have a huge amount of scientific data at their disposal regarding the Opaque Moon thanks to the Cassini-Huygens mission.
In the prospect of understanding the interactions between the atmosphere, the crust and the presumed subsurface ocean, the team of researchers has had the opportunity to benefit from the science results obtained by Conor Nixon and his collaborators at NASA Goddard. The astronomers used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to analyze the chemistry of Titan's atmosphere. The planetologists need to know the exact composition of the haze or gas layer in order to develop a comprehensive photochemical model of that exotic atmosphere and to identify the potential compounds, hydrocarbons or organics that can fall to the surface or that can reach the presumed subsurface ocean. Titan's atmosphere has been well studied on the basis of data obtained from various instruments of the Cassini orbiter and in particular the CIRS infrared spectrometer instrument. However, some types of molecules found in Titan's atmosphere were too faint in the infrared spectrum to be identified from CIRS. Those compounds can be discerned with ALMA. Conor Nixon argued that the researchers have identified, on the basis of ALMA, several cyanide molecules (CH3CN, C2H3CN and C2H5CN) which represent key molecules incorporating nitrogen, the most abundant element in the atmosphere of the giant moon. The Titanian atmosphere is really rich in hydrocarbons or organics.
ALMA was able to detect spatial variations in trace organic gases produced in the photochemical soup of molecules where methane and molecular nitrogen can break up to form new molecules or compounds under the action of UV light from the Sun. Those trace gases can go down toward the surface. During their migration, they can interact with other organics to form new molecules whose structure is more complex. The surface may be rich in complex organics since heavier molecules tend to fall toward the surface. Can those organics infiltrate and reach the presumed subsurface ocean ? The Cassini orbiter has monitered the evolution of Titan's atmosphere from the Winter period in the northern hemisphere to the Summer period in the northern hemisphere. Thus, the spacecraft has acquired data for about half a Titanian year. ALMA allows us to continue the analytical work upon the evolution of Titan's atmosphere. Will the atmospheric composition above the south polar region significantly change for instance ? On the basis of Cassini data, the team of Rosaly Lopes has identified, for instance, seasonal variations in the C3Hx hydrocarbons such as propane and propyne in the opaque atmosphere of the giant moon.
A team led by Alex Hayes at Cornell University is studying the potential ways molecules resulting from precipitation processes can be transported across the surface. The planetologists want to determine the potential chemical interactions involving organics at the level of the soil. They also want to determine whether the organics can migrate toward the presumed subsurface ocean. Thus, we need to understand the potential mechanisms of interaction between the compounds of the soil and the compounds of the presumed subsurface ocean. A research work proposed by Kelly Miller, Hunter Waite and NAI team-member Christopher Glein of the Southwest Research Institute in Texas has revealed that Titan's atmosphere, mainly composed of nitrogen, may be fuelled by organics or compounds found inside the moon when the world was young. The compounds containing nitrogen may have been heated so that nitrogen migrated to the surface and the atmosphere. The study suggests that there are already organics inside Saturn's largest moon beneath the hypothetical subsurface ocean. Therefore, organics can go upward to reach the ocean and fuel a potential biosphere even if there are no interactions between the subsurface ocean and the surface or the atmosphere.
Rosaly Lopes advanced : « These organics may actually be able to percolate up through cryovolcanism. » Cryovolcanism has not been clearly identified yet on Titan but signs of tectonic activity have been observed. The dense atmosphere may be closely related to the internal activity of the moon. A major goal of the team is, obviously, to determine whether there are locations on the moon or inside the moon that can host an exotic biosphere or that are habitable. The degree of habitability will be closely related to the concentration of organics or hydrocarbons. Therefore, the presumed subsurface ocean or the presumed icy crust must contain organics and a network or a circuit of organics or hydrocarbons must exist between the presumed liquid layer and the surface. Biologists focus their attention on organisms that can withstand high pressures or that can survive low environmental temperatures in particular. Planetologists have to try to determine the exact composition of the presumed subsurface ocean. Mike Malaska pointed out : « What we don't know is the exact composition of the ocean, its density, its thermal profile, the overall structure of the icy crust on top of it. » The scientists envisage several configurations of composition for the subsurface ocean and then, on the basis of those hypotheses, they develop theoretical models.
The planetologists intend to resort to theoretical models as well as laboratory experiments in order to deduce the potential environment that can exist beneath the external crust or above the rocky core as well as the various interactions between the atmosphere, the surface, the presumed subsurface ocean and the presumed rocky core. Can we encounter microbes that benefit from the flow of oxidants and reductants between the various layers ? The group of Rosaly Lopes will have to determine, via theoretical models, the potential level of energy or chemical energy present in the presumed ocean on the basis of the various interactions between the layers, on the basis of the different organics present in the environment and on the basis of the composition of the ocean. The researchers will have to determine the potential metabolisms that can be generated in that exotic environment as well. What kind of life or biodiversity can develop in that hypothetical ocean which is probably rich in organics and where the environmental temperature is particularly low and where the pressure is particularly high ?
The planetologists have in mind a microbe known as Pelobacter acetylenicus which can use acetylene as a source of energy to metabolize. Mike Malaska pointed out : « Our goal is to think of Pelobacter acetylenicus as the model organism, something that could exist in the deep sub-surface on Titan. » The scientists will perform laboratory experiments involving microbes like Pelobacter acetylenicus in the types of environment envisaged in their theoretical models to evaluate the ability of the microbes or the extremophiles to survive, to thrive or to adapt in the harsh environment. It will be interesting, in particular, to determine what types of biomolecules are produced in that soup of chemicals or what types of biosignatures or molecular traces of life are produced in the experiments. The researchers will have to be in a position to understand the potential biomarkers. That's why they will generate a database of potential biosignatures. The list will contain isotopes of carbon, nitrogen and oxygen as well as relatively complex molecules related to life such as the lipids in cell membranes.
The fourth objective of the group of researchers represents the detection of potential biosignatures. We must determine the various ways organics or biomarkers from the presumed subsurface ocean can migrate to the surface. One can imagine cryovolcanic phenomena or convective ice rising toward the upper part of the crust. Ice can be slushy or more mobile due to higher environmental temperatures. There may be regular releases of methane from the interior of the moon. Rosaly Lopes argued : « Methane in the atmosphere is destroyed by ultraviolet light, so there has to be some replenishment. » She added : « And there may still be outgassing happening. » Today, we don't have any clear evidence of cryovolcanoes on Saturn's largest moon but we have identified several locations which may potentially represent cryovolcanoes. Rosaly Lopes advanced : « We're already studying theoretical ways that cryovolcanism can transport material. » Mike Malaska considers that the whole crust as well as the subsurface ocean are biologically interesting. One can imagine reservoirs of liquid water in pockets within the external crust. Those environments may be rich in organics.
Mike Malaska advanced that one could find microbes like Pelobacter acetylenicus in tiny spaces, at a depth between 7 and 30 kilometers, at the boundary between the hard, brittle ice and the more ductile, softer ice. In those environments, the temperatures and pressures can be comparable to the temperatures and pressures found at a depth of about 2 or 3 kilometers beneath Antarctica. The extremophiles may thrive in tiny spaces located between the ice grains and the ice shell. If the pockets of life are found relatively close to the surface, one may be in a position to identify biomarkers. Planetologists also have to anticipate the way biosignatures can be chemically altered or modified during the migration process toward the surface. Various environments like an environment of liquid water, of slushy water or solid water can engender complex interactions with organics or biosignatures. We'll have to be in a position to identify the potential biomarkers with the probes, drones, boats or submarines we'll send to Titan. Mike Malaska pointed out : « This is our big objective, to try and evaluate Titan as a potentially habitable system. » He concluded : « We're going to create a list of potential biomarkers and try and indicate where on the surface might be a good place to look for them. » Titan clearly represents a chemical soup of organics and hydrocarbons which can tell us a lot about biology even if we don't find any lifeform in the environment of that world.
The image above represents a portion of a radar swath obtained from the Radar Mapper of the Cassini spacecraft during the T3 Flyby on February 15, 2005. One can clearly notice bright sinuous channels that may be related to cryovolcanism or an impact event. Are those channels dried-up rivers related to meteorological phenomena ? Each side of the view represents 100 km. Image credit: NASA/JPL/Cassini RADAR team/Jason Perry.
- To get further information on that news, go to: https://www.astrobio.net/news-exclusive/the-habitability-of-titan-and-its-ocean.