May 6, 2022: New Experiments Reveal The Potential Freezing Point Of Subsurface Oceans Like The Potential Subsurface Ocean Of Europa Or Titan
A new research work entitled "On the pressure dependence of salty aqueous eutectics", published on April 12, 2022 in Cell Reports Physical Science and proposed by a team of researchers involving Baptiste Journaux reveals the potential limits of subsurface oceans like the potential subsurface ocean found beneath the crust of several icy worlds present in the Outer Solar System. Moons like Europa, Ganymede, Mimas, Tethys or Titan may contain a subsurface ocean beneath their external crust. If the subsurface ocean is dominated by water, one has to determine the potential limits of that ocean or the right combination of pressure and environmental temperature to allow the stable presence of a liquid layer beneath the external crust. The level of pressure above the potential subsurface ocean must be extreme so that any potential liquid can remain liquid even if the environmental temperature is extremely low. The presence of salt in the liquid dominated by water makes the freezing point of the liquid lower and a higher level of pressure can lower the freezing point of the potential liquid. How deep will we have to dig in order to reach the potential subsurface ocean inside Europa, Ganymede or even Titan ?
The new experiments upon the physical limits of any potential subsurface ocean were led by scientists from the University of Washington and from the University of California, Berkeley. The identification of any potential subsurface ocean in the Outer Solar System is fundamental in terms of exobiology in particular. The potential subsurface ocean can be in direct contact with the surface like on the tiny moon Enceladus, a surprising world evolving around the Gas Giant Saturn. There are geysers in the fractures of the south polar region of Enceladus. Those geysers demonstrate that a subsurface ocean dominated by water is clearly possible. Tidal forces must play a key role in the level of stability of the potential subsurface ocean. Baptiste Journaux who is an acting assistant professor of Earth and space sciences at the University of Washington pointed out: "The more a liquid is stable, the more promising it is for habitability." He added: "Our results show that the cold, salty, high-pressure liquids found in the deep ocean of other planets' moons can remain liquid to much cooler temperature than they would at lower pressures. This extends the range of possible habitats on icy moons, and will allow us to pinpoint where we should look for biosignatures, or signs of life."
The image above reveals, from the left to the right, Ganymede, Europa and Titan at scale. Those worlds evolving in the Outer Solar System may contain a subsurface ocean. Planetologists try to determine the nature, the dynamics or the characteristics of those hypothetical internal oceans. What is the exact composition of the liquid or what is the freezing point of the liquid ? Key questions must be answered ! Credit for the original view of Ganymede: NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill. Credit for the original view of Europa: NASA/JPL/DLR. Credit for the original view of Titan: NASA/JPL-Caltech/Space Science Institute. Montage credit: Marc Lafferre, 2022.
April 26, 2022: A New Research Work Reveals The Potential Sedimentary Cycle Of Titan
A new study "entitled "The Role of Seasonal Sediment Transport and Sintering in Shaping Titan's Landscapes: A Hypothesis", published on April 1st, 2022 in Geophysical Research Letters and proposed by a team of researchers involving Mathieu Lapôtre from Stanford University brings us key information regarding the various potential formation processes of the landscape of Saturn's largest moon Titan. The research work unveils a model that can help us better understand the sedimentary cycle of the giant moon. There are clearly some similarities between the landscape of Titan and the landscape of the Earth even if the molecules involved at the level of the surface of Titan and at the level of the surface of the Earth can be radically different. The atmosphere of Titan, dominated by molecular nitrogen like the atmosphere of the Earth, contains a relatively significant fraction of methane. In the harsh environment of Titan, methane can form rivers, lakes and seas in the high latitudes of each hemisphere. Methane can also form clouds that can engender rainfall events on that giant moon. The exotic meteorology of Titan can appear familiar to us to a certain extent. Water can be present on Titan in its solid form due to the extremely low environmental temperature at the level of the surface.
The relatively dark areas found in the low or middle latitudes of Titan tend to be dominated by Seif Dunes or linear and parallel dunes extending over long distances. The shape of those dunes is closely linked to prevailing winds. The composition of the Titanian dunes can appear really exotic since they must be dominated by hydrocarbons or organics. The chemistry and the dynamics of the Titanian dunes must be quite different from the typical dunes we regularly encounter on Earth. The mean density of the dunes found on Earth must be much higher than the mean density of the dunes found on Titan. On the Blue Planet, the dunes will tend to be rich in silicate-based substances whereas they will tend to be rich in carbon-based substances on the Opaque Moon. The team of Mathieu Lapôtre has managed to determine the processes that can lead to the formation of the dunes, the formation of the sand grains or the formation of the bedrock one can find on Titan. Winds, rainfall events and streams flow play a key role in the various potential processes leading to the landscape one can see today. Several types of landscape features can be identified on the Orange Moon, from dunes to plains, lakes, seas or labyrinth terrains.
The annotated image above represents a raw view of Titan's surface acquired from the Huygens probe on January 14, 2005. The landing site can be found at a low latitude in the southern hemisphere of Saturn's largest moon in the area of the relatively bright Adiri and of the relatively dark Shangri-La. One can observe eroded stones or pebbles which implies that the Huygens probe may have landed onto an ancient stream, brook or river. Image credit: ESA/NASA/JPL/University of Arizona .
February 22, 2022: A New Study Reveals How The Radiative Heating Of The Haze Of Titan Engenders A Superrotation Of Its Stratosphere
A new research work proposed by Motoki Sumi, Shin-ichi Takehiro, Wataru Ohfuchi, Hideko Nomura and Yuka Fujii, entitled "Superrotation of Titan's stratosphere driven by the radiative heating of the haze layer" and published in Arxiv shows the influence of the radiative heating of Titan's haze layer on the superrotation phenomenon of its stratosphere. The atmosphere of Saturn's largest moon is known to move faster than its surface. A phenomenon of superrotation is observed for the atmosphere of Titan or for the atmosphere of Venus. Both worlds contain a completely opaque atmosphere that prevents us from discerning surface features in the visible spectrum from outer space. The atmosphere of Titan is extremely cold whereas the atmosphere of Venus is extremely warm but the dynamics of both atmospheres presents some similarities. The atmosphere of the Opaque Moon rotates many times faster than its surface does for instance. That's also the case for Venus where the surface rotates extremely slowly compared to the surface of the Earth for instance. The atmosphere of Titan is particularly complex with a multitude of layers in its outer limits and with a thick haze layer containing various molecules like hydrocarbons or organics. The dynamics and the composition of that haze layer are closely related to solar radiations.
The researchers of the new study carried out numerical experiments mobilizing a General Circulation Model (GCM) to determine or evaluate the effects of the haze layer on the stratospheric superrotation. They resorted to a semi-gray radiation model of the atmosphere of the giant moon of Saturn following McKay et al. (1999) which incorporates the factor of sunlight absorption by haze particles. The simulations of the team did not incorporate the factor of the phase change of methane or the factor of the seasonal changes. The model based on the action of solar radiations on the thick haze layer of the Opaque Moon produced a configuration in which there is a global eastward wind around the equator with higher speeds at higher altitudes except at about 70 kilometers above the ground after 10^5 Earth days. The model predicts zonal wind profiles that are in line with the observations of that type of phenomenon even if the atmosphere does not appear to be in an equilibrium state. The analytical work of the team in its experiments implies that the quasi-stationary stratospheric superrotation is stabilized by the balance between the meridional circulation disconnected from the surface and the eddies that push angular momentum towards the equator.
The image in the upper part of the table which represents a raw view reveals a crescent of Titan. The view whose file name is N00199171.jpg was acquired from the Cassini orbiter on the basis of the CL1 filter and of the CB3 filter on December 16, 2012. The image had not been validated or calibrated at the time of the observation and a validated or calibrated version was going 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. One can notice a type of vortex above the south polar region in this view in particular. Credit for the original image: NASA/JPL-Caltech/Space Science Institute. Credit for the colorization process of the original view: Marc Lafferre, 2022.
February 19, 2022: A New Study Reveals That Craters Whose Diameter Is Smaller Than 50 Kilometers Tend To Completely Disappear Due To Erosion Over Geologic Time Scales On Titan
A new research work entitled "Crater production on Titan and surface chronology", proposed by N.L. Rossignoli, R.P. Di Sisto and M.G. Parisi and published in Arxiv reveals the key action of erosion on relatively small craters on Saturn's largest moon. Big craters whose diamater is over 50 kilometers can persist or can be well discerned over the Solar System age whereas smaller craters tend to completely vanish due to the erosion processes related to winds, rainfall events or even snowfall events. Titan is the only world evolving around the Gas Giant Saturn that contains a significant atmosphere implying a relatively strong action of erosion on its surface. The other moons of Saturn contain numerous craters that are likely to allow us to determine the potential age or history of those worlds. Worlds like Tethys, Dione or Rhea are rich in impact craters on their bright surface likely rich in water ice. The researchers can analyze those craters in order to deduce the age of the crust or surface. They are in a position to determine constraints on the population of the impactors that can be comets or asteroids. Regarding the impactors, the planetologists focus their attention on centaur objects or objects that evolve in the area of the Gas Giants.
During the long Cassini-Huygens mission in the Saturn System, from 2004 to 2017, the Cassini orbiter has captured a huge amount of data regarding the surface of the giant moon. We have collected images obtained in the infrared or near-infrared spectrum as well as radar images so that the surface could be discerned despite the presence of a completely opaque atmosphere in the visible spectrum. Researchers have had the opportunity to evaluate the concentration of potential impact craters on that exotic world and it turns out that impact craters are remarkably scarce despite a relatively dry environment in the low or mid-latitudes. The relatively big craters can appear strongly degraded or eroded. The planetologists mobilized the previous research works on impact cratering rates on the various moons of the Ringed Planet Saturn and modeled the cratering process on the Orange Moon to constrain the surface chronology of that world and to evaluate the role of centaur objects that are considered the main impactors of Saturn's largest moon. The Outer Solar System is far from being empty in terms of asteroids, comets or even Dwarf Planets. Numerous objects can be identified between Jupiter and Neptune.
The image above reveals a portion of Ganesa Macula, a landscape feature that may represent an impact crater or even a cryovolcano. The view represents a portion of a radar swath of Saturn's largest moon Titan obtained during the flyby of the giant moon performed on October 26, 2004 by the Cassini-Huygens spacecraft. Each side of the view represents about 100 kilometers. In the upper left part of the image, one can notice, in particular, a relatively round feature that appears relatively bright. That feature may represent the central peak of the presumed impact crater. Credit for the original view: NASA/JPL/Cassini Radar Team/Jason Perry. Montage credit: Marc Lafferre, 2022.
January 14, 2022: A New Study Reveals That The Haze Particles Of Pluto Are Bimodally Distributed And Confirms The Key Role Of Photochemistry In The Formation Of The Upper Haze Of Titan
A new study entitled "A bimodal distribution of haze in Pluto's atmosphere" and proposed by a team of researchers involving Siteng Fan and Peter Gao reveals that the haze particles of the Dwarf Planet Pluto are bimodally distributed and that the formation of the upper haze of the giant moon Titan is apparently related to photochemical phenomena. Several worlds of the Outer Solar System contain atmospheres that look alike to a certain extent. Those worlds are Titan, Triton and Pluto. Their atmosphere unveils a haze that is closely related to the action of the light emitted by our star the Sun. The atmosphere of Titan is completely opaque and is particularly deep and thick. On the other hand, the atmosphere of the largest moon of Neptune Triton and of the biggest Dwarf Planet Pluto is very thin with a very limited atmospheric pressure on the surface. The atmosphere of the three worlds is dominated by molecular nitrogen like the atmosphere of our own planet. One can notice that the atmosphere of Mars or Venus is part of another category of atmospheres since the atmosphere of both worlds is dominated by carbon dioxide rather than molecular nitrogen. However, molecular nitrogen appears to be the second most abundant compound of the atmosphere of those worlds.
The atmosphere of Pluto or Triton can be compared to the upper atmosphere of Titan where photochemistry tends to play a key role. In the upper atmosphere of the largest moon of the Ringed Planet, several atmospheric layers can be discerned. That's also the case in the atmosphere of Pluto which had been clearly observed from the New Horizons spacecraft during its historic flyby of Pluto and Charon on July 14, 2015. The icy or rocky worlds of the Outer Solar System tend to be rich in molecular nitrogen, carbon monoxide, methane or water ice. A molecule like methane can appear in its solid form on the surface of Triton or Pluto whereas that molecule appears as a gas in the typical environment of the Earth. Methane can also form clouds in the atmosphere of Titan. Rainfall events can be encountered on Saturn's largest moon. Therefore, rivers, lakes or seas can be encountered on that enigmatic world. On Pluto or Triton, the atmosphere is too thin for the presence of liquid methane or liquid nitrogen on the surface. A heavier atmosphere is needed on Pluto for the formation of lakes or seas of nitrogen. Strong cryovolcanic events on Pluto may allow the presence of transient lakes or rivers of nitrogen from time to time.
The image in the upper part of this table reveals a portion of Titan's atmosphere in which the upper atmosphere can be clearly discerned. The image whose file name is N00099119.jpg represents a raw view acquired from the Cassini spacecraft on December 20, 2007 on the basis of the CL1 filter and of the UV3 filter. At the time of the observation, the view had not been validated or calibrated and a validated or calibrated version was going to be archived with the Planetary Data System proposed by NASA. The view in the lower part of the table represents a colorized version of the original image. Credit for the original view: NASA/JPL-Caltech/Space Science Institute. Credit for the colorization process of the original view: Marc Lafferre, 2022.