August 18, 2020 : The Opaque Moon Titan Or The Dwarf Planet Ceres Are Part Of The List Of Potential Ocean Worlds
At first sight, the Earth appears exceptional in the Solar System because its surface is dominated by liquid water. No other world in the Solar System contains any ocean of liquid water on its surface. Most moons in the Solar System are devoid of any atmosphere. Therefore, they can't contain any stable pool of liquid compounds on their surface. On our planet, there are the right combinations of atmospheric pressure and environmental temperature for the presence of stable pools of liquid water and we live in an environment where liquid water is widespread. The oceans represent, approximately, 70 percent of the surface of the Earth. Mercury, the nearest planet to the Sun, is devoid of any atmosphere. That's why its surface is heavily cratered and devoid of any lake, sea or river. Venus looks like the Earth and may have contained seas or oceans of liquid water in the past but today, the surface of Venus is a hell where water can't appear in its liquid form. The environmental temperature is so high on the surface of Venus that lead could be encountered in its liquid form on the surface. The only extraterrestrial world where stable pools of liquid can be found on the surface is the giant moon Titan.
The moons Titan and Io clearly demonstrate the potential diversity of worlds beyond the Solar System. Io is a major moon of Jupiter where the tidal forces exerted by the Gas Giant and the other moons of that planet engender an extremely strong volcanic activity. The volcanic eruptions on Io can reach high altitudes which can represent several hundred kilometers. Giant lakes of volcanic lava can regularly form. In fact, Io is the most active world in the Solar System. Titan has also pools of liquids but the lakes, seas or rivers found on the Opaque Moon are related to a meteorological cycle. To a certain extent, Titan looks like the Earth. Titan's atmosphere is dominated by molecular nitrogen like our atmosphere. The main difference in terms of atmospheric composition is that the second most abundant gas in Titan's atmosphere is methane whereas the second most abundant gas in our atmosphere is oxygen. On Titan, there is a meteorological cycle involving methane whereas on our planet, there is a meteorological cycle involving water. There can't be stable pools of liquid water on Titan's surface but water is widespread in the System of Saturn and subsurface oceans of liquid water may in fact exist beneath the external crust of Titan or Enceladus.
Thanks to the Cassini-Huygens mission, we have been in a position to determine that the tiny moon Enceladus has a particularly young surface in its south polar region and that there are fractures where geysers or active cryovolcanoes spewing water ice or water vapor can be observed in that remarkable area. The bright, icy moon Enceladus is in fact an active world probably containing a subsurface ocean dominated by liquid water. The internal activity of Enceladus is probably related to tidal forces engendered by the Ringed Planet Saturn and by the other moons evolving around the Gas Giant. Several moons of Saturn which are rich in water ice may contain a subsurface layer of liquid water or at least pockets of liquid water beneath their icy crust that is generally heavily cratered. That's the case for Tethys, Dione or Rhea. Some clues gathered by the various instruments of the Cassini orbiter also suggest that Titan may contain a subsurface ocean. Researchers are eager to determine the composition of the presumed subsurface ocean of the giant moon. How deep must we dig to find the exotic subsurface ocean of the Orange Moon ? What is the composition of that hypothetical subsurface ocean ?
Prior to the atmospheric descent of the Huygens probe toward the surface of Saturn's largest moon Titan, some researchers had hypothesized that the dark areas which mark a sharp contrast with brighter areas represented oceans of methane or ethane. The aerial views acquired from the Huygens module as well as its famous view obtained from the surface of that hazy moon have clearly demonstrated that the relatively dark areas found at low or mid-latitudes are not seas or oceans of methane or ethane. The aerial views captured from the Huygens probe have revealed bright hills containing systems of dark channels which may represent drainage channels. The rivers found in the low or mid-latitudes may be active during heavy rainfall events or under the action of phenomena comparable to monsoon events on Earth. The dark area where the Huygens probe had landed appears relatively dark and uniform. That dark area may represent an ancient sea or may become a small sea or a pond from time to time. Curiously, the infrared or near-infrared views as well as the radar images obtained from the Cassini orbiter during its long mission in the Saturn System from 2004 to 2017 have allowed us to determine that the humid areas can be found at a high latitude of each hemisphere or in the polar regions.
How can we explain the fact that lakes, seas or rivers are concentrated in the high latitudes of the Hazy Moon ? The radar views or the infrared or near-infrared views clearly show that stable pools of liquids are mostly found in the high latitudes of the northern hemisphere. The dichotomy in the distribution of lakes and seas between the northern hemisphere and the southern hemisphere may be related to orbital or physical phenomena. Methane is a volatile molecule that may be less volatile in the polar areas where the environmental temperature tends to be weaker than in the low or mid-latitudes. On January 14, 2005, the Huygens probe had recorded, at a low latitude in the southern hemisphere, a surface temperature of -179 degrees Celsius, -290 degrees Fahrenheit or 94 Kelvin and an atmospheric pressure on the surface of 1,467 hPa which is much higher than the atmospheric pressure on the surface of the Earth at sea level. Therefore, there is the right combination of environmental temperature and atmospheric pressure for the presence of liquid methane, liquid ethane or liquid propane on the surface. Water can only appear in its solid form on the surface of the Opaque Moon.
The size and the level of the lakes, seas or rivers on Titan may vary depending on meteorological factors. If there are more condensation processes or rainfall events, the size or the level of the lakes or seas will tend to increase. If the evaporation processes are stronger, the size or the level of the lakes or seas will tend to decrease. During the Cassini-Huygens mission in the Saturn System, the infrared or near-infrared views of the area of Ontario Lacus during the Summer season in the southern hemisphere had clearly revealed a multitude of dynamic clouds or storms implying relatively strong evaporation processes in the area. Over a full Titanian year, the size or the level of Ontario Lacus may be higher during the Winter season in the southern hemisphere than during the Summer season in the same hemisphere because condensation processes must be stronger than evaporation processes during the Winter season in the area. The Cassini mission in the Saturn System came to an end at the start of the Summer season in the northern hemisphere. Therefore, we have not been in a position to identify any signs of net evaporation processes in the high latitudes of the northern hemisphere where most lakes, seas and rivers are located on Titan.
Some pools of liquids in the high latitudes of the northern hemisphere or in the north polar region may in fact represent volcanic lakes or volcanic depressions where lakes of hydrocarbons have taken shape. Some researchers advance that there may be subsurface pockets of liquid methane or liquid ethane feeding some lakes or seas in the north polar region. The relatively high concentration of methane in Titan's atmosphere may be explained by the presence of a subsurface layer of methane or hydrocarbons. The dense atmosphere of the giant moon may be regularly fuelled by a subsurface reservoir rich in ammonia or nitrogen because the gas blanket of Titan is dominated by molecular nitrogen and ammonia contains nitrogen. Ammonia can be encountered in the atmosphere of the Gas Giant Jupiter for instance. Some planetologists have also advanced that the Opaque Moon may contain a subsurface ocean composed of a mixture of water and ammonia. Geysers or cryovolcanoes can spew those compounds from time to time like on Enceladus. However, no clear signs of active cryovolcanism on Titan have been identified so far.
In fact, many moons in the Solar System may contain subsurface oceans. A major target today is Europa, the fractured moon of Jupiter. Some researchers advance that there may be geysers or active cryovolcanoes on that world at the present time. The hypothetical subsurface ocean of Europa may be dominated by liquid water. That's why astrobiologists are particularly interested in the exploration of the icy moon. Ganymede which has its own magnetic field may also contain a subsurface ocean. Closer to us, the Dwarf Planet (1) Ceres, evolving in the Asteroid Belt, may contain a subsurface ocean as well because the famous crater known as Occator Crater contains bright patches representing deposits of sodium carbonate (Na2CO3). Therefore, pockets of liquid water or a subsurface layer of liquid water may exist beneath the dark, cratered surface of the small planetary body. Triton, the largest moon of Neptune, is also part of the list of potential ocean worlds. The Voyager 2 spacecraft had revealed that Triton unveils a multitude of geysers or cryovolcanoes in particular. The Dwarf Planet Pluto and its moon Charon are also serious candidates for the potential presence of a subsurface ocean dominated by liquid water. Potential cryovolcanoes were clearly observed from the New Horizons spacecraft during its historic flyby of July 14, 2015. An area like Sputnik Planitia may also contain a subsurface layer of liquids at the present time.
The image above reveals three worlds of the Solar System that may contain a subsurface ocean dominated by liquid water. The upper part of the view reveals the Opaque Moon Titan. The bright, tiny disk is Enceladus, a moon where geysers or cryovolcanoes have been clearly observed. The disk in the lower right part of the view represents the Dwarf Planet (1) Ceres unveiling its famous bright patch. The three celestial bodies are represented at scale. Credit for the original view of Titan: NASA/JPL-Caltech/Space Science Institute. Credit for the original view of Enceladus: NASA/JPL-Caltech/Space Science Institute. Credit for the original view of (1) Ceres: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Credit for the montage: Marc Lafferre, 2020.
- To get further information on that news, go to: https://solarsystem.nasa.gov/news/1440/ocean-worlds-resources/?page=0&per_page=40&order=created_at+desc&search=&tags=Enceladus%2CEuropa%2CEuropa+Clipper%2CGanymede%2CTitan%2CTriton&category=324.