June 8, 2021: What Is The Closest World To Titan In Terms Of Nature Or Appearance In The Solar System ?
Planetologists often say that the environment of Titan, the largest moon of the Ringed Planet Saturn, looks like the environment of the Early Earth to a certain extent. Titan contains a significant atmosphere like the Earth. That's not the case for most moons of the Solar System. Our moon, The Moon, is devoid of any significant atmosphere. The Galilean Moons of Jupiter, Io, Europa, Ganymede and Callisto are also devoid of any significant atmosphere. Among the numerous moons orbiting Saturn, Titan is clearly the only moon containing a significant atmosphere. The atmospheric pressure on the surface of Titan is even higher than the atmospheric pressure on the surface of the Blue Planet at sea level. The moons of Uranus are also devoid of any significant atmosphere. Triton, the largest moon of Neptune, is in fact the only other moon in the Solar System to contain a visible atmosphere. However, that atmosphere is particularly thin since the atmospheric pressure on the surface of Triton is much lower than the atmospheric pressure on the surface of Mars for instance. Charon, the largest moon of Pluto, is devoid of any significant atmosphere as well. Surprisingly, Pluto which represents a Dwarf Planet and which is much smaller than Ganymede or Titan contains a thin atmosphere.
Among the Terrestrial worlds or the icy worlds of the Solar System, the worlds which are covered in a relatively significant atmosphere are quite rare. Mercury, the nearest world to the Sun, is devoid of any significant atmosphere for instance. Yet, the small planet Mercury is quite massive and is almost as big as Titan or Ganymede. Mercury may have had an atmosphere in the past but today that is not the case anymore. Mercury is dominated by impact craters like the Moon. One can notice that the mass and the level of energy received from the Sun play a key role for the stability of any atmosphere around a Terrestrial world or icy world. The likelihood for the presence of a significant atmosphere is higher if the world is heavier or more massive. The likelihood for the presence of a significant atmosphere is also higher if the distance between the Sun and the planet, the Dwarf Planet or the moon is higher. In fact, there must be the right combination of gravity and energy received from the Sun for the presence of a significant atmosphere. Venus, The Earth, Mars, Titan, Triton or Pluto appear to have the right combination of gravity and environmental temperature for the stable presence of any atmosphere around those worlds.
Titan's atmosphere is dominated by molecular nitrogen like our own atmosphere. By contrast, the atmosphere of Venus and the atmosphere of Mars are dominated by carbon dioxide. The atmosphere of those planets also contains molecular nitrogen but that molecule only represents a small fraction of the atmospheric composition. The atmosphere of Titan looks like the atmosphere of the Earth or the atmosphere of Triton or Pluto to a certain extent since the atmosphere of those worlds is dominated by molecular nitrogen. The major difference between the atmosphere of Titan and the atmosphere of the Earth is related to the significant presence of methane in the opaque atmosphere of Titan. Methane is the second most abundant gas in Titan's atmosphere and is almost absent in the atmosphere of the Blue Planet. The second most abundant gas of the atmosphere of our planet is oxygen whereas the second most abundant gas of the atmosphere of Titan is methane. A parallel has been drawn between the environment of Titan today and the environment of the Early Earth. In terms of composition or chemistry, the atmosphere of the Orange Moon may look like the atmosphere of our planet a long time ago.
Can we say that the hazy atmosphere of Saturn's largest moon looks like the atmosphere of our planet prior to the emergence of life ? Researchers often say that Titan unveils a prebiotic chemistry, a chemistry that can lead to life or prebiotic molecules. Under the action of ultraviolet light from the Sun, complex chemical reactions involving hydrocarbons, organics, radicals or molecules containing nitrogen take shape in the upper atmosphere of the giant moon. Heavier molecules can fall to the surface to form a dark or reddish mud or sludge called tholin. To a certain extent, Titan can be regarded as a natural laboratory for the study of the chemistry of hydrocarbons or organics which can lead to relatively complex molecules. Can the chemistry of hydrocarbons or organics on Titan lead to prebiotic molecules or biological molecules like amino acids, proteins, lipids, sugars, RNA or DNA ? We'll have to send probes, drones, boats or submarines in order to gather news clues regarding the mysterious chemistry encountered in the harsh environment of Titan. Thanks to the Cassini-Huygens mission in the Saturn System, we know that Titan looks like the Earth to a certain extent.
The Cassini spacecraft and the Huygens probe have clearly unveiled a varied world unveiling erosional processes or familiar landscapes. The Huygens probe which had plunged into Titan's atmosphere on January 14, 2005 had captured remarkable aerial views of the landscape as well as a captivating view of the horizon taken from the ground. The aerial views clearly show bright hills containing a network of dark channels and a dark or reddish plain which marks a sharp contrast with those bright hills. Is the dark or reddish plain an ancient sea or ocean ? That's a relevant question ! The network of dark channels likely represents a network of rivers or streams related to meteorological processes. From time to time, the area must undergo heavy rainfall events. Rainfall events seem to be quite rare in the low or mid-latitudes of Titan on the basis of the data gathered from the Cassini orbiter during its long mission in the Saturn System from 2004 to 2017. Rainfall events in the low or mid-latitudes must be closely related to seasonal factors. The image of Titan's soil acquired from the ground with the Huygens module suggests that the Huygens probe may have landed onto an ancient brook, stream or river. One can clearly notice pebbles or stones which are strongly eroded.
The radar views as well as the infrared or near-infrared views obtained from the Cassini spacecraft during its long mission in the Saturn System have clearly unveiled the remarkable diversity of the Opaque Moon. Titan has really familiar landscape features like mountains, hills, tectonic formations, large dune fields as well as rivers, lakes or seas. Curiously, the lakes, seas and rivers are mostly concentrated in the high latitudes or in the polar areas. The infrared or the near-infrared images as well as the radar images of Titan have clearly shown that the most humid area on the giant moon is found in the high latitudes of the northern hemisphere. The clouds, the seas, the lakes or the rivers identified from the Cassini orbiter demonstrate that there is a meteorological cycle on Titan like on Earth. However, the meteorological cycle of Titan is not based on water. Water can only appear in its frozen state on the surface of Titan due to the extremely low environmental temperature at the level of the surface. Water can form stones or pebbles on Titan. In fact, the meteorological cycle of Titan is based on methane or CH4. Methane can appear in its liquid form on Titan's surface.
A parallel can be drawn between the meteorological cycle of Titan and the meteorological cycle of the Earth. Bright clouds can form on Titan. Those clouds can be composed of methane (CH4) or ethane (C2H6). They can engender rainfall events like the cumulonimbus on Earth. Due to the relatively low gravity of Titan compared to the gravity of the Earth, droplets will tend to fall slower than on Earth. The mean density of the lakes, seas or rivers dominated by methane will tend to be lower than the mean density of the oceans, seas, lakes or rivers on Earth. The infrared or near-infrared views as well as the radar views acquired from the Cassini orbiter regarding the first pool identified on Titan have clearly shown that the surface of any pool of liquid on Titan can be remarkably flat. Does it mean that the liquid can be particularly viscous ? Ontario Lacus which is located in the south polar region of Saturn's largest moon and whose shape is reminiscent of a foot may look like a typical pond on our planet. Does Ontario Lacus look like the pond "Etang du Grec" in the area of Palavas-les-Flots in France for instance ? The seas or lakes found in the high latitudes of the northern hemisphere may be more dynamic than Ontario Lacus.
The infrared or near-infrared views obtained from the Cassini orbiter have clearly revealed the sharp contrast between relatively bright areas and relatively dark areas. The relatively dark areas found in the low or mid-latitudes tend to be dominated by linear and parallel dunes extending over long distances. Thoses dunes which are reminiscent of the Seif Dunes found in the Namib Desert on Earth may be rich in hydrocarbons or organics. A parallel could be drawn between Titan and Pluto. Pluto unveils dark or reddish areas as well as bright areas like Titan. The dark or reddish areas of Pluto may be rich in tholins. The nature of those dark or reddish areas on Pluto may be close to the nature of the dark or reddish areas of Titan. Titan is relatively rich in methane or hydrocarbons which is also the case for Pluto. However, lakes or seas of methane can't be encountered on the surface of Pluto because there is not the right combination of environmental temperature and atmospheric pressure at the level of the surface. However, there can be lakes or seas of nitrogen or methane beneath the icy crust of the Dwarf Planet. Titan may in fact be closer to Pluto or the Earth than Ganymede. We still have a lot to learn about the various worlds of the Solar System.
The image above represents Titan and Pluto at scale. The original view of Titan was obtained on February 26, 2006 from the Cassini spacecraft on the basis of the CB3 filter and of the CL2 filter. The original view of Pluto was acquired on July 13, 2015 from the New Horizons spacecraft. Artificial colors were incorporated into the disk of Titan and into the disk of Pluto. A contrast between bright areas and dark or brown areas can be observed on both worlds. Credit for the original view of Titan: NASA/JPL-Caltech/Space Science Institute. Credit for the original view of Pluto: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute. Montage credit: Marc Lafferre, 2021.
- To get further information on that news, go to: https://en.wikipedia.org/wiki/Titan_(moon) .