December 31, 2021: What Are The Fundamental Differences Between Titan And The Earth ?

During the formation process of the Solar System, several planetary bodies have taken shape around our star The Sun. One can imagine a multitude of circumstellar disks from which the planets or moons we know today have progressively taken shape via accretion or aggregation processes. The Proto-Earth was probably devoid of any moon. The "Blue Planet" we know today is likely the outcome of a collision between the Proto-Earth and a planetary body around the size of Mars. The collision engendered the Moon and the Earth. The Moon is probably composed of a mixture of the planetary body that collided with the Proto-Earth and of the crust of the Proto-Earth. The core of the Moon is in fact relatively small compared to the core of the Earth. Heavier materials or elements tend to be found within the core. The core of the Earth is likely dominated by iron and nickel. The composition of any planetary body in the Solar System is closely linked to the distance or to the level of energy received from the Sun. In the Inner Solar System, the planets appear to be rocky worlds. In the Outer Solar System, the planets represent Gas Giants and tend to be dominated by hydrogen and helium. Their moons like Europa, Titan or Enceladus tend to be icy or rocky worlds.

Around Jupiter, the largest planet in the Solar System, there are four major moons which appear relatively different from each other. Io is the most volcanically active world in the Solar System and looks like a pizza to a certain extent. Europa appears remarkably icy with a multitude of fractures. Some researchers advance that Europa may contain a subsurface ocean dominated by liquid water. Ganymede which is the largest moon in the Solar System appears quite different from Io and Europa. Ganymede looks like Europa to a certain extent because that world also contains a multitude of fractures but the surface appears less young or less dynamic at first sight. Callisto appears older than Ganymede because that moon is heavily cratered. There are also several major moons around the Ringed Planet Saturn which is the second largest Gas Giant in the Solar System. One can mention Tethys, Dione, Rhea, Titan and Iapetus. Those moons may have taken shape during the formation process of Saturn. From Tethys, the closest moon to Saturn among those worlds, to Titan, the largest moon among those worlds, the size and the mass of the moon appear higher as we go away from Saturn up to the Orange Moon.

Iapetus which is located much further from Saturn than Titan appears much smaller than the Opaque Moon. That particular configuration demonstrates that those moons have likely taken shape during the formation process of Saturn in the multitude of circumplanetary disks. The system of Saturn is often regarded as a miniature Solar System. The current rings of Saturn are probably the outcome of the presence of an ancient moon which moved too close to Saturn and disintegrated due to the gravitational influence of the Gas Giant in particular. The rings of Saturn tend to be dominated by dust and water ice. Most moons of Saturn appear relatively uniform and bright and are heavily cratered. However, a moon like Enceladus unveils a particularly young surface in its south polar region in particular. The tiny moon appears remarkably round in spite of its relatively small mass or size. Enceladus is remarkably bright and contains fractures in its south polar region where geysers or cryovolcanoes have been clearly identified. The surface of Enceladus appears to be dominated by water ice and the geysers or cryovolcanoes of that moon tend to spew a mixture of water ice, hydrocarbons or organics.

Planetologists believe that there may be a subsurface ocean dominated by liquid water beneath the external crust of Enceladus. Can there be a lifeform based on liquid water and involving chemosynthesis in that hypothetical subsurface ocean ? The geysers or cryovolcanoes of Enceladus can potentially spew complex hydrocarbons, organics or even archaea or bacteria. Those complex molecules or extremophiles could be encountered by future in-situ probes in the fractures found in the south polar region of Enceladus. We would not have to dig deep to find clues regarding a potential biosphere or ecosystem inside the external crust of that tiny world evolving relatively close to Saturn. In fact, several moons of Saturn may contain a subsurface ocean or subsurface pockets dominated by liquid water. The external crust of worlds like Tethys, Dione, Rhea or Titan may evolve above a relatively deep layer rich in liquid water. However, Enceladus is the only world evolving around the Ringed Planet where geysers or active cryovolcanoes have been clearly identified on the Surface. The major fractures of Enceladus demonstrate that the external crust of that world is relatively unstable.

The exploration of the Solar System has revealed that several worlds located in the Outer Solar System may contain a subsurface ocean or a layer dominated by liquid water beneath the external crust. That's potentially the case for Europa or Ganymede, for the major moons of Saturn, for the tiny moon Enceladus, for Titania or Oberon around the Gas Giant or Ice Giant Uranus, for Triton around the Gas Giant or Ice Giant Neptune or for the Dwarf Planet Pluto. Our planet also contains several layers with a relatively viscous layer beneath the external crust dominated by silicate rocks. The composition of the external crust of the planetary bodies located in the Inner Solar System appears to be fundamentally different from the composition of the major moons, the Dwarf Planets or the planets located in the Outer Solar System. The upper layer of the four Gas Giants is dominated by hydrogen and helium like the Sun. The mean density of the planetary bodies found in the Outer Solar System appears lower than the mean density of the planetary bodies located in the Inner Solar System which implies the presence of lighter elements in the external crust of the worlds found in the Outer Solar System.

So far, we have only sent probes to the surface of the Moon, Venus, Mars and Titan. That's why our knowledge regarding the composition of the soil of the various worlds of the Solar System is relatively limited. Venus contains a huge amount of major volcanoes. That's why the composition of the surface of Venus is generally closely related to the Venusian volcanism. Water can't appear in its liquid form on the surface of Venus due to the extremely high environmental temperature. Thus, the surface of Venus is expected to be extremely dry. The crust of Venus may be remarkably stable over time because the typical plate tectonics we know on Earth may be absent on Venus today. Let's point out however that nothing is sure regarding the properties of the Venusian crust. Some researchers have revealed clues that Venus may still be volcanically active today. Our moon, The Moon, may have been volcanically active several billion years ago. The relatively dark areas of the disk of the Moon that one can observe from the Blue Planet represent ancient volcanic areas. Mars appears volcanically inactive today but the Red Planet has been volcanically active long ago. That planet contains the highest volcano in the Solar System.

We have not found clear signs of cryovolcanism on Titan yet but the crust of Saturn's largest moon may be relatively unstable because the Orange Moon contains a significant atmosphere rich in molecular nitrogen and methane. Methane is expected to disappear over time. The relatively significant presence of methane in Titan's atmosphere may be explained by the presence of a liquid layer of methane or by subsurface reservoirs of liquid methane beneath the external crust of that enigmatic world. The surface or the soil of Titan may be relatively rich in water ice, in hydrocarbons like methane, ethane, acetylene and benzene or in organics. The global haze of that giant moon can produce relatively complex hydrocarbons or organics that will tend to fall to the surface to form a brown material called tholin. The molecules falling from that haze can engender dunes. That's what the radar views obtained from the Cassini orbiter during its long mission in the Saturn System from 2004 to 2017 have shown. The relatively dark areas found in the low or mid-latitudes of Saturn's largest moon tend to be dominated by Seif Dunes or linear and parallel dunes extending over long distances.

Titan looks like the Earth to a certain extent because that world contains a relatively significant atmosphere where clouds can take shape and engender rainfall events. The Cassini orbiter has clearly revealed the presence of lakes, seas or rivers in the high latitudes of Titan. The north polar region appears to be the most humid area of that intriguing world. The lakes, seas or rivers must be dominated by liquid methane or liquid ethane. The pools or the rivers found on the surface of Titan must contain a mixture of methane, ethane and dissolved nitrogen. In the harsh environment of Saturn's largest moon, molecules like methane, ethane and propane can appear in their liquid form on the surface. Are there internal sources to the major pools found in the high latitudes of the northern hemisphere of Titan ? Meteorology plays a key role in the high latitudes of each hemisphere but there may also be a network of liquid methane or liquid ethane between the surface and the interior of the giant moon. In the harsh environment at the level of Saturn where the level of energy received from the Sun is much lower than that received at the level of the Earth, the elements mobilized or interacting at the level of the surface of Titan clearly appear to be lighter elements than the elements mobilized at the level of the surface of the Earth.

The image above reveals Titan and the Earth at scale. The original view of Titan represents a raw view whose file name is N00253709.jpg and which was obtained on December 30, 2015 from the Cassini spacecraft on the basis of the BL1 filter and of the CL2 filter. The original view of the Earth was acquired from the Deep Space Climate Observatory (DSCOVR) on November 22, 2021. Titan appears completely opaque from outer space in the visible spectrum. In this view, the colors of Titan are artificial. Credit for the original view of Titan: NASA/JPL-Caltech/Space Science Institute. Credit for the original view of the Earth: DSCOVR/EPIC. Montage credit: Marc Lafferre, 2021.

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