September 14, 2020 : The Atmosphere Of Titan And The Atmosphere Of Saturn Represent Key Atmospheres To Study In Our Solar System
Most worlds in the Solar System are devoid of any atmosphere because they don't have the right combination of gravity and environmental temperature. A world like Mercury is not heavy enough at its distance from the Sun to retain a significant atmosphere for instance. At the level of Mercury, the level of solar energy received by Mercury's surface appears too high for the presence of a stable atmosphere. Venus, the Earth and Mars which evolve farther from the Sun and which are bigger than Mercury contain an atmosphere whose weight is very different. Beyond the Asteroid Belt, there are the four Gas Giants which are dominated by an extremely deep and thick atmosphere. Those worlds which evolve in a particularly harsh environment are heavy enough to retain a significant atmosphere. Jupiter, the largest planet in the Solar System, is well known for its numerous hurricanes or vortices. Saturn is well known for its rings and for its hexagonal structure in the north polar region. Uranus is considered an Ice Giant because it is an extremely cold world that tends to appear featureless in the visible spectrum. Neptune represents another Ice Giant and another blue planet where giant clouds can be found and where winds blow at an extremely high speed.
In the Outer Solar System, beyond the Asteroid Belt, several rocky or icy worlds contain an atmosphere. That's the case for Titan, the largest moon of Saturn. The surface of Titan has long represented a mystery because the atmosphere is completely opaque from outer space in the visible spectrum. The Voyager 1 spacecraft and the Voyager 2 spacecraft had captured images of the hazy world but we had not been in a position to discern surface features. The Cassini-Huygens mission has clearly revolutionized our level of understanding of the Opaque Moon. Triton, the largest moon of Neptune, also contains an atmosphere but that gas layer is very thin compared to the atmosphere of the Earth or Mars. To a certain extent, the atmosphere of Triton can be compared to the atmosphere of the Dwarf Planet Pluto which unveils several atmospheric layers. The atmosphere can have a strong impact on the landscape due to erosional processes related to winds, snowfall or rainfall events. Dunes can take shape for instance. That's the case on Venus, on Earth, on Mars, on Titan or on Pluto. Clouds imply precipitation phenomena that can fuel rivers, lakes or seas. That's the case on Earth and on Titan.
The study of other atmospheres in the Solar System is likely to bring significant clues regarding the dynamics of our atmosphere, the evolution of our atmosphere or even the future of our atmosphere. Our atmosphere is unique and fundamental for the balance of our biosphere. There is liquid water on the surface because there are the right combinations of atmospheric pressure and environmental temperature at sea level. If the atmospheric pressure increases, the level of greenhouse effects is likely to rise and to engender higher levels of evaporation. If there are more clouds, the environmental temperature is likely to increase. Venus whose size is comparable to that of the Earth may have resembled the Earth in an ancient past but today that planet which evolves closer to the Sun is a hell due to the huge greenhouse effects encountered in the environment of that bright planet. Water vapor, methane and carbon dioxide are known to represent strong greenhouse gases. Their concentration in any atmosphere must not exceed a particular limit. Otherwise, the stability or the balance of the atmosphere can be broken. That's probably why Venus may have become the hell the planetary body represents today.
The atmosphere of Venus is dominated by carbon dioxide like the atmosphere of Mars. Thus, greenhouse effects are observed on both worlds but the greenhouse effects are obviously much stronger on Venus where the atmospheric pressure is about 91 times higher than that of our planet at sea level. Mars doesn't have the right combination of atmospheric pressure and environmental temperature for the stable presence of liquid water on the surface. That's also the case for Venus where the environmental temperature at the level of the surface is much too high for the presence of liquid water or water ice on the surface at any place on the globe. The polar regions of Mars contain frozen carbon dioxide as well as water ice. Can there be pockets of liquid water beneath those ice blankets ? On Venus, the environmental temperature at the level of the surface is around 464 degrees Celsius so that lead can appear in its liquid form. The clouds of Venus are dominated by sulfuric acid. They can produce rain but that rain rapidly vaporizes at a lower altitude due to the extreme environmental temperatures related to greenhouse effect phenomena.
The numerous giant volcanoes of Venus may regularly fuel the atmosphere. That's probably why the clouds of that exotic world are relatively rich in sulfur. Our own atmosphere is unique because it contains oxygen and a very limited concentration of carbon dioxide. The atmosphere of the Earth is dominated by molecular nitrogen like the atmosphere of Titan or Pluto. A parallel can be drawn between the meteorological cycle of the Earth and the meteorological cycle of Titan. Oxygen that is closely related to our biosphere is absent or almost absent in the atmosphere of Titan. However, the atmosphere of Titan contains a relatively significant concentration of methane that can be compared to water vapor or water ice in the atmosphere of our planet. In fact, a parallel can be drawn between the water cycle of the Earth and the methane cycle of Titan. On the Opaque Moon, methane can produce clouds via evaporation processes and condensation processes. From time to time, the clouds of methane or ethane can generate rainfall events like the clouds of water vapor or water ice on Earth. During the Cassini-Huygens mission in the Saturn System, from 2004 to 2017, the Cassini orbiter and the Huygens probe had clearly revealed the presence of lakes, seas or rivers on the surface of Titan.
In the harsh environment of Titan where the atmospheric pressure on the surface is much higher than that of the Earth at sea level, the ambient temperature is sufficiently low to allow the presence of liquid methane, liquid ethane or liquid propane on the surface. Curiously, the stable pools of liquids appear in the high latitudes of the giant moon. The first stable extraterrestrial lake or sea ever identified was found in the high latitudes of Titan's southern hemisphere. Ontario Lacus, a pool whose shape looks like the shape of a foot, was identified in infrared or near-infrared data obtained from the Cassini orbiter during its orbital dance in the Saturn System. Later, radar data and infrared or near-infrared data acquired from the Cassini spacecraft revealed that the north polar region appears to be the most humid area on Titan. The area unveils three major pools of liquids, Kraken Mare, Ligeia Mare and Punga Mare. Many drainage channels have been found in the high latitudes of the northern hemisphere as well. Clouds tend to be mostly concentrated in the polar regions or in the high latitudes of the Hazy Moon.
Planetologists try to better understand the meteorological cycle of Titan in the prospect of a better understanding of our own meteorology. The distribution of clouds, lakes, seas and rivers is likely to tell us a lot regarding the climate or the meteorological cycle of Saturn's largest moon. Seasonal factors must play a key role on Titan. During the Winter season in the southern hemisphere, one can assume or imagine that condensation processes are higher than evaporation processes so that the lakes, seas or rivers unveil a higher level or a higher size. On the other hand, during the Summer season in the southern hemisphere, the evaporation processes may be higher than the condensation processes and the level or the size of the lakes, seas or rivers may decrease. In the regions found at low or mid-latitudes, strong rainfall events related to seasonal factors may take shape from time to time during the long Titan year or Saturn year which represents almost 30 Earth years. In the dark or brown plain where the Huygens probe had landed, the surface can become saturated with liquid methane when there are strong rainfall events comparable to monsoon events on Earth. Thus, the dark areas found at low or mid-latitudes can become transient seas or even oceans from time to time.
From outer space, Titan looks like a Gas Giant because its atmosphere is completely opaque and almost uniform like the disk of Uranus. However, the four Gas Giants follow another physical logic because their atmosphere is extremely deep. The exact nature of their internal structure is still unknown but the models help us anticipate what we can find below the upper atmosphere of those giant planets. Saturn and Jupiter may contain a subsurface ocean of metallic hydrogen. Those worlds unveil in their upper layers remarkable vortices, cyclones or hurricanes. The north polar region of Saturn is unique due to the presence of a hexagon. Planetologists try to understand the mechanisms that lead to that type of structure. The speed of the winds depends on the latitude and at lower latitudes, they blow at a slower speed. When air masses collide with each other, they can engender vortices or hurricanes. Those vortices or hurricanes can influence the path of the jet streams above those vortices or hurricanes. That type of configuration is likely to produce the type of hexagon identified in the north polar region of the Ringed Planet Saturn. The presence of an atmosphere is likely to boost the development of any lifeform on the surface if there are solvents on the surface like liquid water, liquid methane or liquid nitrogen.
The image above reveals Saturn and Titan at scale. The original view of Saturn was generated on the basis of data obtained on April 25, 2016 with the Wide-Angle Camera of the Cassini spacecraft. The original view of Titan was produced on the basis of data acquired on January 30, 2012 with the Wide-Angle Camera of the Cassini spacecraft. The atmosphere of Saturn is remarkably dynamic with multiple vortices. The atmosphere of Titan reveals a captivating photochemical haze rich in hydrocarbons or organics. Credit for the original view of Saturn: NASA/JPL-Caltech/Space Science Institute. Credit for the original view of Titan: NASA/JPL-Caltech/Space Science Institute. Credit for the montage: Marc Lafferre, 2020.
- To get further information on that news, go to: https://www.space.com/what-makes-earths-atmosphere-special.html and https://en.wikipedia.org/wiki/Atmosphere_of_Titan.