January 24, 2019 : A New Study Suggests That The Presence Of A Thick Atmosphere On Titan May Be Intimately Related To The Heating Of Organic Molecules In The Interior Of The Giant Moon

A new research work entitled  Contributions from Accreted Organics to Titan's Atmosphere: New Insights from Cometary and Chondritic Data , released online in The Astrophysical Journal on January 22, 2019 and led by the Doctor Kelly Miller who is a research scientist in SwRI's Space Science and Engineering Division, reveals that the presence of a thick atmosphere on Saturn's largest moon may be closely related to the heating of organics in the interior of the Hazy Moon. Researchers are really fascinated by the thick, dense or deep atmosphere of Titan because most moons in the Solar System are devoid of any significant atmosphere. The other moons of Saturn are devoid of any significant atmosphere and are generally heavily cratered. The gravity of Titan is much weaker than that of the Earth but the air on the surface of the Opaque Moon is much denser than that of the Earth at sea level. In other words, the atmospheric pressure on the surface of Titan is much higher than that of the Earth at sea level. The Titanian atmosphere appears to be largely dominated by molecular nitrogen today.

The team of Kelly Miller advances that there may be an outgassing process involving organics from Titan's interior which fuels, sustains or maintains Titan's atmosphere. The secrets of Titan's atmosphere may be found in the heating or  cooking  of organic material inside the giant moon of the Gas Giant Saturn. During the Cassini mission in the Saturn System, we have had the opportunity to realize that Titan is rich in hydrocarbons or organics. The concentration of methane in Titan's atmosphere can reach about 5 percent at the level of the surface. There are lakes, seas and rivers of hydrocarbons, dominated by methane and ethane, in the polar areas or in the high latitudes of the Opaque Moon. Kelly Miller pointed out :  Titan is a very interesting moon because it has this very thick atmosphere, which makes it unique among moons in our solar system.  She added :  It is also the only body in the solar system, other than Earth, that has large quantities of liquid on the surface. Titan, however, has liquid hydrocarbons instead of water. A lot of organic chemistry is no doubt happening on Titan, so it's an undeniable source of curiosity.  Due to the harsh environment, water can only appear in its solid form on the surface of Titan.

The atmosphere of Titan is opaque like the atmosphere of Venus and unlike the atmosphere of the Earth, Mars, Triton or Pluto. One can see the surface of the Earth in the visible spectrum from outer space but our clouds of water or ice generally appear opaque. The haze of the Orange Moon makes the atmosphere of Saturn's largest moon completely opaque from outer space in the visible spectrum. Solar radiations play a key role in the chemistry of that exotic haze rich in hydrocarbons or organics. UV light from the Sun engenders complex interactions between molecules, particles or ions in the upper atmosphere of Titan. The atmospheric pressure on the surface of the Opaque Moon is close to 1.5 Bar which is remarkably high compared to the atmospheric pressure on the surface of the Earth at sea level. And the atmospheric pressure on the surface of Mars appears ridiculous compared to the atmospheric pressure on the surface of the Earth at sea level. Ganymede which is the largest moon of the Gas Giant Jupiter and which is the largest moon in the Solar System is devoid of any significant atmosphere for instance.

Researchers are intrigued regarding the origin, the stability and the dynamics of Titan's atmosphere. Kelly Miller pointed out :  Because Titan is the only moon in our solar system with a substantial atmosphere, scientists have wondered for a long time what its source was.  Researchers regularly put forward the role of comets in the development or in the formation of Titan's atmosphere because comets tend to be rich in ammonia (NH3) which can engender nitrogen or molecular nitrogen via photochemistry or via impact events in particular. Kelly Miller argued :  The main theory has been that ammonia ice from comets was converted, by impacts or photochemistry, into nitrogen to form Titan's atmosphere. While that may still be an important process, it neglects the effects of what we now know is a very substantial portion of comets: complex organic material.  Comets may have brought a lot of organics, hydrocarbons or even prebiotic molecules to the giant moon of the Ringed Planet Saturn.

The Huygens probe which performed a parachuted descent of Titan's atmosphere and which performed a soft landing on that intriguing moon on January 14, 2005, has collected key data regarding the composition, the structure or the altitude profile of the atmosphere of that exotic world. During the landing process, a relatively significant concentration of methane was identified from the Huygens probe. The data gathered from the probe clearly demonstrated that the environment at the level of the soil is rich in methane, hydrocarbons or organics. Planetologists were surprised to observe relatively high concentrations of methane in Titan's atmosphere because methane tends to disappear over time or to react quickly, in astrophysical terms, to form new molecules or organics which are likely to gradually fall to the surface. If the concentration of methane in Titan's atmosphere remains, roughly, at the same level over geological time scales, there may be internal sources, geysers or cryovolcanoes spewing organics, methane or other hydrocarbons which regularly fuel the atmosphere.

Thanks to the radar data obtained with the Radar Mapper of the Cassini orbiter during the Cassini mission in the Saturn System from 2004 to 2017, we have clearly demonstrated that the humid areas or that the land of lakes, seas and rivers appear in the high latitudes of the Opaque Moon. The dark areas of the low latitudes tend to be dominated by Seif dunes which may be rich in hydrocarbons or organics and which may be closely related to the complex haze of Titan. That haze represents a soup of molecules where various interactions occur to engender new molecules which can become heavy enough or complex enough to fall to the surface. The haze of Titan's atmosphere may engender an exotic type of sand over time. That sand is likely to generate Seif dunes or linear and parallel dunes extending over long distances under the influence of prevailing winds. It is too soon to say that Titan's atmosphere is in a stable state with a level or a concentration of methane and nitrogen which remains, roughly, the same over time.

In fact, Titan may be in an exceptional period regarding its atmosphere if that atmosphere is closely related to an increase in the level of its presumed cryovolcanism or if the giant moon has recently undergone a significant impact event stimulating its internal activity. Kelly Miller and her team based their analysis on the data obtained from the Rosetta spacecraft, a probe proposed by the European Space Agency in collaboration with NASA, which had studied the distant comet known as 67P/Churyumov-Gerasimenko. Researchers had determined that the composition of the irregular comet was about 50 percent ice, about 25 percent rock and about 25 percent organic material. They were surprised to find such a concentration of organics. Kelly Miller pointed out :  Comets and primitive bodies in the outer solar system are really interesting because they're thought to be leftover building blocks of the solar system.  She added :  Those small bodies could be incorporated into larger bodies, like Titan, and the dense, organic-rich rocky material could be found in its core.  Some planetologists also suspect the presence of an internal ocean of hydrocarbons beneath its icy crust. That subsurface ocean could regularly fuel the Titanian atmosphere via cracks or fractures for instance.

In her research work regarding the origin and the nature of Titan's atmosphere, Kelly Miller combined the data available regarding the organic material located in meteorites with previous thermal models of the interior of the giant moon in order to determine how much gaseous material could be generated and to determine whether that process could account for the characteristics of the Titanian atmosphere at the present time. By following the principle according to which  If you cook something, it will produce gases,  Kelly Miller came to the conclusion that about half of the nitrogen atmosphere, and potentially the whole amount of methane, could be the outcome of the  cooking  of these hydrocarbons or organics that have been transported to Titan via comets or meteorites at the very beginning of Titan's geological history. Like our oceans of liquid water which may be intimately linked to the rain of comets and asteroids at the beginning of the geological history of our planet, nitrogen and methane may also have been brought by comets and meteorites in the formation process of the Opaque Moon. A better understanding upon the origin, the degree of stability and the dynamics of Titan's atmosphere is likely to help us better understand the origin, the degree of stability and the dynamics of our atmosphere over geological time scales.

The image above reveals a portion of the Gas Giant Saturn as well as the disk of its largest moon Titan approaching the Gas Giant. One can notice in particular the sharp contrast between the color of Titan's atmosphere, dominated by nitrogen and methane, and the color of Saturn's atmosphere, dominated by hydrogen and helium. This natural color view was generated on the basis of several images taken on January 29, 2008 with the Narrow-Angle Camera of the Cassini spacecraft using red, green and blue spectral filters. The views were captured at a distance of about 2.3 million kilometers or 1.4 million miles from the Opaque Moon. Planetologists try to determine whether the presence of the massive atmosphere of Titan might be related to the release of gases from the interior of the giant moon where heating processes may be encountered. Image credit: NASA/JPL/Space Science Institute.

- To get further information on that news, go to: https://www.swri.org/press-release/origin-nitrogen-atmosphere-titan-saturn-moon and https://iopscience.iop.org/article/10.3847/1538-4357/aaf561.



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