June 30, 2018 : New Laboratory Experiments Reveal The Potential Mechanisms In The Development Of Titan's Haze

A new study entitled  Evolution of Titan's high-altitude aerosols under ultraviolet irradiation  and proposed by a team involving Nathalie Carrasco, Sarah Tigrine, Lisseth Gavilan, Laurent Nahon and Murthy S. Gudipati shows the potential evolution in the chemistry of Titan's aerosols from high altitudes where they form to lower altitudes. The team of Nathalie Carrasco performed laboratory experiments to unveil the influence of solar vacuum-ultraviolet (VUV) irradiation on the chemistry of Titan's haze. The Cassini-Huygens mission has allowed us to determine that the thick brownish haze of Saturn's largest moon starts to develop at an altitude of about 1000 km. Under the action of UV light from the Sun, chemical reactions occur in that haze and complex organics or hydrocarbons can develop. Molecules which are heavy enough migrate down toward the surface. The soil of Titan may be rich in organics, hydrocarbons or tholins which are closely related to the complex haze of the Opaque Moon. The flow of aerosols toward the surface may be relatively progressive and slow.

During its atmospheric descent of the giant moon on January 14, 2005, the Huygens probe, proposed by the European Space Agency, had recorded key data regarding the chemical composition of the haze close to the ground at altitudes below 130 km. Planetologists want to analyze the potential photochemical evolution of the aerosols encountered in the haze or smog during their descent. The group of scientists resorted to infrared spectroscopy to analyze the influence of solar vacuum-ultraviolet light on the aerosol optical properties. The laboratory experiments allow us to study the atmospheric aerosol aging process. The outcome of the simulation reveals that there is a significant evolution in the chemistry of the aerosols. Can it account for the apparent contradiction between the data gathered by the Cassini spacecraft which had unveiled a relatively low level of nitrogen below an altitude of 600 km via spectroscopy in the deep atmosphere of the Orange Moon and the data gathered by the Aerosol Collector and Pyroliser of the Huygens probe at the surface of the giant moon ?

The Cassini-Huygens mission has clearly shown the remarkable complexity of Titan's atmosphere that is deep, thick and completely opaque from outer space. The atmospheric pressure on the surface of that enigmatic world is largely higher than that of the Earth at sea level. The Huygens probe had recorded an atmospheric pressure of 1,467 hPa on Titan's surface close to the equator. Thus, the atmospheric pressure on Titan's surface is about 1.5 times higher than that of the Earth at sea level. There are similarities between Earth's atmosphere and Titan's atmosphere in terms of composition. The atmosphere of both worlds is dominated by molecular nitrogen. The atmosphere of the Orange Moon is devoid of any oxygen as opposed to the atmosphere of our planet where the concentration of oxygen is around 21 percent of the overall composition. In fact, in the harsh environment of Titan where temperatures are around minus 180 degrees Celsius, water can't be present in its liquid form on the surface and carbon dioxide can't be present in the atmosphere. The atmosphere of Venus and the atmosphere of Mars are dominated by carbon dioxide but that's not the case in the Outer Solar System for the atmosphere of Titan, Triton or Pluto. To a certain extent, Titan looks like Venus from outer space but the composition of its atmosphere is radically different from that of the atmosphere of Venus.

The atmosphere of Titan is reminiscent of the atmosphere of the Earth because there are meteorological processes involving evaporation processes, condensation processes with cloud formation and precipitation processes on both worlds. Methane appears to be the second most abundant gas in Titan's atmosphere and this hydrocarbon plays a key role in the meteorological cycle of the Opaque Moon. The meteorological cycle of the Earth involves water whereas the meteorological cycle of Titan involves methane. There are seasons on Titan because the rotation axis of the giant moon of Saturn is inclined by about 27 degrees relative to the normal of its orbital plane around the Sun. One can notice that the obliquity of Titan is higher than that of the Earth. Moreover, the orbit of Titan around the Sun is elliptical which implies that the distance between the Opaque Moon and the Sun can strongly vary during a Titanian year which lasts almost 30 Terrestrial years. The Cassini-Huygens spacecraft entered the Saturn System in 2004 during the Summer season in the southern hemisphere of Titan and Saturn or during the Winter season in the northern hemisphere of Titan and Saturn. The study of the atmospheric dynamics of Titan is not obvious since each season on Titan lasts approximately 7 Terrestrial years.

During the long mission of the Cassini spacecraft which started in mid-2004 and ended in the second half of 2017, we have been in a position to determine that Titan's atmosphere is really complex with a meteorology that is closely related to seasonal factors. Infrared or near-infrared views of the high latitudes of the southern hemisphere experiencing the Summer season at the beginning of the Cassini-Huygens mission have clearly revealed a multitude of dynamic clouds. On the basis of those observations, one could imagine that rainfall must occur from time to time in the area. And the first pool of liquids identified on Titan was Ontario Lacus, a kidney-shaped lake located in the south polar region of the Opaque Moon and clearly observed in the infrared or near-infrared spectrum as soon as the year 2005. We had imagined that the dark areas found at low latitudes on Titan likely represented seas of methane or ethane but the aerial views from the Huygens probe as well as the radar views obtained from the Cassini spacecraft have clearly shown that it is not the case. In fact, the dark areas contrasting with bright areas at low latitudes are dominated by linear and parallel dunes extending over long distances under the influence of prevailing winds. Clouds are quite rare at low or mid-latitudes paradoxically.

A giant ethane cloud engulfing the north polar region had been observed during the Winter season in the northern hemisphere. In parallel, radar views acquired from the Radar Mapper of the Cassini spacecraft during the Winter season in the northern hemisphere have clearly revealed the presence of a myriad of lakes, seas and rivers in the high latitudes of the northern hemisphere. Kraken Mare, Ligeia Mare and Punga Mare are the major pools of liquids found in the polar area of the northern hemisphere. Therefore, the humid areas on Titan appear at high latitudes at the present time. How can we explain that surprising distribution of liquids on the globe of Titan ? Is it related to the fact that methane is extremely volatile and extremely sensitive to the level of solar radiation ? Is it related to internal sources or is it exclusively related to meteorological phenomena ? Is the south polar region more humid during the Winter season due to a higher level of condensation in the area ? A polar vortex developing at a high altitude over the south polar region was observed in 2012 revealing that the atmosphere of Titan is far from being monotonous.

When the polar vortex evolving above the south polar region was observed in 2012, the area was experiencing the Autumn season. Some researchers wondered whether the cyclone would continue to grow significantly due to a progressive decrease in the level of solar radiation reaching the south polar region. The start of the Winter season in the southern hemisphere with the Solstice occurred in 2017. Some researchers anticipate that the level and the size of Ontario Lacus will progressively grow as the Winter season develops in the southern hemisphere. Will there be the same kind of cyclone over the south polar region as the cyclone observed earlier over the north polar region ? The complex atmosphere of Saturn's largest moon has unveiled many surprises during the Cassini mission and the atmosphere has only been studied during a fraction of a complete Titanian year. That's why we can only speculate on that kind of phenomenon. Like in the atmosphere of the Earth, there are several layers of gas in the atmosphere of Titan and complex interactions between UV light from the Sun and molecules or ions present in the upper atmosphere occur.

Planetologists have determined that the concentration of methane is close to 2 percent in the Stratosphere of the Opaque Moon between the altitude of 40 km and the altitude of 320 km. Most of the mass of the organic haze or smog is located in the Stratosphere. The UVIS spectrometer of the Cassini spacecraft has allowed us to determine that the aerosols are generated in the Ionosphere, a portion of the Thermosphere where positive and negative ions take shape thanks to ultraviolet light from the Sun and electrons from the magnetosphere of the Gas Giant Saturn. The large negative ions turn out to be the embryos of the aerosols. The ionized molecules tend to aggregate to form bigger molecules which engender the haze we observe today. If the molecules produced in the haze become heavy enough, they can fall to the surface to produce a dark or red sludge known as tholin. Remarkably complex organics or hydrocarbons have already been found in the haze of Titan's atmosphere. Thanks to data acquired from the Cassini spacecraft, we know, now, that the embryos of the aerosols are found at an altitude of about 1000 km. Those aerosols go down to an altitude of about 600 km, in the Thermosphere, via photochemistry related to UV light from the Sun. The experiments clearly show that Titan's haze is clearly a photochemical soup which can engender remarkably complex molecules.

The image above shows the disk of Saturn's largest moon Titan as well as a dark hood or atmospheric banding evolving above the north polar region. The image was obtained from the Wide-Angle Camera of the Cassini spacecraft with a spectral filter sensitive to wavelengths of visible violet radiation on June 21, 2010. One can notice that the atmosphere appears slightly darker in the northern hemisphere up to the dark polar cap than in the southern hemisphere. The northern hemisphere was experiencing the start of the Spring season and the southern hemisphere was experiencing the start of the Autumn season at the time of the observation. The haze of the Orange Moon makes the atmosphere completely opaque from outer space. Image Credit: NASA/JPL/Space Science Institute.

- To get further information on that news, go to: https://arxiv.org/abs/1806.09160.



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