January 17, 2019 : Researchers Identify Clues Or Evidence Of Rainfall And Changing Seasons In The North Polar Region Of Titan

A new study entitled  Observational evidence for summer rainfall at Titan's north pole , accepted for a release in Geophysical Research Letters, a journal of the American Geophysical Union, and produced by a team of researchers led by Rajani Dhingra, a doctoral student in physics at the University of Idaho in Moscow, reveals clues or evidence of rainfall and changing seasons in the north polar region of the Opaque Moon Titan, on the basis of data acquired from the Cassini orbiter. The planetologists based their analyses on a particular image obtained with the Visual and Infrared Mapping Spectrometer (VIMS) of the Cassini spacecraft which can see in the infrared or near-infrared spectrum. Surface features on Titan can't be observed in the visible spectrum from outer space but with an infrared eye or a near-infrared eye, one can see through the hazy atmosphere of Titan and one can discern surface features such as lakes, seas, volcanoes, mountains or craters.

The team of Rajani Dhingra clearly identified a reflective feature close to the north pole of Saturn's largest moon in a view acquired on June 7, 2016 with the Visual and Infrared Mapping Spectrometer of the Cassini orbiter. The reflective feature could be identified in the infrared or near-infrared spectrum, in a wavelength which allows researchers and the general public to discern landscape features and which brings an idea regarding the composition or the dynamics of the surface. The reflective feature represented, in surface area, about 46,332 square miles which represents, in surface area, approximately 119,999 square kilometers. That is the equivalent of the surface area of a square whose side is about 215 miles long or 346 kilometers long. In fact, the area of the reflective surface represents, roughly, half the size of the Great Lakes. That reflective feature didn't appear to be a permanent feature since it had not been observed in previous images of the area. Moreover, subsequent views of the region, taken from the Cassini spacecraft, didn't show the reflective feature anymore. As a result, that bright or reflective feature appeared to be transient.

Rajani D. Dhingra, who collaborated with several scientists like Jason W. Barnes, Jason E. Perry and Elizabeth P. Turtle to produce the research work and to find the clues regarding rainfall in the north polar region of the giant moon, pointed out :  The whole Titan community has been looking forward to seeing clouds and rains on Titan's north pole, indicating the start of the northern summer, but despite what the climate models had predicted, we weren't even seeing any clouds.  She added :  People called it the curious case of missing clouds.  Thanks to the long mission of the Cassini orbiter in the Saturn System, from the year 2004 to the year 2017, we have been in a position to monitor and to study the evolution of Titan's meteorology on each hemisphere of that smoggy moon. When the Cassini orbiter and the Huygens lander reached the Saturn System in mid-2004, the southern hemisphere of Saturn's largest moon was experiencing the Summer season whereas the northern hemisphere was experiencing the Winter season. The mission of the Cassini orbiter ended in the second half of 2017 in a completely different configuration, at the start of the Summer season in the northern hemisphere and at the start of the Winter season in the southern hemisphere.

During the Summer season in the southern hemisphere, in the first half of the Cassini mission, researchers had clearly identified dynamic and transient cloud systems above the south polar region. Ontario Lacus, the famous lake or sea located in the high latitudes of the southern hemisphere of that  orange ball , was the first stable pool of liquids identified on Saturn's largest moon and in the Solar System beyond the Earth. The lakes, seas or rivers are mainly composed of relatively light hydrocarbons. Methane, ethane and propane can appear in their liquid form on the surface of Titan. Methane and ethane can form clouds via evaporation and condensation processes. Those processes can be compared to the water cycle of the Blue Planet which implies evaporation processes, condensation processes and precipitation processes. Water can only appear in its solid form on the surface of Titan. The pebbles, the stones or the rocks of the Opaque Moon may be rich in water. One can imagine pebbles, rocks or stones of water ice in the harsh environment of Titan where the ambient temperature is around minus 180 degrees Celsius, minus 292 degrees Fahrenheit or 93 Kelvin.

Thanks to the observations of the south polar region of Saturn's largest moon during the Summer season in the southern hemisphere, researchers, astronomers or planetologists have been in a position to determine that there is a methane cycle between the surface or the soil and the deep and thick atmosphere that is relatively rich in methane. Like the atmosphere of the Earth, the atmosphere of Titan is dominated by molecular nitrogen. However, the second most abundant gas of Titan's atmosphere is methane as opposed to the second most abundant gas of our atmosphere that is oxygen. Oxygen is absent or almost absent in Titan's atmosphere. A Titanian year is very long since it lasts almost 30 Terrestrial years and each season on Titan lasts approximately 7 Earth years. That's why researchers have had to wait for a particularly long time to study the meteorology of the north polar region of Titan. The Winter season in the northern hemisphere ended in 2009 and the Summer season in the northern hemisphere started in 2017.

The image of June 7, 2016, unveiling the north polar region of Titan in particular, revealed the north polar region at the end of the Spring season in the northern hemisphere. Rajani Dhingra and her collaborators believe that the ephemeral reflective feature observed in the high latitudes of the northern hemisphere is likely the outcome of solar radiations reflecting off a wet soil or surface. The analysis suggests that the reflection is closely related to a methane rainfall event, likely followed by a period of evaporation. Rajani Dhingra pointed out :  It's like looking at a sunlit wet sidewalk.  The bright or reflective surfaces or the specular reflections may correspond to the signature or the first clues of Summer rainfall in the high latitudes of Titan's northern hemisphere. Let's point out, however, that the area was still in the Spring season at the time of the observation but the Summer season in the northern hemisphere took shape a Terrestrial year later with the Solstice. A Terrestrial year on Titan is clearly a small portion of a Titanian season since a Titanian season lasts no less than 7 Terrestrial years.

During the Cassini/Huygens mission, planetologists have gathered a large amount of data or clues regarding the dynamics of Titan's meteorology. During the transition between the Winter season in the northern hemisphere and the Spring season in the northern hemisphere, we've seen large cloud systems taking shape and evolving at low or mid-latitudes on Titan. Those elongated clouds represented transient clouds. Researchers were in a position to determine that they had produced rainfall on the basis of several images of the same area which showed changes in appearance or reflectivity. The presence of clouds in the region of Ontario Lacus in the high latitudes of the southern hemisphere during the Summer season in the southern hemisphere clearly implied the presence of evaporation and condensation processes. As a result, one could guess that there are rainfall events, monsoon events or storms from time to time in the area.

Climate models of the Opaque Moon predicted a similar configuration with storms or rainfall in the northern hemisphere in the years leading up to the Solstice of 2017 which represented the end of the Spring season and the beginning of the Summer season in the northern hemisphere. The radar views of Titan, obtained with the Radar Mapper of the Cassini orbiter, have clearly revealed that the pools of hydrocarbons are mostly concentrated in the high latitudes of the northern hemisphere. Is the dichotomy in the distribution of lakes, seas and rivers exclusively related to the meteorological cycle of Titan ? The observations have demonstrated that the previous climate models had not been accurate since the expected cloud cover in the northern hemisphere, by 2016, was not identified for instance. Planetologists have collected a huge amount of data regarding the dynamics of Titan's atmosphere or regarding the meteorology and the climate of Titan.

The analyses upon the dynamics and the chemistry of Titan's atmosphere and upon the seasons of Titan are continuing. The atmosphere, the soil and the internal structure of Titan are like a puzzle which implies new clues, speculations or anticipations. Our understanding of that intriguing world is clearly incomplete. Rajani Dhingra pointed out :  We want our model predictions to match our observations. This rainfall detection proves Cassini's climate follows the theoretical climate models we know of.  She added :  Summer is happening. It was delayed, but it's happening. We will have to figure out what caused the delay, though.  Rajani Dhingra explained that a new analytical work suggests that the methane rain reached a relatively pebble-like surface. A rougher surface produces an amorphous or irregular pattern since the liquid tends to go down to crevasses or gullies. By contrast, if the liquid reaches a smooth surface, it will tend to puddle in a relatively circular pattern. Rajani Dhingra is resorting to the analysis of the sidewalk effect to look for similar phenomena on Saturn's largest moon as part of her research work. Titan is clearly a dynamic world with seasons.

The image above reveals a portion of the Titanian disk generated on the basis of data obtained with the Visual and Infrared Mapping Spectrometer (VIMS) of the Cassini orbiter. The lower view shows an enlarged view of the area corresponding to the area of the blue box and of the orange box displayed in the upper view. One can notice the  wet sidewalk , ephemeral region likely representing the outcome of a past rainfall event and likely representing the sign of changing seasons. One can also notice several specular reflections as well as clouds. The mirror-like reflection of a pool of liquids known as Xolotlan Lacus can be identified. In the left part of the lower view, the famous sea called Kraken Mare can be discerned as well. The north pole of the Orange Moon is indicated by a black dot in the lower view. Image credit: NASA/JPL/University of Arizona/University of Idaho.

- To get further information on that news, go to: https://news.agu.org/press-release/new-study-finds-evidence-of-changing-seasons-rain-on-titans-north-pole.



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