December 4, 2019 : A New Study Proposed By Ellen C. Creecy Reveals Relatively Strong Seasonal Variations In The Energy Budget Of Titan

A new research work entitled  Seasonal Variations of Titan's Brightness , recently published in the journal Geophysical Research Letters and proposed by a team involving Ellen C. Creecy, reveals that Titan unveils relatively strong seasonal variations in its energy budget. The huge amount of data acquired from the Cassini spacecraft during its long mission in the Saturn System from 2004 to 2017 has allowed us to perform multiple studies regarding the dynamics of the Titanian atmosphere or regarding the climate and the meteorology of Saturn's largest moon. Due to the remarkable dynamics of its atmosphere, Titan appears to be a really complex world where the geology or the landscape can be reminiscent of our own geology or landscape. The study of the energy budget of Titan can have major implications for the study of our own climate or meteorology, here on Earth. The planetologists who analyze the weather and the climate of the Opaque Moon evaluate the amount of solar energy absorbed by the giant moon as well as the thermal energy it emits.

The study of Saturn's largest moon can bring key information regarding our own world simply because there a remarkable similarities between the meteorological cycle of Titan and the meteorological cycle of the Earth. Titan unveils lakes, seas and rivers like the Earth and clouds can take shape and produce rainfall events or even snowfall events on Titan like on Earth. Titan is to a certain extent a mini-Earth where evaporation processes, condensation processes and rainfall processes can be encountered. However, the hydrological cycle of Titan is based on methane rather than water. In the harsh environment of Titan, water can only appear in its solid form on the surface and hydrocarbons like methane, ethane and propane can appear in their liquid form. Ellen C. Creecy who is a doctoral student in the Department of Earth and Atmospheric Sciences at the University of Houston and who is the first author of the study upon seasonal variations in the energy budget of the Orange Moon pointed out :  By studying Titan, we can learn a lot about Earth.  We can better understand our atmosphere or our climate by studying other atmospheres in the Solar System. Titan and Venus are among those worlds which can bring key information upon the level of understanding of our atmosphere.

The Earth and Titan evolve in a completely different environment. Titan evolves about ten times farther from the Sun than the Earth which implies a particularly low level of energy received from our star. Thus, the environmental temperature on the surface of Saturn's largest moon is very low even though its deep, dense or thick atmosphere is likely to engender greenhouse effects like on Mars, on Earth or on Venus. Titan's atmosphere is dominated by molecular nitrogen like our atmosphere but oxygen is absent or almost absent in the Titanian atmosphere. That's a major difference between our atmosphere and the atmosphere of the Opaque Moon. The Titanian atmosphere contains a relatively significant fraction of methane, a compound which represents the second most abundant gas in the harsh environment of that exotic moon. Pools of methane or ethane can be encountered in the high latitudes of Titan. That is not possible on Earth due to the relatively high environmental temperatures which prevent methane or ethane from appearing in their liquid form on the surface. In the harsh environment of Titan, water can be found in its solid form on the surface but it won't be found in its liquid form and in a stable state on the surface.

The year in the system of Saturn is extremely long compared to the well-known Terrestrial year which represents approximately 365 days. As a result, each season on Titan or on Saturn whose rotation axis is tilted relative to the normal of the orbital plane is particularly long. A season on Titan represents about 7 Earth years and the Titanian year represents approximately 29 Earth years. Therefore, if we want to clearly understand the dynamics and the chemistry of Titan's atmosphere, we have to perform a long-term study of that moon, from the study of its landscape or topography to the study of its haze or atmosphere. Liming Li who is a physics professor at the University of Houston and who is a corresponding author of the research work pointed out that the orbital period of the Saturn System around the Sun is extremely long. He also argued that our better understanding of the energy budget of the Orange Moon is likely to bring an improvement or new clues to our level of understanding of climate change on Earth.

Studying other worlds or better understanding other worlds can allow us to better understand our own world. Liming Li advanced that previous analyses have detected a small energy imbalance on the Blue Planet. He pointed out :  Earth's small energy imbalance has significant effects on its global warming and climate change.  He added :  We expect that the dynamically-varying energy budget and the possible energy imbalance have important impacts on the weather and climate systems on Titan.  During the long mission of the Cassini orbiter in the Saturn System from 2004 to 2017, the various instruments of the spacecraft have collected a large amount of information regarding the dynamics and the chemistry of Titan's atmosphere. We know that seasonal factors play a key role for the meteorology or the climate of the giant moon. In the low or mid-latitudes, dynamic and transient clouds take shape and develop from time to time depending on seasonal factors or variations in the amount of energy received from the Sun. For instance, that was the case in 2010 when transient and elongated clouds took shape and developed in the low or mid-latitudes of Titan due to seasonal factors.

Thanks to the Cassini orbiter, we have gathered data upon Titan for three different Titanian seasons. When the Cassini-Huygens probe reached the system of Saturn and its moons, the southern hemisphere of Titan was experiencing the Summer season whereas the northern hemisphere of Titan was experiencing the Winter season. Then, in 2009, the Autumn season in the southern hemisphere began whereas, in parallel, the Spring season began in the northern hemisphere. Later, in 2017, at the end of the Cassini mission inside the Saturn System, the Summer season in the northern hemisphere began whereas, in parallel, the Winter season in the southern hemisphere began. Thus, we have had the opportunity to obtain data upon Titan for three different Titanian seasons. The multiple flybys of Titan performed by the Cassini spacecraft have allowed us to acquire major clues regarding the climate, the meteorology or seasonal phenomena on Titan in particular. We have become aware that surface features can tell us a lot regarding the climate or the meteorology of Titan. One can mention the famous  Cat Scratches  which represent linear and parallel dunes extending over long distances and whose shape is influenced by prevailing winds.

Prior to the new research work proposed by Ellen C. Creecy and her collaborators, some studies had suggested that the energy budget of Saturn's largest moon was in balance. However, the new analyses reveal that the emitted thermal energy and the absorbed energy emanating from the Sun went down over the 14-year period studied by the team. But thermal emissions decreased less since they diminished by approximately 6.8 percent compared with an 18.6 percent decrease in solar energy at the level of the Opaque Moon. The phenomenon varied between the northern hemisphere and the southern hemisphere of that giant moon. The phenomenon appears to be closely linked to the distance between Titan and the Sun during the elliptical orbit of the Hazy Moon around our star. Xun Jiang who is a professor of atmospheric science at the University of Houston pointed out that the discoveries imply that the distance between the Earth and the Sun may play a role in the energy balance of the Earth. Xun Jiang and Liming Li act as co-doctoral advisors for Ellen C. Creecy whose future studies will be supported by a new grant from NASA, representing 433,210 dollars. The Doctor Liming Li represents the principal investigator on that grant.

Xun Jiang is convinced that the variations in the energy received from the Sun during the Saturn year in the elliptical orbit of the Saturn System around our star have an impact on the climate or the meteorology of Titan. She pointed out :  Such a mechanism has not been examined for Earth's energy imbalance.  She also added that the future studies will focus on the analytical comparison between Titan and Earth's energy budgets in the prospect of better understanding the climate system on each world. Ellen C. Creecy, Liming Li and Xun Jiang collaborated with C.A. Nixon of the NASA Goddard Space Flight Center, with R.A. West and with M.E. Kenyon of the Jet Propulsion Laboratory at the California Institute of Technology to produce the new research work. Ellen C. Creecy argued that more work will be required in order to better understand the potential implications related to the observed energy imbalance unveiled by Titan. She pointed out :  The possible energy imbalance means that Titan's atmospheric system keeps evolving.  And she concluded :  There are complicated interaction between the energy budget and atmospheric system, which is being explored by our research.  Is the atmosphere of Titan stable over time ? Or is it evolving in an unstable way ?

The image above reveals the disk of Titan at the beginning of the Cassini-Huygens mission in the Saturn System. The image appearing with artificial colors was produced on the basis of data obtained on October 24, 2004 from the Narrow-Angle Camera of the Cassini orbiter with a filter sensitive to near-ultraviolet light. One can notice in particular the detached haze layer found in the upper atmosphere of the Hazy Moon. At the time of the observation, the southern hemisphere was experiencing the Summer period whereas the northern hemisphere was experiencing the Winter season. Image credit: NASA/JPL/Space Science Institute.

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