May 10, 2023: Star Occultations Involving Titan Can Tell Us A Lot Regarding Wind Patterns On That Moon

During the mission of the Cassini spacecraft and of the Huygens probe in the Saturn System, researchers have gathered a huge amount of data regarding the atmosphere, the climate or the meteorology of Titan. The end of the Huygens mission in the atmosphere and on the surface of Titan occurred on January 14, 2005 whereas the end of the mission of the Cassini orbiter occurred on September 15, 2017 with the crash of the Cassini probe into the atmosphere of the Ringed Planet Saturn. Now, we can continue to study Saturn or Titan via telescopes on Earth or via telescopes located in space like the Hubble Space Telescope or the James Webb Space Telescope. The atmosphere of Saturn's largest moon is of particular interest because it looks like the atmosphere of the Earth to a certain extent and because it can tell us a lot regarding our own atmosphere. Like on Earth, winds on Titan can engender large dune fields. A parallel can be drawn between the meteorological cycle of Titan and the meteorological cycle of the Earth. Let's note however that the meteorological cycle of Titan involves methane whereas the meteorological cycle of the Earth involves water. In the harsh environment of Titan, methane and ethane molecules can form lakes or seas !

The Cassini-Huygens mission in the Saturn System has allowed us to determine that the landscape of Titan can be reminiscent of the landscape on Earth. There are lakes, seas, rivers, large dune fields, mountains or fractured landscapes on Titan. Cloud systems can be regularly found in the high latitudes of each hemisphere on the Opaque Moon. Those clouds are not composed of water ice or water vapor like in the atmosphere of the Earth. Those clouds will tend to be dominated by methane or ethane. Planetologists have the opportunity to study another type of meteorological system and to better understand our own atmosphere thanks to the study of the Titanian atmosphere. The atmosphere of the giant moon appears completely opaque in the visible spectrum from outer space. That's not the case in the infrared or near-infrared spectrum which allows to discern surface features or cloud systems. Since the end of the Cassini-Huygens mission in the Saturn System, planetologists continue to study Titan thanks to data acquired from the level of the orbit of the Earth via star occultations or via Doppler wind measurements for instance. Thanks to the data gathered by our telescopes, astronomers or researchers can analyze the dynamics or the chemistry of Titan.

Researchers have had the opportunity to study the atmosphere of the giant moon of the Gas Giant Saturn on the basis of star occultations or Doppler wind measurements. The star occultation involving Titan takes shape when the star passes behind Titan from the observer on Earth so that the disk of gas of Titan blocks the light from the star. Astronomers can measure the light curve from the start of the occultation to the end of the occultation. They can deduce atmospheric features such as the depth of the atmosphere or the direction and the strength of winds from the shape of the light curve and from the refracted light. An occultation performed in 1989 had unveiled that the winds on Saturn's largest moon are fastest at mid-northern latitudes and mid-southern latitudes and slowest near the equator. However, there is a discrepancy between those results and the results obtained in 2016 and 2017 with the Atacama Large Millimeter/submillimeter Array (ALMA) in the Atacama Desert in Chile on the basis of the method of the Doppler wind measurements. In this method, the speed and the direction of the winds are intimately linked to the Doppler shift of the frequency of light emitted by various molecules in the atmosphere.

The Doppler wind measurements allowed the team of Martin Cordiner, a researcher in astrochemistry and planetary science at the NASA Goddard Space Flight Center, to determine that there were fast winds at the level of the equator and that there were slow winds in the northern latitudes and in the southern latitudes. That's the opposite of the conclusion of the occultation event in 1989 ! Martin Cordiner shared the surprising results with other researchers including Juan Lora who is an assistant professor of planetary science at Yale University and Eliot Young who is a principal scientist at the Southwest Research Institute's Space Studies Department. The researchers realized that our level of understanding of Titan is quite limited and that we must compare the various observations with a model. Eliot Young started to plan observations of Titan's atmosphere during upcoming occultation events. The planetologists focused on the occultation event of September 5, 2022 when the shadow of Titan would be passing right over Hawai'i where the NASA Infrared Telescope Facility (IRTF), W. M. Keck Observatory, Faulkes Telescope North, and Gemini North, one half of the International Gemini Observatory, managed by NSF's NOIRLab, are found.

The researchers were going to mobilize several telescopes to analyze the light from Titan during the occultation event of September 5, 2022. A great collaboration took shape for this rare event ! The length of the occultation event was just six and a half minutes. ALMA was also going to be mobilized to acquire Doppler wind measurements of the Titanian atmosphere. Is the wind profile predicted by the analytical work related to ALMA data in line with the wind profile predicted by the analytical work related to the occultation event ? That's a crucial question that researchers will try to answer ! The potential difference can be related to human errors, limitations related to the quality of the instrument or can be related to complex meteorological phenomena related to seasonal factors for instance. Any outcome must be compared to the model and the model must be compared to the experimental work. The researchers had another opportunity to observe Titan during an occultation event. That occultation event took shape on November 30, 2022. Its length turned out to be shorter since it lasted only 200 seconds. Several telescopes were mobilized such as SOAR and the VLT.

The researchers managed to obtain precious data regarding the occultation event thanks to a remarkable collaborative work. The dynamics of Titan's atmosphere is captivating because the seasons must play a strong role and each season on Titan is extremely long. In fact, a season on Titan represents about seven and a half Terrestrial years. The Titanian year is extremely long as well since it lasts almost 30 Terrestrial years. Planetologists try to get a good picture of the dynamics of Titan's atmosphere during a complete Titan year. Currently, the northern hemisphere of Titan is experiencing the Summer season whereas the southern hemisphere of that world is experiencing the Winter season. The next equinox will occur in 2025 which implies changes in the structure of environmental temperatures. Even if the level of energy received from the Sun at the level of Saturn and Titan is particularly low, seasonal changes can be significant on Titan. The concentration of clouds can rapidly change in certain areas. The images observed from the Cassini orbiter during the seasonal transition have clearly shown the potential meteorological changes in the low or middle latitudes.

Planetologists anticipate that, in 2025, the flow of winds on Saturn's largest moon will switch directions and begin circulating from the north to the south. That's a configuration that has not been encountered since the year 1993. The new occultation observations will be useful to anticipate the behavior of the atmospheric winds on the Opaque Moon and to build a better model. Researchers must try to remove the noise of our atmosphere and the noise from the light of Titan in order to better discern the image of the star on the edges of the disk of the moon. Ultimately, the planetologists will be in a position to refine the existing global circulation models of the atmospheric winds of Saturn's largest moon. The planetologists will not hesitate to take advantage of other occultation events involving Titan in order to monitor or to study that captivating atmosphere. The James Webb Space Telescope as well as the VLT can obtain remarkable views of Titan's atmosphere or Titan's surface. One can discern atmospheric features such as clouds from the level of the Earth for instance. The distribution of clouds is likely to tell us a lot regarding the seasonal phenomena on the giant moon of Saturn.

Curiously, the atmosphere of Titan looks more like the atmosphere of Venus than the atmosphere of the Earth from outer space but its composition is closer to the composition of our own atmosphere. The Titanian atmosphere is dominated by molecular nitrogen like our own atmosphere. In the harsh environment of Titan, nitrogen can appear as a gas like in the atmosphere of the Earth. Let's point out that on Titan, the environmental temperatures are around -179 degrees Celsius, -290 degrees Fahrenheit or 94 Kelvin. A little colder and nitrogen could appear in its liquid form on the surface. Nitrogen can probably mix with liquid methane or liquid ethane within the high latitude lakes or seas found on that intriguing moon. There is a relatively high concentration of methane in the troposphere of Titan. Methane on Titan must play the same type of role as water on Earth. Methane can evaporate, condense, form clouds and fall as rain like water on Earth. Relatively complex hydrocarbons or organics can be found in the haze of the atmosphere of that enigmatic world. The heavier molecules will tend to fall to the surface to form dunes or a type of brown mud known as tholin. Prevailing winds in the low or middle latitudes tend to influence the shape of the dunes which tend to appear linear and parallel.

The image in the upper part of the table represents a raw view of Titan obtained from the Cassini orbiter on May 28, 2007. The view whose file name is W00028906.jpg was acquired on the basis of the CB3 filter and of the CL2 filter. The image had not been validated or calibrated at the time of the observation and a validated or calibrated version was going to be archived with the Planetary Data System proposed by NASA. The image in the lower part of the table represents a colorized version of the original view. Credit for the original image: NASA/JPL-Caltech/Space Science Institute. Credit for the colorization process of the original image: Marc Lafferre, 2023.

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