December 2, 2022: Large Cloud Systems Identified On The Disk Of Titan Thanks To Data Gathered By The JWST And The Keck Telescope

The James Webb Space Telescope as well as the Keck Telescope have recently obtained new views of the disk of Saturn's largest moon Titan. Those views obtained in the infrared spectrum reveal surface features as well as major clouds in the upper part of the disk. Those clouds bring clues to the complex meteorology of that intriguing world whose atmosphere appears completely opaque from outer space. The infrared or near-infrared eyes of the JWST and of the Keck Telescope can unveil key features regarding the surface of the Orange Moon and regarding the meteorology of that giant moon. Titan has been intensively studied during the Cassini-Huygens mission in the Saturn System from 2004 to 2017. Thanks to radar data and infrared or near-infrared data captured with the eyes of the Cassini spacecraft, we know, now, that there are lakes, seas and rivers on Titan. A probe known as the Huygens probe had performed a parachuted descent into Titan's atmosphere on January 14, 2005 capturing several panoramic images of the landscape. The parachuted descent was carried out at a low latitude in the southern hemisphere in the area of Shangri-La and Adiri.

The aerial views acquired from the Huygens module during the atmospheric plunge clearly showed a major contrast between relatively bright hills and a relatively dark plain. The bright hills contained a network of dark channels that may represent drainage channels related to heavy rainfall events that must be quite scarce in the low or middle latitudes. In the harsh environment of Titan, water can only be present in its solid form or in the form of water ice on the surface. The stable pools or rivers present on the surface of Titan must be dominated by methane, ethane or propane because those molecules can appear in their liquid form on the surface. The radar images taken from the Cassini orbiter during its long mission in the Saturn System have revealed that the pools or rivers tend to be concentrated in the high latitudes or in the polar area of each hemisphere. The composition of the pools or rivers can vary depending on the latitude, on the area or on the season. Planetologists believe that the pools of that surprising world are generally composed of a mixture of methane, ethane and dissolved nitrogen. Those pools can engender clouds via typical evaporation processes.

The new views acquired with the James Webb Space Telescope and with the Keck Telescope bring new clues to the complex puzzle of the meteorological cycle of Titan. The atmosphere of that intriguing world is mainly composed of molecular nitrogen like the atmosphere of the Earth but the second most abundant gas of the atmosphere is not oxygen on Titan. The second most abundant gas in the atmosphere of that giant moon of the Gas Giant Saturn is methane, a molecule composed of one carbon atom and of 4 hydrogen atoms. Methane on Titan can be compared to water on Earth because methane on Titan can evaporate, condense to form clouds and fall as rain like water does on Earth. There is a meteorological cycle involving methane on Saturn's largest moon. That exotic meteorological cycle can be compared to the meteorological cycle of water on Earth. The study of the meteorological cycle of Titan is hard because, during the Cassini-Huygens mission in the Saturn System, we have obtained a relatively limited amount of atmospheric data during a fraction of a Titanian year, from 2004 to 2017. A Titanian year is in fact particularly long since it lasts almost 30 Terrestrial years.

Currently, we don't have any orbiter specifically devoted to the study of Titan's atmosphere or meteorology. We have gathered a large amount of information regarding that exotic atmosphere but that's far from being enough to decipher the complex dynamics or chemistry of Titan's atmosphere or to perform accurate simulations regarding the dynamics or the chemistry of that atmosphere. The new views taken from the JWST and from the Keck Telescope reveal that Titan's atmosphere is relatively dynamic at the present time. The large clouds identified in the upper part of the disk appear dynamic, imply the presence of relatively strong evaporation processes and can potentially engender strong rainfall events. The northern hemisphere of Titan is currently experiencing the Summer season whereas the southern hemisphere is currently experiencing the Winter season. At the beginning of the Cassini-Huygens mission in the Saturn System, the northern hemisphere was experiencing the Winter season and a giant ethane cloud system was evolving above the north polar region. The cloud recently identified in the right part of the disk seems to evolve in the area of Kraken Mare, the largest pool found on the surface of Titan.

Engineers and researchers are now preparing a new mission to study the environment of Titan. The mission known as Dragonfly is very ambitious because it involves a rotorcraft that will evolve in an environment of dunes and craters. The launch is planned for the year 2027 and the rotorcraft is expected to start its work several years later in the 2030s. The area in which the rotorcraft will evolve may be relatively dry and the presence of lakes or rivers in that area is unlikely. However, the rotorcraft may gather major clues regarding the chemistry of hydrocarbons and organics or regarding the potential for a prebiotic chemistry or for a methane-based life on that world. What types of hydrocarbons or organics are there on Titan ? Will we find the unexpected ? Since the observations of the Pioneer 11 spacecraft and of the Voyager 1 and of the Voyager 2 spacecraft, we have advanced in our understanding of Titan but that world still remains captivating and intriguing. The observations continue thanks to researchers like Imke de Pater from the University of California, Berkeley. The new views of the disk of Titan fuel the work of planetologists like Imke de Pater or Conor Nixon.

The image of Titan obtained with the JWST on November 4, 2022 revealed two major clouds that demonstrated the relatively significant dynamics of the atmosphere in the northern hemisphere. Conor Nixon observed that image and quickly asked Imke de Pater and Katherine de Kleer, who is a UC Berkeley Ph.D. and who is now an assistant professor of planetary science and astronomy at the California Institute of Technology, to mobilize the Keck Telescope to visualize those clouds in order to monitor their dynamics, their movement or their evolution. A series of Keck views acquired approximately 30 and 54 hours later unveiled relatively similar clouds and those clouds which likely represented the clouds initially observed had moved on the disk due to the rotation of the moon relative to our planet in particular. Imke de Pater who is a UC Berkeley Professor of the Graduate School pointed out: "We were concerned that the clouds would be gone when we looked at Titan one and two days later with Keck, but to our delight there were clouds at the same positions, looking like they might have changed in shape." Have the size and the level of the lakes and seas diminished in the northern hemisphere since the end of the Cassini mission in the Saturn System ?

The relative quality of the images obtained from the JWST and from the Keck Telescope is remarkable because Titan is extremely far away from the Blue Planet. Imke de Pater pointed out that the James Webb Space Telescope can tell us more than the Keck Telescope regarding the altitude of clouds and hazes on Saturn's largest moon. The infrared or near-infrared eye of the JWST can identify more features in the atmosphere than the optical eye of the Keck Telescope. We can combine data from the Keck Telescope and from the JWST in order to better understand what we see. We can get information regarding the height of the clouds, the optical thickness of the atmosphere and the elevation of the haze in that atmosphere. The infrared or near-infrared eye of the JWST can identify features in the lower atmosphere and on the surface of Titan. Imke de Pater and Katherine de Kleer have taken part in early September and this week in an international observing mission to acquire the occultation by Saturn's largest moon of a distant star. That mission was led by Eliot Young, a senior program manager at the Southwest Research Institute in Boulder, Colorado.

The occultation of a star by the Opaque Moon allowed us to study the structure of the atmosphere of that particular world in more detail. The campaign was performed with the Keck Telescope and with the Very Large Telescope in Chile. Those observations are combined with the observations of occultations acquired from other large telescopes and from Doppler wind retrievals on the giant moon from the Atacama Large Millimeter Array, a radio telescope also located in Chile. The work can be combined with recent wind modeling results in order to better understand the various atmospheres one can encounter in the Solar System or beyond. Elizabeth "Zibi" Turtle of Johns Hopkins University who is Dragonfly's principal investigator pointed out: "This is some of the most exciting data we have seen of Titan since the end of the Cassini-Huygens mission in 2017, and some of the best we will get before NASA's Dragonfly arrives in 2032." Clues regarding the dynamics and the chemistry of the lakes, seas and rivers found in the high latitudes of Titan's northern hemisphere can be obtained thanks to the study of the clouds observed by the JWST and the Keck Telescope.

The composite view above reveals new images of Titan obtained, respectively from left to right, with the James Webb Space Telescope and with the Keck Telescope. The view acquired with the NIRCam instrument of the JWST on November 4, 2022 clearly shows the presence of large clouds in the upper part of the disk. The view obtained with the NIRC-2 instrument of the Keck Telescope on November 6, 2022 likely reveals a different position of the same clouds on the disk due to the relative rotation of the moon in particular. Image credit: NASA/ESA/CSA/Webb Titan GTO Team/Alyssa Pagan (STScl).

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