May 15, 2025: Cloud Convection Identified In The Northern Hemisphere Of Titan Thanks To Data From The JWST And From The Keck II Telescope
A new study entitled "The atmosphere of Titan in late northern summer from JWST and Keck observations", published in Nature Astronomy on May 14, 2025 and proposed by a team of researchers involving Conor A. Nixon and Heidi B. Hammel reveals the identification of cloud convection in the northern hemisphere of Titan as well as the methyl radical on that world thanks to data acquired with the JWST and with the Keck II telescope. The observations of the James Webb Space Telescope and of the Keck II telescope were performed in 2022 and 2023 during the late northern summer of the giant moon of the Gas Giant Saturn. The year on Titan or Saturn is very long because both worlds evolve at a much higher distance from the Sun than the Earth so that a year on Titan, Enceladus, Tethys, Dione, Rhea, Iapetus or Titan represents 29.45 Earth years. Thanks to the Cassini-Huygens mission in the Saturn System, we have had the opportunity to study the atmosphere and the surface of Titan and to obtain key data upon that world during a relatively long period, from the Saturn Orbit Insertion in 2004 to the crash of the Cassini orbiter into the atmosphere of Saturn in 2017 but that period only represents less than half a year on Titan !
The various images of the Opaque Moon acquired from the Cassini orbiter in particular clearly show that Titan is a complex world with a complex meteorology and with seasonal phenomena. We need to study Titan in the very long run in order to try to understand the dynamics and the chemistry of its atmosphere in particular. Even if the Cassini-Huygens mission in the Saturn System is over, we can continue to study the atmosphere or the surface of Titan from the region of the Earth thanks to powerful telescopes like the James Webb Space Telescope or the Keck II telescope. Planetologists were particularly interested in the evolution of cloud activity in the northern hemisphere and in the north polar region in particular in the years after the end of the Cassini-Huygens mission in the Saturn System because a new season was starting in the northern hemisphere as well as in the southern hemisphere in 2017. In 2017, the Summer season was starting in the northern hemisphere whereas the Winter season was starting in the southern hemisphere. Were we going to identify major changes in the northern hemisphere, in the north polar region, in the southern hemisphere or in the south polar region ?
The James Webb Space Telescope and the Keck II telescope allow us to obtain key data regarding the dynamics and the chemistry of Titan's atmosphere during the Summer season in the northern hemisphere and during the Winter season in the southern hemisphere. One has to keep in mind that seasons are very long on Titan. In fact, each season represents around 7 Earth years on Titan. The new season starting in 2017 would last around 7 Earth years ! The new views were acquired during this new season a few Earth years later, in 2022 and in 2023. On the basis of the mid-infrared instrument onboard the JWST, the researchers were in a position to spectroscopically identify the methyl radical (CH3). CH3 can result from the break-up of methane (CH4) and can engender more complex molecules like ethane (C2H6). Methane (CH4), ethane (C2H6) and propane (C3H8) can be found in their liquid form on the surface of Titan due to the extremely low environmental temperatures. The planetologists were also in a position to identify several non-local thermodynamic equilibrium CO and CO2 emission bands on the basis of the near-infrared spectrograph of the James Webb Space Telescope (JWST). The team of researchers managed to measure these species over a large altitude range.
The near-infrared camera of the James Webb Space Telescope and the near-infrared camera of the Keck II telescope also allowed the planetologists to identify tropospheric clouds evolving in altitude in the northern hemisphere of Saturn's largest moon. That's a sign of convection phenomena in the lower part of the atmosphere or in the troposphere of Titan. Those observations bring some new information or new clues regarding the meteorology or the climate of the giant moon. At the start of the Cassini-Huygens mission in the Saturn System, we had clearly observed dynamic clouds in the high latitudes of the southern hemisphere of Titan. The southern hemisphere was experiencing the Summer season whereas the northern hemisphere was experiencing the Winter season. The current seasonal configuration represents the opposite of that configuration. Can we expect the same types of clouds in the north polar region as the types of clouds found in the south polar region at the start of the Cassini-Huygens mission ? The new observations suggest that cloud activity in the high latitudes of the northern hemisphere is relatively dynamic during the Summer season in the northern hemisphere. Storms can be expected due to relatively high levels of evaporation related to higher levels of energy received from the Sun.
If the level of evaporation is higher than the level of precipitation, one can expect a decrease in the size or in the level of the potential lakes, seas and rivers in the area. The numerous radar views obtained from the Cassini orbiter during its long mission in the Saturn System had clearly unveiled an asymmetry in the distribution of the lakes and seas on Titan. The north polar region appeared to be the most humid area of the moon with major pools like Kraken Mare, Ligeia Mare and Punga Mare in particular. The south polar region also unveiled a major pool known as Ontario Lacus but the area was clearly not as wet or humid as the north polar region. How can we explain that dichotomy between the south polar region and the north polar region ? Was it related to seasonal factors ? Can we expect smaller lakes or seas during the Summer season in the polar areas due to extremely strong evaporation processes ? The new observations of cloud activity are likely to help us evaluate the potential evolution of the lakes, seas or rivers in terms of size, shape or level. New pools or rivers could be expected in the south polar region during the Winter season in the southern hemisphere for instance. In parallel, a decrease in the size or in the level of the lakes, seas and rivers found in the north polar region could be expected during the Summer season in the northern hemisphere.
Prior to the Cassini-Huygens mission in the Saturn System, some planetologists had anticipated that there could be seas or oceans of methane or ethane on the surface of Titan. The Cassini orbiter and the Huygens module have clearly demonstrated that there are lakes, seas and rivers on Titan. However, the low and middle latitudes of the giant moon are devoid of any major sea or lake. The relatively dark areas of the low or middle latitudes are in fact dominated by linear and parallel dunes extending over long distances. Strong rainfall events or storms may occur from time to time in the low or middle latitudes. Planetologists or climate experts must continue gathering data regarding the dynamics and the chemistry of Titan's atmosphere in order to better understand the methane cycle of Titan. Are there internal sources to the methane found in the atmosphere of that intriguing world ? Methane is supposed to disappear over time due to the action of ultraviolet light from the Sun in particular. Yet, the concentration of methane in the atmosphere of the giant moon is relatively high. That's why some researchers advance that there may be pockets of methane or a subsurface layer of methane beneath the external crust.
The atmosphere of Titan is likely to tell us a lot regarding our own atmosphere because a parallel can be drawn between the methane cycle of Titan and the water cycle of the Earth. Methane can form lakes, seas or rivers. Methane can evaporate and condense to form clouds in the troposphere. Those clouds can generate rain. The clouds of Titan can be composed of several types of hydrocarbons. Clouds of methane and clouds of ethane can be encountered in that exotic world. The clouds recently identified in the northern hemisphere of the Opaque Moon appeared to move to higher altitudes over time demonstrating a convection phenomenon. The clouds take shape in the troposphere, the lower layer of the atmosphere whose upper limit is around 7 miles or 12 kilometers on Earth and around 27 miles or 45 kilometers on Titan. That upper limit is higher on Saturn's largest moon due to its much lower gravity. The various views of the James Webb Space Telescope and of the Keck II telescope clearly suggest that the clouds follow an upward motion between July 11, 2023 and July 14, 2023. The views also reveal that the stratosphere is far from being uniform. The view from the Keck II telescope acquired on July 14, 2023 clearly reveals bright patches in the polar areas in particular.
During the Cassini-Huygens mission in the Saturn System, a giant ethane cloud engulfing the north polar region had already been identified during the harsh period in the area. Is there also a giant ethane cloud engulfing the south polar region during the Winter period in the southern hemisphere ? That's a possibility if the giant ethane cloud represents a seasonal phenomenon. The composition of the lakes, seas and rivers of Titan may vary depending on the area or depending on the season. The pools can be composed of a mixture of methane, ethane and dissolved nitrogen. One can imagine lakes or seas containing layers of liquid ethane and layers of liquid methane, the lighter type of liquid appearing in the upper part of the pool. Astrobiologists are particularly interested in the chemistry of the haze of Titan's atmosphere because that haze is rich in organics and hydrocarbons that can interact to form more complex molecules like sugars, lipids or amino acids. In the upper atmosphere of the giant moon, methane, molecular nitrogen and radicals can interact to form new molecules like methyl radicals (CH3) under the action of sunlight or energetic electrons related to the magnetosphere of the Ringed Planet Saturn. The chemistry of the environment of Titan is clearly captivating !
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The mosaic above reveals several views of Titan acquired with the James Webb Space Telescope on July 11, 2023 and with the Keck II telescope on July 14, 2023. The white arrows indicate the clouds of methane identified in the north polar region of the giant moon. The different views reveal that those clouds can be found at different altitudes from the troposphere to the stratosphere. In the left part of the mosaic, in the view obtained by the JWST, light at 1.4 microns is colored blue, light at 1.5 microns is colored green and light at 2.0 microns is colored red (filters F140M, F150W and F200W, respectively). In the left part of the mosaic, in the view acquired by the Keck II telescope, light at 2.13 microns is colored blue, light at 2.12 microns is colored green and light at 2.06 microns is colored red (H2 1-0, Kp, and He1b, respectively). In the middle part of the mosaic, the images represent single-wavelength views captured by the JWST and by the Keck II telescope at 2.12 microns. That's a wavelength that is sensitive to emission from the lower troposphere of that world. The images found in the right part of the mosaic show emission at 1.64 microns (JWST) and at 2.17 microns (Keck II telescope). Those wavelengths reveal features at higher altitudes in the upper troposphere and in the stratosphere. Those views suggest an upward motion of the clouds because the clouds appear at a higher altitude in the observation of July 14, 2023. Image credit: NASA, ESA, CSA, STScl, and W.M. Keck Observatories. |
- To get further information on that news, go to: https://science.nasa.gov/missions/webb/webbs-titan-forecast-partly-cloudy-with-occasional-methane-showers/ and https://www.nature.com/articles/s41550-025-02537-3 .