February 2, 2023

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The James Webb Space Telescope observes Saturn's moon Titan

The James Webb Space Telescope observes Saturn’s moon Titan

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On the morning of Saturday, November 5, an international team of planetary scientists woke up with great joy to the first web images of Saturn’s largest moon, Titan. Principal investigator Connor Nixon and others from the GTO 1251 (Guaranteed Time Observation) program who are using Webb to investigate Titan’s atmosphere and climate describe their initial reactions upon seeing the data here.

Titan is the only moon in the solar system with a dense atmosphere, and it is also the only planetary body outside Earth that currently has rivers, lakes, and seas. However, unlike Earth, the liquid on Titan’s surface is made up of hydrocarbons, including methane and ethane, not water. The atmosphere is filled with a thick haze that blocks visible light bouncing off the surface.

We’ve waited years to use Webb’s infrared vision to study Titan’s atmosphere, including its fascinating weather patterns and gaseous composition, and also to look through the nebula to see the albedo features (light and dark spots) on its surface. Titan’s atmosphere is incredibly interesting, not only because of its methane clouds and storms, but also because of what it can tell us about Titan’s past and future — for example, whether it ever had an atmosphere. We were thrilled with the initial results.

Team member Sebastien Rodriguez of Universite Paris Cité was the first to see the new footage and alerted the rest of us via email: “What a wake up this morning (Paris time)! So many warnings in my inbox! I went straight to my computer and immediately started to Download the data. At first glance, it’s extraordinary! I think we’re seeing a cloud!” Heidi Hammel, Webb Solar System GTO project leader, of the Association of Universities for Research in Astronomy (AURA), had a similar comment: “Wow! Love seeing the cloud and the signs Crystal clear whiteness. Looking forward to the spectra! Congratulations, everyone! Thank you!”

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Image: NASA, European Space Agency, Canadian Space Agency, A.R. Bagan (STScI)

Thus began a day of frantic activity. By comparing several images taken by the Near Infrared Webcam (NIRCam), we quickly confirmed that the bright spot in Titan’s northern hemisphere was, in fact, a large cloud. Not long after that we saw a second cloud. The discovery of the clouds is exciting because it confirms long-standing predictions of computer models about Titan’s climate, namely that clouds will readily form in the northern hemisphere in late summer, when the surface is warmed by the sun.

Then we realized it was important to see if the clouds were moving or changing shape, which could provide information about airflow in Titan’s atmosphere. So we quickly reached out to our colleagues to request follow-up observations using the Keck Observatory in Hawaii that evening. Webb Titan team lead at NASA’s Goddard Space Flight Center Connor Nixon wrote to Imke de Pater of the University of California, Berkeley, and Kathryn de Claire of Caltech, who have extensive experience using the Keck: “We have just received our first images of Titan from Webb, captured last night. Very exciting! There appears to be a large cloud, which we believe is over the Arctic near Kraken Mare. We were wondering if we could do a quick follow-up observation on Keck in order to see if the cloud was developing.

After negotiations with Keck’s staff and observers already scheduled to use the telescope that night, Imke and Katherine quickly put together a series of observations. The goal was to probe Titan from the stratosphere to the surface, to try to capture the clouds we saw with Webb. Views were successful! Imke de Pater commented, “We were worried that the clouds would disappear when we looked at Titan with Keck two days later, but to our delight there were clouds in the same positions, which seemed to have changed shape.”

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Image: NASA, ESA, CSA, W.M. Keck and A. Pagan Observatory (STScI)

After we had the Keck data, we turned to atmospheric modeling experts to help interpret it. One of those experts, Juan Laura of Yale University, commented, “Really exciting! I’m glad we’re seeing this because we expected a lot of cloud activity for the season! We can’t be sure that the clouds on November 4 and 6 are the same clouds, but they are confirmation of seasonal weather patterns.”

The team also collected spectra using Webb’s Near-Infrared Spectrograph (NIRSpec), which gives us access to many wavelengths that Earth’s atmosphere blocks out with ground-based telescopes like Keck. With this data, which we are still analyzing, we can examine the composition of Titan’s lower atmosphere and surface even better than the Cassini spacecraft, and we can learn more about the cause of the bright spot over the south pole.

We expect additional data on Titan from NIRCam and NIRSpec in May or June 2023, and preliminary data from Webb’s Mid-Infrared Instrument (MIRI). MIRI data will show a larger portion of Titan’s spectrum, including some wavelengths not seen before. This will give us information about the complex gases in Titan’s atmosphere and provide important clues for deciphering why Titan is the only moon in the solar system with a dense atmosphere.

source: NASA