Free Astronomy Magazine July-August 2025

49 ASTRO PUBLISHING water on Earth when it comes to weather. It evaporates from the sur- face and rises into the atmosphere, where it condenses to form meth- ane clouds. Occasionally it falls as a chilly, oily rain onto a solid surface where water ice is hard as rocks. “Titan is the only other place in our solar system that has weather like Earth, in the sense that it has clouds and rainfall onto a surface,” ex- plained lead author Conor Nixon of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. The team observed Titan in Novem- ber 2022 and July 2023 using both Webb and one of the twin ground- based W.M. Keck Observatories tel- escopes. Those observations not only showed clouds in the mid and high northern latitudes on Titan — the hemisphere where it is currently summer — but also showed those clouds apparently rising to higher altitudes over time. While previous studies have observed cloud convec- tion at southern latitudes, this is the first time evidence for such convec- tion has been seen in the north. This is significant because most of Titan’s lakes and seas are located in its northern hemisphere and evapora- tion from lakes is a major potential methane source. Their total area is similar to that of the Great Lakes in North America. On Earth the lowest layer of the at- mosphere, or troposphere, extends up to an altitude of about 7 miles (12 kilometers). However, on Titan, T hese images of Titan were taken by NASA’s James Webb Space Telescope on July 11, 2023 (top row) and the ground-based W.M. Keck Observatories on July 14, 2023 (bottom row). They show methane clouds (denoted by the white ar- rows) appearing at different altitudes in Titan’s northern hemisphere. On the left side are representative-color images from both telescopes. In the Webb image light at 1.4 microns is colored blue, 1.5 microns is green, and 2.0 microns is red (fil- ters F140M, F150W, and F200W, respectively). In the Keck image light at 2.13 mi- crons is colored blue, 2.12 microns is green, and 2.06 microns is red (H2 1-0, Kp, and He1b, respectively). In the middle column are single-wavelength images taken by Webb and Keck at 2.12 microns. This wavelength is sensitive to emission from Titan’s lower troposphere. The rightmost images show emission at 1.64 microns (Webb) and 2.17 microns (Keck), which favor higher altitudes, in Titan’s upper tro- posphere and stratosphere (an atmospheric layer above the troposphere). It demonstrates that the clouds are seen at higher altitudes on July 14 than earlier on July 11, indicative of upward motion. [NASA, ESA, CSA, STScI, Keck Observatory]