Observations of Clouds on Titan with JWST/NIRCam and Keck/NIRC-2

Titan, Saturn’s largest moon, is the only body in the solar system other than Earth to exhibit a ‘terrestrial’ meteorological cycle, with evaporation from a solid surface leading to condensation clouds and rainfall. On Titan however, methane takes on the role of terrestrial water as the environmental evaporate and condensate. On November 4th 2022 the newly-commissioned James Webb Space Telescope made its first observations of Titan on the trailing hemisphere, using the NIRCam and NIRSpec instruments. These observations constituted the first 40% of Guaranteed Time Observations (GTO) project #1251, part of a larger GTO Solar System campaign to observe all the planets and many smaller bodies from Mars to the Kuiper belt and beyond. Clouds were seen in the northern hemisphere in filters (e.g. 2.12 μm) that can probe the lower atmosphere and surface, between the strong methane absorption bands. A large cloud was seen at ~45° north on the evening (west) limb, and a less distinct cloud at around 60° north on the morning (east) limb. Global circulation models (GCMs) predict that solar heating in the northern hemisphere at this epoch (late northern summer) drives upward convection of methane-rich air at mid-latitudes in the troposphere, leading to methane condensation into clouds at altitudes of 15-30 km. To observe subsequent cloud evolution, the JWST team requested rapid follow-up observations with Keck/NIRC-2 adaptive optics, which were conducted on November 6th and 7th. The Keck images showed clouds at the same locations, indicating that clouds are persistent at these latitudes at this seasonal epoch, in agreement with model predictions. Analysis of the NIRCam data in multiple spectral filters and the November 4th JWST/NIRSpec observations is ongoing, which will help to constrain the cloud vertical location as well as providing new information on the gas composition of the atmosphere and the ice composition of the surface. In summer 2023 the team looks forward to the second tranche of Titan observations with NIRCam and NIRSpec of Titan’s leading hemisphere, as well as the first MIRI spectra of Titan at 5-28 μm.