Letter to the Editor

A patchy CO₂ exosphere on Ganymede revealed by the James Webb Space Telescope

¹ LESIA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CNRS, 92195 Meudon, France
² Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
³ LATMOS/CNRS, Sorbonne Université, UVSQ, Paris, France
⁴ Department of Astronomy, University of California, 22 Berkeley, CA 94720, USA
⁵ Department of Earth and Planetary Science, University of California, 22 Berkeley, CA 94720, USA
⁶ Space and Plasma Physics, KTH Royal Institute of Technology, Stockholm, Sweden
⁷ Institute of Geophysics and Meteorology, University of Cologne, Albertus Magnus Platz, 50923 Cologne, Germany
⁸ University of Wisconsin, Madison, WI 53706, USA
⁹ Department of Astronomy & Astrophysics, University of California, San Diego, La Jolla, CA 92093, USA
¹⁰ Division of Geological and Planetary Sciences, Caltech, Pasadena, CA 91125, USA
¹¹ Johns Hopkins University Applied Physics Laboratory, 11001 Johns Hopkins Rd, Laurel, MD 20723, USA
¹² Faculty of Aerospace Engineering, Delft University of Technology, Delft, The Netherlands
¹³ School of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
¹⁴ Istituto Nazionale di AstroFisica – Istituto di Astrofisica e Planetologia Spaziali (INAF-IAPS), 00133 Rome, Italy
¹⁵ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

Received: 22 July 2024
Accepted: 19 September 2024

Abstract

Jupiter’s icy moon Ganymede has a tenuous exosphere produced by sputtering and possibly sublimation of water ice. To date, only atomic hydrogen and oxygen have been directly detected in this exosphere. Here, we present observations of Ganymede’s CO₂ exosphere obtained with the James Webb Space Telescope. CO₂ gas is observed over different terrain types, mainly over those exposed to intense Jovian plasma irradiation, as well as over some bright or dark terrains. Despite warm surface temperatures, the CO₂ abundance over equatorial subsolar regions is low. CO₂ vapor has the highest abundance over the north polar cap of the leading hemisphere, reaching a surface pressure of 1 pbar. From modeling we show that the local enhancement observed near 12 h local time in this region can be explained by the presence of cold traps enabling CO₂ adsorption. However, whether the release mechanism in this high-latitude region is sputtering or sublimation remains unclear. The north polar cap of the leading hemisphere also has unique surface-ice properties, probably linked to the presence of the large atmospheric CO₂ excess over this region. These CO₂ molecules might have been initially released in the atmosphere after the radiolysis of CO₂ precursors, or from the sputtering of CO₂ embedded in the H₂O ice bedrock. Dark terrains (regiones), more widespread on the north versus south polar regions, possibly harbor CO₂ precursors. CO₂ molecules would then be redistributed via cold trapping on ice-rich terrains of the polar cap and be diurnally released and redeposited on these terrains. Ganymede’s CO₂ exosphere highlights the complexity of surface-atmosphere interactions on Jupiter’s icy Galilean moons.