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Jupiter as an Exoplanet: UV to NIR Transmission Spectrum Reveals Hazes, a Na Layer, and Possibly Stratospheric H2O-ice Clouds.
Author/s: Montañés-Rodríguez, P.; González-Merino, B.; Pallé, E.; López-Puertas, M.; García-Melendo, E.
Reference: 2015ApJ, 801L, 8M | Link
Figure caption: The transmission spectrum of Jupiter during the penumbra (a) and the umbra (b)phases. Gray shaded regions mark the deeply absorbed telluric bands of H2O, which cannot be observed from the ground. The locations of the major CH4 absorption bands are marked with a brown background shadow. On both panels, the thin gray line is the brightness ratio of the light reflected off Ganymede when fully illuminated by the Sun at two different airmass (equivalent to the airmass difference for the two spectra used to extract the transmission spectrum). It serves to illustrate the contamination from telluric lines to be expected during our measurements. All spectra have been binned to a lower resolution for display purposes. Panel (a): two of our penumbra spectra, taken at different airmass, which are almost identical except for the stronger effect of Jupiter's haze absorption as the eclipse progresses. Panel (b):three measured umbra spectra, normalized to have unityflux at 1.6 cm. The different relative depth of the bands is due to different timing within the eclipse umbra, which allows probing different altitudes in Jupiter's upper atmosphere. The first two spectra (green) show features of gaseous CH4 and H2O ices, while in the third spectrum (red), the H2O ice features diminish indicating that we are probing below the H2O-ice cloud layer. In the third deep umbra spectra, the spectra are dominated by the CH4 absorption and by the haze.
Currently, the analysis of transmission spectra is the most successful technique to probe the chemical composition of exoplanet atmospheres. However, the accuracy of these measurements is constrained by observational limitations and the diversity of possible atmospheric compositions. Here, we show the UV-VIS-IR transmission spectrum of Jupiter as if it were a transiting exoplanet, obtained by observing one of its satellites, Ganymede, while passing through Jupiter's shadow, i.e., during a solar eclipse from Ganymede. The spectrum shows strong extinction due to the presence of clouds (aerosols) and haze in the atmosphere and strong absorption features from CH4. More interestingly, the comparison with radiative transfer models reveals a spectral signature, which we attribute here to a Jupiter stratospheric layer of crystalline H2O ice. The atomic transitions of Na are also present. These results are relevant for the modeling and interpretation of giant transiting exoplanets. They also open a new technique to explore the atmospheric composition of the upper layers of Jupiter's atmosphere.