Transmission spectrum of Jupiter’s atmosphere reflected off Ganymede.


When trying to understand the transmission spectra of exoplanets, we are lead to wonder what these would look like for Solar-System planets. Surprisingly, it is quite difficult to find out, transmission spectra for Solar-System atmospheres have generally not been measured.

A group led by Pilar Montañes-Rodriguez at the IAC in Tenerife has just published a measurement of the transmission spectrum of Jupiter.

We cannot see the Sun through Jupiter’s atmosphere from Earth, because our planet is situated within Jupiter’s orbit. The authors used a trick: they observed the shadow of Jupiter’s atmosphere on its moon Ganymede, during an eclipse of the moon by the giant planet. Although the geometry is not exactly that of a planetary transit, it offers a good proxy of a transmission spectrum, since the light in the penumbra on Ganymede has partially crossed Jupiter’s atmosphere.

Transmission spectroscopy of Jupiter using an eclipse of Ganymede [photomontage]

Transmission spectroscopy of Jupiter using an eclipse of Ganymede [photomontage]

 Ganymede’s entry into Jupiter’s shadow lasts only a few minutes (4 minutes if my quick calculation is correct). The IAC team used the fast “XSHOOTER” camera on the VLT, combining rapid exposure times with large telescope size to obtain enough accuracy on the reflected spectrum.

Actual picture of an eclipse of Ganymede by Jupiter.

Actual picture of an eclipse of Ganymede by Jupiter.

The resulting spectrum shows the signature of methane vapor, water-ice crystals, and haze. It looks remarkably similar to the transmission spectrum of hot Jupiters – in spite of the vastly different temperatures –  with a prevalence of haze scattering at visible wavelengths, and molecular bands in the near infrared.

Spectrum of Jupiter's penumbra on Ganymede, compared to transmission spectrum of hot Jupiter HD 189733b. [Lower panel adapted from Fig. 3 of Montañes-Rodriguez et al.]

Spectrum of Jupiter’s penumbra on Ganymede, compared to the transmission spectrum of exoplanet HD 189733b. [Lower panel adapted from Fig. 3 of Montañes-Rodriguez et al.]

There is good news and bad news for exoplanet transmission spectroscopy in this nice result.

The good news is that it gives some more credence to the present interpretation of hot Jupiter transmission spectra. Jupiter’s penumbra shows the same balance of features, with haze scattering dominant at short wavelengths, and molecular bands prevalent in the near infrared.

The bad news is that in the case of Jupiter, the causes of the different signatures in the spectrum reflected off Ganymede are far from obvious. According to the Montañes-Rodriguez et al. study, the abundance of water required to account for the water-ice features in the spectrum are far higher than viable values for the upper atmosphere of Jupiter, and the exact nature of the hazes is not known. In addition, the prominent features in Jupiter’s atmosphere – ammonia and ammonium sulphide clouds – leave no trace in the transmission spectrum. Conversely the component dominating the transmission spectrum have little impact on the atmosphere globally.

This might apply to hot Jupiters as well: the hazes, elements and molecules that mark the transmission spectrum might reflect the conditions near 1 mbar but may have little relation to conditions in the main bulk of the atmosphere.

At this point, however, we cannot afford to be choosy about the information we get on exoplanet atmospheres. We have to infer the most we can from what we can measure.


Further reading

– Montañes-Rodriguez et al. 2015, “Jupiter as an exoplanet: UV to NIR transmission spectrum reveals hazes, a Na layer and possibly stratospheric H2O-ice clouds“, Astrophysical Journal Letters

– Transmission spectrum of Venus as an extra-solar planet 



About Author

I am a professor of planetary science at the University of Exeter. My specialty is the study of exoplanets, in particular the observation and modelling of exoplanet atmospheres. I have done my PhD a the University of Geneva and worked in Chile, France and Switzerland.