Gabriel Tobie, 18/08/2023
The Galileo mission (1996-2003) provided clear evidence that the Galilean moons, Europa, Ganymede and Callisto, harbour a salted water ocean underneath their cold icy shells. Various salt and simple organic compounds have been identified at their surface by infrared spectroscopy. But it is still unclear how the surface composition is linked to the oceanic composition. The spectro-imager MAJIS onboard JUICE will map the surface composition with unprecedented spatial resolution, providing clues on the exchange processes between the ocean and the surface and the ocean habitability.
MAJIS (Moons and Jupiter Imaging Spectrometer) is one of the remote-sensing on JUICE. As a spectro-imager, its main purpose is to take both spectra and images in the visible and infrared wavelength.
Similar to a camera, it will take pictures of the targets, but not only in a few color band like most camera, but with more than 1000 “colors” (wavelength) between 0.4 micron (visible blue) to 5.7 microns (infrared). For each pixel on an image, it will acquire a full spectra from the visible to the infrared, providing direct constraints on the chemical composition of the observed area.
MAJIS will observe various targets in the Jupiter system, including the Galilean moons, Jupiter’s atmosphere, rings, minor satellites. In particular, it will acquire high-resolution spectro-images during close flybys of the icy moons, and during the orbital phase around Ganymede, with spatial resolution as high as 30-40m/pixel. As a comparison, the best resolution obtained with the Galileo mission was 4km/pixel (see Fig. 1b).
In the same way that the color of a fruit in the visible is controlled by the concentration in various pigments, the shape of the infrared spectra of any surface material will vary depending on its composition. By analyzing precisely such a spectra, notably by identifying any hollow and bump and by comparing with reference spectra acquired in the laboratory, one can determine the composition of the observed area. Using such an approach, scientists working the analysis of the NIMS Galileo data were able to identify at the surface of Europa and Ganymede areas dominated by water ice, while other areas were associated to non-icy compounds (Fig. 1). In the case of Europa, the non-icy compounds appear to be consistent with spectra of various salts and the fact that these salt-rich material is located along tectonic features suggests a direct transport from the underlying ocean.
For the moment, at the exception of a very few areas on Europa, most of the infrared data has been acquired at very low spatial resolution either from the space missions (Galileo, New Horizons, JUNO, cf. Fig. 2) or from ground-based and space telescopes (VLT, Keck, Subaru, JWST). They have, however, already suggested some chemical diversity at the surface of the icy moons, indicating areas with highest proportion of salts and other with some organic compounds. JUICE and in particular the MAJIS instrument will revolutionize our vision of these icy moons by providing chemical information with unprecedented resolution. As an example, Fig. 2 shows a typical area at Ganymede acquired in the visible by the Galileo mission, with a resolution comparable to the expected resolution of MAJIS during the orbital phase at 500 km altitude. For each small detail on this image MAJIS will get a full spectrum, which then will be used to identify the different chemical compounds and assess their concentration. Any subtle variation in the infrared spectra will provide key information on the variability of chemical composition, their correlation with the geological features, and thus indirectly their link with the ocean composition. Any change in chemical composition associated with surface ages may also inform about its past evolution.