JUICE instrument detects four atmospheric gases during Earth fly-by

The spacecraft’s infrared spectrometer matched measurements from another satellite and showed sensitivity to methane, ozone and two other gases.

Editorial illustration — not from the study.

Researchers assessed MAJIS using infrared observations collected over tropical parts of the Pacific Ocean during JUICE’s lunar and Earth gravity assist in August 2024. They compared the data with measurements from the Infrared Atmospheric Sounding Interferometer, or IASI, over the wavelength range shared by the instruments: 3.6 to 5.56 micrometres.

After removing observations affected by clouds or reflected light from the ocean, the instruments showed an excellent match in their measured infrared brightness. Comparisons with computer-generated atmospheric spectra also revealed signals associated with methane, nitrous oxide, ozone and carbon monoxide.

What the instrument detected

The comparison suggests that MAJIS’s infrared measurements are calibrated to better than 10%, with accuracy declining to about 15% between 5.25 and 5.56 micrometres. The researchers identified a small wavelength offset of about 4 nanometres beyond 4 micrometres. They estimated a signal-to-noise ratio of 200–300, in line with the instrument’s expected performance, and found that its spectra were sensitive to methane, nitrous oxide, ozone and carbon monoxide.

Why the test matters

The Earth fly-by provided an in-flight test of MAJIS before JUICE reaches Jupiter in 2031. Its agreement with IASI and its sensitivity to several atmospheric gases indicate that the instrument is performing well enough to support planned studies of Jupiter’s atmosphere, where MAJIS is intended to examine atmospheric composition.

Evidence and caveats

The study is an instrument-performance assessment based on Earth observations, not a measurement of Jupiter. Researchers compared MAJIS with 36 co-located IASI observations after filtering out clouds and ocean glint, and also compared the data with spectra produced by a radiative-transfer model. The analysis did not retrieve gas concentrations; some model profiles for methane, carbon monoxide and nitrous oxide were scaled to improve the fit. The observations covered part of the tropical Pacific, and the two instruments do not have identical spectral coverage or resolution.

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