JUICE: A Look at our Blue Planet

Evidence of life, ozone hole and magnetic shield: during its flyby of Earth, the space probe got to know our home from an unusual perspective.

Approximately four weeks after the successful Moon and Earth flyby of ESA’s space probe Jupiter Icy Moons Explorer (JUICE), the scientific and technical teams at the Max Planck Institute for Solar System Research (MPS) in Germany have analyzed the first observational data from their two instruments on board. The data not only reveal that both the Submillimetre Wave Instrument (SWI) and the Jovian Electron and Ion Sensor (JEI) of the Particle Environment Package (PEP) are functioning as expected. The results also invite a kind of cosmic control experiment: What could an extraterrestrial space probe, equipped like JUICE and with no prior knowledge of Earth and its inhabitants, find out about our planet? Could it recognize the structure of the magnetosphere, our magnetic protective shield? And would it consider Earth to be inhabited?

Seeing Earth from space is something special - regardless of whether it is through the eyes of astronauts or through the instruments of an unmanned space probe. Such images, such as the first color photograph of the entire globe taken by the Apollo 8 mission crew in 1968 or the Pale Blue Dot photograph captured by the space probe Voyager 1 34 years ago, have achieved an almost iconic status. About four weeks ago, JUICE's flyby of the Moon and Earth provided an opportunity for a similar change of perspective.

In order to adjust its flight speed and direction on its eight-year journey to Jupiter, JUICE first flew past the Moon at a distance of 750 kilometers on 19 August 2024 and then past Earth at a distance of 6840 kilometers on 20 August 2024. The scientific instruments, which were developed and built under the lead of the MPS, were also switched on during the maneuver. While SWI investigates the composition, temperature and dynamics of atmospheres, PEP-JEI determines the energy and distribution of charged particles in the vicinity of planets.

A promising candidate for the existence of life

During the Earth flyby, SWI “listened” for the signals of more than a hundred molecules in Earth's atmosphere. For example, the instrument searched for water vapor, for the elements carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur, which are considered to be the basic building blocks of life, as well as for methane and other molecules that are metabolic products of life. “So far, we have only evaluated a small part of the data,” says MPS scientist Dr. Paul Hartogh, who heads the SWI team, summarizing the current status. “But there is already a lot to suggest that the Earth is a promising candidate for the existence of life,” he adds with a smile. He expects that the SWI team will come to a similar conclusion as the astronomer Carl Sagan, the famous pioneer of the search for extraterrestrial life. In 1993, the researcher analyzed measurements from the Galileo space probe's Earth flyby and found clear indications of life-friendly environmental conditions and even life itself.

Unlike previous space instruments, SWI relies on a new technology. The heterodyne spectrometer superimposes a reference wave on the received signal and thus shifts it into the range of classic radio waves. “SWI is a pioneering instrument that has never been used in deep space before,” says Hartogh. The instrument's very high spectral resolution is crucial to its performance. This means that signals from very rare trace gases in the atmosphere can be detected even from a great distance.

During the flyby, the team was able to make spectroscopic observations of the Earth's atmosphere at frequencies around 1200 gigahertz for the first time. The Earth's atmosphere is expected to show signatures of ozone and oxygen molecules, among others, in this frequency window. "Our measurements show a particular spatial variation of ozone over the South Pole region," says SWI team member Dr. Christopher Jarchow. "This could be consistent with an ozone hole that usually begins to develop at this time of year," adds Dr. Ladislav Rezac, also a member of the SWI team at MPS. For the first time, the team has obtained data of sufficient quality to carry out an analysis of wind speeds in the Earth's atmosphere. Wind measurements are an important part of SWI and will be performed routinely in Jupiter's atmosphere once the spacecraft reaches its final destination.

Flight through the magnetopause

JUICE’s visit home was also an important test for the PEP-JEI instrument from Göttingen. The instrument measures the energy of the ions and electrons that surround the space probe. In the Jovian system, one of its aims is to find out more about the structure and dynamics of Jupiter's enormous magnetosphere.

During the four time-windows in which the instrument was switched on during the maneuver, JUICE apparently flew through different areas of Earth's magnetosphere. This can be seen in the data. The space probe crossed the magnetopause, the interface between the Earth's magnetosphere and the solar wind, at least four times and passed the bow shock wave on August 21, 2024. At the bow shock wave, the solar wind, which flows around the Earth at supersonic speed, is slowed down to subsonic speed. The data initially show particles from the sphere of influence of the Earth's magnetosphere, then abruptly protons and helium ions from the solar wind.

In addition, JUICE penetrated the innermost part of the magnetosphere during the flyby. The so-called plasmasphere surrounds around the Earth in a torus shape and reaches at least 25500 kilometers into space. The comparatively cool plasma that prevails there consists of electrons, protons, helium and oxygen ions. “The composition of the plasmasphere has rarely been measured before,” says MPS scientist and PEP team member Dr. Markus Fränz. “The current observations of PEP-JEI can best be explained by a very high proportion of oxygen ions,” he adds.

“During the Earth flyby, we succeeded in revealing the basic features of the structure of Earth's magnetosphere with just a few measurements. We now know that our instrument is well prepared for measurements near Jupiter,” says MPS scientist Dr. Norbert Krupp from the PEP team. JUICE will, of course, spend much longer in the Jovian system and orbit the planet in many different orbits. The current data therefore makes the members of the PEP-JEI team confident that a complete picture of the gas giant's plasma environment will emerge.