New information on Ganymede from the old Galileo mission data

Ganymede (Image NASA)
Ganymede (Image NASA)

An article published in the journal “Geophysical Research Letters” describes a new analysis of data collected by NASA’s Galileo space probe during its flybys of Ganymede, one of Jupiter’s great moons. Glyn Collinson of NASA’s Goddard Space Flight Center and some colleagues reused the old flight software to process the data discovering new information on its magnetic field, in particular on its auroras and on the magnetic reconnection phenomena.

With a diameter of 5,268 kilometers (3,273 miles), Ganymede is the largest of Jupiter’s moons and is also the most massive. It has an iron core that probably has internal convective movements which generate a magnetic field, the only known case of a satellite that has one in the solar system. Discovered by Galileo Galilei, it was studied by the NASA’s space probe named after him, which among other things detected its magnetosphere.

The Galileo space probe’s Plasma Subsystem (PLS) instrument was designed to collect charged particles to be analyzed and to measure their mass and energy. In the course of 6 Ganymede flybys carried out since 1996, it was the main instrument for the analysis of the plasma – a gas composed of charged particles – of the moon’s magnetosphere.

These measurements also allowed to obtain information on the interaction between Ganymede’s magnetosphere and Jupiter’s plasma, whose particles are directed towards the moon’s surface by the local magnetic field. In some ways it’s an interaction similar to that between the Earth’s magnetosphere and solar wind, even if the Jovian plasma moves much more slowly than solar wind.

The Galileo space probe’s mission ended in 2003 but the data sent to Earth are available in the archive. A new analysis and a comparison with observations of Ganymede conducted with the Hubble Space Telescope revealed new information, such as the precise location of the auroras, useful to try to understand what generated them.

The data also showed an event of magnetic reconnection, a common phenomenon in magnetospheres but still not entirely understood, so much so that on March 12, 2015, NASA launched the four MMS space probes exactly to study it in the Earth’s magnetosphere. The Galileo space probe detected strong plasma flows pushed between Jupiter and Ganymede due to an event of magnetic reconnection occurred between their magnetospheres.

Research on magnetospheres is important because the Earth’s one is a shield that protects life forms on the planet from radiation that would be lethal. Understanding the mechanisms of certain phenomena connected to magnetospheres can help to better understand the consequences of events such as solar flares, which in extreme cases can still be harmful, also for electronic equipment on Earth and for satellites in orbit.

This research based on data collected by the Galileo space probe shows how these missions can help the research even several years after their end. The information gathered by the PLS instrument will be analyzed again to look for new clues to the possible underground ocean of Ganymede.

Infographic about Ganymede's magnetosphere (Image NASA's Goddard Space Flight Center/Mary Pat Hrybyk-Keith)
Infographic about Ganymede’s magnetosphere (Image NASA’s Goddard Space Flight Center/Mary Pat Hrybyk-Keith)

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