More than two decades ago, NASA's Galileo spacecraft did its first flyby of Ganymede. The Galileo mission lasted for almost eight years, from 1996 to 2003, during which time the spacecraft made six targeted flybys of Jupiter's biggest moon.
But the most exciting discoveries were made during the spacecraft's maiden voyage, dated June 27, 1996. Galileo's first journey far across the solar system uncovered that Ganymede, the system's largest moon, is far more unusual than astronomers had expected.
On that initial flyby, the Galileo spacecraft, which is about the size of a full-grown giraffe, carried an instrument called the Plasma Subsystem (PLS). This instrument ended up revealing that Ganymede acts just like a regular planet and generates its own magnetic field, separate from that of Jupiter.
Years later, NASA researchers have now taken a fresh look at the original flight software, analyzing Galileo PLS data that has never been published before. Their endeavor yielded even more surprising discoveries and helped piece together the puzzle of Ganymede's unique environment.
"We are now coming back over 20 years later to take a new look at some of the data that was never published and finish the story," said Glyn Collinson, from NASA's Goddard Space Flight Center in Greenbelt, Maryland.
"We found there's a whole piece no one knew about," he revealed.Last month, Collinson's team published a paper on Ganymede's magnetosphere, which is the magnetic bubble around Jupiter's moon. This plasma of electrically charged gas particles surrounding Ganymede is constantly being shaped by Jupiter's larger magnetic field, making it completely "different from other magnetospheres in the solar system," NASA notes in a news release.
The new analysis of the PLS-recorded data, published in Geophysical Research Letters, exposed that plasma particles raining down from Jupiter are being channeled by the moon's magnetosphere toward its icy surface. Furthermore, the 20-year-old Galileo observations recorded that Jupiter's plasma rain is blasting charged water‐based particles off Ganymede's surface.
"There are these particles flying out from the polar regions, and they can tell us something about Ganymede's atmosphere, which is very thin," said study co-author Bill Paterson, also from Goddard.
Paterson, who served on the PLS team during the Galileo mission, believes these observations can hold important clues into how Ganymede's auroras form.As luck would have it, the team discovered that the Galileo spacecraft passed right over Ganymede's auroral regions — something which wasn't previously known. The spacecraft serendipitously flew over the exact location where these regions have also been spotted by the Hubble Space Telescope and managed to record ions falling down on the surface of Ganymede's polar cap.
These revelations have now helped researchers pinpoint the exact location of the moon's auroral zone. Going forward, the team hopes to find out how Ganymede's auroras are created and why the moon's northern and southern lights are so bright.
One explanation could be connected to Ganymede's magnetic reconnection, an event that describes the explosive interactions between Jupiter's magnetic field lines and the magnetosphere of its moon. On its initial flyby, the Galileo spacecraft recorded strong flows of plasma pushed between Jupiter and Ganymede.
"We also observe evidence for the acceleration of plasmas by magnetic reconnection, wherein magnetic energy is converted into kinetic and thermal energy in space plasmas," the NASA Goddard team wrote in their paper.