On June 27, 1996, NASA’s Galileo spacecraft made humanity’s first flyby of Ganymede, Jupiter’s largest moon, discovering that it is the only moon known to possess an internally generated magnetic field.
As at Earth, Ganymede’s magnetic field projects a magnetic bubble around it called a magnetosphere. Galileo carried a package called the Plasma Subsystem (PLS) that was designed to measure charged particles. Although PLS collected data during the flyby, the results were never published. Resurrecting the original flight software, Glyn Collinson of NASA’s Goddard Space Flight Center and colleagues processed the Galileo PLS data and present them for the first time.
“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. We found there’s a whole piece no one knew about,” Dr. Collinson said.
Published in the journal Geophysical Research Letters, the new results showed a stormy scene: particles blasted off the moon’s icy surface as a result of incoming plasma rain, and strong flows of plasma pushed between Jupiter and Ganymede due to an explosive magnetic event occurring between the two bodies’ magnetic environments.
These observations could be key to unlocking the secrets of the moon, such as why Ganymede’s auroras are so bright.
“We know that Earth’s magnetosphere is key to in sustaining life on our planet, because it helps protect our planet from radiation coming from space,” Dr. Collinson and co-authors explained.
“Some think Earth’s magnetosphere was also essential for the initial development of life, as this harmful radiation can erode our atmosphere.”
“Studying magnetospheres throughout the Solar System not only helps us learn about the physical processes affecting this magnetic environment around Earth, it helps us understand the atmospheres around other potentially habitable worlds, both in our own Solar System and beyond.”
Ganymede’s magnetosphere offers the chance to explore a unique magnetic environment located within the much larger magnetosphere of Jupiter. Nestled there, it’s protected from the solar wind, making its shape different from other magnetospheres in the Solar System.
Typically, magnetospheres are shaped by the pressure of supersonic solar wind particles flowing past them. But at Ganymede, the relatively slower-moving plasma around Jupiter sculpts the moon’s magnetosphere into a long horn-like shape that stretches ahead of the moon in the direction of its orbit.
Flying past Ganymede, Galileo was continually pummeled by high-energy particles.
Plasma particles accelerated by the Jovian magnetosphere, continually rain down on Ganymede’s poles, where the magnetic field channels them toward the surface.
The new analysis of Galileo PLS data showed plasma being blasted off the moon’s icy surface due to the incoming plasma rain.
“There are these particles flying out from the polar regions, and they can tell us something about Ganymede’s atmosphere, which is very thin. It can also tell us about how Ganymede’s auroras form,” said co-author Dr. Bill Paterson, also from NASA’s Goddard Space Flight Center.
Ganymede has auroras, or northern and southern lights, just like Earth does. However, unlike our planet, the particles causing Ganymede’s auroras come from the plasma surrounding Jupiter, not the solar wind.
When analyzing the data, the team noticed that during its first Ganymede flyby, Galileo fortuitously crossed right over Ganymede’s auroral regions, as evidenced by the ions it observed raining down onto the surface of the moon’s polar cap.
By comparing the location where the falling ions were observed with data from the NASA/ESA Hubble Space Telescope, the researchers were able to pin down the precise location of the auroral zone, which will help them solve mysteries, such as what causes the auroras.
As it cruised around Jupiter, Galileo also happened to fly right through an explosive event caused by the tangling and snapping of magnetic field lines.
This event, called magnetic reconnection, occurs in magnetospheres across our Solar System.
For the first time, Galileo observed strong flows of plasma pushed between Jupiter and Ganymede due to a magnetic reconnection event occurring between the two magnetospheres. It’s thought that this plasma pump is responsible for making Ganymede’s auroras unusually bright.
Future study of the PLS data from that encounter may yet provide new insights related to subsurface oceans previously determined to exist within the moon using data from both Galileo and Hubble.
Source: Sci News