It has been orbiting and observing Jupiter and its moons for four years, but the scientific spacecraft Juno still has some surprises to share. For the first time, he has photographed the north pole of one of the strangest objects in the Solar System, Jupiter Ganymede’s moon.
There, the constant rain of plasma from Jupiter’s magnetosphere has dramatically altered the moon’s icy surface, the new images reveal.
Ganymede is quite surprising, actually. It is the largest and most massive moon in the entire Solar System. At 5,268 kilometers (3,273 miles) in diameter, it outweighs the entire category of dwarf planets, even registering larger than Mercury (but not more massive – Mercury is dense. It’s like a fruitcake from the planet.)
It consists of water ice and silicate rock, with a frozen shell wrapped around a liquid ocean, wrapped around a core of liquid iron. This nucleus is believed to give Ganymede another point of distinction: it is the only moon in the Solar System with its own magnetosphere, generated by convection in the nucleus.
Because Ganymede orbits Jupiter within the planetary magnetic field, the moon’s magnetosphere is also embedded within it. This creates impressively strong plasma waves as the plasma particles, primarily electrons, accelerate along the complex lines of the magnetic field.
This acceleration along the lines of the magnetic field also has another effect: the aurora. Here on Earth, those particles are channeled into the polar regions, where they interact with atoms high in the atmosphere to generate impressive light shows.
Ganymede, however, has a negligible atmosphere, so a large proportion of the plasma is poured directly onto the surface of the moon. In the newly released infrared images, taken by Juno’s Jovian Infrared Auroral Mapper (JIRAM) instrument, the effect of that constant plasma rain is clear.
“The JIRAM data shows that the ice in and around the north pole of Ganymede has been modified by plasma precipitation,” said planetary scientist Alessandro Mura, Juno co-researcher at the National Institute of Astrophysics in Italy.
“It is a phenomenon that we have been able to learn for the first time with Juno because we can see the North Pole in its entirety.”
The ice at the two Poles of Ganymede has a different infrared signature than the ice at the moon’s equator. And analysis has revealed that this is because constant plasma rain has altered the very structure of ice crystals.
On Earth, and on most of Ganymede’s surface, the molecules in most of the ice are arranged in a highly ordered hexagonal pattern. But under certain conditions, this orderly network can fall into structural disorder. This messy shape is called amorphous ice; While rare on Earth, it is actually quite common in space, in dust grains in interstellar clouds, in comets, and in icy bodies.
Three of Jupiter’s moons, Europa, Callisto and Ganymede, are frozen; And oddly enough, they all have different ice profiles. Calisto’s ice is crystalline. Europe is amorphous. And Ganymede is a strange mixture.
Previous research found that this may have something to do with proximity to Jupiter; Europe is the closest and is therefore subject to the highest level of radiation from the radiation belts generated by Jupiter’s magnetosphere. Callisto is the furthest and is subject to the least radiation.
(NASA / JPL-Caltech / SwRI / ASI / INAF / JIRAM)
Ganymede is in the middle, and scientists have previously suggested that its magnetic field would channel radiation to its poles, leading to a higher concentration of amorphous ice at those locations.
This has now been validated by Juno’s data. The JIRAM team believes that constant bombardment of charged particles in the polar regions prevents ice from forming a crystalline structure.
Future observations could reveal more about this fascinating phenomenon. Juno’s main mission is to observe Jupiter, but more dedicated missions are underway. The European Space Agency plans to launch an orbiter called JUpiter ICy Moons Explorer (JUICE) in 2022 to verify not only Ganymede, but also Europe and Callisto.
These comparative observations should reveal much more about Ganymede ice and the effect of Jupiter’s radiation belts than just looking at Ganymede alone.
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