Ganymede in the spotlight

via Astronomy Now.

Ganymede is the largest moon in the solar system, and also the largest moon of the largest planet, Jupiter. With a diameter of 5,262 kilometres, larger than that of Mercury, and its own region of magnetic influence, this is a world in its own right and has been the subject of the scrutiny of two recent studies.

Wes Patterson of the Johns Hopkins University Applied Physics Laboratory and colleagues have worked for the past seven years to piece together images from the Voyager and Galileo missions. The result is a map of the satellite that joins it to only two other moons (our own and Callisto, another of Jupiter’s satellites) to have had maps constructed of the surface.

Credit: Patterson et al

Credit: Patterson et al

The map, shown above, provides information on geological features and the cratering history, which provides details on processes occurring on the moon since the time of its formation. Four series of grooves and deposits of light material have been mapped, each linked to a different global event in the history of the moon.

A bit further away from the surface now and into the region surrounding Ganymede. The planets of the solar system have various amounts of magnetic influence over their immediate region – either due to their own magnetic fields or due to plasma turbulence around them. These are known as magnetospheres, magnetic bubbles within the flow of charged plasma coming off the surface of the Sun, which carries the solar magnetic field to the distant reaches of the solar system.

Interactions between the magnetospheres and the solar wind magnetic field power releases of charged particles into some planetary atmospheres, leading to the aurora. In Jupiter’s case, the magnetosphere is absolutely enormous due to the strength of the Jovian magnetic field. It is so large that the effects of the solar wind are confined to a small region of variable ‘polar emissions’. The main auroral oval on Jupiter, which exists in a ring around the polar emissions and is one hundred times as powerful as Earth’s aurorae, is created by an entirely different route.

The Jovian moon Io is volcanic. It lies closest of all the Galilean satellites to the body of the planet and is pulled back and forth by the other satellites. Its orbit therefore brings it closer and farther from the giant planet altering the tidal forces (the closer half of the satellite is pulled harder to the planet than the far side, the difference between the pulls alters as the distance between the satellite and planet does, stretching and compressing the body). The result is massive jets of material thrown from the satellite into orbit of Jupiter. This Io torus is then subjected to the particles in the Jovian radiation belts as well as high energy solar radiation, such as UV and x-rays. These ionise the Io torus, creating a plasma sheet. Plasma is made up of equal numbers of positive and negative particles. Charged particles tend to follow directions supplied by magnetic fields and as these ones are created inside Jupiter’s magnetic field, they must follow its rotation. Jupiter rotates quickly and the further the plasma is from the planet, the faster it must move to make a full rotation in one Jovian day. Up to a point, this holds, but past that point the energy required to accelerate the particles round simply isn’t there and they end up spinning round at a lower rate. This break from ‘corotation’ leaves two magnetic discs spinning one inside the other. The result is an electric circuit that runs from one disc, along the field line to the atmosphere, then back up to another disc. This circuit accelerates the electrons into Jupiter’s atmosphere.

As well as the polar emissions and the main auroral oval, the aurorae also contain ‘hot spots’. These are satellite footprints. Io itself does not corotate with the magnetic field – it has no need to. It does however have something of a tail due to the material coming off it. These material gets ionised and as it hasn’t yet entered corotation, some of it also forms a circuit and ends up accelerated into the atmosphere.

Looking at the auroral images shown below, there’s another hot spot shown – that relating to Ganymede. This time there is no tail. Ganymede has its own magnetosphere cutting a swathe inside the Jovian one. The Hubble Space Telescope has seen aurora happening on the satellite, its thin O, O2 and possibly ozone atmosphere glowing due to the impact of plasma from the giant planet’s magnetosphere. But it is the auroral footprint that interests a recent study by Denis Grodent of the University of Liege in Belgium.

He used HST images from 2007 to study the ultraviolet emissions from the Jovian aurorae (see at the bottom of the article). Looking at the tail of Io’s footprint for example, Grodent saw that the emissions were all from the same altitude in Jupiter’s atmosphere, suggesting the accelerated particles reached the same energy at all points of the tail. He also saw the spectroscopic signature of methane higher than expected. As a relatively heavy molecule, methane and the other hydrocarbons tend to concentrate at lower altitudes.

Turning to the dimmer footprint created by Ganymede’s path round the planet, Grodent found it to correspond well with the diameter of the magnetosphere rather than the moon itself. He also saw periodic variations in the brightness of the footprint – one showing brightnings every five hours, one varying on a timescale of 10-40 minutes and a final one varying every 100 seconds.

The five hour, or half a Jovian day, variations may be due to the satellite passing through the plasma disc created by ionisation of the Ion torus. This is tilted to the orbit of Ganymede and so the satellite punches through it twice a Jovian day. Ganymede takes a week to get round its orbit, meaning the plasma disc interaction is regulated by the much faster rotation of the planet below.

The 10-40 minute variations are thought to relate to general plasma populations hitting the magnetosphere. Finally the 100 second variations are thought to be the result of the magnetosphere of Ganymede interacting with that of Jupiter, forcing Jovian field lines to drape over it, get stretched over it and reconnect behind it. But Grodent stresses nothing is certain at this stage of the investigation.


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