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New views of Solar System's largest moon
Posted: September 17, 2009

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In two separate studies, scientists have created the first global geological map of Jupiter's moon Ganymede and offered new insight into the complex electromagnetic interactions between Jupiter, Ganymede and Io.

The seven year mapping effort was led by Wes Patterson of the Johns Hopkins University Applied Physics Laboratory and is only the third ever map completed of a moon, along with Earth's Moon and Ganymede's cratered neighbour Callisto. The new map combines low-resolution Voyager images with high resolution Galileo mission images.

Global image mosaic of Ganymede with four episodes of light material formation superposed (red, yellow, green, and blue). Variability of the orientations with latitude and longitude of groove sets within each episode are shown. Image: Patterson et al.

“The map really gives us a more complete understanding of the geological processes that have shaped the moon we see today,” says Patterson, whose team presented the map at the European Planetary Science Congress this week. The map details geologic features of the moon that formed and evolved over much of our Solar System’s history, including evidence of its internal evolution, its dynamical interactions with the other Galilean satellites, and of its impact cratering history.

“By mapping the entirety of Ganymede’s surface, we can more accurately address scientific questions regarding the formation and evolution of this truly unique moon,” says Patterson. Vast swaths of grooved terrain cover the surface of the satellite, and formed in a specific sequence that tells planetary scientists about the forces that must have been necessary to form those swaths. The new map will help piece together this aspect of the moon's history and will provide a reference for the future exploration of the Jovian moons, such as the joint NASA-ESA mission Europa Jupiter System Mission which includes a Ganymede orbiter.

“A primary goal of the next flagship mission to the Jupiter system will be to characterise, in detail, the geophysical, compositional, geological, and external processes that affect icy satellites,” he says. “This map will be an invaluable tool in determining how best to address those goals for Ganymede.”

HST/ACS FUV image of auroral region in Jupiter's Northern hemisphere, showing Io and Ganymede auroral footprints. Image: Grodent/Hubble Space Telescope Team HST program GO-10862, date: 17 May 2007.

With a diameter of 5,262 kilometres, Ganymede is the largest moon in the Solar System, and is even larger than both planet Mercury and dwarf planet Pluto. It is also the only satellite in the Solar System known to have its own magnetosphere. Also presented at EPSC this week are the results of a study of Jupiter's aurorae, which has given new insight into the complex interactions between the giant planet, Ganymede and Io.

As these two moons orbit Jupiter, they interact with regions of plasma and generate electromagnetic waves that are projected along Jupiter’s magnetic field lines towards its poles, generating auroral bright spots. In a new study based on images taken by the Hubble Space Telescope, scientists from the University of Liege in Belgium have monitored these auroral features in ultraviolet wavelengths in unprecedented detail.

“Each of these auroral structures is telling an ongoing story about vast transfers of energy taking place far away from the planet,” says Denis Grodent, who presented the results at EPSC on Thursday. “By analysing the exact locations of these features and how their shape and brightness changes as Io and Ganymede move in their orbit around Jupiter, we have created the most detailed picture to date of how Jupiter and these moons are electromagnetically interconnected.”

HST/ACS FUV image of auroral region in Jupiter's Southern hemisphere, showing Io and Ganymede auroral╩footprints. The moon╩in the background is Callisto. Image: Grodent/Hubble Space Telescope Team HST program GO-10862, date 19 May 2007.

Ganymede's magnetic field is so strong as to carve a protective magnetic bubble within Jupiter’s powerful magnetosphere. By analysing the Hubble images, Grodent and colleagues could measure the size of Ganymede's auroral footprint for the first time, finding it is too big to be a simple projection of Ganymede’s cross-section. Using a three-dimensional computer model to map the footprint back along the field lines, the team has found that it corresponds well with the diameter of Ganymede’s mini-magnetosphere.

In addition, the sequences of Hubble images revealed unexpected brightness variations of Ganymede’s auroral footprint at three different timescales: 100 seconds, 10 to 40 minutes, and 5 hours.

“Each of these timescales appears to refer to a specific aspect of the Ganymede-Jupiter interaction and allows us to identify possible actors of this interaction,” says Grodent. “The 5 hour variation appears to be linked to the rotational period of Jupiter’s magnetic field and the movement of Ganymede through the tilted plasma sheet that surrounds the planet. The 10-40 minute variations could be due to sudden changes in energy due to plasma being injected into the system and the 100 second pulses may be linked to bursts of magnetic energy being suddenly released when Jupiter and Ganymede’s magnetic field lines connect. However, we are not sure at this stage.”

The same team also mapped the positions of all possible locations of the auroral footprint of Jupiter’s volcanically active moon, Io, mapping the altitude of Io's tail with great accuracy for the first time. “We found that the tail is at an altitude of approximately 900 kilometres above Jupiter’s cloud tops," says Bertrand Bonfond. "Interestingly, although the brightness of the tail decreases as it gets further away from the main spot, the altitude remains relatively constant. We also saw spectral absorption indicating that methane is present, which is unexpected at such a high altitude.”

The Planets
From tiny Mercury to distant Neptune and Pluto, The Planets profiles each of the Solar System's members in depth, featuring the latest imagery from space missions. The tallest mountains, the deepest canyons, the strongest winds, raging atmospheric storms, terrain studded with craters and vast worlds of ice are just some of the sights you'll see on this 100-page tour of the planets.

Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.

3D Universe
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