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Io's Atmosphere Detected During Jovian Mutual Events

At 5h UT of 20090807 Jupiters moons Io and Europa were involved in back to back mutual events. First the shadow from Io eclipsed Europa, followed shortly by Io occulting Europa. An anomalous event was detected in the resulting lightcurve.
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Solar system's most volcanic body to go dormant
The most volcanically active body in the solar system has just received a death sentence. Jupiter's moon Io, whose surface erupts with active volcanoes, will one day become dormant, a new study analysing more than 100 years of observations suggests.

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Io proche de l'équilibre thermique
Io est-il en équilibre thermique? En utilisant des observations astrométriques des quatre satellites galiléens de Jupiter (Io, Europe, Ganymède et Callisto) couvrant la période 1891-2007, un groupe de chercheurs de l'Observatoire de Paris (IMCCE) et de l'Observatoire royal de Belgique a montré que la chaleur induite par les marées à l'intérieur de Io correspond au flux de chaleur observé en surface. La dissipation dans Jupiter induite par les effets de marées créés par Io a également été trouvée proche de sa limite supérieure attendue. C'est la première fois que la dissipation par effet de marée a été mesurée dans une planète géante grâce à l'astrométrie. Ces résultats sont publiés dans le journal Nature du 18 juin 2009.

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Animated GIF of volcano on Io.

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In 1610, using one of the worlds first telescopes, Galileo discovered Jupiters moons: Io, Europa, Ganymede and Callisto. For the next several hundred years, astronomers assumed the four moons were similar to our own icy, cold, pockmarked and geologically dead. But then in 1979, the Voyager probe flew by Io, the closest moon to Jupiter, and returned images of volcanic eruptions hundreds of kilometres high, larger than any ever seen in the solar system. In the 30 years since then, a series of observations has revealed Io to be an inferno more volatile than Dante himself imagined.

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Astronomers have witnessed a spectacular eruption on Io, the fiery moon of Jupiter. NASA's New Horizons probe snapped pictures (right) of a volcanic plume 350 kilometres high 40 times the height of Mount Everest when the probe passed by Jupiter in February and March on its way to Pluto.

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Jupiter's volcanic moon Io is veiled by a thin atmosphere, but how much its volcanoes and chunks of frozen gas contribute to its atmosphere has puzzled scientists for decades.
The New Horizons spacecraft recently documented the moon's glowing aurora, however, giving researchers a chance to solve the atmospheric mystery.
Io is the most volcanically active object in the solar system. The moon's pockmarked and colorful appearance is not unlike a pepperoni pizza.

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The missions investigations of Jupiters four largest moons focused on Io, the closest to Jupiter and whose active volcanoes blast tons of material into the Jovian magnetosphere (and beyond). New Horizons spied 11 different volcanic plumes of varying size, three of which were seen for the first time and one a spectacular 200-mile-high eruption rising above the volcano Tvashtar that offered an unprecedented opportunity to trace the structure and motion of the plume as it condensed at high altitude and fell back to the moons surface. In addition, New Horizons spotted the infrared glow from at least 36 Io volcanoes, and measured lava temperatures up to 1,900 degrees Fahrenheit, similar to many terrestrial volcanoes.

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New Horizons global map of Ios surface backs the moons status as the solar systems most active body, showing more than 20 geological changes since the Galileo Jupiter orbiter provided the last close-up look in 2001. The remote imagers also kept watch on Io in the darkness of Jupiters shadow, noting mysterious glowing gas clouds above dozens of volcanoes. Scientists suspect that this gas helps to resupply Ios atmosphere.

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Dramatic changes in the atmospheric density of Jupiter's moon Io and its interaction with Jupiter's magnetosphere during solar eclipse were observed through Io's aurora on four occasions this past spring as the New Horizons spacecraft rounded Jupiter for a gravity assist on its way to Pluto.
Scientists using New Horizons' Southwest Research Institute (SwRI)-developed Alice ultraviolet spectrograph, which is designed to image ultraviolet emissions, noted auroral brightness and morphology variations as the spacecraft entered and then exited the eclipse zone revealing changes in the relative contribution of sublimation and volcanic sources to the atmosphere. The findings were supported by concurrent Hubble Space Telescope ultraviolet imaging and will be published in the Oct. 12 issue of Science.
FUV (far-ultraviolet) aurora morphology also reveals the plumes effect on Io's electrodynamic interaction with Jupiter's magnetosphere. Comparisons to simulations of Io's aurora indicate that volcanoes supply 1 percent to 3 percent of Io's dayside atmosphere.
Aurora observations, particularly while Io is in solar eclipse by Jupiter, can provide information on both Io's atmosphere and its interaction with Jupiter, the paper states.
An aurora is a luminous phenomenon in the upper atmosphere of a planet caused by the emission of light from atoms excited by electrons accelerated along magnetic field lines. Most planetary aurorae occur in the polar regions, but Io's aurora is brightest near its equator as well as in volcanic plumes distributed across the satellite.

"Io is volcanically active, and that volcanism ultimately is the source material for Io's sulphur-dioxide atmosphere, but the relative contributions of volcanic plumes and sublimation of frosts deposited near the plumes have remained a question for almost 30 years" - Dr. Kurt Retherford, a senior research scientist in the Space Science and Engineering Division at the Institute.

The interaction between Io's atmosphere and the Io plasma torus produces displays of auroral emissions on Io, supplies plasma to Jupiter's magnetosphere and physically links Io to Jupiter, according to the paper.

"When Io goes into solar eclipse, and during the night, its surface temperature drops significantly, causing diminished sublimation of surface material into the atmosphere. The atmosphere at that point collapses down so that all that is left supplying the atmosphere are the volcanoes" - Dr. Kurt Retherford.

A dramatic difference between Io's dayside and nightside atmospheric density best explains the aurorae observations, he added.
Alice provides spectral images in the extreme- and far-ultraviolet (EUV and FUV) passbands. S. Alan Stern, NASA's associate administrator for the Science Mission Directorate and former executive director of the Space Science and Engineering Division at SwRI, is the principal investigator of New Horizons, Alice and Ralph, a visible and infrared camera onboard the spacecraft. Prof. Joachim Saur at the University of Cologne, Germany, conducted the simulations.

The paper is titled, "Io's Atmospheric Response to Eclipse: UV Aurorae Observations," by K.D. Retherford, J.R. Spencer, S.A. Stern, J. Saur, D.F. Strobel, A.J. Steffl, G.R. Gladstone, H.A. Weaver, A.F. Cheng, J.Wm. Parker, D.C. Slater, M.H. Versteeg, M.W. Davis, F. Bagenal, H.B. Throop, R.M.C. Lopes, D.C. Reuter, A. Lunsford, S.J. Conard, L.A. Young and J.M. Moore.

Source Southwest Research Institute

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Title: Cassini UVIS Observations of the Io Plasma Torus. IV. Modelling Temporal and Azimuthal Variability
Authors: A.J. Steffl, P.A. Delamere, F. Bagenal

In this fourth paper in a series, we present the results of our efforts to model the remarkable temporal and azimuthal variability of the Io plasma torus during the Cassini encounter with Jupiter. The long-term (months) temporal variation in the average torus composition observed by the Cassini Ultraviolet Imaging Spectrograph (UVIS) can be modelled by supposing a factor of ~4 increase in the amount of material supplied to the extended neutral clouds that are the source of torus plasma, followed by a gradual decay to more "typical" values. On shorter timescales, the observed 10.07-hour torus periodicity and azimuthal variation in plasma composition, including its surprising modulation with System III longitude, is reproduced by our model using the superposition of two azimuthal variations of suprathermal electrons: a primary hot electron variation that slips 12.5 degrees/day relative to the Jovian magnetic field and a secondary variation that remains fixed in System III longitude.

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