* Astronomy

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.

Read more  (French)

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.

Read more

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.

Read more

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