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New Horizons' Alice UV spectrograph observes Io's atmosphere change in response to eclipse events

SAN ANTONIO -- Oct. 9, 2007 -- 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 sulfur-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," said 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," Retherford said.

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 of NASA Headquarters, a 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.

New Horizons is the first mission in NASA's New Frontiers program. The Johns Hopkins University Applied Physics Laboratory manages the mission and operates the spacecraft for the NASA Science Mission Directorate. SwRI leads the SWAP instruments and hosts the Tombaugh Science Operations Center.

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.

For more information contact Robert Leibold at (210) 522-2258, Communications Department, Southwest Research Institute, PO Drawer 28510, San Antonio, TX 78228-0510.

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