July 18, 1997
"While this discovery does not relate to the question of possible life on Europa, it does show us there is a surface process occurring there, and Europa is not just some dead hunk of material," said lead investigator Dr. Arvydas Kliore of NASA's Jet Propulsion Laboratory, Pasadena, CA. Kliore reports his findings in the July 18 issue of Science magazine.
The ionosphere was detected through a series of six occultation experiments performed during Galileo's encounters with Europa in December 1996 and February 1997. During occultation, Europa was positioned between the spacecraft and Earth, causing interruption in the radio signal. Measurements of the Galileo radio signal received at the Deep Space Network stations in Goldstone, CA, and Canberra, Australia, showed that the radio beam was refracted by a layer of electrons, or charged particles, in Europa's ionosphere.
An ionosphere is a layer of charged particles (ions and electrons) found in the upper levels of an atmosphere, created when gas molecules in the atmosphere are ionized. On Europa, this ionized layer can be caused either by the Sun's ultraviolet radiation or by energetic particles trapped in Jupiter's magnetic field, known as the magnetosphere. Europa and the other Jovian satellites are immersed in this magnetosphere. "Most likely the charged particles in Jupiter's magnetosphere are hitting Europa's icy surface with great energy, knocking atoms of water molecules off the moon's surface," Kliore said.
Europa's ionosphere has a maximum density of 10,000 electrons per cubic centimeter, which is significantly lower than the average density of 20,000 to 250,000 electrons per cubic centimeter found in Jupiter's ionosphere. This indicates that Europa's ionosphere is tenuous; nonetheless, it is strong enough for scientists to infer the presence of an atmosphere.
The latest Galileo findings follow last year's observations by NASA's Hubble Space Telescope of oxygen emissions on Europa, a strong hint that an atmosphere might exist on that moon.
The existence of an ionosphere and, by inference, an atmosphere, on another Jovian moon, Io, was observed in 1973 during a radio occultation conducted by NASA's Pioneer 10 spacecraft and confirmed by recent Galileo occultations. Io is believed to have an unusual atmosphere affected by sulfur dioxide spewing from the moon's volcanic vents. Kliore and his colleagues are currently studying two of Jupiter's other largest moons, Ganymede and Callisto, to determine whether they also have ionospheres and atmospheres.
"You could say an ionosphere or some kind of atmosphere has been found on most solar system bodies studied so far," said Kliore.
Participating with Kliore in the Europa radio occultation experiments were Dr. David Hinson, professor at Stanford University, Stanford, CA; Dr. Michael Flasar of NASA's Goddard Space Flight Center, Greenbelt, MD; Dr. Andrew Nagy, professor at the University of Michigan, Ann Arbor, MI, and Dr. Thomas Cravens, professor at the University of Kansas, Lawrence, KS.
The Galileo mission is managed by JPL for NASA's Office of Space Science, Washington, DC. The spacecraft entered the Jovian system on December 7, 1995, and its primary mission will end in November of this year. However, the mission has been extended for two more years so the craft can conduct an intensive study of Europa, with additional flybys of Callisto and Io, depending on spacecraft health.