Designed and built by scientists in the University of Maryland's space physics group, the CHEMS (CHarge Energy Mass Spectrometer) sensor measures ions -- positively charged atoms -- in Saturn's magnetosphere. A planet's magnetosphere is the magnetic field and charged particle environment that surrounds it.

The magnetosphere traps ions produced in and around a planet. And it shields a planet from, and interacts with, the solar wind - the high-speed stream of ionized particles flowing out in all directions from the Sun.

"By determining the elemental composition and charge state of the ions within and around Saturn's magnetosphere, CHEMS will identify the sources of the plasma found there and study the processes of plasma acceleration," says Douglas C. Hamilton, a professor of physics at the University of Maryland and leader of the space physics team that designed and built the CHEMS sensor.

"CHEMS has already yielded data indicating the plasma in Saturn's magnetosphere consists mostly of hydrogen and oxygen ions and molecular ions derived from water," says Hamilton.

"This suggests that the plasma probably comes from the surfaces of Saturn's icy moons and rings, and not from the atmosphere of Titan, which consists primarily of nitrogen."

Plasmas are the most common form of matter, comprising more than 99 percent of the known visible universe including the Sun and other stars. These ionized gases generate and interact with magnetic and electric fields around planets, stars and other astrophysical environments.

Plasma processes can accelerate some ions to incredible energies. Cosmic rays -- which are some of the highest energy plasma particles -- contain "signatures" of the birth and death of stars. Observing the properties of space plasmas and energetic particles provides scientists a rich source of information about the physical processes that energize these materials and the conditions that exist at the sites where this energizing takes place.

Magnetospheric Imaging Maryland's CHEMS is one of three sensors that make up the Magnetospheric Imaging Instrument, MIMI, aboard NASA's Cassini-Huygens spacecraft. MIMI is one of 12 science instruments on the main Cassini spacecraft and one of six instruments designed primarily to investigate the space environments around Saturn and its satellites.

The Huygens probe, which has six instruments of its own, will investigate Saturn's largest moon, Titan. Titan is the only moon in the solar system with its own atmosphere.

MIMI and its science team are led by Stamatios (Tom) M. Krimigis, head of the space department of The Johns Hopkins University Applied Physics Laboratory. Using MIMI, Krimigis, Hamilton and other members of the international MIMI team will profile the plasma environment of charged particles around Saturn and provide the first visible, global images of Saturn's magnetosphere.

Gaining a better understanding of Saturn's magnetosphere and its interaction with the solar wind and solar storms promises to also help scientists better understand space weather and its interaction with the magnetosphere of our own planet.

MIMI's sensors combine three critical measurements to create that picture. In addition to Maryland's CHEMS, there is the higher-energy particle detector LEMMS, primarily developed by the Max Planck Institute at Lindau, Germany, that looks at the distribution and strength of energetic ions and electrons near the spacecraft.

MIMI's ion and neutral camera, or INCA, uses an APL-developed technique known as energetic neutral atom imaging to provide a global view of the entire magnetosphere - a deep-space mission first. All of MIMI's sensors are linked together by a central computer.

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