Radiation-driven reactions on Jupiter's moons
Posted: August 30, 2001

By his own admission, Thomas Orlando deals with "weird chemistry." In fact, the Georgia Institute of Technology researcher studies chemical processes that are literally out of this world -- reactions occurring on the moons of Jupiter, driven by extreme radiation at ultra-cold temperatures.

Based on laboratory simulations, work by Orlando and other researchers is helping planetary scientists understand data reported by a NASA spacecraft flying past the Galilean satellites Europa, Ganymede and Callisto. The studies provide new insight into the unique chemical reactions that take place on extremely cold icy surfaces under high vacuum, driven by high-energy electrons and ions rather than normal thermal processes.

The moons, which are gravitationally locked to Jupiter, co-rotate with Jupiter and lie within Jupiter's intense magnetosphere. Here they are constantly bombarded by radiation with the trailing sides receiving a greater radiation dose than the leading sides.

"When the magnetospheric particles (ions and electrons) are smashing into the surface of the moons, strange things happen even though the surface is about as cold as cold can be. Radicals are produced, ionization occurs and reactive species produce materials that wouldn't normally be produced," explained Orlando, a professor in Georgia Tech's School of Chemistry and Biochemistry. "The bottom line is that weird chemistry goes on when there is too much energy."

Orlando discussed aspects of this "weird chemistry" at the 222nd national meeting of the American Chemical Society on August 29th in Chicago. His presentation will be part of a chemical education section on the importance of radiation and high-energy chemistry in both the laboratory and the real world -- which includes the outer reaches of our solar system.

Near-infrared data sent from the Galileo spacecraft in 1997 indicated the presence of frozen brine on the surface of Europa. This suggestion was mainly discussed by McCord and co-workers (Science 280, 1242-45, 1998) and many planetary scientists believed the brine could have originated in a subsurface ocean beneath Europa's frozen crust. Brought to the surface by cryo-volcanic action, the brine would have been flash-frozen in the extreme cold (below 130 degrees Kelvin, or minus 145 degrees Celsius) and ultra-high vacuum (less than 10 -10 Torr).

To test that hypothesis, a team of scientists led by Orlando (formerly of Pacific Northwest National Laboratory) and Prof. Tom McCord of the University of Hawaii, duplicated the freezing of brine under similar conditions of temperature and vacuum, then cycled the samples through the thermal changes that occur on the surface of Europa. Near-infrared analysis of the resulting samples showed characteristics similar to what the spacecraft reported, supporting the brine theory.

"We made some pretty good connections to what the planetary scientists had seen on the surface of these moons," said Orlando. "We thought about flash freezing from the chemical physics standpoint because if you freeze the brine fast enough, you can 'lock' the waters of hydration into their local positions. These water molecules should have a different optical signature than the rest of the water molecules in ice."

Spacecraft have also measured oxygen molecules (O2) as part of a tenuous atmosphere on the moons. To understand how oxygen could be produced and liberated from extremely cold ice on the moons, Orlando's research team at Pacific Northwest National Laboratory bombarded ice samples with an electron beam much like those used in the microelectronics industry. The result was an unexpected reaction that involved the production of a stable precursor molecule that would not form under conditions seen by most chemists.

Simulations may also help scientists construct a time line for tracking the evolution and transformation of the moons' surfaces. Since the high radiation is constantly changing the ice, understanding the rate at which those processes occur might allow researchers to date them -- particularly if changes can be measured from one space mission to the next.

Beyond the Galilean satellites, Orlando's interest extends to Mars, comets, asteroids and even the dust found in space. "Radiation induced processes are generally the rule in outer space," he said. "They're not limited to just one system. We are just simulating what cosmic rays do. Cosmic rays produce electrons so we study the chemistry these electrons initiate."

A chemical physicist, Orlando began studying chemical reactions driven by radiation while a researcher at Pacific Northwest National Laboratory. There, the interest was in the effects of radiation on production of hydrogen and oxygen from nuclear waste. Transitioning that knowledge to planetary science shows the value of interdisciplinary studies, Orlando says.

"We're working in an interesting area where chemical physics, surface science and radiation chemistry can help planetary scientists address the issues raised by the really superb mission data," he noted. "The planetary science community is getting data so good that we can take a molecular view of what's happening."

At Georgia Tech, Orlando has established a laboratory to continue the study of radiation effects on icy surfaces. Using equipment that can produce ultra-high vacuum and temperatures down to 15 Kelvin, he plans to study the production of hydrogen molecules, and to better understand how small changes in the processing conditions affect the characteristics of the very cold ice -- and what can be driven from it.

"The surface morphology and the surface temperature greatly affect the products you make," he said. "At one temperature, you might make a lot of O2. At another temperature, you may just sputter off water molecules and get water into the gas phase. The general radiation processing of low-temperature water is still not completely characterized."

Also on the agenda: photochemistry studies of iron oxides on Mars, sulfuric acid interaction with radiation -- and possible nanotechnology and medical applications using controlled electron-beam technology.

The research is sponsored by NASA and the Department of Energy. The research team conducting the brine studies included Thomas McCord, Gary Hansen, and Lisa Van Keulen of the University of Hawaii, and Glenn Teeter, Matthew Sieger and William Simpson of the Pacific Northwest National Laboratory. A paper on brine work was published in Volume 106 of the Journal of Geophysical Research. A paper on the oxygen production was published in volume 394 of Nature.