Weird chemistry: Researchers study unique radiation-driven reactions in extreme cold and high vacuum of Jupiter’s moons
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 will discuss 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.
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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.
- Production of O2 on icy satellites by electronic excitation of low-temperature water ice, M. T. Sieger, W. C. Simpson, T. M. Orlando. Nature 394, 554 – 556 (06 August 1998) [abstract. Fee required for access]
“The signature of condensed molecular oxygen has been reported in recent optical-reflectance measurements of the jovian moon Ganymede, and a tenuous oxygen atmosphere has been observed on Europa. The surfaces of these
moons contain large amounts of water ice, and it is thought that O2 is formed by the sputtering ofice by energetic particles from the jovian magnetosphere. ”
Technical Contact: Thomas Orlando (404-894-4012); E-mail:
(thomas.orlando@chemistry. gatech.edu).
Research News & Publications Office
Georgia Institute of Technology
430 Tenth Street, N.W., Suite N-116
Atlanta, Georgia 30318 USA
Contact: John Toon
john.toon@edi.gatech.edu
404-894-6986
Georgia Institute of Technology
