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  • Spectroscopy Technique Reveals Strange Chemistry of a Frozen World

Photonics Spectra
Feb 2002
Brent D. Johnson

Recent data from the Near Infrared Mapping Spectrometer aboard the Galileo spacecraft may support the hypothesis of a subsurface ocean on some of the jovian moon surfaces, such as Europa. Specifically, reflection spectra from the surface of this moon indicate the possible presence of minerals that may be frozen brines and salts. If these species are indeed present, they may have originated from an endogenic source.

In this composite image of the Jupiter-facing hemisphere of Europa, which was taken in 1999 by two instruments aboard NASA's Galileo spacecraft, a superimposed false-color image obtained by Galileo's NIR mapping spectrometer reveals the presence of materials of differing compositions. Scientists disagree about the materials' chemical makeup, but suggest that the sulfuric acid and salty minerals could come from a subsurface ocean.

To further investigate this possibility, a research team led by professors Thomas McCord at the University of Hawaii at Manoa and Thomas Orlando at Georgia Institute of Technology have simulated deposition and flash freezing of brines to see what chemical surrogates might result.

This view of Europa combines images taken in 1998 and 1995 using violet, green and near-IR filters. Where the surface has been disrupted and ice blocks moved around, mottled, reddish terrain can be seen. The red material at the ridges is a non-ice contaminant and could be salts brought up from a possible ocean beneath Europa's frozen surface.

Orlando explained that interest in the Galilean moons of Jupiter has increased because recent mission data indicate that they are prototypical systems in extreme environments. The surface temperatures dip to 90 K, and the surfaces are constantly bombarded with radiation from the surrounding magneto- sphere.

The joint project has an interest-ing history. The US Department of Energy originally funded Orlando to investigate chemical transformations in mixed (chemical and radiative) wastes. The physics and chemistry that occur on the surface of Europa are similar to the chemical transformations in radiative wastes. Thus, using surface science tools developed by Orlando, the team began developing experiments to test the Galileo spectrometer's data. These experiments are based on near-infrared reflection and absorption measurements using laboratory light sources, whereas the mission data used the sun as the light source.

Orlando also employs resonance enhanced multiphoton ionization (REMPI), a spectroscopy technique developed largely by the atomic, molecular and optical physics community to see what is "leaving" the surfaces of very cold ices.

Lasers are used to excite the sample species, and the resulting ionized particles are introduced into an ionization chamber, where they are subjected to an electrical potential and accelerated into a mass spectrometer. By carefully selecting the laser wavelength, only the atomic particles of interest are affected. This technique is sensitive to concentrations as low as 10 ppb.

The team analyzed many of the fragments, such as atomic oxygen and hydrogen, that are produced from ice when hit with radiation. Orlando uses an Nd:YAG pumped dye laser from Continuum that is frequency-tripled to about 200 nm and a Spectra-Physics Quanta Ray MOPO UV laser that is tunable from 220 to 750 nm. In addition, he uses rare-gas jets to produce vacuum- ultraviolet radiation to generate Lyman-a (121.6 nm) radiation.

"None of our initial work on radiation effects would have worked as well without the lasers," he said. The lasers enabled the researchers to measure with much greater sensitivity. The researchers also used a quadrupole mass spectrometer that has good mass resolution but is generally less sensitive than the laser techniques.

"When you can combine REMPI with time-of-flight mass spectrometry, you can get a good picture of the surface chemistry," he said. "The mission data is so good spectrally and spatially that it requires a molecular view of what's happening."

A new mission to Europa that is being planned by NASA could use these same laser-based techniques to help measure the thickness of the moon's ice crust.

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