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A rocky road to Mercury molded into disks. The researchers to address issues of planetary formation. Can’t get to Mercury? Bring monitored four incident photon enerFor BepiColombo, the scientific comMercuryOor least a rocky gies: 9.2, 7, 4, and 2.5 keV. The radiamunity decided this was totally unacapproximationOto the laboratory. In a tion excites K-level transitions, which ceptable, and they needed a better specnew AC paper (DOI 10.1021/ correspond to peaks in the spectra that ac8009627), Alan Owens and colleagues trometer.” are unique to each element in the Specifically, the community wanted of the European Space Agency (ESA); sample. Furthermore, the count rates (a an X-ray fluorescence spectrometer with Physikalisch-Technische Bundesanstalt measure of a peak’s amplitude) were at least 5 ⫻ better energy resolution (Germany); Leicester University (U.K.); plugged into an equation to Istituto Nazionale di Fisica Nudetermine the concentracleare; Sezione di Genova tion of each element within (Italy); Technische Universita¨t a rock. The values calcuWien (Austria); and Bruker AXS lated from the spectral data (Germany) describe the collecagreed well with the extion of X-ray fluorescence data pected concentrations. from planetary-analog rock The researchers then samples. Their experiments will used GEANT4, a standard help ESA prepare for a planned computational tool in 2014 mission to Mercury on a nuclear physics experiments craft called the BepiColombo. X-ray fluorescence spectra. Left: measured from the asteroid Eros durfor making predictions, to X-rays provide scientists with ing a solar flare with a low-resolution gas counter system. Right: measimulate the spectra from information about the elements sured from a sample of Hawaiian basalt with a high-resolution system their synchrotron experithat constitute the top few miat BESSY II. The excitation energy was 9.5 keV. The GEANT4 simulated ments. GEANT4 was able crometers of a rocky planet. spectrum is overlaid on the spectrum measured at the synchrotron to reproduce the spectra for These data, combined with radiation source. the most part, but between complementary data on neusimulation and experiment, trons and gamma rays, will help than the previous generation. “This an asymmetry in one particular peak scientists determine how Mercury was technology has moved on enough that emerged. “This indicates something is formed. it’s not considered a risky technology not right with the simulation,” says Rocks on Mercury are stimulated by anymore,” says Owens. The improved Owens. “This is how you find out how solar activity, which varies and can instrument will allow researchers to dethe microphysics is falling down. This is modulate the intensity of X-ray fluorestermine the surface composition of a another reason why the GEANT4 [decence. To determine which spectral amrocky celestial body to a previously unat- velopers are] very keen on collaborating plitudes to expect during the mission, tained level of precision. on this.” Owens ran a simulation of X-ray proBecause the type of spectrometer to According to Owens, the next step duction yields from basaltOthe rock be used on the Mercury mission has yet will be to analyze different grain sizes. predicted to be most prevalent on Merto be tested in the field, the researchers “Most of the rocks are in the form of a cury, on the basis of the limited existing dust or powder covering the surface, dataOfor a variety of different solar flare took a similar instrument on a test drive with planetary-rock analogs. For their with a range of grain sizes. The intensity classes. On BepiColombo, a separate X-ray fluorescence experiments, the inof the fluoresced lines depends on the detector will measure solar emissions vestigators used monochromatized syngrain size to a certain extent,” says directly so they can be time-matched to chrotron radiation, tunable from 1.75 to Owens. “If it makes a big difference, data collected from Mercury with an 10.5 keV, from the BESSY II electron we’ll go back [to the synchrotron] next X-ray fluorescence spectrometer. storage ring in Germany. “The only way year.” “Up until now, NASA and ESA have you can excite the X-ray spectrum in a —Erika Gebel almost exclusively flown gas counters,” says Owens, a researcher in ESA’s Scien- predictable fashion is to go to Mercury or use a synchrotron,” says Owens. tific Projects Department in The NethThey tested a variety of rocks: basalt, erlands. “They have very basic spectroscopy, making it difficult or impossible to obsidian, anorthosite, hematite, gabbro, identify all the elements in the data from and dolerite. To prepare the sample, the the planet. This in turn makes it difficult rocks were pulverized into a powder and 8862
ANALYTICAL CHEMISTRY /
DECEMBER 1, 2008
10.1021/AC802073N 2008 AMERICAN CHEMICAL SOCIETY
Published on Web 10/24/2008
