Until 1974, Comet 81P/Wild 2 orbited beyond Jupiter, but a gravitational kick from that planet altered its orbit transforming it into a short-period comet in the inner solar system. This allowed NASA's solar-powered Stardust spacecraft to intercept the comet's tail within the orbit of Mars. The fly-by was completed at a relative speed of 6.1 km/s passing through the coma of the comet. Microscopic dust grains were captured in low-density silica aerogel tiles and aluminum foils, and these samples were returned successfully to Earth in January 2006. These Stardust samples are unique among extraterrestrial materials on Earth as the first samples returned from an identified parent body originating in the Kuiper belt beyond the gas giants. Due to the very recent orbit change, dust from Comet Wild 2 provides a means of inferring conditions in the Kuiper belt. It is expected that objects preserved in the frozen Kuiper belt are relatively unaltered since our solar system formed ~4.6 billion years ago. As a result, studies of Stardust particles offer a means of looking back in time to the origins of the solar system. First order, fundamental research involves understanding the elemental and isotopic chemistry, the astronomical signatures, the organics and mineralogy of these samples.
We studied the elemental chemistry of Stardust comet dust measured by
synchrotron x-ray fluorescence. Historically, comet composition measurements
have been made remotely or with poor resolution and detection limits due to the
constraints of space-flown instrumentation. Micro-focused synchrotron x-ray
fluorescence measurements (micro-SXRF) of comet dust on Earth offer enormous
improvements over past measurements. 23 aerogel samples containing comet dust
were analyzed using SXRF by the Preliminary Examination Team. This
international collaboration provided the first look at the Stardust samples
after the sample return, and results are presented in several publications by
the Preliminary Examination Team in the December 15 issue of Science.
Figure 1. Optical microscope images of two of the five Stardust comet
dust impact tracks in aerogel keystones analyzed by micro-SXRF at SSRL: a)
Track 4 is a conical, carrot-shaped track, and b) Track 10 is a bulbous-shaped
track. The keystone in a) is approximately 4 mm in length.
At SSRL, five Stardust samples were analyzed. Each sample contained a single
comet dust impact track and was extracted from an aerogel tile in the form of a
wedge-shaped "keystone". Impacting comet dust particles created tracks with a
variety of morphologies and lengths and differing amounts of cometary material
deposited along the path. Figure 1 contains optical micrographs of two of the
keystones studied. Each Stardust comet dust impact track was mapped in the
Beam Line 6-2 hard x-ray scanning fluorescence microprobe. Kirkpatrick-Baez
focusing optics and adjustable virtual source slits produce a beam size of 2 x
2 microns2 with 109 photons/second (ph/s). For efficient
mapping of Stardust impact tracks, the focused beam size was increased to 15 x
19 microns2 with 2x1010 ph/s. The K fluorescence lines
of elements from Si through Br were accessible using 14 keV incident x-rays,
and the microprobe end station is equipped with an optical microscope and low-Z
mirror for x-ray line-of-sight viewing and sample positioning. Full
fluorescence spectra were collected at each map pixel with dwell times of at
least 30 seconds/pixel.
Figure 2. X-ray fluorescence maps for the impact tracks in Figure 1
generated by setting energy windows encompassing relevant Ka lines. Maps of
Track 4 are shown in a) Fe, b) Ni, and c) Cr. Warmer colors indicate higher
fluorescence intensity on a logarithmic scale. Analogous x-ray fluorescence
maps were generated for Track 10 in d) Fe, e) Ni and f) Cr. The Track 4 impact
track contains two distinct Fe-Ni-rich regions at its terminus.
To provide an example of the data collected during these experiments, Figure 2
shows elemental maps of two different comet dust impact tracks. The upper
three maps are from Track 4, a conical track with terminal particles. The maps
show the Fe (top), Ni (middle) and Cr (bottom) abundances. Below those are the
same (Fe, Ni and Cr) maps from Track 10. Track 10 has a bulbous entry region
and a terminal particle well-separated from the initial entry region. From
these data, the summed abundances of various elements can be determined both
for the entire impact track and for the terminal particle. In some tracks, the
total mass deposited is dominated by the terminal particle while in others, a
significant portion is of mass is finely distributed along the length of the
track. Significant differences between the elemental abundances of the whole
track and the terminal particle require measurement of the entire impact track
in order to assess the mean Wild 2 elemental composition.
Micro-SXRF is a powerful tool for the non-destructive analysis of the Stardust
comet dust sample. In total, approximately 180 nanograms of Stardust comet dust
was analyzed for bulk composition measurements during the Preliminary
Examination period. The mean elemental composition of the Stardust Comet
81P/Wild 2 dust measured is generally consistent with the CI meteorite
composition, believed to be representative of the overall composition of the
solar system. A few elements, Cu, Zn and Ga, are enriched suggesting that the
CI meteorites may not be entirely representative of the solar system
composition for these moderately volatile elements. Given all that we have
learned and the experiments planned, we expect to continue studying Stardust
comet dust impact tracks for years to come.
Primary Citation
G. J. Flynn, P. Bleuet, J. Borg, J. P. Bradley, F. E. Brenker, S. Brennan, J.
Bridges, D. E. Brownlee, E. S. Bullock, M. Burghammer, B. C. Clark, Z. R. Dai,
C. P. Daghlian, Z. Djouadi, S. Fakra, T. Ferroir, C. Floss, I. A. Franchi, Z.
Gainsforth, J.-P. Gallien, P. Gillet, P. G. Grant, G. A. Graham, S. F. Green,
F. Grossemy, P. R. Heck, G. F. Herzog, P. Hoppe, F. Hörz, J. Huth, K. Ignatyev,
H. A. Ishii, K. Janssens, D. Joswiak, A. T. Kearsley, H. Khodja, A. Lanzirotti,
J. Leitner, L. Lemelle, H. Leroux, K. Luening, G. J. MacPherson, K. K. Marhas,
M. A. Marcus, G. Matrajt, T. Nakamura, K. Nakamura-Messenger, T. Nakano, M.
Newville, D. A. Papanastassiou, P. Pianetta, W. Rao, C. Riekel, F. J. M.
Rietmeijer, D. Rost, C. S. Schwandt, T. H. See, J. Sheffield-Parker, A.
Simionovici, I. Sitnitsky, C. J. Snead, F. J. Stadermann, T. Stephan, R. M.
Stroud, J. Susini, Y. Suzuki, S. R. Sutton, S. Taylor, N. Teslich, D. Troadec,
P. Tsou, A. Tsuchiyama, K. Uesugi, B. Vekemans, E. P. Vicenzi, L. Vincze, A. J.
Westphal, P. Wozniakiewicz, E. Zinner, M. E. Zolensky, "Elemental Compositions
of Comet 81P/Wild 2 Samples Collected by Stardust", Science
314, 1731-1735 (2006).
| SSRL is supported by the Department of Energy, Office of Basic Energy Sciences. The SSRL Structural Molecular Biology Program is supported by the Department of Energy, Office of Biological and Environmental Research, and by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program, and the National Institute of General Medical Sciences. |