Post mortem of a Comet: analysing Comet Wild 2

Dr John Bridges, University of Leicester

In 2006 the Stardust mission returned to Earth with the first bona fide samples of a short period comet 81P/Wild2. Micron – sized dust samples were collected from the coma of the comet at a closing velocity of 6 km/s. The Stardust sample analyses including the work carried out by us at Diamond has shown that the traditional ideas of all comets being predominantly a mixture of low temperature material with a high proportion of interstellar grains and secondly of a rigid distinction between asteroids (chondrite meteorite parent bodies) and comets are no longer valid. Our analyses carried out on beamline I18 have contributed to the identification of the first sulphide terminal particle (pyrrhotite, identified by Fe-XANES); a better understanding of the effects of capture heating on the Wild2 tracks e.g. leading to slight oxidation of FeNi metal; and identification of V-bearing chromites, hematite. This work, co-ordinated with TEM analyses on parts of the tracks, has already effectively shown carbonaceous chondrite affinities and unexpectedly ruled out CI chondrite (the most primitive type of carbonaceous chondrite) as the closest analogue for Comet Wild2.

Figure 1: X-ray maps of one of the Stardust tracks.wwΣΔ59

The position in the cold, outer Solar System within which comets aggregated means that it is surprising that there are refractory, high temperature assemblages and even some minerals (magnetite) that may have formed by hydrothermal processes in the comet. Material from the hot, inner Solar System has somehow been moved into the outer Solar System during its earliest stages. Understanding how this happened and how Comet Wild 2 compares to other known planetary materials such as carbonaceous chondrites is helping to illuminate the origin of our Solar System.

Since the recovery of the Stardust capsule in January 2006, samples from the coma of Comet 81P/Wild 2 have started to reveal what this Jupiter family comet is made of [1]. Burchell et al. [2] estimated that approximately 1200 particles larger than 1 micron had struck the cometary collector at 6.1 km/s. The samples from the coma of Comet Wild 2 were fragmented during collection by the Stardust spacecraft. The tracks within the aerogel collector tiles are typically a few mm long and contain nanometre to micron size cometary grains of silicate, sulphide and oxide minerals. The closing velocity of capture was 6 km/s and this caused the fragmentation of the cometary grains into micron to nanometre size particles. The largest (tens of microns and least thermally altered by capture), grains are mainly at the terminal ends of the tracks. The fine sample size makes these samples challenging and requires the high spatial resolution (e.g. 2-3 microns) of I18 in order to determine the composition by Fe, Cr, Ni-XANES/EXAFS and X-ray fluorescence (XRF). Our I18 analyses are the most extensive UK synchrotron Microfocus XRF analyses of planetary materials performed in the UK to date and we are one of only a few international laboratories to study the rare samples from Comet Wild2.

During our shifts at I18 we have mapped the composition of heavier elements in cometary tracks #41, #134, #162, allowing the nature of the sample collection process to be characterised. The XRF maps also allowed the identification of larger grains suitable for the XANES/ EXAFS measurements and long acquisition time, X-ray fluorescence analyses. Here the high spectral resolution of I18 is critical for the accurate identification of minerals and oxidation features associated with capture. By using peak fitting routines and comparing our spectra to those we have taken on mineral standards we have identified a range of FeNi metal, sulphide and iron oxide minerals. Often these phases have been slightly oxidised, during capture we believe, and this is shown by characteristic pre and post Fe K-edge features at 7110 – 7111 eV and at around 7185 eV respectively.

On the I18 beamline, a Si (111) double crystal monochromator was used for energy selection with resolution of 10-4. The 9 element germanium based solid state detector was used which is capable of measuring the X-rays of Ca upwards. Our energy calibrations were checked with Mn metal samples.

XRF (fluorescence) maps were generated at 3 areas of track #41 and similarly for tracks #134 and #162. The germanium fluorescence detector was placed 45° to the sample with the beryllium window as close as possible to maximise count rates. 250 x 250 µm maps were produced at 13 keV and a dwell time of 5 seconds per spot. Beam spot sizes were 3-4 mm. Long integration times of 500 s were performed on hotspots identified after mapping. Fe K-edge XANES (X-ray Absorption Near Edge Structure) was performed on ‘hotspots’. Some Ni and Cr XANES were also performed on Ni and Cr bearing phases. Approximately 120 data points were integrated across the XANES pre and post Fe edge. One second integration was performed at each 0.2- 0.4 eV energy step from 6962-7090 eV, followed by 5 s onwards beyond the Fe K-edge (EXAFS, Extended X-ray Absorption Fine Structure) up to 7500 eV. A range of mineral standards similar to the phases studied, including: magnetite (Fe2O3FeO), hematite (Fe2O3), Mg-rich olivine from the Admire pallasite meteorite (Mg/Mg+Fe atomic ratio 0.88), chromite FeCr2O4, Fe-sulphide – pyrrhotite (Fe1-x S) and troilite FeS. XANES spectra were reduced using Pyspline software, XRS data with PyMCA software. The resultant XANES spectra were normalised by the software to the EXAFS region. The I18 analyses of Comet Wild2 so far have been presented at various conferences including the Lunar and Planetary Science conference [4] and are published [3].

Principal Publications and Authors
J. Bridges, H. Changela, S. Gurman J.C. Bridges et al., Fe Oxide Minerals in Comet 81P/Wild2. Meteoritics and Planetary Science, Volume 45 Issue 1, Pages 55 - 72, (2010).

References
[1] D. Brownlee et al., Comet 81P/Wild 2 under a microscope, Science, 314, 1711–1716 (2006).
[2] M.J. Burchell et al., Characteristics of cometary dust tracks in Stardust aerogel and laboratory calibrations. Meteoritics and Planetary Science, 43, 23 – 40. (2008).
[3] J.C. Bridges et al., Fe Oxide Minerals in Comet 81P/Wild2. Meteoritics and Planetary Science, Volume 45 Issue 1, Pages 55 - 72 (2010).
[4] J.C. Bridges, H.C. Changela, J.D. Carpenter, and J.S. Gurman Oxide Minerals in Comet Wild 2: TEM and Synchrotron Characterisation #1889 LPSC XL (2009).

Funding Acknowledgement Science & Technology Facilities Council, UK .