Surface modifications of comet‐exposed aerogel from the Stardust cometary collector     

Hans A. BECHTEL  Zack GAINSFORTH  Ryan C. OGLIORE  Saša BAJT  Andrew J. WESTPHAL 《Meteoritics & planetary science》2012,47(8):1336-1346

Abstract– Keystones removed from the Stardust cometary collector show varying degrees of visible fluorescence when exposed to UV light, with the brightest fluorescence associated with the space‐exposed surface. We investigated the spatial characteristics of this phenomenon further by using fluorescence microscopy, confocal Raman microscopy, and synchrotron Fourier transform infrared (FTIR) spectromicroscopy. Twenty‐four keystones, extracted from the Stardust cometary collector, were analyzed. Fluorescence measurements show two distributions with different excitation characteristics, indicating the presence of at least two distinct fluorophores. The first distribution is confined to within about 10 μm of the space‐exposed surface, whereas the second distribution is much broader with a maximum that is typically about 30–50 μm below the surface. Confocal Raman measurements did not reveal any changes associated with the surface; however, only features associated with aliphatic hydrocarbons were strong enough to be observed. FTIR measurements, on the other hand, show two distinct distributions at the space‐exposed surface: (1) a narrow, surface‐confined distribution originating from ?O₃SiH groups and (2) a broader, sub‐surface distribution originating from ?O₂SiH₂ groups. These functional groups were not observed in keystones extracted from the cometary flight spare or from the Stardust interstellar collector, indicating that they may result at least partially from cometary exposure. The presence of O₃SiH and O₂SiH₂ groups at the comet‐exposed surface suggests that the enhanced surface fluorescence is caused by defects in the O‐Si‐O network and not by organic contamination.  相似文献   

Assessment and control of organic and other contaminants associated with the Stardust sample return from comet 81P/Wild 2     

Scott A. SANDFORD  Saša BAJT  Simon J. CLEMETT  George D. CODY  George COOPER  Bradley T. DEGREGORIO  Vanessa De VERA  Jason P. DWORKIN  Jamie E. ELSILA  George J. FLYNN  Daniel P. GLAVIN  Antonio LANZIROTTI  Thomas LIMERO  Mildred P. MARTIN  Christopher J. SNEAD  Maegan K. SPENCER  Thomas STEPHAN  Andrew WESTPHAL  Sue WIRICK  Richard N. ZARE  Michael E. ZOLENSKY 《Meteoritics & planetary science》2010,45(3):406-433

Abstract– Numerous potential sources of organic contaminants could have greatly complicated the interpretation of the organic portions of the samples returned from comet 81P/Wild 2 by the Stardust spacecraft. Measures were taken to control and assess potential organic (and other) contaminants during the design, construction, and flight of the spacecraft, and during and after recovery of the sample return capsule. Studies of controls and the returned samples suggest that many of these potential sources did not contribute any significant material to the collectors. In particular, contamination from soils at the recovery site and materials associated with the ablation of the heatshield do not appear to be significant problems. The largest source of concern is associated with the C present in the original aerogel. The relative abundance of this carbon can vary between aerogel tiles and even within individual tiles. This C was fortunately not distributed among a complex mixture of organics, but was instead largely present in a few simple forms (mostly as Si‐CH₃ groups). In most cases, the signature of returned cometary organics can be readily distinguished from contaminants through their different compositions, nonterrestrial isotopic ratios, and/or association with other cometary materials. However, some conversion of the carbon indigenous to the flight aerogel appears to have happened during particle impact, and some open issues remain regarding how this C may be processed into new forms during the hypervelocity impact collection of the comet dust.  相似文献   

Infrared spectroscopy of Wild 2 particle hypervelocity tracks in Stardust aerogel: Evidence for the presence of volatile organics in cometary dust     

S. BAJT  S. A. SANDFORD  G. J. FLYNN  G. MATRAJT  C. J. SNEAD  A. J. WESTPHAL  J. P. BRADLEY 《Meteoritics & planetary science》2009,44(4):471-484

Abstract— Infrared spectroscopy maps of some tracks made by cometary dust from 81P/Wild 2 impacting Stardust aerogel reveal an interesting distribution of organic material. Out of six examined tracks, three show presence of volatile organic components possibly injected into the aerogel during particle impacts. When particle tracks contained volatile organic material, they were found to be ‐CH₂‐rich, while the aerogel is dominated by the ‐CH₃‐rich contaminant. It is clear that the population of cometary particles impacting the Stardust aerogel collectors also includes grains that contained little or none of this organic component. This observation is consistent with the highly heterogeneous nature of collected grains, as seen by a multitude of other analytical techniques.  相似文献   

Heating experiments simulating atmospheric entry heating of micrometeorites: Clues to their parent body sources     

ANSGAR GRESHAKE  WOLFGANG KL CK  PETER ARNDT  MISCHA MAETZ  GEORGE J. FLYNN  SASA BAJT  ADDI BISCHOFF 《Meteoritics & planetary science》1998,33(2):267-290

Abstract— Depending on their velocity, entry angle and mass, extraterrestrial dust particles suffer certain degrees of heating during entry into Earth's atmosphere, and the mineralogy and chemical composition of these dust particles are significantly changed. In the present study, pulse-heating experiments simulating the atmospheric entry heating of micrometeoroids were carried out in order to understand the mineralogical and chemical changes quantitatively as well as to estimate the peak temperature experienced by the particles during entry heating. Fragments of the CI chondrites Orgueil and Alais as well as pyrrhotites from Orgueil were used as analogue material. The experiments show that the volatile elements S, Zn, Ga, Ge, and Se can be lost from 50 to 100 μm sized CI meteorite fragments at temperatures and heating times applicable to the entry heating of similar sized cosmic dust particles. It is concluded that depletions of these elements relative to CI as observed in micrometeorites are mainly caused by atmospheric entry heating. Besides explaining the element abundances in micrometeorites, the experimentally obtained release patterns can also be used as indicators to estimate the peak heating of dust particles during entry. Using the abundances of Zn and Ge and assuming their original concentrations close to CI, a maximum heating of 1100–1200 °C is obtained for previously analyzed Antarctic micrometeroites. Thermal alteration also strongly influenced the mineralogy of the meteorite fragments. While the unheated samples mainly consisted of phyllosilicates, these phases almost completely transformed into olivine and pyroxene in the fragments heated to ≥800 °C. Therefore, dust particles that still contain hydrous minerals were probably never heated to temperatures ≥800 °C in the atmosphere. During continued heating, the grain size of the newly formed silicates increased and the composition of the olivines equilibrated. Applying these results quantitatively to Antarctic micrometeorites, typical peak temperatures in the range of 1100–1200 °C during atmospheric entry heating are deduced. This temperature range corresponds to the one obtained from the volatile element concentrations measured in these micrometeorites and points to an asteroidal origin of the particles.  相似文献