Engineering topics

Stephan, Thomas

Publications and source records attributed to Stephan, Thomas.

6 records

DOE OSTI2022

On the provenance of GEMS, a quarter century post discovery

The provenance of GEMS (glass with embedded metal and sulfides) in cometary type interplanetary dust particles is investigated using analytical scanning transmission electron microscopy and secondary ion mass spectrometry. We review the current state of knowledge and closely examine the densities, elemental compositions and distributions, iron oxidation states, and isotopic compositions of a subset of GEMS in chondritic porous interplanetary dust. We find that GEMS are underdense with estimated densities that are 35–65% of compositionally equivalent crystalline aggregates. GEMS low densities result in a lower contribution to the bulk compositions of IDPs than has been assumed based on their volume fraction. We also find that element/Si ratios, assumed to be primary (indigenous), are instead perturbed by contamination and secondary alteration, including pulse heating during atmospheric entry. Fe in pyrrhotite inclusions was oxidized and Mg, S, Ca, and Fe were depleted relative to lithophile Al and Si, resulting in reduction in element/Si ratios. Because they trap outgassing elements, Fe-rich oxide rims that formed on the surface of GEMS are serendipitous “witness plates” to the changes in composition that accompany atmospheric entry. As a result of alteration, GEMS elemental compositions cannot reliably inform about their provenance. Except for highly anomalous oxygen isotope ratios measured in some large GEMS grains that indicate a contribution from circumstellar dust, oxygen isotope compositions are generally poor indicators of provenance. Prior work indicates that most GEMS fall close to the terrestrial oxygen isotope composition, which, however, does not exclude a presolar interstellar origin. Nitrogen isotopic compositions are more diagnostic. Elevated 15 N/ 14 N ratios indicate that GEMS accreted in conjunction with formation of organic matter by ion–molecule reactions in a cold (<50 K) presolar environment like the extreme outer nebula or interstellar medium. Considering all observations, we conclude that GEMS are most likely processed interstellar silicates.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2013

Stardust Interstellar Preliminary Examination III: Infrared Spectroscopic Analysis of Interstellar Dust Candidates

Under the auspices of the Stardust Interstellar Preliminary Examination, picokeystones extracted from the Stardust Interstellar Dust Collector were examined with synchrotron Fourier transform infrared (FTIR) microscopy to establish whether they contained extraterrestrial organic material. The picokeystones were found to be contaminated with varying concentrations and speciation of organics in the native aerogel, which hindered the search for organics in the interstellar dust candidates. Furthermore, examination of the picokeystones prior to and post X-ray microprobe analyses yielded evidence of beam damage in the form of organic deposition or modification, particularly with hard X-ray synchrotron X-ray fluorescence. From these results, it is clear that considerable care must be taken to interpret any organics that might be in interstellar dust particles. For the interstellar candidates examined thus far, however, there is no clear evidence of extraterrestrial organics associated with the track and/or terminal particles. However, we detected organic matter associated with the terminal particle in Track 37, likely a secondary impact from the Al-deck of the sample return capsule, demonstrating the ability of synchrotron FTIR to detect organic matter in small particles within picokeystones from the Stardust interstellar dust collector.

Interstellar↗

NASA NTRS2008

Comparing Wild 2 Particles to Chondrites and IDPS

We compare the observed composition ranges of olivine, pyroxene and Fe-Ni sulfides in Wild 2 grains, comparing these with chondritic IDPs and chondrite classes to explore whether these data suggest affinities to known hydrous materials in particular. Wild 2 olivine has an extremely wide composition range, from Fo4-100 with a pronounced frequency peak at Fo99. The composition range displayed by the low-calcium pyroxene is also very extensive, from En52 to En100, with a significant frequency peak centered at En95. These ranges are as broad or broader than those reported for any other extraterrestrial material. Wild 2 Fe-Ni sulfides mainly have compositions close to that of FeS, with less than 2 atom % Ni - to date, only two pentlandite grains have been found among the Wild-grains suggesting that this mineral is not abundant. The complete lack of compositions between FeS and pentlandite (with intermediate solid solution compositions) suggests (but does not require) that FeS and pentlandite condensed as crystalline species, i.e. did not form as amorphous phases, which later became annealed. While we have not yet observed any direct evidence of water-bearing minerals, the presence of Ni-bearing sulfides, and magnesium-dominated olivine and low-Ca pyroxene does not rule out their presence at low abundance. We do conclude that modern major and minor element compositions of chondrite matrix and IDPs are needed.

Zolensky, Michael↗

NASA NTRS2007

Stardust in STARDUST - the C, N, and O Isotopic Compositions of Wild 2 Cometary Matter in Al foil Impacts

In January 2006, the STARDUST mission successfully returned dust samples from the tail of comet 81P/Wild 2 in two principal collection media, low density silica aerogel and Al foil. While hypervelocity impacts at 6.1 km/s, the encounter velocity of STARDUST, into Al foils are generally highly disruptive for natural, silicate-dominated impactors, previous studies have shown that many craters retain sufficient residue to allow a determination of the elemental and isotopic compositions of the original projectile. We have used the NanoSIMS to perform C, N, and O isotope imaging measurements on four large (59-370 microns diameter) and on 47 small (0.32-1.9 microns diameter) Al foil impact craters as part of the STARDUST Preliminary Examination. Most analyzed residues in and around these craters are isotopically normal (solar) in their C, N, and O isotopic compositions. However, the debris in one large crater shows an average 15N enrichment of approx. 450 %, which is similar to the bulk composition of some isotopically primitive interplanetary dust particles. A 250 nm grain in another large crater has an O-17 enrichment with approx. 2.65 times the solar O-17/O-16 ratio. Such an O isotopic composition is typical for circumstellar oxide or silicate grains from red giant or asymptotic giant branch stars. The discovery of this circumstellar grain clearly establishes that there is authentic stardust in the cometary samples returned by the STARDUST mission. However, the low apparent abundance of circumstellar grains in Wild 2 samples and the preponderance of isotopically normal material indicates that the cometary matter is a diverse assemblage of presolar and solar system materials.

Stadermann, Frank J.↗

NASA NTRS2006

TOF-SIMS Analysis of Crater Residues from Wild 2 Cometary on Stardust Aluminum Foil

Impact residues of cometary particles on aluminum foils from the Stardust mission were investigated with TOF-SIMS for their elemental and organic composition. The residual matter from comet 81P/Wild 2 shows a wide compositional range, from nearly monomineralic grains to polymict aggregates. Despite the comparably small analyzed sample volume, the average element composition of the investigated residues is similar to bulk CI chondritic values. Analysis of organic components in impact residues is complicated, due to fragmentation and alteration of the compounds during the impact process and by the presence of contaminants on the aluminum foils. Nevertheless, polycyclic aromatic hydrocarbons (PAHs) that are unambiguously associated with the impact residues were observed, and thus are most likely of cometary origin.

Leutner, Jan↗

NASA NTRS2006

SIMS Studies of Allende Projectiles Fired into Stardust-type Aluminum Foils at 6 km/s

We have explored the feasibility of C-, N-, and O-isotopic measurements by NanoSIMS and of elemental abundance determinations by TOF-SIMS on residues of Allende projectiles that impacted Stardust-type aluminum foils in the laboratory at 6 km/s. These investigations are part of a consortium study aimed at providing the foundation for the characterization of matter associated with micro-craters that were produced during the encounter of the Stardust space probe with comet 81P/Wild 2. Eleven experimental impact craters were studied by NanoSIMS and eighteen by TOF-SIMS. Crater sizes were between 3 and 190 microns. The NanoSIMS measurements have shown that the crater morphology has only a minor effect on spatial resolution and on instrumental mass fractionation. The achievable spatial resolution is always better than 200 nm, and C- and O-isotopic ratios can be measured with a precision of several percent at a scale of several 100 nm, the typical size of presolar grains. This clearly demonstrates that presolar matter, provided it survives the impact into the aluminum foil partly intact, is recognizable even if embedded in material of Solar System origin. TOF-SIMS studies are restricted to materials from the crater rim. The element ratios of the major rockforming elements in the Allende projectiles are well characterized by the TOF-SIMS measurements, indicating that fractionation of those elements during impact can be expected to be negligible. This permits information on the type of impactor material to be obtained. For any more detailed assignments to specific chondrite groups, however, information on the abundances of the light elements, especially C, is crucial.

Hoppe, Peter↗