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At least 541 records · Page 30Linked to original sources
PubMed2004

Chondrules with peculiar REE patterns: implications for solar nebular condensation at high C/O.

Rare earth element (REE) data from two ordinary chondrite chondrules show distinct negative chondrite-normalized concentration anomalies of samarium, europium, and ytterbium. The peculiar patterns may be the result of REE gas/solid fractionation at an oxygen fugacity lower than has been assumed for the canonical solar nebula. We suggest that the two ordinary chondrite chondrules acquired the fractionated REE patterns by incorporation of highly reduced, ultrarefractory condensates in their precursors. This interpretation implies that high-temperature condensation processes occurred in nebular environments with a strong deficit in oxygen, such as regions with an enhanced carbon/oxygen ratio.

Carbon↗
PubMed2004

Bedout: a possible end-Permian impact crater offshore of northwestern Australia.

The Bedout High, located on the northwestern continental margin of Australia, has emerged as a prime candidate for an end-Permian impact structure. Seismic imaging, gravity data, and the identification of melt rocks and impact breccias from drill cores located on top of Bedout are consistent with the presence of a buried impact crater. The impact breccias contain nearly pure silica glass (SiO2), fractured and shock-melted plagioclases, and spherulitic glass. The distribution of glass and shocked minerals over hundreds of meters of core material implies that a melt sheet is present. Available gravity and seismic data suggest that the Bedout High represents the central uplift of a crater similar in size to Chicxulub. A plagioclase separate from the Lagrange-1 exploration well has an Ar/Ar age of 250.1 +/- 4.5 million years. The location, size, and age of the Bedout crater can account for reported occurrences of impact debris in Permian-Triassic boundary sediments worldwide.

Argon↗
PubMed2004

Discovery of ancient silicate stardust in a meteorite.

We have discovered nine presolar silicate grains from the carbonaceous chondrite Acfer 094. Their anomalous oxygen isotopic compositions indicate formation in the atmospheres of evolved stars. Two grains are identified as pyroxene, two as olivine, one as a glass with embedded metal and sulfides (GEMS), and one as an Al-rich silicate. One grain is enriched in 26Mg, which is attributed to the radioactive decay of 26Al and provides information about mixing processes in the parent star. This discovery opens new means for studying stellar processes and conditions in various solar system environments.

Aluminum↗
PubMed2004

Surface of young Jupiter family comet 81P/Wild 2: view from the Stardust Spacecraft.

Images taken by the Stardust mission during its flyby of 81P/Wild 2 show the comet to be a 5-kilometer oblate body covered with remarkable topographic features, including unusual circular features that appear to be impact craters. The presence of high-angle slopes shows that the surface is cohesive and self-supporting. The comet does not appear to be a rubble pile, and its rounded shape is not directly consistent with the comet being a fragment of a larger body. The surface is active and yet it retains ancient terrain. Wild 2 appears to be in the early stages of its degradation phase as a small volatile-rich body in the inner solar system.

Cosmic Dust↗
PubMed2004

Modeling the nucleus and jets of comet 81P/Wild 2 based on the Stardust encounter data.

We interpret the nucleus properties and jet activity from the Stardust spacecraft imaging and the onboard dust monitoring system data. Triangulation of 20 jets shows that 2 emanate from the nucleus dark side and 16 emanate from sources that are on slopes where the Sun's elevation is greater than predicted from the fitted triaxial ellipsoid. Seven sources, including five in the Mayo depression, coincide with relatively bright surface spots. Fitting the imaged jets, the spikelike temporal distribution of dust impacts indicates that the spacecraft crossed thin, densely populated sheets of particulate ejecta extending from small sources on the rotating nucleus, consistent with an emission cone model.

Algorithms↗
PubMed2004

Comet or asteroid shower in the late Eocene?

The passage of a comet shower approximately 35 million years ago is generally advocated to explain the coincidence during Earth's late Eocene of an unusually high flux of interplanetary dust particles and the formation of the two largest craters in the Cenozoic, Popigai and the Chesapeake Bay. However, new platinum-group element analyses indicate that Popigai was formed by the impact of an L-chondrite meteorite. Such an asteroidal projectile is difficult to reconcile with a cometary origin. Perhaps instead the higher delivery rate of extraterrestrial matter, dust, and large objects was caused by a major collision in the asteroid belt.

Earth, Planet↗
PubMed2004

Dust measurements in the coma of comet 81P/Wild 2 by the Dust Flux Monitor Instrument.

We present measurements of the dust particle flux and mass distribution from the Stardust Dust Flux Monitor Instrument (DFMI) throughout the flyby of comet 81P/Wild 2. In the particle mass regime from 10(-14) to 10(-7) kilograms, the spacecraft encountered regions of intense swarms of particles, together with bursts of activity corresponding to clouds of particles only a few hundred meters across. This fine-scale structure can be explained by particle fragmentation. We estimate that 2800 +/- 500 particles of diameter 15 micrometers or larger impacted the aerogel collectors, the largest being approximately 6 x 10(-7) kilograms, which dominates the total collected mass.

Cosmic Dust↗
PubMed2004

The Cometary and Interstellar Dust Analyzer at comet 81P/Wild 2.

The CIDA (Cometary and Interstellar Dust Analyzer) instrument on the Stardust spacecraft is a time-of-flight mass spectrometer used to analyze ions formed when fast dust particles strike the instrument's target. In the spectra of 45 presumably interstellar particles, quinone derivates were identified as constituents in the organic component. The 29 spectra obtained during the flyby of Comet 81P/Wild 2 confirm the predominance of organic matter. In moving from interstellar to cometary dust, the organic material seems to lose most of its hydrogen and oxygen as water and carbon monoxide. These are now present in the comet as gas phases, whereas the dust is rich in nitrogen-containing species. No traces of amino acids were found. We detected sulfur ions in one spectrum, which suggests that sulfur species are important in cometary organics.

Amino Acids↗
PubMed2004

Pinpointing the source of a lunar meteorite: implications for the evolution of the Moon.

The lunar meteorite Sayh al Uhaymir 169 consists of an impact melt breccia extremely enriched with potassium, rare earth elements, and phosphorus [thorium, 32.7 parts per million (ppm); uranium, 8.6 ppm; potassium oxide, 0.54 weight percent], and adherent regolith. The isotope systematics of the meteorite record four lunar impact events at 3909 +/- 13 million years ago (Ma), approximately 2800 Ma, approximately 200 Ma, and <0.34 Ma, and collision with Earth sometime after 9.7 +/- 1.3 thousand years ago. With these data, we can link the impact-melt breccia to Imbrium and pinpoint the source region of the meteorite to the Lalande impact crater.

Crystallization↗
PubMed2004

Detection of methane in the atmosphere of Mars.

We report a detection of methane in the martian atmosphere by the Planetary Fourier Spectrometer onboard the Mars Express spacecraft. The global average methane mixing ratio is found to be 10 +/- 5 parts per billion by volume (ppbv). However, the mixing ratio varies between 0 and 30 ppbv over the planet. The source of methane could be either biogenic or nonbiogenic, including past or present subsurface microorganisms, hydrothermal activity, or cometary impacts.

Archaea↗
PubMed2005

Supernova olivine from cometary dust.

An interplanetary dust particle contains a submicrometer crystalline silicate aggregate of probable supernova origin. The grain has a pronounced enrichment in 18O/16O (13 times the solar value) and depletions in 17O/16O (one-third solar) and 29Si/28Si (<0.8 times solar), indicative of formation from a type II supernova. The aggregate contains olivine (forsterite 83) grains <100 nanometers in size, with microstructures that are consistent with minimal thermal alteration. This unusually iron-rich olivine grain could have formed by equilibrium condensation from cooling supernova ejecta if several different nucleosynthetic zones mixed in the proper proportions. The supernova grain is also partially encased in nitrogen-15-rich organic matter that likely formed in a presolar cold molecular cloud.

Cosmic Dust↗
PubMed2005

Photochemical mass-independent sulfur isotopes in achondritic meteorites.

Sulfides from four achondrite meteorite groups are enriched in 33S (up to 0.040 per mil) as compared with primitive chondrites and terrestrial standards. Stellar nucleosynthesis and cosmic ray spallation are ruled out as causes of the anomaly, but photochemical reactions in the early solar nebula could produce the isotopic composition. The large 33S excess present in oldhamite from the Norton County aubrite (0.161 per mil) suggests that refractory sulfide minerals condensed from a nebular gas with an enhanced carbon-oxygen ratio, but otherwise solar composition is the carrier. The presence of a mass-independent sulfur effect in meteorites argues for a similar process that could account for oxygen isotopic anomalies observed in refractory inclusions in primitive chondrites.

Carbon↗
PubMed2005

Martian surface paleotemperatures from thermochronology of meteorites.

The temporal evolution of past martian surface temperatures is poorly known. We used thermochronology and published noble gas and petrographic data to constrain the temperature histories of the nakhlites and martian meteorite ALH84001. We found that the nakhlites have not been heated to more than 350 degrees C since they formed. Our calculations also suggest that for most of the past 4 billion years, ambient near-surface temperatures on Mars are unlikely to have been much higher than the present cold (<0 degrees C) state.

Extraterrestrial Environment↗
PubMed2005

Deep Impact: excavating comet Tempel 1.

Deep Impact collided with comet Tempel 1, excavating a crater controlled by gravity. The comet's outer layer is composed of 1- to 100-micrometer fine particles with negligible strength (<65 pascals). Local gravitational field and average nucleus density (600 kilograms per cubic meter) are estimated from ejecta fallback. Initial ejecta were hot (>1000 kelvins). A large increase in organic material occurred during and after the event, with smaller changes in carbon dioxide relative to water. On approach, the spacecraft observed frequent natural outbursts, a mean radius of 3.0 +/- 0.1 kilometers, smooth and rough terrain, scarps, and impact craters. A thermal map indicates a surface in equilibrium with sunlight.

Jupiter↗