83 research outputs found
High-temperature Dust Condensation around an AGB Star: Evidence from a Highly Pristine Presolar Corundum
Corundum (-AlO) and amorphous or metastable
AlO are common components of circumstellar dust observed around
O-rich asymptotic giant branch (AGB) stars and found in primitive meteorites.
We report a detailed isotopic and microstructural investigation of a unique
presolar corundum grain, QUE060, identified in an acid residue of the Queen
Alexandra Range 97008 (LL3.05) meteorite. Based on its O and Mg isotopic
compositions, this 1.4 m diameter grain formed in a low- or
intermediate-mass AGB star. It has four developed rhombohedral 011
faces of corundum and a rough, rounded face with cavities. High Mg contents
(Mg/Al 0.004) are due to the decay of radioactive Al. No spinel
(MgAlO) inclusions that might have exsolved from the corundum are
observed, but there are several high-Mg domains with modulated structures. The
subhedral shape of grain QUE060 is the first clear evidence that corundum
condenses and grows to micrometer sizes in the extended atmospheres around AGB
stars. The flat faces indicate that grain QUE060 experienced little
modification by gas-grain and grain-grain collisions in the interstellar medium
(ISM) and solar nebula. The Mg distribution in its structure indicates that
grain QUE060 has not experienced any severe heating events since the exhaustion
of Al. However, it underwent at least one very transient heating event
to form the high-Mg domains. A possible mechanism for producing this transient
event, as well as the one rough surface and cavity, is a single grain-grain
collision in the ISM. These results indicate that grain QUE060 is the most
pristine circumstellar corundum studied to date
Probing the Depths of Space Weathering: A Cross-sectional View of Lunar Rock 76015
The term "space weathering" refers to the cumulative effects of several processes operating at the surface of any solar system body not protected by a thick atmosphere. These processes include cosmic and solar ray irradiation, solar wind implantation and sputtering, as well as melting and vaporization due to micrometeorite bombardment. Space weathering discussions have generally centered around soils but exposed rocks will also incur the effects of weathering. Rocks have much longer surface lifetimes than an individual soil grain and thus record a longer history of exposure. By studying the weathering products which have built up on a rock surface, we can gain a deeper perspective on the weathering process and better assess the relative importance of various weathering components. The weathered coating, or patina, of the lunar rock 76015 has been previously studied using SEM and TEM. It is a noritic breccia with both "glazed" (smooth glassy) and "classic" (microcratered and pancake-bearing) patina coatings. Previous TEM work on 76015 relied on ultramicrotomy to prepare cross sections of the patina coating, but these sections were limited by the "chatter" and loss of material in these brittle samples. Here we have used a focused ion beam (FIB) instrument to prepare cross sections in which the delicate stratigraphy of the patina coating is beautifully preserved
- …