A. Morlok

CIRCUMSTELLAR DUST CREATED BY TERRESTRIAL PLANET FORMATION IN HD 113766

We present an analysis of the gas-poor circumstellar material in the HD 113766 binary system (F3/F5, 10-16 Myr), recently observed by the Spitzer Space Telescope. For our study we have used the IR mineralogical model derived from observations of the Deep Impact experiment. We find the dust dominated by warm, fine (~1 μm) particles, abundant in Mg-rich olivine, crystalline pyroxenes, amorphous silicates, Fe-rich sulfides, amorphous carbon, and colder water ice. The warm dust material mix is akin to an inner main-belt asteroid of S-type composition. The ~440 K effective temperature of the warm dust implies that the bulk of the observed material is in a narrow belt ~1.8 AU from the 4.4 L☉ central source, in the terrestrial planet-forming region and habitable zone of the system (equivalent to 0.9 AU in the solar system). The icy dust lies in two belts, located at 4-9 and 30-80 AU. The lower bound of warm dust mass in 0.1-20 μm, -->dn/da ~ a−3.5 particles is very large, at least -->3 × 1020 kg, equivalent to a 320 km radius asteroid of 2.5 g cm−3 density. Assuming 10 m particles are the largest present, the lower bound of warm dust mass is at least 0.5 MMars. Neither primordial nor mature, the dust around HD 113766A originates from catastrophic disruption of terrestrial planet embryo(s) and subsequent grinding of the fragments or from collisions in a young, extremely dense asteroid belt undergoing planetary aggregation. The persistence of the strong IR excess over the last two decades argues for a mechanism to provide replenishment of the circumstellar material on yearly timescales.

SPITZER EVIDENCE FOR A LATE-HEAVY BOMBARDMENT AND THE FORMATION OF UREILITES IN η CORVI At ∼1 Gyr

We have analyzed Spitzer and NASA/IRTF 2-35 μm spectra of the warm, ~350 K circumstellar dust around the nearby MS star η Corvi (F2V, 1.4 ± 0.3 Gyr). The spectra show clear evidence for warm, water- and carbon-rich dust at ~3 AU from the central star, in the systems terrestrial habitability zone. Spectral features due to ultra-primitive cometary material were found, in addition to features due to impact produced silica and high-temperature carbonaceous phases. At least 9 × 1018 kg of 0.1-100 μm warm dust is present in a collisional equilibrium distribution with dn/da ~ a –3.5, the equivalent of a 130 km radius Kuiper Belt object (KBO) of 1.0 g cm3 density and similar to recent estimates of the mass delivered to the Earth at 0.6-0.8 Gyr during the late-heavy bombardment. We conclude that the parent body was a Kuiper Belt body or bodies which captured a large amount of early primitive material in the first megayears of the systems lifetime and preserved it in deep freeze at ~150 AU. At ~1.4 Gyr they were prompted by dynamical stirring of their parent Kuiper Belt into spiraling into the inner system, eventually colliding at 5-10 km s–1 with a rocky planetary body of mass ≤M Earth at ~3 AU, delivering large amounts of water (>0.1% of M Earth s Oceans) and carbon-rich material. The Spitzer spectrum also closely matches spectra reported for the Ureilite meteorites of the Sudan Almahata Sitta fall in 2008, suggesting that one of the Ureilite parent bodies was a KBO.

Stable isotope composition of impact glasses from the Nordlinger Ries impact crater, Germany

Abstract The Ries impact, which today is represented by a 24-km-diameter complex crater, occurred at 15 Ma. It is estimated that a projectile of about 1 km diameter formed a transient crater of 6 to 7 km radius and 2.8 km depth. Impact melt glasses investigated are found within the suevite, the breccia that forms the uppermost layers of the ejecta blanket around the crater. The glasses are considered to represent quenched melts produced as a result of the impact. The oxygen isotope compositions of several glasses sampled from widely spaced localities are very homogeneous with δ 18 O values in the range of 6.7 to 7.4‰ and δ 17 O of 3.3 to 3.7‰. With increasing devitrification and alteration the δ 18 O values increase up to 15.8‰, δ 17 O up to 8.1‰. Water content also increases with devitrification from fresh glasses with about 1.3 wt.% to 3.8 wt.% in devitrified glass. δD values decrease with increasing water content from about −87 to −127‰. Whole-rock oxygen isotope compositions of the sedimentary cover sequence range from 17 to 27‰, and crystalline basement rocks range from 8.8 to 13.5‰ for granites and gneisses, whereas amphibolites have δ 18 O values of 5.2 and 6.1‰. Models suggesting that the glass in the suevite represents a mixed melt derived from all the rocks present in the suevite in proportion of their occurrence (Engelhardt, 1997) do not agree with the oxygen isotope composition of the glass. The simplest explanation for the homogeneous chemical and oxygen isotope composition is that the glass represents quenched melts from a few closely spaced lithologies only. High Fe and Ni contents in the glass, Cr/Ni and Cr/Co ratios, and relatively low δ 18 O values indicate involvement of amphibolites during melting. The balance of the melt was made up of spatially associated granites or gneisses as indicated by the major element and rare-earth element geochemistry. The best agreement between major element and oxygen isotope composition of the glass and model melts is obtained by mixtures of amphibolite and granite in proportions similar to their average occurrence in the fallout suevite.