The Stardust spacecraft collected thousands of particles from comet 81P/Wild 2 and returned them to Earth for laboratory study. The preliminary examination of these samples shows that the nonvolatile portion of the comet is an unequilibrated assortment of materials that have both presolar and solar system origin. The comet contains an abundance of silicate grains that are much larger than predictions of interstellar grain models, and many of these are high-temperature minerals that appear to have formed in the inner regions of the solar nebula. Their presence in a comet proves that the formation of the solar system included mixing on the grandest scales.

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.

Particles emanating from comet 81P/Wild 2 collided with the Stardust spacecraft at 6.1 kilometers per second, producing hypervelocity impact features on the collector surfaces that were returned to Earth. The morphologies of these surprisingly diverse features were created by particles varying from dense mineral grains to loosely bound, polymineralic aggregates ranging from tens of nanometers to hundreds of micrometers in size. The cumulative size distribution of Wild 2 dust is shallower than that of comet Halley, yet steeper than that of comet Grigg-Skjellerup.

The Ries Crater, an impact structure of 26 km diameter in south Germany, is the largest terrestrial crater where substantial amounts of ejecta are preserved, on occasion >100 m deep. Further, the target stratigraphy is well known, and it is possible to relate specific clasts and breccia lithologies to initial target depth. As a consequence the continuous deposits of the Ries, also known as Bunte Breccia, may be studied with exceptional field control. We report field observations and laboratory analyses obtained from 560‐m core materials, taken at nine different locations that range from 16 to 37 km in radial distance from the impact center. The objective is to relate the Ries observations to ejection, and to emplacement processes of large‐scale, planetary crater deposits. The observations regarding the modal‐stratigraphic characteristics of the Bunte Breccia may be summarized as follows: only <0.15% (weight) of the total deposit consists of crystalline clasts larger than 1 cm that are derived from depths of >600 m; some 0.7% is composed of Triassic clasts, originating from 300 to 600‐m depths; Lower and Middle Jurassic horizons (approximately 300–150 m) constitute some 2.3%, and Upper Jurassic (0–150 m) makes up some 31.5%. In addition, the Bunte Breccia contains Tertiary materials in the form of >1‐cm clasts (29.1%) and as highly comminuted, fine‐grained “matrix” (<1 cm) accounting for the remaining 36.3%; these Tertiary materials constituted the immediate crater environment, i.e., a substrate, onto which the Ries ejecta were deposited. These substrate materials were thoroughly mixed into the continuous deposits. The ratio of “primary crater ejecta” to local substrate components decreases with increasing radial range. There is, however, no vertical stratification with regard to modal‐stratigraphic composition at any specific location; modal‐stratigraphic composition is highly variable on meter scales; Bunte Breccia appears to be a chaotic mixture resulting from a highly turbulent depositional environment. Also, the orientation of clasts larger than 1 cm is random. Detailed grain size data reveal progressively decreasing grain sizes with increasing radial range of both primary crater ejecta and local substrate materials. In addition, progressive comminution of primary ejecta related to increasing target depth is observed. Components shocked to >10 GPa constitute <0.1% (weight) of the entire deposit, which indicates that Bunte Breccia was emplaced at essentially ambient temperatures. When possible, the above observations are quantified via linear regressions throughout the text. All of these observations are consistent with, if not predicted by, a ballistic emplacement scenario as postulated by Oberbeck and co‐workers: primary crater ejecta are expelled ballistically and will form secondary craters in the local substrate; a mixture of primary and secondary ejecta results and combines into a highly turbulent, ground‐hugging debris surge as the final phase of ejecta emplacement. Total emplacement time for the Bunte ...

Abstract-The cometary tray of the NASA Stardust spacecraft's aerogel collector was examined to study the dust captured during the 2004 flyby of comet 81P/Wild 2. An optical scan of the entire collector surface revealed 256 impact features in the aerogel (width >100 μm). Twenty aerogel blocks (out of a total of 132) were removed from the collector tray for a higher resolution optical scan and 186 tracks were observed (track length >50 μm and width >8 μm). The impact features were classified into three types based on their morphology. Laboratory calibrations were conducted that reproduced all three types. This work suggests that the cometary dust consisted of some cohesive, relatively strong particles as well as particles with a more friable or low cohesion matrix containing smaller strong grains. The calibrations also permitted a particle size distribution to be estimated for the cometary dust. We estimate that approximately 1200 particles bigger than 1 μm struck the aerogel. The cumulative size distribution of the captured particles was obtained and compared with observations made by active dust detectors during the encounter. At large sizes (>20 μm) all measures of the dust are compatible, but at micrometer scales and smaller discrepancies exist between the various measurement systems that may reflect structure in the dust flux (streams, clusters etc.) along with some possible instrument effects.

Abstract-Velocities of ejecta from seven impacts of aluminum projectiles into coarse-grained sand have been measured with a laser-based apparatus that produces stroboscopic photographs of individual grains in ballistic flight. Speeds and angles of the majority of the ejecta can then be measured very precisely. There appears to be little effect of impact velocity on the functional relationship between the scaled, radial launch position and either the speed or angle of ejection; the seven experiments covered a range of impact velocities from 0.8 to 1.9 km s-1. The measured ejection speeds follow power-law distributions, as predicted by dimensional analysis, but the angle of ejection is not constant throughout a given event as predicted. Indeed, the angle of ejection declines gradually with increasing radial distance from the impact point, but there are indications that the angle increases again as the position of the final crater's rim is approached. The exponents determined from scaled crater dimensions and ejection-speed distributions are substantially different. Although this might imply that assumptions used in the dimensional analysis are not valid, it is also possible that the coarse sand, whose component grains were comparable in dimension to the diameter of the impactors, instead presented a target that was more of an inhomogeneous aggregate of large fragments than a uniform, continuous medium.

Stardust, launched in 1999, is the first mission designed to bring samples from a known, recently deflected comet, 81P/Wild 2, on 2 January 2004 and is also the first to capture newly discovered contemporary interstellar dust streaming through our solar system. The Stardust aerogel collector accomplishes Stardust's primary science and will be returned to Earth with its captured samples on 15 January 2006 in a reentry capsule. Wild 2 samples will be captured at 6.12 km/s and represent well‐preserved relics of the outer regions of our solar nebula and fundamental building blocks of our planetary system. Interstellar grains captured at velocities of less than 10 km/s are expected to survive intact and represent the main repositories of condensable elements that permeate the galaxy. These solid cometary and interstellar samples will be captured in two back‐to‐back sample collection trays filled with variable‐density aerogel. There are 132 silica aerogel capture cells of 3 cm and 1 cm thickness for the cometary and the interstellar sides, respectively. The aerogel capture cells were wedged into the sample collection trays and wrapped on all four sides with 100‐μm‐thick pure aluminum foil to facilitate aerogel cell removal. The total exposed Wild 2 surface area is 1039 cm2 of aerogel and 153 cm2 of aluminum foil. Results from a preliminary examination for the Wild 2 samples will be reported within 9 months of sample return and for the interstellar samples a year later. After preliminary examination the samples will be transferred to the NASA Office of the Curator and made available to the general science community.

[1] The feldspar and pyroxene mineralogies on Mars revealed by the Thermal Emission Spectrometer (TES) on Mars Global Surveyor likely record a variety of shock effects, as suggested by petrologic analyses of the Martian meteorites and the abundance of impact craters on the planet's surface. To study the effects of shock pressures on thermal infrared spectra of these minerals, we performed shock recovery experiments on orthopyroxenite and anorthosite samples from the Stillwater Complex (Montana) over peak pressures from 17 to 63 GPa. We acquired emissivity and hemispherical reflectance spectra (350-1400 cm À1; $7-29 mm) of both coherent chips and fine-grained powders of shocked and unshocked samples. These spectra are more directly comparable to remotely sensed data of Mars (e.g., TES) than previously acquired absorption or transmission spectra of shocked minerals. The spectra of experimentally shocked feldspar show systematic changes with increasing pressure due to depolymerization of the silica tetrahedra. For the spectra of chips, this includes the disappearance of small bands in the 500-650 cm À1 region and a strong band at 1115 cm À1 , and changes in positions of a strong band near 940 cm À1 and the Christiansen feature near 1250 cm À1. Spectra of the shocked powders show the gradual disappearance of a transparency feature near 830 cm À1. Fewer changes are observed in the pyroxene spectra at pressures as high as 63 GPa. Spectra of experimentally shocked minerals will help identify more precisely the mineralogy of rocks and soils not only from TES but also from Mars instruments such as miniTES and THEMIS.