The future of Stardust science

Recent observations indicate that >99% of the small bodies in the solar system reside in its outer reaches—in the Kuiper Belt and Oort Cloud. Kuiper Belt bodies are probably the best‐preserved representatives of the icy planetesimals that dominated the bulk of the solid mass in the early solar system. They likely contain preserved materials inherited from the protosolar cloud, held in cryogenic storage since the formation of the solar system. Despite their importance, they are relatively underrepresented in our extraterrestrial sample collections by many orders of magnitude (~10¹³ by mass) as compared with the asteroids, represented by meteorites, which are composed of materials that have generally been strongly altered by thermal and aqueous processes. We have only begun to scratch the surface in understanding Kuiper Belt objects, but it is already clear that the very limited samples of them that we have in our laboratories hold the promise of dramatically expanding our understanding of the formation of the solar system. Stardust returned the first samples from a known small solar system body, the Jupiter‐family comet 81P/Wild 2, and, in a separate collector, the first solid samples from the local interstellar medium. The first decade of Stardust research resulted in more than 142 peer‐reviewed publications, including 15 papers in Science. Analyses of these amazing samples continue to yield unexpected discoveries and to raise new questions about the history of the early solar system. We identify nine high‐priority scientific objectives for future Stardust analyses that address important unsolved problems in planetary science.     

Combined noble gas and trace element measurements on individual stratospheric interplanetary dust particles

Abstract— The trace element compositions and noble gas contents of 32 individual interplanetary dust particles (IDPs) collected in the Earth's stratosphere were measured. Trace element compositions are generally similar to CI meteorites, with occasional depletions in Zn/Fe with respect to CI. Noble gases were detected in all but one of the IDPs. Noble gas elemental compositions are consistent with the presence of fractionated solar wind. A rough correlation between surface‐normalized He abundances and Zn/Fe ratios is observed; Zn‐poor particles generally have lower He contents than the other IDPs. This suggests that both elements were lost by frictional heating during atmospheric entry and confirms the view that Zn can serve as an entry‐heating indicator in IDPs.     

Dust Production from the Hypervelocity Impact Disruption of Hydrated Targets

More than half of the C-type asteroids, which are the dominant type of asteroid in the outer half of the main belt, show evidence of hydration in their reflectance spectra. In order to understand the collisional evolution of asteroids, the production of interplanetary dust, and to model the infrared signature of small particles in the Solar System it is important to characterize the dust production from primary impact disruption events, and compare the disruption of hydrous and anhydrous targets. We performed impact disruption experiments of three “greenstone” targets, a hydrothermally metamorphosed basalt, and compared the results of these disruptions to our previous disruption experiments on porous, anhydrous basalt targets and to literature data on the disruption of non-porous, anhydrous basalt targets. The greenstone targets were selected because their major hydrous alteration phase is serpentine, the same hydrous alteration phase found in hydrous CM meteorites, like Murchison. The porous, anhydrous basalt targets were selected because their structure, consisting of millimeter-size olivine phenocrysts in a more porous, anhydrous matrix is similar to the structure of anhydrous chondritic meteorites, which consist of millimeter-size olivine chondrules embedded in a more porous, anhydrous matrix. The disruption measurements indicate the threshold collisional specific energy, Q _D^*, is 570 J/kg for the greenstone, which is lower than the literature values for non-porous basalt targets, and significantly lower than the value of 2500 J/kg that we have measured for porous anhydrous basalt targets. We determined the mass-frequency distribution of the debris from the disruption of the greenstone targets, which ranged in mass from 80 to 280 g, over a nine order-of-magnitude mass range, from ~10⁻⁹ g to the mass of the largest fragment. The cumulative mass-frequency distribution from the greenstone targets is fit by two power–law segments, one for masses >10⁻² g, which is significantly steeper than the corresponding segment from the disruption of similar-sized anhydrous basalt, and one in the range from 10⁻⁹ to 10⁻² g, which is significantly flatter than the corresponding segment from the disruption of similar size anhydrous basalt. These hydrous greenstone targets overproduce small fragments (10⁻⁴ to 10⁰ g) compared to anhydrous basalt targets, but underproduce dust-size grains (10⁻⁹ to 10⁻⁴ g) compared to anhydrous basalt targets.     

Impacts into porous foam targets: possible implications for the disruption of comet nuclei

We have conducted a series of impact experiments to examine the response of very porous foam targets to various impacts. Under near-vacuum conditions, closed-pore and open-pore foam targets were subjected to ∼1 km s⁻¹ impacts from aluminum and foam projectiles. We found that open-pore targets absorbed the impacts with little or no global fragmentation or noticeable cratering, exhibiting only local damage along the path of the projectile, which tunneled through the target. Closed-pore targets exhibited nearly explosive disruption, apparently resulting from stresses built up within the target due to internal pressurization from air that could not escape the target interior during evacuation of the impact chamber. These results suggest that build-up of internal volatile pressure within the nuclei of collisionally or dynamically unevolved comets could allow comparatively small impacts onto their surfaces to result in disproportionately disruptive outcomes.     

Physical properties of the stone meteorites: Implications for the properties of their parent bodies

The physical properties of the stone meteorites provide important clues to understanding the formation and physical evolution of material in the Solar protoplanetary disk as well providing indications of the properties of their asteroidal parent bodies. Knowledge of these properties is essential for modeling a number of Solar System processes, such as bolides in planetary atmospheres, the thermal inertia of atmosphereless solid body surfaces, and the internal physical and thermal evolution of asteroids and rock-rich icy bodies. In addition, insight into the physical properties of the asteroids is important for the design of robotic and crewed reconnaissance, lander, and sample return spacecraft missions to the asteroids. One key property is meteorite porosity, which ranges from 0% to more than 40%, similar to the range of porosities seen in asteroids. Porosity affects many of the other physical properties including thermal conductivity, speed of sound, deformation under stress, strength, and response to impact. As a result of the porosity, the properties of most stone meteorites differ significantly from those of compact terrestrial rocks, whose physical properties have been used in many models of asteroid behavior. A few physical properties, such as grain density, magnetic susceptibility, and heat capacity are not functions of porosity. Taken together, the grain density and the magnetic susceptibility can be used to classify unweathered or minimally weathered ordinary chondrites. This provides a rapid screening technique to identify heterogeneous samples, classify new samples, and identify misclassified meteorites or interlopers in strewn fields.