The OSIRIS‐REx target asteroid (101955) Bennu: Constraints on its physical,geological, and dynamical nature from astronomical observations          下载免费PDF全文

D. S. Lauretta  A. E. Bartels  M. A. Barucci  E. B. Bierhaus  R. P. Binzel  W. F. Bottke  H. Campins  S. R. Chesley  B. C. Clark  B. E. Clark  E. A. Cloutis  H. C. Connolly  M. K. Crombie  M. Delbó  J. P. Dworkin  J. P. Emery  D. P. Glavin  V. E. Hamilton  C. W. Hergenrother  C. L. Johnson  L. P. Keller  P. Michel  M. C. Nolan  S. A. Sandford  D. J. Scheeres  A. A. Simon  B. M. Sutter  D. Vokrouhlický  K. J. Walsh 《Meteoritics & planetary science》2015,50(4):834-849

We review the results of an extensive campaign to determine the physical, geological, and dynamical properties of asteroid (101955) Bennu. This investigation provides information on the orbit, shape, mass, rotation state, radar response, photometric, spectroscopic, thermal, regolith, and environmental properties of Bennu. We combine these data with cosmochemical and dynamical models to develop a hypothetical timeline for Bennu's formation and evolution. We infer that Bennu is an ancient object that has witnessed over 4.5 Gyr of solar system history. Its chemistry and mineralogy were established within the first 10 Myr of the solar system. It likely originated as a discrete asteroid in the inner Main Belt approximately 0.7–2 Gyr ago as a fragment from the catastrophic disruption of a large (approximately 100‐km), carbonaceous asteroid. It was delivered to near‐Earth space via a combination of Yarkovsky‐induced drift and interaction with giant‐planet resonances. During its journey, YORP processes and planetary close encounters modified Bennu's spin state, potentially reshaping and resurfacing the asteroid. We also review work on Bennu's future dynamical evolution and constrain its ultimate fate. It is one of the most Potentially Hazardous Asteroids with an approximately 1‐in‐2700 chance of impacting the Earth in the late 22nd century. It will most likely end its dynamical life by falling into the Sun. The highest probability for a planetary impact is with Venus, followed by the Earth. There is a chance that Bennu will be ejected from the inner solar system after a close encounter with Jupiter. OSIRIS‐REx will return samples from the surface of this intriguing asteroid in September 2023.  相似文献   

Pristine stratospheric collection of interplanetary dust on an oil‐free polyurethane foam substrate          下载免费PDF全文

Scott Messenger  Keiko Nakamura‐Messenger  Lindsay P. Keller  Simon J. Clemett 《Meteoritics & planetary science》2015,50(8):1468-1485

We performed chemical, mineralogical, and isotopic studies of the first interplanetary dust particles (IDPs) collected in the stratosphere without the use of silicone oil. The collection substrate, polyurethane foam, effectively traps impacting particles, but the lack of an embedding medium results in significant particle fragmentation. Two dust particles found on the collector exhibit the typical compositional and mineralogical properties of chondritic porous interplanetary dust particles (CP‐IDPs). Hydrogen and nitrogen isotopic imaging revealed isotopic anomalies of typical magnitude and spatial variability observed in previous CP‐IDP studies. Oxygen isotopic imaging shows that individual mineral grains and glass with embedded metal and sulfide (GEMS) grains are dominated by solar system materials. No systematic differences are observed in element abundance patterns of GEMS grains from the dry collection versus silicone oil‐collected IDPs. This initial study establishes the validity of a new IDP collection substrate that avoids the use of silicone oil as a collection medium, removing the need for this problematic contaminant and the organic solvents necessary to remove it. Additional silicone oil‐free collections of this type are needed to determine more accurate bulk element abundances of IDPs and to examine the indigenous soluble organic components of IDPs.  相似文献   

Connecting atmospheric science and atmospheric models for aerocapture at Titan and the outer planets     

C.G. Justus  Vernon W. Keller  Mary Kae Lockwood 《Planetary and Space Science》2005,53(5):601-605

Many atmospheric measurement systems, such as the sounding instruments on Voyager, gather atmospheric information in the form of temperature versus pressure level. In these terms, there is considerable consistency among the mean atmospheric profiles of the outer planets Jupiter through Neptune, including Titan. On a given planet or on Titan, the range of variability of temperature versus pressure level due to seasonal, latitudinal, and diurnal variations is also not large. However, many engineering needs for atmospheric models relate not to temperature versus pressure level but atmospheric density versus geometric altitude. This need is especially true for design and analysis of aerocapture systems. Drag force available for aerocapture is directly proportional to atmospheric density. Available aerocapture “corridor width” (allowable range of atmospheric entry angle) also depends on height rate of change of atmospheric density, as characterized by density scale height. Characteristics of hydrostatics and the gas law equation mean that relatively small systematic differences in temperature versus pressure profiles can integrate at high altitudes to very large differences in density versus altitude profiles. Thus, a given periapsis density required to accomplish successful aerocapture can occur at substantially different altitudes (∼150-300 km) on the various outer planets, and significantly different density scale heights (∼20-50 km) can occur at these periapsis altitudes. This paper will illustrate these effects and discuss implications for improvements in atmospheric measurements to yield significant impact on design of aerocapture systems for future missions to Titan and the outer planets. Relatively small-scale atmospheric perturbations, such as gravity waves, tides, and other atmospheric variations can also have significant effect on design details for aerocapture guidance and control systems. This paper will discuss benefits that would result from improved understanding of Titan and outer planetary atmospheric perturbation characteristics. Details of recent engineering-level atmospheric models for Titan and Neptune will be presented, and effects of present and future levels of atmospheric uncertainty and variability characteristics will be examined.  相似文献   

Evidence of space weathering in regolith breccias I: Lunar regolith breccias     

Sarah K. Noble  Lindsay P. Keller  Carl M. Pieters 《Meteoritics & planetary science》2005,40(3):397-408

Abstract— We have analyzed a suite of lunar regolith breccias in order to assess how well space weathering products can be preserved through the lithification process and therefore whether or not it is appropriate to search for space weathering products in asteroidal regolith breccia meteorites. It was found that space weathering products, vapor/sputter deposited nanophase‐iron‐bearing rims in particular, are easily identified in even heavily shocked/compacted lunar regolith breccias. Such rims, if created on asteroids, should thus be preserved in asteroidal regolith breccia meteorites. Two additional rim types, glass rims and vesicular rims, identified in regolith breccias, are also described. These rims are common in lunar regolith breccias but rare to absent in lunar soils, which suggests that they are created in the breccia‐forming process itself. While not “space weathering products” in the strictest sense, these additional rims give us insight into the regolith breccia formation process. The presence or absence of glass and/or vesicular rims in asteroidal regolith breccias will likewise tell us about environmental conditions on the surface of the asteroid body on which the breccia was created.  相似文献   

On the visibility of nuclei of dusty comets     

R. Hellmich  H.U. Keller 《Icarus》1981,47(3):325-332

The problem of visibility of a cometary nucleus discussed in general terms for single scattering by dust grains. The ratio of radiatio scattered in the dust column above the surface and that reflected from the nucleus determines the visibility of features on the nuclear surface. A contrast parameter characterizing the ration of radiation foming from the nuclear surface and that of the nuclear vicinity describes the visibility of the full nucleus against the dust fore- and background. These quantities and the intensity distribution of scattered solar radiation across the nucleus and its vicinity are calculated for the case of comet Halley at a heliocentric distance of 0.9 AU after perihelion (Giotto encounter). The scattering calculations are based on an isotropic dust distribution derived from hydrodynamics gas-dust interactions resulting in a steep densiity increase right above the surface. For Newburn's nominal model of comet Halley, an optical depth of about 0.5 impairs the visibility of the nucleus somewhat.  相似文献   

Some effects of long-term fumigations with ozone on spruce (Picea abies [L.] Karsten)     

Theodor Keller Dr.  Rudolf Häsler Dr. 《GeoJournal》1988,17(2):277-278

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MAJOR ELEMENT COMPOSITIONAL VARIATION WITHIN AND BETWEEN DIFFERENT LATE EOCENE MICROTEKTITE STREWNFIELDS     

Steven L. D'Hondt  Gerta Keller  Robert F. Stallard 《Meteoritics & planetary science》1987,22(1):61-79

Major element compositional overlap exists between microspherules of different microtektite layers or strewnfields. For this reason, microspherules of similar composition cannot, a priori, be assumed to belong to the same microtektite event and those of different compositions cannot, a priori, be assumed to result from different events. Nevertheless, despite major element compositional overlap between microspherules of different strewnfields, multivariate factor analysis shows microtektites and related microspherules of three stratigraphically different late Eocene layers to follow recognizably different compositional trends. The microtektite population of the North American strewnfield (Globorotalia cerroazulensis Zone) follows compositionally well defined trends and is characterized by high concentrations of SiO₂, Al₂O₃, and TiO₂. The microspherule population of the slightly older crystal-bearing Globorotalia cerroazulensis Zone microspherule layer is more heterogeneous and characterized by microspherules which are relatively enriched in FeO and MgO and relatively impoverished in SiO₂ and TiO₂. The microspherule population of the oldest microspherule layer in the uppermost Globigerapsis semiinvoluta Zone is highly heterogeneous and characterized by microspherules which are relatively enriched in CaO and impoverished in Al₂O₃ and Na₂O. Individual microspherules of this oldest late Eocene horizon often exhibit major element compositions similar to those of the lower Gl. cerroazulensis Zone layer and occasionally exhibit major element compositions similar to North American layer microtektites. Nevertheless, late Eocene microspherule occurrences can be assigned to appropriate late Eocene microtektite horizons on the basis of major element compositional trends.  相似文献   

Yaxcopoil-1 and the Chicxulub impact     

Wolfgang?StinnesbeckEmail author  Gerta?Keller  Thierry?Adatte  Markus?Harting  Doris?Stüben  Georg?Istrate  Utz?Kramar 《International Journal of Earth Sciences》2004,93(6):1042-1065

CSDP core Yaxcopoil-1 was drilled to a depth of 1,511 m within the Chicxulub crater. An organic-rich marly limestone near the base of the hole (1,495 to 1,452 m) was deposited in an open marine shelf environment during the latest Cenomanian (uppermost Rotalipora cushmani zone). The overlying sequence of limestones, dolomites and anhydrites (1,495 to 894 m) indicates deposition in various carbonate platform environments (e.g., sabkhas, lagoons). A 100-m-thick suevite breccia (894–794 m) identifies the Chicxulub impact event. Above the suevite breccia is a dolomitic limestone with planktic foraminiferal assemblages indicative of Plummerita hantkeninoides zone CF1, which spans the last 300 ky of the Maastrichtian. An erosional surface 50 cm above the breccia/dolomite contact marks the K/T boundary and a hiatus. Limestones above this contact contain the first Tertiary planktic foraminifera indicative of an upper P. eugubina zone P1a(2) age. Another hiatus 7 cm upsection separates zone P1a(2) and hemipelagic limestones of planktic foraminiferal Zone P1c. Planktic foraminiferal assemblages of Zone Plc to P3b age are present from a depth of 794.04 up to 775 m. The Cretaceous carbonate sequence appears to be autochthonous, with a stratigraphic sequence comparable to late Cretaceous sediments known from outside the Chicxulub crater in northern and southern Yucatan, including the late Cenomanian organic-rich marly limestone. There is no evidence that these sediments represent crater infill due to megablocks sliding into the crater, such as major disruption of sediments, chaotic changes in lithology, overturned or deep dipping megablocks, major mechanical fragmentation, shock or thermal alteration, or ductile deformation. Breccia units that are intercalated in the carbonate platform sequence are intraformational in origin (e.g., dissolution of evaporites) and dykes are rare. Major disturbances of strata by the impact therefore appear to have been confined to within less than 60 km from the proposed impact center. Yaxcopoil-1 may be located outside the collapsed transient crater cavity, either on the upper end of an elevated and tilted horst of the terrace zone, or even outside the annular crater cavity. The Chicxulub site thus records a large impact that predates the K/T boundary impact and mass extinction.  相似文献   

Direct statistical simulation of the near-surface layers of the cometary atmosphere. I. A spherical nucleus     

Yu. V. Skorov  G. N. Markelov  H. U. Keller 《Solar System Research》2004,38(6):455-475

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Personality type differences between Ph.D. climate researchers and the general public: implications for effective communication     

C. Susan Weiler  Jason K. Keller  Christina Olex 《Climatic change》2012,112(2):233-242

Effectively communicating the complexity of climate change to the public is an important goal for the climate change research community, particularly for those of us who receive public funds. The challenge of communicating the science of climate change will be reduced if climate change researchers consider the links between personality types, communication tendencies and learning preferences. Jungian personality type is one of many factors related to an individual’s preferred style of taking in and processing information, i.e., preferred communication style. In this paper, we demonstrate that the Jungian personality type profile of interdisciplinary, early career climate researchers is significantly different from that of the general population in the United States. In particular, Ph.D. climate researchers tend towards Intuition and focus on theories and the “big picture”, while the U.S. general population tends towards Sensing and focuses on concrete examples and experience. There are other differences as well in the way the general public as a group prefers to take in information, make decisions, and deal with the outer world, compared with the average interdisciplinary climate scientist. These differences have important implications for communication between these two groups. We suggest that climate researchers will be more effective in conveying their messages if they are aware of their own personality type and potential differences in preferred learning and communication styles between themselves and the general public (and other specific audiences), and use this knowledge to more effectively target their audience.  相似文献