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71.
We present the results of spectral modeling of 17 Trojan asteroids. The surface composition of this group of objects (located just beyond the main belt, trapped in Jupiter's stable Lagrange points) remains uncertain due to an absence of diagnostic absorption features in their spectra. We quantitatively analyze spectra of these objects covering the range 0.3-4.0 μm using the formulation for scattering in a particulate medium developed by Hapke. Since the widest spectral range possible is desired to provide the most robust results, recently measured near-IR spectra are combined with previously published visible and near-IR data. These composite spectra are converted to and modeled in terms of geometric albedo to provide the additional constraint of the absolute brightness of the asteroids. It is important that this modeling is performed for a large number of objects, and results are derived based on trends among best-fit models. Under this rigorous examination, we find that it is unlikely that the red spectral slope is a result of organics on the surfaces, due mainly to the lack of absorptions in the L-band. Instead, anhydrous silicates adequately describe the spectral characteristics of this group of objects. A significant fraction of carbonaceous material is also likely present, but is not responsible for the red spectral slope in these models. Also, using these models, we estimate that these surfaces contain at most a few wt% of H2O ice and no more than 10-30 wt% of hydrated silicates. 相似文献
72.
73.
R.H. Brown K.H. Baines J.-P. Bibring F. Capaccioni R.N. Clark D.P. Cruikshank V. Formisano Y. Langevin T.B. McCord V. Mennella P.D. Nicholson C. Sotin M.A. Chamberlain G. Hansen M. Showalter 《Icarus》2003,164(2):461-470
The Cassini Visual and Infrared Mapping Spectrometer (VIMS) is an imaging spectrometer covering the wavelength range 0.3-5.2 μm in 352 spectral channels, with a nominal instantaneous field of view of 0.5 mrad. The Cassini flyby of Jupiter represented a unique opportunity to accomplish two important goals: scientific observations of the jovian system and functional tests of the VIMS instrument under conditions similar to those expected to obtain during Cassini's 4-year tour of the saturnian system. Results acquired over a complete range of visual to near-infrared wavelengths from 0.3 to 5.2 μm are presented. First detections include methane fluorescence on Jupiter, a surprisingly high opposition surge on Europa, the first visual-near-IR spectra of Himalia and Jupiter's optically-thin ring system, and the first near-infrared observations of the rings over an extensive range of phase angles (0-120°). Similarities in the center-to-limb profiles of H+3 and CH4 emissions indicate that the H+3 ionospheric density is solar-controlled outside of the auroral regions. The existence of jovian NH3 absorption at 0.93 μm is confirmed. Himalia has a slightly reddish spectrum, an apparent absorption near 3 μm, and a geometric albedo of 0.06±0.01 at 2.2 μm (assuming an 85-km radius). If the 3-μm feature in Himalia's spectrum is eventually confirmed, it would be suggestive of the presence of water in some form, either free, bound, or incorporated in layer-lattice silicates. Finally, a mean ring-particle radius of 10 μm is found to be consistent with Mie-scattering models fit to VIMS near-infrared observations acquired over 0-120° phase angle. 相似文献
74.
Peter Brown Robert J. Weryk Daniel K. Wong James Jones 《Earth, Moon, and Planets》2008,102(1-4):209-219
The Canadian Meteor Orbit Radar is a multi-frequency backscatter radar which has been in routine operation since 1999, with an orbit measurement capability since 2002. In total, CMOR has measured over 2 million orbits of meteoroids with masses greater than 10 μg, while recording more than 18 million meteor echoes in total. We have applied a two stage comparative technique for identifying meteor streams in this dataset by making use of clustering in radiants and velocities without employing orbital element comparisons directly. From the large dataset of single station echoes, combined radiant activity maps have been constructed by binning and then stacking each years data per degree of solar longitude. Using the single-station mapping technique described in Jones and Jones (Mon Not R Astron Soc 367:1050–1056, 2006) we have identified probable streams from these single station observations. Additionally, using individual radiant and velocity data from the multi-station velocity determination routines, we have utilized a wavelet search algorithm in radiant and velocity space to construct a list of probable streams. These two lists were then compared and only streams detected by both techniques, on multiple frequencies and in multiple years were assigned stream status. From this analysis we have identified 45 annual minor and major streams with high reliability. 相似文献
75.
Zoë E. Wilbur;Timothy J. McCoy;Catherine M. Corrigan;Jessica J. Barnes;Sierra V. Brown;Arya Udry; 《Meteoritics & planetary science》1962,59(9):2373-2387
Enstatite meteorites, both aubrites and enstatite chondrites, formed under exceptionally reducing conditions, similar to the planet Mercury. Despite being reduced, the MESSENGER mission showed that the surface of Mercury is more enriched in volatiles (e.g., S, Na, K, Cl) than previously thought. To better understand the mineral hosts of these volatiles and how they formed, this work examines the chemistry and petrographic settings of a rare, K-bearing sulfide called djerfisherite within enstatite chondrites and aubrites. The petrographic settings of djerfisherite within aubrites suggest this critical host of Cl formed after both the crystallization of troilite and exsolution of daubréelite. Djerfisherite is commonly observed as a rim on other sulfides and in contact with metal. We present an alteration model for djerfisherite formation in aubrite meteorites, whereby troilite and Fe-Ni metal are altered through anhydrous, alkali- and Cl-rich fluid metasomatism on the aubrite parent body to produce secondary djerfisherite. Moreover, we observe a loss of volatiles in djerfisherite within impact melted regions of the Miller Range 07139 EH3 chondrite and the Bishopville aubrite and explore the potential for impact devolatilization changes to sulfide chemistry on other reduced bodies in the Solar System. Vapor or fluid phase interactions are likely important in the formation of volatile-rich phases in reduced systems. While most Na and K on the mercurian surface is expected to be hosted in feldspar, djerfisherite is likely a minor, but critical, reservoir for K, Na, and Cl. Djerfisherite present on reduced bodies, such as Mercury, may represent sulfides formed via late-stage, primary metasomatism. 相似文献
76.
77.
A number of models which would explain ocean wave imagery taken with a synthetic aperture imaging radar are analyzed analytically and numerically. Actual radar imagery is used to support some conclusions. The models considered correspond to three sources of radar backscatter cross section modulation: tilt modulation, roughness variation, and the wave orbital velocity. The effect of the temporal changes of the surface structure, parametric interactions, and the resulting distortions are discussed. 相似文献
78.
79.
E.D. Brown 《Marine Policy》1981,5(3):282-286
The provisions of the Draft Convention1 on dispute settlement are of considerable length and complexity. Part XV (Settlement of Disputes) alone runs to 21 articles covering eight pages and Annexes V-VIII add a further twenty pages.2 Rather than subject these draft articles to a detailed analysis on a technical level, with a view to proposing improvements in drafting or in the institutional model employed, this article asks basic questions about the utility and practicability of the system incorporated in the Draft Convention. Although much has been written on the details of the system and the various optional models available to the Conference draftsmen,3 there has been a remarkable absence of debate about some of the fundamental assumptions implicit in the Draft Convention's provisions on dispute settlement. 相似文献
80.
Christopher P. Weaver Robert J. Lempert Casey Brown John A. Hall David Revell Daniel Sarewitz 《Wiley Interdisciplinary Reviews: Climate Change》2013,4(1):39-60
In this paper, we review the need for, use of, and demands on climate modeling to support so‐called ‘robust’ decision frameworks, in the context of improving the contribution of climate information to effective decision making. Such frameworks seek to identify policy vulnerabilities under deep uncertainty about the future and propose strategies for minimizing regret in the event of broken assumptions. We argue that currently there is a severe underutilization of climate models as tools for supporting decision making, and that this is slowing progress in developing informed adaptation and mitigation responses to climate change. This underutilization stems from two root causes, about which there is a growing body of literature: one, a widespread, but limiting, conception that the usefulness of climate models in planning begins and ends with regional‐scale predictions of multidecadal climate change; two, the general failure so far to incorporate learning from the decision and social sciences into climate‐related decision support in key sectors. We further argue that addressing these root causes will require expanding the conception of climate models; not simply as prediction machines within ‘predict‐then‐act’ decision frameworks, but as scenario generators, sources of insight into complex system behavior, and aids to critical thinking within robust decision frameworks. Such a shift, however, would have implications for how users perceive and use information from climate models and, ultimately, the types of information they will demand from these models—and thus for the types of simulations and numerical experiments that will have the most value for informing decision making. WIREs Clim Change 2013, 4:39–60. doi: 10.1002/wcc.202 This article is categorized under:
- Climate Models and Modeling > Knowledge Generation with Models
- Vulnerability and Adaptation to Climate Change > Institutions for Adaptation