Asteroidal regoliths     

Kevin R. Housen  Laurel L. Wilkening  Clark R. Chapman  Richard Greenberg 《Icarus》1979,39(3):317-351

We develop a physical model for the evolution of regoliths on small bodies and apply it to the asteroids and meteorite parent bodies. The model considers global deposition of that fraction of cratering ejecta that is not lost to space. It follows the build up of regolith on a typical region, removed from the larger craters which are the source of most regolith blankets. Later in the evolution, larger craters saturate the surface and are incorporated into the typical region; their net ejection of materials to space causes the elevation of the typical region to decrease and once-buried regolith becomes susceptible to ejection or gardening. The model is applied to cases of both strong, cohesive bodies and to bodies of weak, unconsolidated materials. Evolution of regolith depths and gardening rates are followed until a sufficiently large impact occurs that fractures the entire asteroid. (Larger asteroids are not dispersed, however, and evolve mergaregoliths from multiple generations of surficial regoliths mixed into their interiors.) We find that large, strong asteroids generate surficial regoliths of a few kilometers depth while strong asteroids smaller than 10-km diameter generate negligible regoliths. Our model does not treat large, weak asteroids, because their cratering ejecta fail to surround such bodies; regolith evolution is probably similar to that of the Moon. Small, weak asteroids of 1- to 10-km diameter generate centimeter- to meter-scale regoliths. In all cases studied, blanketing rates exceed excavation rates, so asteroid regoliths are rarely, if ever, gardened and should be very immature measured by lunar standards. They should exhibit many of the characteristics of the brecciated, gas-rich meteorites; intact foreign clasts, relatively low-exposure durations to galactic and solar cosmic rays low solar gas contents, minimal evidence for vitrification and agglutinate formation, etc. Both large, strong asteroids and small, weak ones provide regolith environments compatible with those inferred for the parent bodies of brecciated meteorites. But from volumetric calculations, we conclude that most brecciated meteorites formed on the surfaces of, and were recycled through the interiors of, parent bodies at least several tens of kilometers in diameter. The implications of our regolith model are consistent with properties inferred for asteroid regoliths from a variety of astronomical measurements of asteroids, although such data do not constrain regolith properties nearly as strongly as meteoritical evidence Our picture of substantial asteroidal regoliths produced predominantly by blanketing differs from earlier hypotheses that asteroidal regoliths might be thin or absent and that short surface exposure of asteroidal materials is due chiefly to erosion rather than blanketing.  相似文献   

Self-generated aperiodic behaviour in a simple climate model     

JM Salazar  C Nicolis 《Climate Dynamics》1988,3(2):105-114

Climatic variability arising from the coupling between ocean temperature and sea-ice extent is studied in a spatially distributed system. A spatial degree of freedom is crudely introduced by the coupling, through energy transfer, of two box models each of which describes a different space region. The evolution equations are cast into a normal form and some qualitative features of this general class of models are predicted. It is shown, both analytically and numerically, that internally generated complexity in the form of aperiodic behaviour can be a natural consequence of spatially distributed systems.  相似文献   

Eros' Rahe Dorsum: Implications for internal structure     

Richard Greenberg 《Meteoritics & planetary science》2008,43(3):435-449

Abstract— An intriguing discovery of the NEAR imaging and laser‐ranging experiments was the ridge system known as Rahe Dorsum and its possible relation with global‐scale internal structure. The curved path of the ridge over the surface roughly defines a plane cutting through Eros. Another lineament on the other side of Eros, Calisto Fossae, seems to lie nearly on the same plane. The NEAR teams interpret Rahe as the expression of a compressive fault (a plane of weakness), because portions are a scarp, which on Earth would be indicative of horizontal compression, where shear displacement along a dipping fault has thrust the portion of the lithosphere on one side of the fault up relative to the other side. However, given the different geometry of Eros, a scarp may not have the same relationship to underlying structure as it does on Earth. The plane through Eros runs nearly parallel to, and just below, the surface facet adjacent to Rahe Dorsum. The plane then continues lengthwise through the elongated body, a surprising geometry for a plane of weakness on a battered body. Moreover, an assessment of the topography of Rahe Dorsum indicates that it is not consistent with displacement on the Rahe plane. Rather, the topography suggests that Rahe Dorsum results from resistance of the Rahe plane to impact erosion. Such a plane of strength might have formed in Eros' parent body by a fluid intrusion (e.g., a dike of partial melt) through undifferentiated material, creating a vein of stronger rock. Albedo, color and near‐infrared spectra could be consistent with a distinct material composition and such a history, although the instruments' resolution was not adequate for a definitive detection of such a spatially limited component. However the plane of strength formed, such structural reinforcing might have enabled and controlled the elongated irregular shape of Eros, as well as Rahe Dorsum.  相似文献   

How fast do Galilean satellites spin?     

Richard Greenberg  Stuart J. Weidenschilling 《Icarus》1984,58(2):186-196

Each of the Galilean satellites, as well as most other satellites whose initial rotations have been substantially altered by tidal dissipation, has been widely assumed to rotate synchronously with its orbital mean motion. Such rotation would require a small permanent asymmetry in the mass distribution in order to overcome the small mean tidal torque. Since Io and Europa may be substantially fluid, they may not have the strenght to support the required permanent asymmetry. Thus, each may rotate at the unknown but slightly nonsynchronous rate that corresponds to zero mean tidal torque. This behaviour may be observable by Galileo spacecraft imaging. It may help explain the longitudinal variation of volcanism on Io and the cracking of Europa's crust.  相似文献   

Collisional history of asteroids: Evidence from Vesta and the Hirayama families     

Donald R. Davis  Clark R. Chapman  Stuart J. Weidenschilling  Richard Greenberg 《Icarus》1985,62(1):30-53

Collisional evolution studies of asteroids indicate that the initial asteroid population at the time mean collisional velocities were pumped up to ～5 km/sec was only modestly larger than it is today; i.e., the asteroid belt was already depleted relative to the mean surface density elsewhere in the planetary region. Numerical simulations of the collisional evolution of hypothetical initial asteroid populations have been run, subject to three constraints: they must (a) evolve to the present observed asteroid size distribution, (b) preserve Vesta's basaltic crust, and (c) produce at least the observed number of major Hirayama families. A “runaway growth” initial asteroid population distribution is found to best satisfy these constraints. A new model is presented for calculating the fragmental size distribution for the disruption of large, gravitationally bound bodies in which the material strength is increased by hydrostatic self-compression. This model predicts that large asteroid behave as intrinsically strong bodies, even if they have had a history of being collisionally fractured. This model, when applied to the breakup of the Themis and Eos family parent bodies, gives size distributions in reasonably good agreement with those observed.  相似文献   

Asteroids and meteorites: Parent bodies and delivered samples     

Richard Greenberg  Clark R. Chapman 《Icarus》1983,55(3):455-481

Meteorites may be pieces of main-belt asteroids, derived by cratering collisions. The physical strength of an asteroid critically affects the quantity of ejecta that can be placed in orbits (probably resonant) that evolve to cross the Earth's. Asteroid strengths very widely due to initial composition and size (e.g., weak carbonaceous material or strong rock), subsequent geophysical evolution (e.g., formation of a strong iron core), and subsequent collisional evolution (e.g., conversion of a strong rocky body into a weak rubble pile). The meteorite yield on Earth further depends on meteorite strength, which affects longevity in space and survival through the atmosphere. We show that meteorites may be derived mainly by cratering rather than by disruptive fragmentation and from large main-belt asteroids rather than from small, Earth-approaching bodies. The model combines a wide variety of evidence from various disciplines to yield results consistent with meteorite statistics. However, no claim is made for uniqueness of this model, and many elements still admit considerable uncertainty.  相似文献   

There is no horohalinicum     

Lewis E. Deaton  Michael J. Greenberg 《Estuaries and Coasts》1986,9(1):20-30

Species abundance declines to a minimum (the Artenminimum) between 5 and 8‰, not only in estuaries, but in all bodies of brackish water. Khlebovich (1968) examined published hydrochemical data for estuaries and concluded that sharp changes in the ionic composition of seawater diluted with fresh water occur at salinities below 5 to 8‰. He further argued that these ionic changes constitute a physico-chemical barrier between marine and freshwater faunas. Kinne (1971) gave the name “horohalinicum” to the segment of the salinity gradient between 8 and 5‰. We have re-examined the data used by Khlebovich (1968) and found that, in fact, while the ionic composition of diluted seawater changesslightly between 8 and 5‰, the changes in ionic ratios below 2‰ are much larger. Thus, the proposed physico-chemical barrier does not exist between 8 and 5‰; it cannot then explain the Artenminimum; and there is no basis for the horohalinicum concept of Kinne (1971). Two ecological explanations for the occurrence of the Artenminimum—a species-area effect and the stability-time hypothesis—are discussed and found to be inconsistent with published data on species distributions in brackish waters. The low species diversity of brackish water may be explained, in part, by two factors: few animals evolve those physiological mechanisms required for life in the variable habitat; and these species, which are very eurytopic, have low rates of speciation.  相似文献   

Local relief and the height of Mount Olympus     

David R. Montgomery  Harvey M. Greenberg 《地球表面变化过程与地形》2000,25(4):385-396

A three‐dimensional assessment of the net volume of rock differentially eroded from below mountain tops to form valleys yields a range‐wide constraint on feedback between valley development and the height of mountain peaks. The ‘superelevation’ of mountain peaks potentially attributable to differential removal of material from below peaks in the Olympic Mountains, Washington, was constrained by fitting a smoothed surface to the highest elevation points on a 30 m grid digital elevation model of the range. High elevation areas separate into two primary areas: one centred on Mount Olympus in the core of the range and the other at the eastern end of the range. The largest valleys, and hence areas with the greatest volume of differentially eroded material, surround Mount Olympus. In contrast, the highest mean elevations concentrate in the eastern end of the range. Calculation of the isostatic rebound at Mount Olympus attributable to valley development ranges from 500 to 750 m (21 to 32 per cent of its height) for a 5 to 10 km effective elastic thickness of the crust. Comparison of cross‐range trends in mean and maximum elevation reveals that this calculated rebound for Mount Olympus corresponds well with its ‘superelevation’ above the general cross‐range trend in mean elevation. It therefore appears that the location of the highest peak in the Olympics is controlled by the deep valleys excavated in the centre of the range. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献