N-Body simulations of growth from 1 km planetesimals at 0.4 AU

We present N-body simulations of planetary accretion beginning with 1 km radius planetesimals in orbit about a 1 M_⊙ star at 0.4 AU. The initial disk of planetesimals contains too many bodies for any current N-body code to integrate; therefore, we model a sample patch of the disk. Although this greatly reduces the number of bodies, we still track in excess of 10⁵ particles. We consider three initial velocity distributions and monitor the growth of the planetesimals. The masses of some particles increase by more than a factor of 100. Additionally, the escape speed of the largest particle grows considerably faster than the velocity dispersion of the particles, suggesting impending runaway growth, although no particle grows large enough to detach itself from the power law size-frequency distribution. These results are in general agreement with previous statistical and analytical results. We compute rotation rates by assuming conservation of angular momentum around the center of mass at impact and that merged planetesimals relax to spherical shapes. At the end of our simulations, the majority of bodies that have undergone at least one merger are rotating faster than the breakup frequency. This implies that the assumption of completely inelastic collisions (perfect accretion), which is made in most simulations of planetary growth at sizes 1 km and above, is inappropriate. Our simulations reveal that, subsequent to the number of particles in the patch having been decreased by mergers to half its initial value, the presence of larger bodies in neighboring regions of the disk may limit the validity of simulations employing the patch approximation.     

The formation of the Baptistina family by catastrophic disruption: Porous versus non‐porous parent body

Abstract— In this paper, we present numerical simulations aimed at reproducing the Baptistina family based on its properties estimated by observations. A previous study by Bottke et al. (2007) indicated that this family is probably at the origin of the K/T impactor, is linked to the CM meteorites and was produced by the disruption of a parent body 170 km in size due to the head‐on impact of a projectile 60 km in size at 3 km s^?1. This estimate was based on simulations of fragmentation of non‐porous materials, while the family was assumed to be of C taxonomic type, which is generally interpreted as being formed from a porous body. Using both a model of fragmentation of non‐porous materials, and a model that we developed recently for porous ones, we performed numerical simulations of disruptions aimed at reproducing this family and at analyzing the differences in the outcome between those two models. Our results show that a reasonable match to the estimated size distribution of the real family is produced from the disruption of a porous parent body by the head‐on impact of a projectile 54 km in size at 3 km s^?1. Thus, our simulations with a model consistent with the assumed dark type of the family requires a smaller projectile than previously estimated, but the difference remains small enough to not affect the proposed scenario of this family history. We then find that the break‐up of a porous body leads to different outcomes than the disruption of a non‐porous one. The real properties of the Baptistina family still contain large uncertainties, and it remains possible that its formation did not involve the proposed impact conditions. However, the simulations presented here already show some range of outcomes and once the real properties are better constrained, it will be easy to check whether one of them provides a good match.     

Karin cluster formation by asteroid impact

Insights into collisional physics may be obtained by studying the asteroid belt, where large-scale collisions produced groups of asteroid fragments with similar orbits and spectra known as the asteroid families. Here we describe our initial study of the Karin cluster, a small asteroid family that formed 5.8±0.2 Myr ago in the outer main belt. The Karin cluster is an ideal ‘natural laboratory’ for testing the codes used to simulate large-scale collisions because the observed fragments produced by the 5.8-Ma collision suffered apparently only limited dynamical and collisional erosion. To date, we have performed more than 100 hydrocode simulations of impacts with non-rotating monolithic parent bodies. We found good fits to the size-frequency distribution of the observed fragments in the Karin cluster and to the ejection speeds inferred from their orbits. These results suggest that the Karin cluster was formed by a disruption of an ≈33-km-diameter asteroid, which represents a much larger parent body mass than previously estimated. The mass ratio between the parent body and the largest surviving fragment, (832) Karin, is ≈0.15-0.2, corresponding to a highly catastrophic event. Most of the parent body material was ejected as fragments ranging in size from yet-to-be-discovered sub-km members of the Karin cluster to dust grains. The impactor was ≈5.8 km across. We found that the ejections speeds of smaller fragments produced by the collision were larger than those of the larger fragments. The mean ejection speeds of >3-km-diameter fragments were . The model and observed ejection velocity fields have different morphologies perhaps pointing to a problem with our modeling and/or assumptions. We estimate that ∼5% of the large asteroid fragments created by the collision should have satellites detectable by direct imaging (separations larger than 0.1 arcsec). We also predict a large number of ejecta binary systems with tight orbits. These binaries, located in the outer main belt, could potentially be detected by lightcurve observations. Hydrocode modeling provides important constraints on the interior structure of asteroids. Our current work suggests that the parent asteroid of the Karin cluster may have been an unfractured (or perhaps only lightly fractured) monolithic object. Simulations of impacts into fractured/rubble pile targets were so far unable to produce the observed large gap between the first and second largest fragment in the Karin cluster, and the steep slope at small sizes (≈6.3 differential index). On the other hand, the parent asteroid of the Karin cluster was produced by an earlier disruptive collision that created the much larger, Koronis family some 2-3 Gyr ago. Standard interpretation of hydrocode modeling then suggests that the parent asteroid of the Karin cluster should have been formed as a rubble pile from Koronis family debris. We discuss several solutions to this apparent paradox.     

从断裂带构造认识断层滑移习性

对于地震物理学和断裂运动研究而言,探索断裂带构造和地震行为之间的联系是至关重要的。地震代表滑动行为的一个极端;摩擦熔融和假玄武玻璃为动力破裂过程中的局部滑动提供了野外证据[1-3]。无震滑动则     

Contrasting effects of cross-ecosystem subsidies and predation on benthic invertebrates in two Pacific coastal streams

Cross-ecosystem subsidies, such as terrestrial invertebrates and leaf litter falling into water as resources for aquatic communities, can vary across environmental gradients. We examined whether the effect of terrestrial subsidy inputs on benthic invertebrates was mediated by resident coastal cutthroat trout (Oncorhynchus clarki) in two representative streams. We experimentally manipulated the input rates (reduced, ambient) of terrestrial subsidies (terrestrial invertebrates and leaf litter) as well as the presence or absence of cutthroat trout in the two streams. The hypothesis that the reduction of terrestrial subsidies to the stream influences benthic invertebrate assemblages was supported by experimental results. The treatments of terrestrial subsidy reduction and cutthroat trout presence had a significant negative effect on benthic invertebrate community biomass and shredder biomass in East Creek with high natural terrestrial subsidy input and small amount of large wood in channel. In contrast, results from Spring Creek with low subsidy input and large amount of large wood in channel showed that only the terrestrial subsidy reduction significantly reduced the biomass of shredders. The effects of the terrestrial subsidy input and trout predation on benthic invertebrate communities varied between the two streams. Our results indicate that a subsidy effect on benthic communities can vary between nearby streams differing in canopy and habitats. This study, with the major finding of highly context-dependent effects of spatial subsidies, suggests that the interplay of resource subsidies and predators on invertebrate community assemblages can be site-specific and context-dependent on habitat features.     

How Does the Species Used for Calibration Affect Chlorophyll <Emphasis Type="Italic">a</Emphasis> Measurements by In Situ Fluorometry?

We determined how the species used for calibration affects the accuracy of in situ chlorophyll a (chl a) measurements by fluorometry using single-species cultures and natural phytoplankton populations from Winyah Bay, South Carolina, USA. When a diatom was used for calibration, chl a in a dinoflagellate culture was overestimated by 66 ± 7%, whereas concentrations of a cryptophyte, chlorophyte, and cyanobacterium were underestimated by 16 ± 20%, 40 ± 7%, and 71 ± 33%, respectively. In natural populations, the combination of species-specific and environmentally induced variation in the ratio of fluorescence to chl a (F Chl⁻¹) led to an overestimate by the in situ fluorometer of 40–169% for an April experiment and an underestimate of 4–50% in July. Even when field samples were dominated by diatoms, environmental effects resulted in highly variable predictions of chl a. Thus, while a carefully selected calibration species can improve estimates of in vivo chl a in the laboratory, calibration of in situ fluorometers should be done with natural communities collected from the site of interest.     

Non-stationary Generation of Weak Turbulence for Very Stable and Weak-Wind Conditions

Turbulence measurements for very stable conditions near the surface are contrasted among three sites: a high altitude basin during winter with grass or snow-covered grass, a broad valley with complex agricultural land use, and a more narrow valley that is influenced by a valley cold pool and cold air drainage. In contrast to previous studies, this investigation emphasizes the very weak turbulence with large bulk Richardson number occurring during extensive periods between brief mixing events. The relationship of the turbulence to the non-stationary wind and stratification is examined along with the impact of short-term flow accelerations, directional shear and downward diffusion of turbulence from higher levels. The failure of the turbulence for strong stratification to decrease with further increase of stratification is explored. Additional analyses are applied to weak-wind cases for the entire range of stratification, including weak stratification associated with cloudy conditions.     

The global effects of impact-induced seismic activity on fractured asteroid surface morphology

Impact-induced seismic vibrations have long been suspected of being an important surface modification process on small satellites and asteroids. In this study, we use a series of linked seismic and geomorphic models to investigate the process in detail. We begin by developing a basic theory for the propagation of seismic energy in a highly fractured asteroid, and we use this theory to model the global vibrations experienced on the surface of an asteroid following an impact. These synthetic seismograms are then applied to a model of regolith resting on a slope, and the resulting downslope motion is computed for a full range of impactor sizes. Next, this computed downslope regolith flow is used in a morphological model of impact crater degradation and erasure, showing how topographic erosion accumulates as a function of time and the number of impacts. Finally, these results are applied in a stochastic cratering model for the surface of an Eros-like body (same volume and surface area as the asteroid), with craters formed by impacts and then erased by the effects of superposing craters, ejecta coverage, and seismic shakedown. This simulation shows good agreement with the observed 433 Eros cratering record at a Main Belt exposure age of 400±200 Myr, including the observed paucity of small craters. The lowered equilibrium numbers (loss rate = production rate) for craters less than ∼100 m in diameter is a direct result of seismic erasure, which requires less than a meter of mobilized regolith to reproduce the NEAR observations. This study also points to an upper limit on asteroid size for experiencing global, surface-modifying, seismic effects from individual impacts of about 70-100 km (depending upon asteroid seismic properties). Larger asteroids will experience only localized (regional) seismic effects from individual impacts.