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11.
Asteroid families are groups of small bodies that share certain orbit and spectral properties. More than 20 families have now been identified, each believed to have resulted from the collisional break-up of a large parent body in a regime where gravity controls the outcome of the collision more than the material strength of the rock. The size and velocity distributions of the family members provide important constraints for testing our understanding of the break-up process, but erosion and dynamical diffusion of the orbits over time can erase the original signature of the collision. The recently identified young Karin family provides a unique opportunity to study a collisional outcome almost unaffected by orbit evolution. Here we report numerical simulations modelling classes of collisions that reproduce the main characteristics of the Karin family. The sensitivity of the outcome of the collision to the internal structure of the parent body allows us to show that the family must have originated from the break-up of a pre-fragmented parent body, and that all large family members formed by the gravitational reaccumulation of smaller bodies. We argue that most of the identified asteroid families are likely to have had a similar history. 相似文献
12.
DNA sequence of both chromosomes of the cholera pathogen Vibrio cholerae 总被引:23,自引:0,他引:23
Heidelberg JF Eisen JA Nelson WC Clayton RA Gwinn ML Dodson RJ Haft DH Hickey EK Peterson JD Umayam L Gill SR Nelson KE Read TD Tettelin H Richardson D Ermolaeva MD Vamathevan J Bass S Qin H Dragoi I Sellers P McDonald L Utterback T Fleishmann RD Nierman WC White O Salzberg SL Smith HO Colwell RR Mekalanos JJ Venter JC Fraser CM 《Nature》2000,406(6795):477-483
13.
Alpine vegetation of Railroad Ridge in the White Cloud Peaks, Custer County, Idaho, was investigated to ascertain native communities and describe their distribution with respect to important environmental factors. Percent cover of all species and environmental information were recorded for 69 plots. Data were analyzed by TWINSPAN and CANOCO, which provided a classification and an ordination, respectively. Predominant communities are dominated by (1) Phlox pulvinata , (2) Carex elynoides , and (3) Ceum rossii , and other communities of lesser importance are dominated by (4) Artemisia campestris , (5) Eriogonum ovalifolium and Silene acaulis , and (6) Ivesia gordonii . Although the Artemisia campestris community occupies a relatively small portion of the study area, it is noteworthy because it is a community type not previously known for Idaho. The community co-dominated by Eriogonum ovalifolium and Silene acaulis is also unique and has not been documented in other Idaho alpine studies. Community distribution showed the strongest correlation with exposure, substrate type, and slope. In general, the study area supports fellfield communities that are representative of alpine areas in Idaho and some Great Basin mountain ranges, and dense turflike communities similar to ones found throughout western North America's temperate alpine regions. 相似文献
14.
W. J. Richardson 《Cellular and molecular life sciences : CMLS》1990,46(4):416-425
Summary Migratory flights are strongly affected by wind, and birds have developed many adaptations to cope with wind effects. By day, overland migrants at high altitudes may often allow crosswinds to drift their tracks laterally from the preferred heading. In contrast, many birds at low altitude adjust their headings to compensate for drift, and may overcompensate to allow for previous drift. The relative motion of landscape features is probably used to sense drift, at least by day. By night, some overland migrants compensate fully for drift but others do not; no pattern is obvious. Over the sea, compensation is rarely if ever total; wave patterns may allow partial compensation. Other adaptations can include reduction of drift by flying at times and/or altitudes without strong crosswinds. Some birds recognize the need to change course to allow for previous wind displacement, and reorient at least roughly toward the original route or destination. Some juveniles en route to previously-unvisited wintering grounds seem to have this ability, but corroboration is needed. Such reorientation may not require a true navigation ability. However, some birds have unexplained abilities to sense the wind while aloft. 相似文献
15.
16.
Three abundance classes in HeLa cell messenger RNA 总被引:68,自引:0,他引:68
17.
Mechanosensitivity of mammalian auditory hair cells in vitro 总被引:1,自引:0,他引:1
Intracellular responses recorded in vitro from the cochleas of anaesthetized mammals have shown that the mechanoreceptive inner and outer hair cells are sharply tuned, accounting for many of the properties of the afferent fibres in the auditory nerve. However, in vivo it has not been possible to measure directly the excitatory mechanical input to these cells (the displacement of their mechanosensitive stereocilia) and thus to determine the relationship between the receptor potentials and displacement of their stereocilia. As a means of circumventing this technical difficulty, we have developed an organ culture of the mouse cochlea and here we describe the receptor potentials generated by the hair cells in response to direct displacement of their stereocilia. 相似文献
18.
Levamisole inactive in treatment of four animal tumours 总被引:3,自引:0,他引:3
19.
Non-random X chromosome expression in female mules and hinnies 总被引:3,自引:0,他引:3
20.