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21.
Bottke WF Vokrouhlický D Minton D Nesvorný D Morbidelli A Brasser R Simonson B Levison HF 《Nature》2012,485(7396):78-81
The barrage of comets and asteroids that produced many young lunar basins (craters over 300 kilometres in diameter) has frequently been called the Late Heavy Bombardment (LHB). Many assume the LHB ended about 3.7 to 3.8 billion years (Gyr) ago with the formation of Orientale basin. Evidence for LHB-sized blasts on Earth, however, extend into the Archaean and early Proterozoic eons, in the form of impact spherule beds: globally distributed ejecta layers created by Chicxulub-sized or larger cratering events4. At least seven spherule beds have been found that formed between 3.23 and 3.47?Gyr ago, four between 2.49 and 2.63?Gyr ago, and one between 1.7 and 2.1?Gyr ago. Here we report that the LHB lasted much longer than previously thought, with most late impactors coming from the E belt, an extended and now largely extinct portion of the asteroid belt between 1.7 and 2.1 astronomical units from Earth. This region was destabilized by late giant planet migration. E-belt survivors now make up the high-inclination Hungaria asteroids. Scaling from the observed Hungaria asteroids, we find that E-belt projectiles made about ten lunar basins between 3.7 and 4.1?Gyr ago. They also produced about 15 terrestrial basins between 2.5 and 3.7?Gyr ago, as well as around 70 and four Chicxulub-sized or larger craters on the Earth and Moon, respectively, between 1.7 and 3.7?Gyr ago. These rates reproduce impact spherule bed and lunar crater constraints. 相似文献
22.
Summary Electrostatic interaction between tropoelastin, the native precursor of elastin, and proteoglycan results in tropoelastin fibrillation. The finding suggests a possible involvement of proteoglycans in elastogenesis. 相似文献
23.
P. Dubový 《Cellular and molecular life sciences : CMLS》1987,43(8):883-884
Summary DPP IV activity was localized in the nerve fascicles of cat glabrous skin at light and electron microscope levels. The observation that the DPP IV end product was restricted to the axon-Schwann cell interface suggests that this enzyme may be involved in the interactions between unmyelinated axons and their Schwann cells.17 November 1986 相似文献
24.
The vector alignments of asteroid spins by thermal torques 总被引:1,自引:0,他引:1
Collisions have been thought to be the dominant process altering asteroid rotations, but recent observations of the Koronis family of asteroids suggest that this may be incorrect. This group of asteroids was formed in a catastrophic collision several billion years ago; in the intervening period their rotational axes should have become nearly random because of subsequent collisions, with spin rates that follow a maxwellian distribution. What is seen, however, is that the observed family members with prograde spins have nearly identical periods (7.5-9.5 h) and obliquities between 42 and 50 degrees, while those with retrograde spins have obliquities between 154 and 169 degrees with periods either <5 h or >13 h. Here we show that these non-random orientations and spin rates can be explained by 'thermal torques' (arising from differential solar heating), which modify the spin states over time. In some cases, the asteroids become trapped in spin-orbit resonances. Our results suggest that thermal torques may be more important than collisions in changing the spin states (and possibly shapes) of asteroids with diameters <40 km. 相似文献
25.
Résumé Après avoir administré i. v. à des rats les solutions des sels Pb++, Hg++, Mn++ et Cu++, on a divisé leur bile par des l'élécrophorèse de disque sur gel de polyacrylamide. Les cations Hg++ se sont montrés surtout dans la fraction contenant des hauts polymères d'albumine spécifique pour la bile; les cations Mn++ et Cu++ furent observés surtout dans la zone pigmentée. Le maximum des cations Pb++ attachés s'est trouvé dans la zone de postalbumine. 相似文献
26.
Zusammenfassung Durch autoradiographische Untersuchung wurde die Einlagerung von Radioaktivität in ein begrenztes Gebiet des Neocortex der Rattenweibchen nach einmaliger Verabreichung von3H-Östradiol im Alter von 5 bis 50 Tagen nachgewiesen. Die reduzierten Silberkörner zeigen eine vorwiegend perinukleare Akkumulation, was für die Zellen der östrogen-Zielgewebe typisch ist. 相似文献
27.
Three new species of the genus Notophthiracarus are described, identified and figured from Uluguru Mountains of Tanzania: Notophthiracarus quasiuluguruensis sp. nov, Notophthiracarus tuberculus sp. nov, and Notophthiracarus uluguruensis sp. nov. A comparison with the most closely related species of the genus Notophthiracarus is also presented.http://zoobank.org/urn:lsid:zoobank.org:pub:3FBF24E7-3E2D-4686-AD57-EFEA25510BBD 相似文献
28.
Zusammenfassung Die Aktivität der Acyl-CoA: Monoglycerid Acyltransferase, eines Enzymes, das in der Esterifizierung der freien Fettsäuren eine wichtige Rolle spielt, wurde während der perinatalen Entwicklungsperiode der Ratte in der Dünndarmschleimhaut bestimmt. Im Foetus ist die Aktivität niedrig, steigt nach der Geburt und zeigt in der Mitte der Säuglingsperiode die höchsten Werte, die 2–3mal höher sind, als bei Erwachsenen. Die Befunde werden im Bezug auf die niedrige Lipaseaktivität des gastrointestinalen Traktes sowie auf die hohe Fettaufnahme während der Säuglingsperiode diskutiert. 相似文献
29.
30.
M. Aušková K. Řežábek V. Zikán M. Semonský 《Cellular and molecular life sciences : CMLS》1974,30(4):393-394
Summary The inhibiting effect of orally administered N-[d-6-methyl-8-isoergolin-I-yl], N, N-diethylurea (VÚFB-6638) on the production of milk was found in rats. The inhibition of lactation was ascertained through the observation of the daily weightgains of the suckled puppies (ED50=0.112 mg/kg/day) and through the observation of the presence of milk in the stomach of these puppies, as seen through their abdomen wall (0.084 mg/kg/day). This inhibition could be abolished with exogenous prolactin, which gives evidence that the drug studied acts via the endocrine system.
Für die technische Assistenz danken wir FrauM. Kommersová und FrauV. Maitová. 相似文献
Für die technische Assistenz danken wir FrauM. Kommersová und FrauV. Maitová. 相似文献