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11.
S. Yabushita 《Astrophysics and Space Science》1983,89(1):159-161
Evolution of cometary orbits by planetary perturbations, weakly hyperbolic original orbits of comets calculated by Marsdenet al. (1978) are taken to indicate the interstellar origin of comets, and the possible formation of cometary nuclei in interstellar globules is discussed. The process is sedimentation of dust grains. It is shown that if a globule is at 40 K, its lifetime is sufficiently long to allow the sedimentation. 相似文献
12.
A number of experimental studies have been carried out to verify the claim that interstellar grains largely consist of organic material, including biological cells. Our spectroscopic studies on biological cells and organic extracts from carbonaceous compounds have failed to identify the well-known 2200 Å interstellar extinction peak with the organic material. 相似文献
13.
S. Yabushita 《Celestial Mechanics and Dynamical Astronomy》1997,69(1-2):31-48
Matsumoto & Kubotani argued that there is a positive and statistically significant correlation between cratering and mass
extinction. This argument is critically examined by adopting a method of Ertel used by Matsumoto & Kubotani but by applying
it more directly to the extinction and cratering records. It is shown that on the null-hypothesis of random distribution of
crater ages, the observed correlation has a probability of occurrence of 13%. However, when large craters are excluded whose
ages agree with the times of peaks of extinction rate of marine fauna, one obtains a negative correlation. This result strongly
indicates that mass extinction are not due to accumulation of impacts but due to isolated gigantic impacts.
Further, correlations between cratering and flood basalts and between mass extinction and flood basalts originally recognized
by Stothers are further investigated. It is shown that for the former, large craters corresponds to 7 of 11 major flood basalts.
Periodicity analyses are carried out for the geomagnetic reversal events and extinction rate peaks recently given by Sepkoski
for the interval up to 540 M yr BP. The former exhibits a significant periodicity at P = 15 M yr, but the latter shows no
significant period. It seems premature to regard the periodicity in geological records established.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
14.
S. Yabushita 《Monthly notices of the Royal Astronomical Society》1998,296(3):662-668
One of the methods discussed in deflecting the orbit of an Earth-colliding asteroid is the use of nuclear explosives. In assessing its feasibility, apart from political considerations, it is important to quantify how effective it is in orbit deflection. The transfer of radiation incident at the surface is governed by a non-linear diffusion equation. For low-yield explosions with a slab geometry ( S 0 ≃108 kJ μs−1 ), the temperature at depth x and time t is well approximated by a similarity solution of the form T ( x , t )= T 0 f (ξ), ξ= x/ ( T n 0 t )1/2 , with T 0 given by ( S 0 /σ)1/4 , where σ is the Stefan–Boltzmann constant, n is an index that specifies the radiation transfer and f (ξ) is the solution of a non-linear differential equation subject to the condition f (0)=1 and limξ→∞ f (ξ)=0. For high-yield explosions ( S 0 ≃1010 kJ μs−1 ), numerical solutions to the non-linear diffusion equation can be obtained. These solutions have properties similar to the case of low-yield explosions. If the duration of the explosion is d ×10−8 s, where d is close to 3, the fraction of energy absorbed by the surface is found to be 7, 12 and 23 per cent for S 0 =108 , 109 and 1010 kJ μs−1 respectively. 相似文献
15.
Extinction efficiency of grains is calculated from the Mie formula on the premise that the grains are of organic composition. The optical constants adopted for the calculations are those ofE. coli, polystyrene and bovine albumin. The grain radiusa is assumed to obey a distribution of the formN(a) a
– and the value of is chosen so as to make the calculated extinction curve match the observed interstellar extinction curve. Although the calculated curve gives a reasonably good fit to the observed extinction curve for wavelength less than 2100 Å, at longer wavelength region agreement is poor. It is concluded that another component is required for the organic model to be viable. 相似文献
16.
It is known that the observed secular accelerations of the Sun and Moon are not consistent with the tidal interactions of the Earth with the Sun and Moon. Following Dicke, the hypothesis of variable constant of gravity is adopted and expressions for the accelerations are derived. It is shown that if the theoretical ratio of the acceleration is equated the observed one, a unique value for —/G can be calculated. Adopting the accelerations obtained by Fotheringham, Newton, Muller and Stephenson, and Stephenson, it is found that — /G ranges from 1.4 × 10–11 to 3.3 × 10–11 yr–1. This estimate is consistent with the one based upon the comparison of the lunar accelerations measured with respect to atomic and ephemis times. 相似文献
17.
Iridium concentration in extra-terrestrial bodies is an important quantity in relation to Ir-rich geological layers. Ir concentration of a Yamato carbonaceous chondrite (Y-793321) has been measured by a neutron activation method. The measurement yields a value (0.57 ± 0.06) g per gramme for the chondrite. 相似文献
18.
S. Yabushita 《Earth, Moon, and Planets》1996,72(1-3):343-356
The controversial topic of periodicity in geological records in relation to astronomical modeling is reviewed. Although impact cratering, frequency distribution of geo-magnetic reversals, and mass extinction of fauna yield periods when certain tests are applied, none of them can be regarded significant in the sense of mathematical statistics. The first two records yield periods of 30 Myr, while the faunal-extinction record yields a period of ~ 26 – 27 Myr. It seems that although catastrophes in the form of large impacts trigger mass extinctions, certain geophysical or geological conditions need be satisfied for mass extinctions to be realized. One should not expect to find an indisputable periodicity in cratering record because random impacts by asteroids are dominant. Thus, the earth-crossing cometary flux modulated by the galactic tidal force appears consistent with the weak detected periodicity. 相似文献
19.
Shin Yabushita 《Monthly notices of the Royal Astronomical Society》2002,334(2):369-373
An analysis is made of the periodicity hypothesis of the ages of large craters, based on the compilation by Grieve with the addition of recently identified craters. A method earlier proposed by Broadbent is used to derive a period, and the significance of the derived period is tested by a Monte Carlo experiment. In accordance with the result of Stothers, the ages of large craters ( D >30 km) are shown to exhibit a period close to 37.5 Myr. Monte Carlo experiments show, however, that the derived period is far from being statistically significant. A subset of crater data earlier adopted by Napier for the purpose of similar investigation is also tested, and it is shown that they also exhibit a similar period at an almost identical level of confidence. A brief discussion is made of the relation between the derived period and that associated with faunal mass extinctions. 相似文献
20.