摘要:

In Solar System astrophysics, isolationism is unfashionable and comparative planetology is all the rage. Astronomers compare Venus with Earth, Mars with Moon and Jupiter with Saturn; so why not compare comets with asteroids? This is the task that I set out to fulfil, but it must be stressed at the outset that, just like the previous examples, comets and asteroids are completely different objects. They have different origins, evolutionary histories, orbits, size ranges, size distributions, compositions, surface features, internal structures, apparent magnitudes, observational histories and end products. Comets and asteroids are often grouped together under the epithet‘minor body'. They are minor because they are smaller than the majority of planetary satellites, and their eccentric orbits often put them on collision courses with the planets. Both comets and asteroids are capable of catering planetary surfaces and, in the present epoch, a minor body > 1 km will hit Earth about every 2 × 106years. This impactor is 20 times more likely to be an asteroid than a comet. Comets decay and asteroids break up. Both these processes produce dust that starts off as a co-orbital meteoroid stream and eventually feeds the general zodiacal dust cloud that surrounds the Sun.

ISSN:0010-7514    

DOI:10.1080/00107519408224452

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The term‘clusters’is usually used to describe aggregates of atoms that are too large to be referred to as molecules and too small to resemble small pieces of crystal. These aggregates generally do not have the same structure or atomic arrangement as a bulk solid and can change structure with the addition of just one or a few atoms. As the number of atoms increases, eventually a crystal-like structure is established, and the addition of new atoms does not change the bulk structure. A motivation for studying clusters is the desire to understand how physical properties evolve in the transition from atom to molecule to cluster to small particle to solid. At present one has a good understanding of the end points of such an evolution, but has still a poor knowledge of the intermediate situations. In particular, while one understands reasonably well why a sodium atom is yellow and why this colour essentially is absent in the electromagnetic spectrum of bulk sodium, we still do not have an accurate theory which explains the spectrum of colours displayed by small sodium clusters. Much progress towards this goal can be achieved by making use of well-established results in nuclear physics. It turns out that there is a close analogy between metal clusters and atomic nuclei, and knowledge acquired during the past 40 years in the study of the response of nuclei to photons helps with the understanding of the absorption of light by microcrystals. The main result of developing such an analogy seems to be that the properties of finite, many-particle systems do not depend so much on the nature of the particles forming the system or the forces acting among them, but on the fact that they move in a confined volume and that they are many of them.

ISSN:0010-7514    

DOI:10.1080/00107519408224453

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

H3+is the simplest poly atomic molecule and as such has always been the subject of extensive theoretical study. It has also long been known to be an important constituent of hydrogen plasmas that are prevalent in many locations in the Universe. This article traces the study of the spectrum of H3+from the first principles quantum mechanical calculation to its observation in the ionosphere of Jupiter and beyond.

ISSN:0010-7514    

DOI:10.1080/00107519408224454

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

ISSN:0010-7514    

DOI:10.1080/00107519408224455

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

ISSN:0010-7514    

DOI:10.1080/00107519408224456

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

A Quantum Measurement Approach to TunnellingBy D. K. Roy 1993, £27–00 (hbk), pp. x + 116. World Scientific, ISBN 981 02 1223 2. Scope: monograph. Level: postgraduate and specialist.

ISSN:0010-7514    

DOI:10.1080/00107519408224457

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

ISSN:0010-7514    

DOI:10.1080/00107519408224458

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor