Characteristics Of Swarms Research Articles

Clusters and packets of grains in comet Halley and the fragmentation of dust

Clusters and packets of dust grains, detected by the VEGA spacecrafts in the coma of comet Halley, are interpreted either as due to microjets arising directly from the nucleus or as due to fragmentation processes of larger conglomerates. After a summary of the characteristics of the particle swarms observed and of the conclusion from their interpretation as microjets, some aspects from the fragmentation scenario of this phenomenon are outlined. From the spatial widths and the total number of grains in a particle swarm the mass of the parent conglomerate can be estimated. For the clusters and packets parent bodies of about 1 to several 100 g can be expected. Their travel time in the coma after release from the nucleus range from 10 to about 100 days before the spacecraft encounters. Finally, possible fragmentation processes are discussed and some comments for further investigations, both in theory and for in-situ data, are given.

Seismogenesis

The study of earthquake generation has been greatly expanded in the present decade through improvements and new developments in observational, mathematical and experimental methods. A high order of accuracy and precision has been attained in seismicity studies. The main characteristics of aftershock sequences and swarms have been established. It is confirmed that earthquakes are not in general triggered by one another nor by earth-tides or other natural phenomena. Certain features of the earthquake phenomenon have been clarified by means of physical models. Spectral and directional properties of the earthquake source can now be determined for periods from a few seconds to infinity, and mathematical representations of the source as a moving point-force system, a dislocation, or an initial departure from equilibrium, have been worked out. Hence the main parameters of the source, regarded as an elastic disturbance, can be evaluated. It is strongly confirmed that the source disturbance is a shearing process. The following mechanisms give a sudden stress-drop in conditions of pressure and temperature corresponding to the depths indicated: ordinary fracture, 0–10 km; stick-slip, 3–20 km; hydrous-anhydrous transition, 20–60 km. Mechanisms involving melting have been proposed for deeper earthquakes. Polymorphic transitions with volume change seem unimportant. Interstitial fluid pressure plays an essential role, as illustrated by earthquake occurrences associated with artificial lakes and with fluid disposal by injection at depth. Although the provenance of seismogenic energy remains hypothetical, the study of earthquake occurrence itself has passed out of the stage of description into that of explanation.