Pagina:Scientia - Vol. VII.djvu/252

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244 scientia

for the action of Jupiter is just as likely to increase as to diminish the period, and in only a few cases would the successive perturbations be all in the same direction.

On the other hand, if the planets be regarded as themselves the parents of these comets, their orbits would necessarily pass through the point of ejection, and consequently the difficulty (2) disappears. The only difficulty is the question whether the giant planets are now in a physical condition capable of expelling matter with a velocity of several miles per second. I do not think that we can put back the epoch of ejection to a date several million years ago, when the giant planets may have been in a quasi-sunlike state; for the life of a short-period comet seems to be measured by centuries, not by millions of years. In the last 130 years Lexell’s and Brooks’ comets have had their orbits greatly modified, two others, Biela’s and Brorsen’s, have definitely perished, while Encke’s comet was extremely faint in 1908, which may indicate its approaching dissolution; to these we may perhaps add the numerous comets for which short periods have been found, but which have never been seen again.

The conclusion is plain that if the giant planets are the parents of their comet families, they must be, at the present time capable of ejecting them. It does not appear to me that the possibility of such ejection can be summarily rejected. We have evidence of disturbances of intense violence in the Jovian atmosphere; the great red spot denotes a mighty cataclysm. Also the long rows of white spots that occur on Jupiter seem to indicate a series of eruptions far below; There are occasional outbursts of white spots on Saturn. Moreover, there is no reason to suppose that the ejection of comets is a common phenomenon; one ejection per century would probably suffice to balance the loss from dissipation and perturbation. Even on the earth we have occasional volcanic outbursts of extraordinary violence, as Skaptar Jokull in 1783, Krakatoa in 1883, etc.; paroxysmal outbursts on a far grander scale may be expected on the giant planets. In 1883 it was computed that over a cubic mile of solid matter was blown to a height of many miles. This is probably comparable with the total mass of the smaller comets. A difficulty has been raised that the ejection of the matter