CONDITIONS OF DISINTEGRATION OF ASTEROIDS FROM OBSERVATIONS OF THE PROPERTIES OF THE ZODIACAL LIGHT

Abstract

R data show that the zodiacal light is characterized by the scattering effect of dust particles in an interplanetary medium largely devoid of free electrons. In fact, its observable properties—its form, position in relation to the ecliptic, absolute brightness, color, polarization, and even its probable connection with the solar corona—can all be explained in terms of the dust hypothesis, that is, in terms of the scattering of solar light by fine particles. The presence in the solar system of a gaseous or even an electronic component cannot be demonstrated photometrically. Fine dust, however, cannot remain long in space, owing to the retarding action of solar radiation and corpuscular emission from the sun. For the first reason alone all such dust should be precipitated on the sun within the comparatively short period of about 100,000 years. Thus the composition of the material of the zodiacal light must be continuously renewed by the uninterrupted arrival of matter from the outside, and this very intensive process is the result of the gradual disintegration of asteroids. The same process is also accompanied by the separation of meteorites, as evidenced by the low values for the cosmic ages of such bodies determined from their He3 content.' Starting from the distribution of asteroids according to the angles of inclination of their orbits, it is possible to calculate the isophotes of the zodiacal light and compare them with observations. This presents known difficulties, since the visible isophotes of the zodiacal light are affected by various components of the night glow of atmospheric and cosmic origin, and, moreover, by zodiacal twilight, i.e., the intensification of parts of the zodiacal light close to the horizon by considerably brighter parts lying directly below it, and, finally, by supplementary scattering in the troposphere. The author's calculations show that the effect of zodiacal twilight is small and incapable of introducing significant distortions into isophote observations; at the same time, the illumination of the troposphere by the zodiacal light itself is quite appreciable, owing to its considerable angular dimensions, and, furthermore, is not uniform with respect to azimuth. This results not only in a certain increase in the observed brightness of the phenomenon, but also in a marked broadening of the characteristic isophotes. It is possible, moreover, that there are other factors working in the same direction. All these influences, however, are much less important when the axis of the zodiacal light is oriented vertically with respect to the horizon, which happens, for example, near the tropics about the equinoctial epoch. We made numerous observations of the zodiacal light in the Libyan Desert, south of Aswan, in October and November 1957, when, in the course of the same night, it was possible to observe this phenomenon normal to the horizon in the east before sunrise and much inclined in the west soon after nightfall. In this way we obtained graphic confirmation of the visible distortion of the isophotes as a function of the angle of inclination of the ecliptic to the horizon. This distortion is largely attributable to the forementioned causes. Even when the zodiacal light is normal to the horizon, however, we find that the true isophotes are comparatively