Observing Chemical Abundances in Comets

The atomic resonance lines of the major elements have been observed in the atmospheres of a few comets, by using vacuum ultraviolet spectrographs on board rockets or orbiting observatories. Dust-to-gas ratios have also been deduced for two comets through a Finson-Probstein's analysis of their dust-tail isophotes. The geometric albedo of the dust for the phase angle a of the observations is not accurately known (A0(a) = 0.20 + 0.05) but, fortunately enough, the dust-to-gas ratio is not overly sensitive to the actual value of this albedo. Next, infrared observations of the dust head of some comets have shown that the bulk of cometary dust must be silicates, although a minor component (5-10 percent) of carbon compounds is rather likely, because of poor dielectric properties of the grains. This interpretation is confirmed by the fact that interplanetary dust probably of cometary origin, that has been collected in the stratosphere by NASA-U2 Spacecraft, is chondritic in nature. Finally, met;al abundances in the head of a sungrazing comet support the chondritic hypothesis. the bulk of Combining the previous data together, and assuming chondritic composition for the cometary dust, it is possible, at least in principle, to deduce the elementary abundances of these volatile compounds of H,C,N,0,S, normalized to (chondritic) silicon and metals. These data give some clues on the origin of comets, in particular on their chemistry before accretion from pristine volatile grains. Unfortunately, present data come from different comets at different times, and their significance for a "mean" comet is rather uncertain, photographs It is urged that a coordination between V.U.V. observations and ground-based of dust tails be established for the same comets, in particular for the incoming passage of Comet Hal ley. Optimum times for improving the accuracy of the dust-to-gas ratio usually are jafter passage to perihelion. 1. The Physical Study of Comets By necessity, the physical study of comets has traditionally been more concentrated on the qualitative understanding of the transient phenomena (coma, dust tail and plasma tail) than on a more quantitative understanding of the underlying permanent features (structure and chemistry of the nucleus). However, the last decade has brought a harvest of quantitative data that can be used as clues for a more fundamental approach about the chemical nature of the nucleus, yielding new insights on its origin and history. If all the recent observations had been properly coordinated to observe the same comets at the proper dates, we would already be several more steps ahead in this direction. This paper is therefore an effort to promote a better understanding of the fundamental problems, in order to encourage a better coordination, so that the proper quantitative data be collected at the proper times. 2. The Two Fractions of the Cometary Nucleus Fundamental chemical data that are clearly connected to the origin of comets, can be derived from the fact that the cometary stuff is a mixture of two constituents with very different properties: a volatile fraction, apparently a mixture of molecules from H,C,N,0,S atoms, and a refractory fraction apparently made up from fine grains of dust. https://ntrs.nasa.gov/search.jsp?R=19820006117 2018-03-26T21:36:41+00:00Z