Polytechnic Institute of Brooklyn

This is a final report for NASA Grant No. NGR33-006-062 for the period 1 January 1972 to 31 December 1972. Principal emphasis of study has been the escape mechanism which results in "blow-off" of an atmosphere. The planets and satellites have been studied in this respect from an evolutionary standpoint. Studies were made of comets and the solar -wind because of similarities with the problem treated here. Both static and dynamic studies have been made of upper atmospheric regions for neutral and for fully ionized media. Variation with more than one dimension is essential and has been incorporated^ This report is the final -report of NASA Grant No. NGR33-006-062, entitled "A Study of the Evolution of the Atmosphere of Venus. " The program •was for one year starting 1 January 1972. The Principal Investigator has been Dr. Stanley H. Gross of the Electrical Engineering and Electrophysics Department of the Polytechnic Institute of Brooklyn. The program has been concerned with the escape of gases from planetary atmospheres, in particular possible primordial atmospheres dominated by light species. Escape of such gases is mainly by the "blow-off" mechanism. One application of interest is the apparent loss of water on Venus and its cosmological significance. Studies under the program were divided into two areas: statics and dynamics. Static-like studies of upper atmospheres have been more, or less traditional, due to the emphasis on solar UV absorption, diffusion and thermal conduction processes. One dimensional problems (with altitude or radius only) are assumed,, and the complication of motion and three dimensions were recognized but not well treated. The structure of the thermosphere and temperature profile are determinable from static models, as well as exospheric temperatures for thermal escape studies. It is reasonable to compare such temperatures with temperatures corresponding to gravitational potentials for estimating escape efficiencies. This was done under this program and much of the contents is described in two papers published in the Journal of the Atmospheric Sciences (Smith and Gross, 1972; Gross, 1972), and these form a part of this report. Further studies were made of the Galilean Satellites of Jupiter and Titan, the largest satellite of Saturn. These also utilize static models that permit some degree of comprehension of observations of these satellites. It is planned to present the results of this study at the DPS meeting of the AAU in March 1973 in Tucson, Arizona. A copy of the Abstract entitled "Evolutionary Aspects of the Atmospheres of Titan and the Galilean Satellites" is attached and forms part of this report. . Dynamical studies have consisted of two parts. One has been concerned with the development of escape flux equations for neutral media. The problem is similar to that originally made for the Solar Wind. Since the latter expands as a result of energy from within, whereas atmospheres expand from the absorption of energy from without, the analysis .of this effort incorporates UV absorption and thermal conductivity rather than resort for simpliticy to a polytrope. Equations have been developed to the point where further analysis will require computer evaluation. Another part of the dynamical study of escape mechanisms has been concerned with ionized media. Here the development has assumed fully ionized gases. The equations are similar to that for neutral media except for the complications of electromagnetic terms. Much more work is required for this effort, particularly for partially ionized media. In view of some similarity of rapid escape due to blow-off and cometary tail formation, some effort during the program was concerned with comets and their properties. One goal of the program is to study and analyze tail flow in this respect as well. Flow from planets, comets or satellites, because of the directional aspect of the sun with respect to solar UV and the solar wind, necessitates treatment of a problem in which there is variation with at least two dimensions. Spherical symmetry is not realistic, and analysis, as contrasted with much of the work on the solar wind (or even the Earth's polar wind), must include the effects of angular variation in the models as well as radial variation. Introduction of the angle coordinate produces considerable complexity. The dynamical studies referred to above have included effects of variations with all