SECRETARY'S REPORT 167 



provided that the initial velocities with respect to the earth are small 

 (about 1.5 km sec"^). The trapping mechanism consists of an initial 

 encounter with the earth's atmosphere followed by conversion to a 

 long-lifetime orbit by radiation pressure. Drs. Colombo and Laut- 

 man have fomid that long-lifetime orbits of particles ejected from 

 the moon cannot contribute significantly to the cloud, nor can particle 

 breakup, since the drag pressure at the relatively high capture altitudes 

 is not high enough to break the particles. 



The particles responsible for the zodiacal light are concentrated 

 near the plane of the ecliptic. Dr. Southworth has performed a cal- 

 culation combining the space-density distribution of the zodiacal dust 

 particles (as observed in the zodiacal light, and as theoretically pre- 

 dicted from the Poynting-Kobertson effect) with the observed redden- 

 ing of the Fraunhofer corona (which is sunlight diffracted by the par- 

 ticles), showing that the mean radius of the observed particles exceeds 

 15 microns. Some invisible submicron particles may also be present, 

 but their total mass will be negligible compared to that of the larger 

 particles. 



Dr. Charles Wliitney has obtained laboratory evidence confirming 

 the suggestion that interstellar bands are produced by resonant absorp- 

 tion in small grains. Experimental work confirms the presence of the 

 band for Na grains, and theory shows that such grains, when coated 

 with ordinary ice, will produce an absorption just at the astronomi- 

 cally observed wavelength. 



Meteorites, solid bodies from space that survive the plunge through 

 the earth's atmosphere, warrant careful attention, since they are the 

 only samples yet available of extraterrestrial material. It is fruitful 

 to study their mineralogy, crystal structure, metallurgy, chemical 

 composition, isotope distribution, and other physical properties. 



In her continuing mineralogical studies Mrs. Marvin has estab- 

 lished zircon as a meteoritic mineral by its positive identification in 

 the Vaca Muerta mesosiderite and the Toluca iron meteorite.^" Zir- 

 con, because it concentrates uranium, thorium, hafnium, and rare 

 earths, is a mineral of choice for age determinations and measure- 

 ments of Zr/Hf ratios and rare-earth distribution in meteorites. The 

 character and mode of occurrence of zircon in Vaca Muerta and 

 Toluca were studied in detail in collaboration with Cornells Klein, of 

 the Harvard University Department of Geological Sciences, who 

 determined chemical compositions by means of electron-probe 

 microanalyses. 



During the past year Dr. Joseph I. Goldstein completed a metallur- 

 gical study of Widmanstatten patterns in metallic meteorites. The 

 object of the project was to establish the roles of pressure, tempera- 



See footnotes, p. 1T7. 



