No abstract

No abstract

No abstract

No abstract

Whipple predicted the existence of micrometeorites, small interplanetary particles which enter the Earth's atmosphere without being melted by frictional heating. During the past 2 yr, large numbers of micrometre sized stratospheric particles have been collected which are believed to be extraterrestrial, because their elemental compositions closely match those of primitive meteorites. We report here the detection of large concentrations of 4He in some of the particles. This not only suggests an exposure to solar wind, but also indicates that these particles are true micrometeorites in the sense that they were not strongly heated by entry into the atmosphere. Strong heating would have caused much of the helium to escape.

No abstract

The Qingzhen (EH3) chondrite contains a population ofspheroidal metal-sulfide nodules, which display textural evidence of reheating and melting. Evidence of metal sulfuration is also present, suggesting replacement of metal by sulfide during melting. This process has led to the nucleation of perryite along metal-sulfide interfaces.Gallium-bearing sphalerite and a Cu-sulfide of composition intermediate between chalcopyrite and cubanite occur as inclusions within the metal of some nodules. Other phases present are: kamacite, troilite, Ga-free sphalerite, niningerite, perryite, schreibersite, oldhamite, Cr-sulfide (minerals A and B), djerfisherite, SiO z , albite and enstatite. The Ga-bearing sphalerite may have formed by injection of molten sulfide droplets into the metal followed by subsolidus diffusion of c«from the metal into the sulfide. The latter may occur because ofGa supersaturation in the metal during progressive sulfuration and its decreased affinity for the metal phase during cooling below the taenite-kamacite transition point.

Luminous efficiency values have been calculated empirically as a function of velocity and altitude for the Lost City, Innisfree, and Pribram meteorites. We have used a combined approach which utilizes the observed light data from the meteorite entries and the appropriate ablation rates via the entry model of ReVelle, which is generally consistent with the results of cosmic ray track analyses. Although our method reduces the absolute value of computed luminous efficiency via a total kinetic power balance equation, our results still exceed the modified Trailblazer results of Ceplecha and McCrosky for chondrites by about a factor of 5 for entry velocities of •<20 km/s. Our values of luminous efficiency (deduced with respect to a detector peaked in the visible) are -,,0.2-0.4% in the vicinity of peak ablation for Lost City and Innisfree, respectively. Results for Pribram are less certain but are -,,0.04% at peak ablation. In terms of numerical values and velocity dependencies our results agree we!! with appropriately modified laboratory simulation data of Givens and Page (1971). We conclude that the deduced photometric mass scale of fireballs should be revised downward by a factor of 5-10 so that a more accurate mass influx determination at the earth's orbit can be made.