FYI -- Here is Dr Melosh's response to my question:

From: "Melosh, Henry J" <jmelosh@purdue.edu> To: Henry Baker <hbaker1@pipeline.com> Date: Tue, 10 Apr 2012 10:06:12 -0400 Subject: Re: Ejecta from Earth ? Venus ?

Dear Dr. Baker:

The atmosphere issue was one of my concerns from the very beginning of such considerations--it is not something that I have overlooked! There is substantial evidence that impacts that produce craters greater than about 10 km diameter on the Earth and about 30 km on Venus breach the atmosphere and eject material directly into space. This evidence is not (yet!) based on the discovery of Earth or Venus meteorites, but rather on the observation of crater ejecta that has traveled immense distances before reentering the atmosphere: That is, material that nearly, but not quite, left the planet. On Venus, this comes in the form of the dark parabolas of ejecta from larger Venusian craters that blanketed the surface with ejecta more than about 10 cm deep (equal to the wavelength of the Magellan radar that detected these deposits) for distances exceeding 5000 km from the impact. On Earth, ejecta from craters like Chicxulub, Popagai, down to Bosumptwi can be found at distances ranging from thousands of km to the antipode of the crater. The 22 km diameter Ries crater in Germany threw intact rocks more than 300 km into Switzerland. These ejecta cannot have traveled this far through the atmosphere and so clearly were thrown above it as part of the plume of vaporized and melted rock, mixed with intact fragments, that expands out of any high speed impact crater.

As to nuclear explosions breaching the atmosphere, see Jones, E. M. and J. W. Kodis (1982). Atmospheric effects of large body impacts: The first few minutes. Geological Implications of Impacts of Large Asteroids and Comets on the Earth. L. T. Silver and P. H. Schultz, Geol. Soc. Amer. Sp. Pap. 190: 175-186.

I have published on this atmosphere breaching in a variety of media, as a google or ADS search would quickly show.

Sincerely, Jay Melosh

########################################################

Jay Melosh Distinguished Professor of Earth and Atmospheric Sciences Departments of Earth and Atmospheric Sciences, Physics and Aerospace Engineering Civil Engineering Building, Room 3237 550 Stadium Mall Drive Tel: (765) 494-3290 Purdue University Fax: (765) 496-1210 West Lafayette IN 47907 email: jmelosh@purdue.edu