At 07:25 AM 4/9/2012, Warren Smith wrote:

--I'm skeptical on some of your/Melosh's claims, but agree re others.

(big atmospheres). Earth has an intermediate size atmosphere and I think it would be difficult to eject earth rocks.

Melosh specifically talks about how stuff can be ejected from the Earth -- in particular, the meteor event of 65 million years ago.

To get a meteor thru an atmosphere, it needs to be large, otherwise it vaporizes.

Which direction are you talking about? Launching or re-entry? According to Melosh, re-entry is relatively easy -- it happens all the time.

However, one might conjecture that an impact big enough to blast huge rocks to well above escape velocity would shatter those rocks, i.e. not giving us big intact ones.

Melosh specifically addresses this issue. Rocks on the surface near by to the collision, but not directly impacted by the collision, will see a huge acceleration _upwards_. His example is if a meteor crashed through the lecture hall and impacted beside him (or more likely some of his bacteria) would cause his body to accelerate upwards at perhaps 100,000 G's. Although he wouldn't survive, some of his bacteria would certainly survive. He also suggests that the upward-expanding fireball would actually help propel the object upwards. I suspect that this effect may be similar to those Soviet-era "supersonic" torpedoes that had small rocket motors facing _forwards_ which pushed the water ahead of the torpedo out of the way. I don't know whether anyone has directly addressed the issue of Venus. I get the impression that this has been a relatively active area of research over the past 10 years, so there may be later results than this 2009 video. Re bacteria surviving: Melosh discusses one of the instruments dropped onto the Moon before one of the Apollo missions brought it back. This instrument had _not_ been sterilized prior to being placed onto the Moon, and was kept sealed when brought back from the Moon. When this instrument was cultured, it showed the presence of live microbes.