[meteorite-list] Is 3He a reliable cometary marker?

From: E.P. Grondine <epgrondine_at_meteoritecentral.com>
Date: Wed, 27 May 2009 15:26:42 -0700 (PDT)
Message-ID: <733942.29333.qm_at_web36908.mail.mud.yahoo.com>

Hi Doug, all,

>Not quite sure I follow what you mean by a long period comet becoming a
>short period comet and apparently circulating for millions of years.
>I thought such comet lifetimes are significantly less than that and if
>the literature I saw is to be believed, along the P-Tr unconformity, a
>two million year interval was hypothesized based on 3-He findings, and
>more recently geologically . Farley, posthumous Shoemaker, et al also
>found about 2 million years of a hypothetical comet swarm at the Eocene
>Chesapeake Bay crater time period.

Follow Clube and Napier's comet injection model to estimate an entry date, then look at the dates for ELE on Earth. The comet impacts on other bodies should be about the same time. For example, Comet Encke and Comet Shoemaker Levy 9.

>It is hard for me to imagine a comet accumulating much of anything from
>the Sun as you conjecture, since it is losing mass very quickly with
>each orbit, though perhaps meteoroid streams could.

Good point, but I also wonder about losing material/gaining material on each pass. It appears to me that comets are composed of accreted cometissimals of around 30 meters, and comet's cores can be quite dense if they've been accreted long enough - hence the irridium.

I've read that our Sun's 3He has accumulated on the Moon.

>I think 3-He is certainly considered an ET marker, especially for small >particles of a cometary nature, bucky balls and the like ... In any case, >I think your questions would be excellent to pose to Dr. Ted Bunch if he >has a moment, a list member who has quite a a bit more experience than me >in this subject ;-), and probably knows the authors you are question about
>personally...

It would be nice. I guess we'll just have to see.

It would be nicer if Weiler and Morrison were relieved, and we could get NASA to fund some serious work on this problem.

Good hunting, taking along your copy of Field Guide to Meteorites,
Ed




      
Received on Wed 27 May 2009 06:26:42 PM PDT