Re: ..not dinosaurs, but comets ...

[Graydon <oak@uniserve.com>]

On Sun, Jan 04, 2009 at 02:31:29PM -0600, jrc scripsit:
> From: "Tommy Tyrberg" <tommy.tyrberg@norrkoping.mail.telia.com>
>> A much better way to explain the absence of a crater would be to
>> postulate an inpact on top of the Laurentide Ice Sheet. You could
>> probably explain away a fairly large crater that way.

Not with the osmium signatures people are finding; if the Younger Dryas
Impactor hypothesis is correct, it's involves a pretty big rock.

> Why so?  Why not run an impact calculator to see how small an impactor 
> would penetrate the Ice Sheet?

There's lots of room in Hudson's bay for a crater; there's a known and
not very well explained gravitic anomaly there, even.  So far as I know,
there isn't a good bottom survey.

While their certainly is a crater in Lake Superior (Slate Islands) that
one is Archean in age, so not useful for an impact hypothesis about the
Younger Dryas.

Given that the evidence for there having been an impact at the start of
the Younger Dryas hasn't been advanced beyond all possibility of
alternative explanations -- see
http://adsabs.harvard.edu/abs/2008AGUFM.V53A2150B or
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1871814 for
examples -- the various discussions in
http://www.agu.org/meetings/sm07/sm07-sessions/sm07_PP42A.html which
appear to assume an impactor may be problematic; the indication that the
Carolina Bays are ejecta impact features is very interesting, though.

Given the lack of a broader arc of ejecta, one can hypothesize either
that the Carolina Bays are the only area where a broad arc is detectable
(I think that one's really tough) or there isn't a broad arc; there was
a directed path of ejecta due to a low angle impactor.

If there's a low angle impact feature in the Lake Superior Basin,
especially one that's been subsequently inundated by a glacial lake or
scoured by glaciers (or both!), it's quite possible that it's going to
be really, really hard to find.

-- Graydon