[meteorite-list] Comets Not Asteroids (and new book METEOR SCIENCE & ENGINEERING)

From: Robert Verish <bolidechaser_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:13 2004
Message-ID: <20010619225537.17253.qmail_at_web10403.mail.yahoo.com>

- ------- Forwarded Message

From: Peiser Benny <B.J.Peiser_at_livjm.ac.uk>
To: cambridge-conference
<cambridge-conference_at_livjm.ac.uk>
Subject: CCNet 80/2001 - 19 June 2001
Date: Tue, 19 Jun 2001 11:57:06 +0100

CCNet 80/2001 - 19 June 2001
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=======================================================
* LETTERS TO THE MODERATOR *
=======================================================

(4) COMETS NOT ASTEROIDS

>From Duncan Steel <D.I.Steel_at_salford.ac.uk>

Dear Benny,

In his nice essay concerning transpermia, Michael
Paine begins by talking about "the ejection of surface
rocks from Mars during impacts by large asteroids".

The feasability of target (planetary) material being
thrown off Mars depends critically upon the impact
speed. Using a sample of over 600 observed
Mars-crossing asteroids I calculated a mean impact
speed of 9.3 km/sec, with less than five percent of
the impacts at greater than 20 km/sec and rather less
than one percent occurring at greater than 30 km/sec.
On the other hand, with an assumed spherical
distribution of near-parabolic comets with a uniform
distribution in perihelion distance (i.e., coming from
the classical
Oort-Opik cloud) I find a mean impact speed of 45
km/sec, a mode of 55-56 km/sec, and a maximum impact
speed just below 60 km/sec, with a very small fraction
having any impact speed *below* 30 km/sec.
(Reference: Duncan Steel,
"Distributions and moments of asteroid and comet
impact speeds upon the Earth and Mars", Planetary and
Space Science, volume 46, pp.473-478, 1998.)

Using the relation given by Melosh
(H.J. Melosh, "The rocky road to panspermia", Nature,
volume 332, pp.687-688, 1988) one finds that an impact
speed in excess of 20 km/sec is necessary to achieve
any substantial ejection from Mars.

Although numerically-speaking large body impacts on
Mars are dominated by asteroids, it seems very likely
that an impact by a comet is a rather better bet for
liberating martian rocks onto heliocentric orbits from
which they may make their way to the Earth. If I
recall correctly, the known Mars meteorites show
groupings in space exposure ages that would indicate a
small number of impacts having ejected these objects
within the last 10-100 million years or so. This would
be consistent with the very occasional high-speed
comet impacts being responsible, rather than the
greater number of lower-speed asteroid impacts.

Duncan Steel

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- ------- End of Forwarded Message

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Date: Tue, 19 Jun 2001 11:44:29 -0700
From: Robert Gardner <rendrag_at_earthlink.net>
Subject: (meteorobs) [Fwd:
         METEOR SCIENCE & ENGINEERING is now in stock]

Psst!

A book on your Notification List at Powells.com is now
in stock. Please note that this book is not on hold
for you; this is merely a notice that the book is now
in stock at Powells.com.

METEOR SCIENCE & ENGINEERING
by D W R Mckinley

in HARDCOVER at $14.00.
To learn more about the book, please proceed to:

<http://www.powells.com/cgi-bin/check_avail?inkey=4-1124077782-0&title=METEOR%20SCIENCE%20%26%20ENGINEERING&author=Mckinley%2C%20D%20W%20R>

The book is available on a first-come, first-served
basis to all customers.
Your best chance to get this book is to order it
online immediately - phone or walk in ordering will
not be as effective as the lightning fast power of the
internet!

Powell's cannot reserve books according to Binding
type (i.e. Hardcover or Trade Paper); notification
requests pertain to title and author only.

Thank you!
Powells.com

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------------------------------

End of meteorobs-digest V3 #661
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Received on Tue 19 Jun 2001 06:55:37 PM PDT