Manuscript & Correspondence Archive

BRITISH MUSEUM (NATURAL HISTORY)
Cromwell Road
London, SW7 5BD
(DEPARTMENT OF MINERALOGY)
Your Ref: AH/aer
Our Ref: RH/MB

15 November 1983

Dr Alan Rubin
University of California, Los Angeles
Institute of Geophysics & Planetary Physics
LOS ANGELES
California 90024
USA.

Dear Alan

Thank you for your letter of 29 September, received on 4 November. I am delighted that you enjoyed my book, and that you took the trouble to make specific comments.

The cratering rate I took from David Hughes' review in Nature, 1979, 281, 11, based on papers by Grieve and Robertson and Grieve and Dence. The figure given for a 1 km crater is once every 1,400 years, so my figure (p.24) of 1,300 years is in error. However, your suggestion of a rate over 20 times greater means that in the past 10,000 years (i.e. post-glacial) over 150 craters would have been formed, 45 of those on land. We know of only one! I can hardly believe that 44 others are still waiting to be discovered.

I agree with your comments about the number of falls represented by the Antarctic meteorites. However, if you take the "fragmentation index" of Ed Olsen at its face value, you end up with a small number of falls, as on my p.26. (See Nature, 1981, 292, 516).

Wood's cooling rate model for the ordinary chondrites is based on the assumption, which I believe to be erroneous, that all metal was, or became, homogenized before cooling through 500°C. See our paper in the "Chondrules" volume, which should be out by now.

Finally, the origin of comets. You may be right that they formed in the vicinity of the outer planets, but this must largely be speculative. Because an ice/dust mixture fits the interpretation of the properties of interstellar grains, and because of the difficulty of holding an Oort cloud against capture during close approaches to giant molecular clouds in passages through galactic spiral arms, more astronomers argue for an interstellar origin for comets. Comets are removed from the solar system by giant molecular clouds. This seems hard to swallow from a meteoritic point of view, but, nevertheless, it is a valid astronomical approach.

Thank you again for your response. If you see anything else in the book that you want to take up, I will be happy to reply.

Yours sincerely

ROBERT HUTCHISON