[meteorite-list] Lunar and Martian Meteorites

From: bernd.pauli_at_paulinet.de <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 10:25:38 2004
Message-ID: <DIIE.0000005C00000A50_at_paulinet.de>

Hello Listees and Listoids,

There is an interesting article in the June 2003 issue of Sky & Telescope about the roughly equal number of lunar and martian meteorites. It discusses several possible explanations for why there are about as many martian meteorites in our collections as there are lunars although there should be many more lunar than martian meteorites. Another plausible, though perhaps less scientific, explanation not mentioned by J. Kelly Beatty is this one: There are still many more lunars ou there (both in Antarctica and the Hot Deserts) awaiting to be found or discovered. They may be covered by several inches of sand, snow or blue ice, and, being achondritic, i.e. poor in free metal, metal detectors can't provide any useful help. See also list member Norbert Classen's article* for further intriguing details.

Something else that has always been a puzzle to me is the relatively small size of lunar and martian meteorites found to date. Why aren't there really big ones like the Jilin main mass (1770 kg), the 408 kg Paragould LL5 stone, or the 91 kg Juvinas eucrite? Does this have to do with orbital dynamics in the Earth-Moon and the Earth-Mars system? Are there tidal forces that literally disrupt martian or lunar meteorites somewhere on their way to Earth?

* CLASSEN N. (2003) The Quest for the Moon - Confessions of
  a Lunatic (METEORITE May 2003, Volume 9, No. 2, pp. 8-13).

Best regards,

Bernd


BEATTY J.K. (2003) The Lost Lunar Meteorites (Sky & Telescope, June 2003, p. 22):

It's been 20 years since planetary scientists first realized that chunks of the Moon and
Mars were practically falling into their laps as meteorites. And, while thankful for the free
samples, they've al-ways puzzled over why these two worlds are represented roughly
equally on Earth (with 24 distinct falls from the Moon and 28 from Mars found to date).
The puzzle arises because the lunar specimens should outnumber their Martian counter-
parts by more than 100 to 1. For one thing, the Moon's weaker gravi-ty means that a
much smaller impact will accelerate lunar debris to escape ve-locity, compared to the
more energetic (and thus rarer) blasts necessary to eject something from Mars. And
calculations performed several years ago by Brett Gladman (University of British
Columbia) show that, once launched into space, a chunk of lunar rock has about a
50-50 chance of ending up on Earth - 10 times better odds than for an arrival from Mars.

So why aren't the meteorite-rich tracts of Antarctica and Saharan Africa littered with
more chunks of Tycho and Mare Imbrium? The answer, according to James N. Head
(Raytheon Missile Sys-tems), may be that most of them have simply disappeared over
the past 100,000 years, eroded to oblivion by wind and water. Head says that most
meteorites from the Moon should reach Earth within only about 10,000 years. So if
by chance there haven't been any recent im-pacts on the Moon, the arrival rate right
now will be in a deep lull, and the old meteorites will be mostly gone. Martian launches,
by contrast, take an average of roughly 10 million years to make their way here, ensuring
a steady trickle of roughly one arrival per month. Notably, four Martian falls have been
witnessed firsthand, whereas no one has seen a piece of the Moon descending to Earth.

However, Head's scheme, which he presented in March at the annual Lunar and Planetary
Science Conference, implies that all lunar meteorites should be recent arrivals, and that's
not the case. Of the 13 with well-established travel times (determined by measuring their
exposure to cosmic rays while in transit), 6 left the Moon between 500,000 and 9 million
years ago. In fact, notes Kunihiko Nishi-izumi (University of California, Berke-ley), lunar
and Martian meteorites share the same basic age distribution.

Nonetheless, Head says, to date the census of the two groups is still "a wash," with 99%
of the expected lunar meteorites somehow staying out of collectors' hands. Gladman agrees,
noting that the near-equal numbers of lunar and Martian meteorites must be a con-sequence
of "transfer dynamics that today deliver few meteorites from an-cient lunar impacts, but a
reasonable flux from larger ancient Martian impacts."
Received on Sun 11 May 2003 02:53:35 PM PDT