[meteorite-list] Where Did The Moon Come From?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:21:01 2004
Message-ID: <200307091730.KAA03201_at_zagami.jpl.nasa.gov>

http://physicsweb.org/article/news/7/7/4

Where did the Moon come from?
Katie Pennicott
Physics Web
3 July 2003

Astronomers believe that the Moon was formed when a Mars-sized body
smashed into the Earth, ejecting matter into orbit and lengthening our day to its
present value of 24 hours. Until recently, however, estimates of much of the
Moon is "impactor material" that came from this impactor object, as opposed
to the Earth, have varied wildly - from 1 to 90%. Now, by comparing the
compositions of lunar and terrestrial rock samples, astronomers in Germany
have calculated that no more than two-thirds of the Moon is impactor
material. Moreover, they estimate that the Moon must be at least 4.5 billion
years old (C Munker et al 2003 Science 301 84).

Carsten Munker and co-workers at the University of Munster compared the ratios
of niobium (Nb) to tantalum (Ta) in samples of rock from the Moon, Earth, Mars
and meteorites. The team found a Nb/Ta ratio of 17 for the Moon, compared with 14
on Earth. The ratio in the other samples was almost 20, which should be consistent
for bodies throughout the solar system - including the object that collided with the
Earth.

According to the researchers, this variation suggests that the impact that formed
the Moon took place during the formation of the Earth's rocky mantle and iron core
- a process that geologists believe was aided by the impact. Under high pressures,
niobium becomes 'siderophile' or iron-loving, so much of the terrestrial niobium
would have become incorporated into the Earth's core when it formed, leaving a
niobium-poor mantle.

If the giant impact occurred while the core and mantle were forming, the Earth would
have contributed little niobium to the Moon. But Munker and colleagues calculated
that the lunar Nb/Ta ratio would be boosted to the observed level if up to 65% of the
Moon consisted of impactor material.

This theory also leads Munker's team to believe that the Moon must be at least 4.5
billion years old, since radioisotope dating shows that the Earth's core and mantle
were fully formed by that time.

Author
Katie Pennicott is a science writer living in Bristol, UK
Received on Wed 09 Jul 2003 01:30:18 PM PDT