Apollo samples reveal Moon's origin

Astronaut Gene Cernan collects soil samples

By BBC News Online science editor Dr David Whitehouse

Measurements of lunar soil have cast new light on the origin of the Moon, and on processes taking place in the Sun's atmosphere.

Analysis of lunar samples returned by the Apollo astronauts confirms previous indications that the ratios of the different types, or isotopes, of oxygen in the Moon's soil are similar to those of Earth. It is a finding that is consistent with the giant impact model for the Moon's formation.

A second examination has measured the amount of beryllium-10 blasted on to the Moon by the solar wind, the stream of particles given off by the Sun.

Its presence in lunar soil indicates that the isotope was ejected directly from the Sun's atmosphere and had never been in the convective zones located deeper inside the star, as some have suggested.

Similar composition

The giant Impact model is the most widely accepted theory for the origin of the Moon. It proposes that the satellite formed from debris thrown into space following a collision between an early-stage Earth and a Mars-sized object.

Thought to be a piece of the Earth

This would have happened when the Solar System was still very young. Recent computer simulations of the impact suggest that the Moon would have been formed mostly from material from the impactor.

If this was the case, then scientists are puzzled as to why the ratios of isotopes of oxygen for Earth and Moon rocks are almost identical.

The best way to explain this, according to a team led by Ernst Wiechert of ETH Zentrium, Zurich, Switzerland, is that the impactor was almost identical in composition to the proto-Earth, having formed a similar distance from the Sun as our own planet.

Cosmic rays

Another analysis, also published in the journal Science, looks at the amount of the isotope beryllium-10 in the lunar soil.

Beryllium-10 can be produced in the Sun when highly energetic protons bombard the nuclei of other elements like carbon and oxygen.

This isotope can be delivered to the Moon by galactic cosmic rays.

But, a detailed analysis of the lunar samples by Kuni Nishiizumi, from the University of California at Berkeley, and Marc Caffee, from Purdue University, suggests that far more beryllium exists in the soil than can be accounted for by cosmic-ray delivery alone.

The researchers say their work provides strong evidence that the beryllium and, therefore, other constituents produced in the Sun's atmosphere are being ejected shortly after they are produced - to hit the Moon.

Some researchers have suggested that materials produced in the Sun's atmosphere are pulled into the star's interior, where they circulate in convection currents for millions of years before making their way back to the outer atmospheric layers.

Nishiizumi and Caffee say their new findings contradict this theory.