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Twin probes reveal Moon's violent past

Paul Sutherland, Feature writer
Dec 7, 2012, 8:00 UTC

Sen—Two space probes have produced a detailed gravity field map of the Moon that will provide scientists with a fund of information about its violent history.

The map, which is more complex than any generated before of any celestial body, was obtained by the twin Gravity Recovery and Interior Laboratory (GRAIL) craft, each the size of a washing machine, as they orbited the Moon.

It identifies many features never seen before in such high resolution. They include tectonic structures, volcanic landforms, basin rings, crater central peaks and numerous simple, bowl-shaped craters caused in an extended battering by asteroids.

The two probes, launched in September 2011 and later renamed Ebb and Flow, have been orbiting the Moon in a near-polar, near-circular orbit, flying at a height of about 55 km (34 miles).

Their measurements allow planetary scientists to make new discoveries about the nature of the Moon's composition and internal structure. That can help them to understand better how the Earth and the other rocky inner planets of the Solar System formed and evolved.

The GRAIL mission's Principal Investigator Maria Zuber, of the Massachusetts Institute of Technology, says that the Moon's gravity field preserves the record of impact bombardment that all terrestrial planetary bodies suffered billions of years ago. It reveals evidence for fracturing of the interior extending to the deep crust and possibly the mantle.

Zuber said: "What this map tells us is that more than any other celestial body we know of, the Moon wears its gravity field on its sleeve. When we see a notable change in the gravity field, we can sync up this change with surface topography features such as craters, rilles or mountains."

Gravity field measured by GRAIL and Apollo

This image depicts the porosity of the lunar highland crust using data from GRAIL mission, Apollo moonrock samples and orbital remote-sensing data. Credit: SA/JPL-Caltech/ IPGP

The probes revealed the bulk density of the Moon's highland crust is substantially lower than has generally been assumed. This finding supports data obtained by the final Apollo lunar missions in the early 1970s, indicating that local samples returned by astronauts are representative of the crust in general.

The result also support the generally accepted idea that the Moon was formed from a chunk of the Earth following a huge cosmic impact.

Mark Wieczorek, GRAIL co-investigator at the Institut de Physique du Globe de Paris, said: "With our new crustal bulk density determination, we find that the average thickness of the Moon's crust is between 21 and 27 miles (34 and 43 kilometers), which is about 6 to 12 miles (10 to 20 kilometers) thinner than previously thought.

"With this crustal thickness, the bulk composition of the Moon is similar to that of Earth. This supports models where the Moon is derived from Earth materials that were ejected during a giant impact event early in solar system history."

The two spacecraft built up their map by transmitting radio signals to define precisely the distance between them as they flew in formation around the Moon. The differing gravitational tug as they passed over features like mountains and craters and hidden masses below the surface, caused the distance between the two spacecraft to change slightly.

These first scientific results from the prime phase of the mission are published in the journal Science. Meanwhile the probes, flying at lower altitudes, continue to study the Moon on an extended mission that is due to conclude on 17 December.

Movie showing the variations in the lunar gravity field as measured by GRAIL during the primary mapping mission from March to May 2012. Credit: NASA/JPL-Caltech/MIT/GSFC