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New Data: Asteroid Bombardment of Moon

Credit: Marchi/Bottke/Kring/Morbidelli

The Moon is a witness plate of a time when asteroids pummeled the Earth and Moon 4 billion years ago.

New lunar data has revealed evidence about that process so long ago.

Using information gleaned from the Lunar Orbiter Laser Altimeter — an instrument on NASA’s Lunar Reconnaissance Orbiter (LRO) now operating in orbit around the Moon — the team has taken a fresh look at the Moon’s ancient surfaces and measured the sizes of the impact craters on them.

Thanks to that appraisal, the researchers have detected a subtle shift in those crater sizes that can best be explained by an increase in the velocities of the asteroids that produced them.

The findings stem from a team of NASA Lunar Science Institute (NLSI) scientists and an international partner, Observatoire de la Cote d’Azur in France, have reported their findings in the current issue of Earth and Planetary Science Letters.

The collaborative scientific investigation involved measuring the sizes of the most ancient impact craters on the Moon.

Together, they discovered a pattern that indicates the energy of impacts associated with the formation of immense 300 to greater than 1000 kilometer diameter impact basins increased for the youngest ones. They interpret that pattern to indicate an increase in impact velocities generated by the movement of Jupiter and other outer solar system planets.

Gravitational disturbances

Roughly 400 million years after solar system formation, the orbits of Jupiter, Saturn, and the other giant planets moved. Jupiter moved slightly closer to the Sun while Saturn moved farther away.

This caused a series of gravitational disturbances to ripple through the asteroid belt. That event dynamically excited or stirred up the asteroids, sending many of them into higher inclination and more elliptical orbits that pushed them into the inner solar system.

That dramatic flux of near-Earth asteroids caused a much higher impact cratering rate on the Earth and Moon in what scientists often call the lunar cataclysm.

Impact velocities of these objects appear to have doubled when the asteroids were stirred up, causing a shift in the crater size distribution that can be detected today in the most ancient of the lunar highlands.

The shifting gravitational disturbances also led to the formation of gaps within the asteroid belt that are seen today.

Study results include:

– Some of the oldest impact basins produced on the Moon were not part of the lunar cataclysm. – The largest impact basins on the lunar nearside, visible from backyards around the world, were produced after the giant planets moved and the velocities of impacting asteroids doubled. – This doubling of velocity is consistent with computer models that suggest a dramatic shift in the configuration of outer solar system planets to their present positions roughly four billion years ago.

Testing the hypothesis

The team has been testing the lunar cataclysm hypothesis, which suggests the Earth and Moon were severely bombarded by a brief pulse of impacting objects that flooded the inner solar system about 4 billion years ago.

“The bombardment of asteroids was an exciting event in solar system history,” said David Kring, the NLSI Principal Investigator at USRA’s Lunar and Planetary Institute. “It reshaped the surfaces of the Earth and Moon at virtually the same time that life was emerging nearly 4 billion years ago.”

The oldest and largest impact basin on the Moon, called the South Pole-Aitken Basin, was produced during that epoch of solar system history. The age of the South Pole-Aitken Basin is, however, still a mystery and is one of the scientific targets for future lunar exploration.

Along with Kring, others on the research team were Simone Marchi, an NLSI Postdoctoral Fellow, William Bottke, the NLSI Principal Investigator at Southwest Research Institute, and Allessandro Morbidelli at the Observatoire de la Cote d’Azur.

By Leonard David

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