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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

Comments

  • Joemaddy March 14, 2012

    Ifound that the earth travel aornud the sun once avery year and it spins aornud once every 24 hours.The Earth also spins on its own axis. The axis is an imaginary line through the centre of the Earth from the North Pole to the South Pole.The Earth is just one of the nine planets that travel aornud the Sun. The other planets are called Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune.Earth is a small, rocky planet which supports a variety of life. As far as we know, Earth is unique from all other planets in this respect.The Moon travels aornud the Earth , It goes round once every 28 days.We only see the part of the Moon that is lit by the Sun.The Moon takes about 27 days (27 days, 7 hours, 43 minutes, 11.6 seconds) to go all the way aornud the Earth and return to its starting position.Most rocks on the surface of the Moon seem to be between 4.6 and 3 billion years old.The Moon travels aornud Earth at a speed of 2,288 miles per hour (3,683 kilometers per hour).The Moon has no air because its gravity is too weak to hold an atmosphere. The Sun is a star and gives out heat and light.It is roughly spherical in shape and is much, much bigger than the Earth.The surface of the Sun is much cooler than its atmosphere.The Sun is by far the largest object in the Solar System.Because the Sun is so large compared to everything else, it is easily able to hold on to the rest of the matter, causing everything else to orbit aornud it.The Sun is a star it is not a planet. Our Sun is just like the stars we see in the night sky. The Sun is also the only star we see during the daytime.