Meteor impacts have shaped the Earth and the moon since early in the history of the solar system.

Both worlds have been and continue to be bombarded by meteorites. Up until a couple billion years ago, they were regularly pummeled by huge space rocks that carved massive craters; many of the wandering rocks left after the solar system's formation 4.6 billion years ago have since been scooped up by the planets and the sun, and things have calmed down.

But why isn?t the Earth as pockmarked as the moon?

On Earth, wind, water, and vegetation rapidly (on geologic times scales) erase craters. With few exceptions, even the largest craters are eventually destroyed by the processes of plate tectonics. On the Moon, however, craters are virtually permanent. The only weathering is caused by later impacts and the solar wind. A ?fresh? crater on the Moon can be hundreds of millions of years old.

This week NASA released two images that compare recent impact craters on the Moon and Earth. Each has a resolution of about 6 feet (2 meters) per pixel, and illumination is from the right.

The bright ?rays? surrounding the lunar crater show that the impact was relatively recent ? less than 500 million years ago. Fresh material (ejected by the impact) on the lunar surface is initially bright, and it darkens over time. Boulders formed from compressed lunar dust and soil litter the crater floor.

In contrast, a hole in the Earth called Meteor Crater (also named Barringer Crater) is only 50,000 years old. Despite the weathering of terrestrial craters, this one is fairly well preserved in the arid climate of the Colorado Plateau in Arizona.

Meteor Crater formed from the impact of an iron-nickel asteroid about 46 meters (150 feet) across, NASA states. Debris from the asteroid melted or vaporized on impact. The collision initially formed a crater more than 1,200 meters (4,000) feet across and 210 meters (700 feet) deep. Subsequent erosion has partially filled the crater, which is now only 150 meters (550 feet) deep.

While both planetary bodies get pelted by small meteorites continuously, though most are too small to reach the ground on our planet. Asteroids measuring roughly 50 meters (160 feet) across strike the Earth every 1,000?2,000 years. Bigger asteroids larger than 1,000 meters (3,300 feet) across typically strike every 100,000 years.