Bombardment by asteroids and comets is much heavier than previously thought, creating 180 sizeable new craters every year, say scientists
The moon is bombarded by so much space rock that its surface gets a complete facelift every 81,000 years, according to a study of Nasa data.
This churn – affecting the top 2cm (nearly an inch) of mostly loose moon dust – happens 100 times more frequently than previously thought, scientists have reported.
The study also estimates that asteroids and comets crashing into the moon create on average 180 new craters at least 10 metres (33ft) in diameter every year.
The findings, published in Nature, come from “before and after” pictures taken by Nasa’s Lunar Reconnaissance Orbiter spacecraft, which has been mapping the moon since 2009.
By comparing images of the same area at regular intervals, a team of scientists led by Emerson Speyerer from Arizona State University in Tempe was able to tally the number of new craters and extrapolate to the entire surface of the moon.
“We detected 222 new impact craters and found 33% more craters with a diameter of at least 10m than predicted” by earlier models, the researchers concluded.
The scientists also found thousands of subtler disturbances on the surface, which they described as “scars” from smaller secondary impacts that – over thousands of years – churned up the top layer of the moon without creating craters.
Earth is also constantly pelted by asteroids and meteors but is protected by a thick atmosphere.
More than 100 tonnes of dust and sand-sized particles rain down on the planet every day. But even those up to 25m across (80ft) usually explode and disintegrate in the upper layers of our atmosphere, causing little or no damage, according to Nasa.
The moon has next to no atmosphere – only contains about 100 molecules of gases and elements per cubic centimetre.
Earth’s atmosphere at sea level, by contrast, has about 100 billion billion molecules per cubic centimetre.