Researchers have pieced together the violent cosmic event at the dawn of the solar system that gave rise to the right eye of the fabled Man in the Moon.
A massive asteroid that was large enough to be classed as a protoplanet slammed into the lunar surface at 22,000 mph to form the distinctive dark patch four billion years ago, they say.
The lump of rock, which was at least 150 miles wide, thumped into the moon at an angle of 30 degrees. Having struck with such force, it fractured into chunks that gouged scars in the lunar surface for up to 300 miles.
The impact punched a 750-mile wide crater into the surface creating the right eye of the Man in the Moon, or left eye as seen from Earth, said Peter Schultz, a planetary scientist who led the research at Brown University in Rhode Island. “It’s one of the biggest and last craters to form on the moon,” he added.
While astronomers have long known about the crater, known as the Imbrium basin, the dark feature is surrounded by grooves that have left researchers scratching their heads. While some of the grooves spread out radially – the result of chunks of moon rock being kicked out by the incoming asteroid – other grooves appear to come from the northwest of the crater, along the asteroid’s trajectory. No one was quite sure where they came from.
“Any kid with a telescope can see the basin and you can see the grooves too, they are very very obvious,” Schultz said.
To investigate the markings, known as the Imbrium Sculpture, Shultz and others turned to a 14-foot cannon based at Nasa’s Ames Research Center in California. The Vertical Gun Range fires projectiles at up to 16,000 miles per hour into plates to study the violent consequences.
In a series of experiments with the cannon, Schultz found that in a high speed, oblique collision, an asteroid would strike the moon’s surface uprange of the final crater, and break into fragments that scarred the surface heading towards the basin.
“The sculpturing has been known for well over 100 years,” Schultz said. “The mystery has been why didn’t the grooves all come from the centre of the basin?”
Having worked out where the grooves came from, the scientists could measure the features and work backwards to calculate the minimum size of the asteroid that thumped into the moon. Most of the grooves are tens of miles long, but others stretched much farther. “This has allowed us for first time to give an estimate of how big the asteroid was. It was 150 miles across at a minimum, which is almost three times greater in diameter and 30 times more massive than previous estimates,” Schultz said. Details of the study are reported in Nature.
Understanding the sizes of objects that struck the moon and other heavenly bodies early in Earth’s history will help scientists work out the nature of the impactors. While some could be asteroids, or fragments of asteroids, others would be comets arriving from the outer solar system. “This tells us maybe bigger things were striking the moon than we thought, and that gives us a sense of what was breaking up, and the larger scale processes at work in the solar system,” she said. “What we don’t know is what happened to all these objects that hit the moon. Did they evaporate, or mix into moon, or are there big chunks still out there?”