The moon’s surface isn’t nearly as geologically active as the Earth’s, despite their similarities in formation, so the craters left by asteroids that have bombarded it over the last 4.5 billion years have taken much longer to erode than they would down here on the planet.
Previously, astronomers estimated the age of the surface by “crater counting,” or tallying up all of the impacts they could find and then estimating how long it would have taken to accumulate them. However, the returning of physical samples from the lunar surface has created problems for that method.
Like here on Earth, lunar rocks can be dated by measuring the quantity of certain radioactive isotopes of potassium, argon, and uranium. In their research, the scientists painstakingly went through all of the lunar samples, corresponding the result of the radioactive dating of the rocks with the sites where they were collected, effectively correcting the errors in estimation from earlier studies.
The project began in 2014 and involved using data from India’s Chandrayaan-1 orbiter, which mapped most of the lunar surface in 3D in 2008, to verify the rock samples were all of lunar origin and not pieces of asteroid deposited there by an impact.
One of those areas was Mare Imbrium, a vast lava plain that forms the “right eye” of the “man in the moon,” an illusion of a human face formed by a grouping of craters and ridges on the side of the moon that faces the Earth. Samples of this crater were brought back by Apollo 15 in August 1971.
The scientists found that while earlier estimates had pegged Mare Imbrium as being 3.9 billion years old, it is in fact 4.1 billion years old - 200 million years older than previously thought.
Their research has been accepted for publication in the Planetary Science Journal.