The moon was born when a Mars-sized body impacted Earth during its very early days. The debris from both objects spewed into space and ultimately coalesced into our moon. Scientists trying to date this moon-forming impact have come up with a wide range of ages: from as early as 30 million years after the birth of the solar system to as late as 100 million years after the Milky Way was formed.
By creating a “geologic clock," researchers now show that the moon-forming impact must have occurred 95 million years after the start of the solar system, plus or minus 32 million years.
That makes the moon millions and millions of years younger than some previous estimates, which were based on radioactive dating of elements like uranium. When you have an element with a known rate of radioactive decay, you can back-calculate a time for when collected moon rocks were formed. But geochemists disagree about these numbers a lot.
To achieve this revised date, an international team led by Seth Jacobson from Observatoire de la Côte d’Azur in Nice, France, devised a new method based on measurements of the Earth’s interior combined with computer simulations of the protoplanetary disk from which the Earth and other planets formed.
They started by simulating the growth of the “terrestrial planets” Mercury, Venus, Earth, and Mars from the planetary building blocks orbiting the Sun. By analyzing the growth history of those planets from 259 simulations, they discovered a relationship between the time the Earth was impacted and the amount of material added to our planet after that collision.
You see, after the big moon-forming impact, proto-Earth was smashed into by other, smaller objects -- these caused the Earth to gain some mass later on. So they constrained their model with the concentration of highly “siderophile” elements (elements like platinum and iridium that like to be chemically associated with iron). Previous work has shown that these are proportional to the mass accumulated on Earth post-impact.
"When the moon-forming event occurs, this melts the entire surface of the Earth," Jacobson tells National Geographic. All the iron present near the surface sinks into the Earth's core, taking iron-loving, siderophile elements along with it. So, any of these metals present on the surface arrived on objects that hit our planet after its core formed.
Together, all of these revealed a relationship that works like a “geologic clock” to date the moon-forming event. Basically, when was the last time Earth was completely molten?
Their calculations show that the moon formed between 63 million and 127 million years after the beginning of the solar system. It seems like wide range, yes, but it rules out an early moon-forming event.
Plus, this is the first geologic clock of early solar system history that doesn’t rely on interpretations of the radioactive decay. “We were excited to find a ‘clock’ for the formation time of the Moon that didn’t rely on radiometric dating methods. This correlation just jumped out of the simulations and held in each set of old simulations we looked at,” Jacobson says in a news release.
The work was published in Nature earlier this week.
Image: NASA/Don Davis