The Moon is much older than previously thought. Its crust was reheated after its formation - and has thus misled researchers in determining its age
After its formation, the Moon may have been the scene of such immense volcanic activity that its entire crust melted several times and was completely churned through. At that time, the Moon orbited significantly closer to Earth than today. The resulting tidal forces heated up its interior and thus powered the violent volcanism. Only Jupiter's moon Io, by far the most volcanically active body in the Solar System, offers comparable conditions. The new considerations presented today in the journal Nature by an international team of researchers from the University of California Santa Cruz, the Max Planck Institute for Solar System Research (MPS) and the Collège de France resolve previous contradictions and inconsistencies regarding the age of the Moon. According to the researchers, the Moon was formed between 4.43 and 4.51 billion years ago. Its crust, however, appears around 80 to 160 million years younger.
A few hundred million years after its formation, the Moon was subject to intense volcanic activity. The distance between the Earth and the Moon was much smaller back then than it is today.
© MPS/Alexey Chizhik
The Moon is apparently quite reluctant to reveal its age. Attempts to uncover its secret have yielded estimates that lie several hundred million years apart: While some researchers suggest that our cosmic companion was formed 4.35 billion years ago, others date its birth to 4.51 billion years ago. One of the most striking inconsistencies is of a stony nature: almost all lunar rock samples point to the younger age. But a few rare crystals of zirconium silicate, known as zircons, are significantly older. How is this possible? In the current study, the researchers succeed in resolving this contradiction. According to their calculations, the Moon's crust was mostly melted again after its formation; only a few zircons were able to withstand these extreme conditions unchanged.
A collision and its consequences
The history of the Moon begins with a massive collision. In the early days of the Solar System, a chunk about the size of Mars crashed into the still young Earth. The collision generated so much heat that our planet melted completely and hurled a huge amount of material into space. By and by this material clumped together to form the Moon, initially covered by a huge ocean of hot, liquid rock. In the millions of years that followed, the newly formed body cooled and moved further and further away from the Earth until it reached its current orbit at a distance of around 384400 kilometers.
"We are particularly interested in the phase when the distance between the Earth and the Moon was about one third of today's distance," explains Francis Nimmo from the University of California Santa Cruz, first author of the new study. At this time, there were various changes in the position and shape of the Moon's orbit. Among other things, it became more elliptical, so that the speed of the Moon and its distance from the Earth varied noticeably within each orbit. The forces acting in this way "churned" through the Moon's interior to such an extent that it heated up. A similar situation is still known today from Jupiter's moon Io, which travels around the gas giant also in a slightly elliptical orbit. The enormous tidal forces of Jupiter make the small moon the most volcanically active body in the Solar System. Earth's early Moon was likely a match even to Io.
As the researchers' calculations show, the heat flow from the lunar interior was sufficient to melt and churn through the entire mantle. While at no time during this phase a magma ocean covered the entire Moon, over the course of several million years the heat from the interior gradually reached every part of the surface liquifying most of the crustal rock - possibly even several times. In some places the hot lava penetrated to the surface, in others magma was injected beneath the surface, heating the rocks around it.
Resetting the geological clock
This volcanic history is decisive for determining the age of the crustal rock. Since their formation, lunar rocks (like terrestrial rocks) contain radioactive isotopes. Isotopes are variations of certain atoms that differ only in the number of neutrons in the atomic nucleus. As the isotopes' decay times are known, it is possible to deduce the age of the rock from their current concentration. The decisive factor: As long as the rock is hot, it can exchange isotopes with its surroundings. When it cools, it locks in its composition. The trapped radioactive isotopes begin to decay - and the geological clock starts ticking.
"The strong volcanism likely reset the Moon's geological clock," explains Thorsten Kleine, Director at the MPS and co-author of the study. "Lunar rocks samples therefore don't reveal their original age, but only when they were last strongly heated", he adds. Only a few heat-resistant zircons provide evidence of the more distant past, the researchers show in their calculations. In some places where the lava did not reach the surface, the zircon grains remained cool so that their internal clock was not affected.
"The lunar rock samples tell us the Moon's entire, turbulent history. They tell us about its formation and its later violent volcanism. Until now, we just didn't read these clues correctly," says Kleine. According to the researcher's results, the Moon itself is between 4.43 and 4.51 billion years old. The violent volcanism shaped its crust around 4.35 billion years ago.
The solution to the riddle
The new findings also resolve many other contradictions that had previously puzzled scientists. For example, the comparatively few craters on the Moon argued against its old age. In such a long time, our cosmic neighbor should have witnessed more impacts. Volcanism now offers an explanation. "Lava from the Moon's interior could have filled the early impact basins and thus made them unrecognizable," says co-author Alessandro Morbidelli from the Collège de France. The composition of the lunar mantle posed another puzzle for researchers. This is the layer of rock that lies directly beneath the Moon's crust. Its list of ingredients differs from that of the Earth in key respects. However, if the Moon's interior was molten a second time around, some substances could have escaped from the mantle into the iron core below. "The new results mean that all the pieces of the puzzle that previously didn't fit together now form a coherent overall picture of the Moon's formation," says Kleine.
