Under the giant impact hypothesis, the Moon formed when a Mars-sized object (called Theia) collided with Earth a billion years ago, at a time when Earth was still a ball of magma. This event not only led to the Earth-Moon system that we recognize today, but is also believed to have led to the differentiation of Earth’s central region into a molten outer core and a solid inner core.
However, there has been an ongoing debate about the timing of this impact and how long the Moon’s subsequent formation took place. According to a new study by a team of German researchers, the Moon formed from an ocean of magma that took up to 200 million years to solidify. This means that the Moon ended up forming about 4.425 million years ago, or 100 million years later than previously thought.
The study, which recently appeared in the magazine. Scientific advances (titled “A long-lived magma ocean on a young Moon”), it was conducted by planetary geophysicists from the German Aerospace Center (DLR), along with researchers from the Technical University of Berlin and the Institute of Planetology at the University of Münster.
When the Earth was still in the process of forming about four and a half billion years ago, the Solar System was a rather chaotic place. At that time, planetesimals that had also formed from the protoplanetary disk were shaken and occasionally collided with a planet. In the case of Earth, this had the effect of increasing its mass and causing its central region to become denser and hotter.
Over time, heavier elements sank into the center of the Earth, leading to the formation of its iron-nickel core. At the same time, larger and larger parts of Earth’s mantle melted to form an ocean of magma. When Theia collided with Earth, this ocean reached several thousand kilometers deep and much of it was ejected into space. This material was either reabsorbed by Earth or orbited around it to form the Moon.
While most scientists today believe that this is an exact scenario of how the Moon formed, there has been disagreement over the details and timing of the process. As Maxime Maurice, DLR researcher and lead author of the study said:
“The results of our latest model suggest that young Earth was hit by a protoplanet about 140 million years after the birth of the Solar System 4,567 million years ago. According to our calculations, this happened 4,425 million years ago, with an uncertainty of 25 million years, and the Moon was born. “
One of the reasons why there has been disagreement about the Big Impact Hypothesis has to do with what inspired it in the first place: the moon rocks. Basically of all the lunar rocks brought to Earth by the six Apollo missions and the three soviets Moon landers, none provide a direct record of the Moon’s age. As a result, scientists have had to rely on indirect methods to obtain estimates of the lunar age.
Furthermore, the energy obtained by the accumulation of material also led to the formation of an ocean of magma on the Moon, which covered the entire surface and was over 1000 km (620 miles) deep at that time. Also like Earth, this ocean of magma began to cool and crystallize rapidly to form a crust, which had the effect of isolating the magma below it and slowing down the cooling process.
Until now, scientists have been unable to determine how long it took for the ocean of magma to fully crystallize, making it difficult to determine when the Moon originally formed. To arrive at their estimates, Maurice and his colleagues used a new computer model that comprehensively considered the processes involved in magma solidification, similar to what Maurice did (albeit in greater detail) for his Ph.D. thesis.
This consisted of calculating how the composition of silicate minerals in the Moon’s crust that are rich in magnesium and iron (which formed during the solidification of magma) changed over time. What they found was evidence that as the solidification progressed, there was a drastic change in the composition of the remaining magma ocean.
This finding allowed the team to link the formation of different types of rocks on the Moon to a certain stage in the solidification process. Ultimately, this led them to conclude that the Moon’s ocean of magma took nearly 200 million years before it completely solidified to form the Moon’s crust. This contradicts what scientists previously thought, which was that it only took 35 million years to solidify.
As summarized by Sabrina Schwinger, DLR researcher and co-author of the study:
“By comparing the measured composition of the Moon’s rocks to the predicted composition of the magma ocean in our model, we were able to trace the evolution of the ocean back to its starting point, the time the Moon formed.”
Last but not least, the results of this study are consistent with previous age estimates obtained with the uranium-lead method. Based on the rate at which uranium decays to lead, scientists discovered that Earth’s metal core ended up forming at roughly the same time. Therefore, this study is the first to directly link the age of the Moon to an event that occurred at the end of Earth’s formation.
As always, determining how a body formed in the Solar System can shed light on how others did too. And when it comes to the Earth-Moon system, the origins of one are inextricably linked to the other.
Further reading: DLR, Scientific advances
