The moon is born. Credit: Ron Miller
- Earth’s natural satellite was formed from debris created by a collision between young Earth and a protoplanet.
- During this process, the Moon became so hot that an ocean of magma more than 1,000 kilometers deep formed, which then took 200 million years to solidify.
- Using a new numerical model, the DLR scientists and the University of Münster They have been able to link these events to the time of the Moon’s formation.
- They discovered that the Moon formed 4,425 million years ago, almost 100 million years later than previously thought.
- Focus: planetary research, planetary geophysics, modeling, space
The Moon formed a little later than previously assumed. When a Marsof large protoplanets was destroyed in a collision with the young Earth, a new body was created from the debris ejected during this catastrophe: the Moon. Planetary geophysicists at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), led by Maxime Maurice, together with researchers from the University of Münster, have used a new numerical model to reconstruct the time when the event occurred: 4,425 millions of dollars. years ago. Previous assumptions about the Moon’s formation were based on an age of 4.51 billion years, that is, 85 million years earlier than the new calculations reveal. Scientists report on this today in the scientific journal Science Advances.
Magmatic ocean and first rocky crust on the Moon. Credit: NASA / Goddard Space Flight Center
Four and a half billion years ago, the Solar System was still a rather chaotic place. Earth was still growing to its current size, collecting matter in the form of what is known as ‘planetesimals’. These had previously formed in the dust and gas disk orbiting the early Sun. The young Earth consolidated, becoming hotter inside. Increasing parts of the rocky mantle melted and formed an ocean of magma. It is at this time that Earth gained the natural satellite that continues to orbit around it to this day. A massive cosmic collision between Earth and a protoplanet resulted in the ejection of rocks from the young Earth. Eventually, these debris clumped together to form a new planetary body: the Moon.
Schematic structure of the interior of the early moon. Credit: DLR / Maxime Maurice
In principle, most scientists agree on how the Moon formed, but not on the details of the process and especially on when it happened. “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,” says Maxime Maurice, summarizing the team’s research. “According to our calculations, this happened 4,425 million years ago, with an uncertainty of 25 million years, and the Moon was born.”
At that time, Earth had just become a planet. During this development, the heavy metal components sank into its center and formed a core of iron and nickel, which was surrounded by a thick blanket of silicate rocks. The rocks in the mantle became increasingly hotter due to the ‘accretion’ process, the agglomeration of matter, and through the heat of the decomposition of the radioactive elements. This allowed the separation of metals and silicates inside the Earth in a few tens of millions of years.
Early moon anatomy. Credit: DLR / Maxime Maurice
A planetary target caused the Moon to form.
At this stage, Theia hit Earth, a protoplanet that was perhaps the size of Mars. Theia was one of the Titans in Greek mythology, and the mother of the moon goddess Selene. In the early days of the Solar System, there would have been many such bodies. Some were ejected from the Solar System, while others were destroyed by collisions with other bodies. However, Theia hit Earth and caused such a large amount of material to be ejected from the planet’s mantle that the Moon could form from it. During this violent impact, an ocean of magma several thousand kilometers deep from molten, hot rock formed. Today, there are no traces of Theia after this collision.
Rock and dust samples from the Moon. Credit: NASA / JSC
Reconstructing how the Moon’s formation was triggered by this event requires a great deal of imagination and creativity. The collision of the two bodies, with their enormous energy, also vaporized a large amount of rock from Earth’s early mantle. This was ejected and collected in a ring of dust around Earth before reassembling there to form rock. “From this, the Moon formed in a short time, probably in a few thousand years,” explains Doris Breuer, Head of the Department of Planetary Physics at the DLR Institute for Planetary Research and co-author of the study.
The oldest moon rock is not old enough
Scientists largely agree on the history of the Moon’s formation. However, they have not been able to date it exactly, as none of the moon rocks brought to Earth by astronauts from the six Soviet Apollo missions and the three Soviet moon robotic missions directly record the age of Earth’s natural satellite. Researchers from DLR and the University of Münster have determined when the Moon formed using a new indirect method. “Our calculations show that this probably occurred at the end of Earth’s formation,” says Sabrina Schwinger, another co-author of the study, who describes the chronological sequence of events.
One of the oldest moon rocks. Credit: NASA / JSC / AACO
It was not just Earth that had an ocean of magma in its early youth. The energy obtained from the accumulation also led to the formation of an ocean of magma on the Moon. The Moon almost completely melted and, similar to Earth, was covered by an ocean of magma more than 1,000 kilometers deep. This ocean of magma quickly began to solidify and formed a crust of floating, lightweight crystals on the surface, its ‘interface’ with cold space. But under this insulating crust, which slowed the cooling and solidification of the ocean of magma, the Moon remained molten for a long time. Until now, scientists couldn’t determine how long it took for the magma ocean to fully crystallize, so they couldn’t conclude when the Moon originally formed.
To calculate the life span of the Moon’s ocean of magma, scientists used a new computer model, which for the first time comprehensively considered the processes involved in magma solidification. “The model results show that the Moon’s ocean of magma was long-lasting and took almost 200 million years to fully solidify in the mantle rock,” says Maxime Maurice. “The time scale is much longer than calculated in previous studies,” adds DLR colleague Nicola Tosi, second author of the study and adviser to Maxime Maurice’s doctoral thesis, which was the basis of this condensed scientific report. “The oldest models gave a solidification period of only 35 million years.”
Solidification models reveal the age of the Moon and Earth
To determine the moon’s age, scientists had to go one step further. They calculated how the composition of the magnesium-iron-rich silicate minerals that formed during the solidification of the magma ocean changed over time. The researchers discovered a drastic change in the composition of the remaining magma ocean as solidification progressed. This finding is significant because it allowed the authors to link the formation of different types of rocks on the Moon to a certain stage in the evolution of its magma ocean. “By comparing the measured composition of the rocks on the Moon with 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,” explains Sabrina Schwinger
The Moon on Earth: extremes of planetary evolution. Credit: Rolf Hempel
The study results show that the Moon formed 4,425 ± 0.025 billion years ago. The exact age of the Moon is in remarkable agreement with a previously determined age for the formation of Earth’s metal core with the uranium-lead method, the point at which the formation of planet Earth was completed. “This is the first time that the age of the Moon can be directly linked to an event that occurred at the end of Earth formation, that is, the formation of the nucleus,” says Thorsten Kleine of the University’s Institute of Planetology. from Münster
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Reference: “An ocean of long-lived magma on a young moon” by M. Maurice, N. Tosi, S. Schwinger, D. Breuer, and T. Kleine, July 10, 2020, Scientific advances.
DOI: 10.1126 / sciadv.aba8949
