10. July 2020
Earth's natural satellite almost 100 million years younger than previously thought

A slight­ly younger Moon

Magma ocean and first rocky crust on the Moon
Mag­ma ocean and first rocky crust on the Moon
Image 1/7, Credit: NASA/Goddard Space Flight Center

Magma ocean and first rocky crust on the Moon

When the Moon formed in­to a sphere ap­prox­i­mate­ly 1700 kilo­me­tres in ra­dius 4.425 bil­lion years ago, its in­te­ri­or heat­ed up con­sid­er­ably due to the en­er­gy re­leased when it ac­cret­ed. The rock melt­ed and an ocean of mag­ma, pos­si­bly more than 1000 kilo­me­tres deep, formed. Lat­er, light rocks crys­tallised, which rose to the sur­face and formed a first crust on the Moon. This crust in­su­lat­ed the Moon from space, and the mag­ma ocean be­neath it cooled down slow­ly. Two hun­dred mil­lion years would pass be­fore the Moon com­plete­ly so­lid­i­fied.
The Moon is born
The Moon is born
Image 2/7, Credit: Ron Miller

The Moon is born

The plan­ets in the So­lar Sys­tem de­vel­oped with­in a few tens of mil­lions of years af­ter its for­ma­tion, 4.567 bil­lion years ago. Dur­ing that time, many pro­to­plan­ets were still wan­der­ing through the young So­lar Sys­tem, in some cas­es on a col­li­sion course with the young plan­ets. Earth was al­so hit by one of these pro­to­plan­ets 4.425 bil­lion years ago. This led to the birth of the Moon, the tim­ing of which sci­en­tists from the Ger­man Aerospace Cen­ter (DLR) and the Uni­ver­si­ty of Mün­ster have now been able to de­ter­mine us­ing mod­el cal­cu­la­tions. This means that the Moon is slight­ly younger than pre­vi­ous­ly thought. The im­pact­ing pro­to­plan­et could have been the size of Mars and caused the ejec­tion of enor­mous amounts of rock from Earth’s man­tle, some of which even evap­o­rat­ed, in­to space. The Moon was formed from this ma­te­ri­al in the course of a few thou­sand years.
Schematic structure of the early Moon’s interior
Schemat­ic struc­ture of the ear­ly Moon’s in­te­ri­or
Image 3/7, Credit: DLR/Maxime Maurice

Schematic structure of the early Moon’s interior

Af­ter the Moon was formed 4.425 bil­lion years ago, heavy ma­te­ri­al from the so­lid­i­fy­ing mag­ma ocean set­tled. The heav­i­est com­po­nents formed a metal­lic core, and the lighter ones a sil­i­cate man­tle. Such rocks formed from the mag­ma ocean are al­so called cu­mu­lates and are heav­ier than the re­main­ing melt. The ex­cep­tion – in a late phase of evo­lu­tion of the mag­ma ocean the min­er­al pla­gio­clase, an alu­mini­um-cal­ci­um-sil­i­cate, which be­longs to the group of feldspars, was formed. Pla­gio­clase was lighter than the melt and rose to the mag­ma ocean’s sur­face, where it formed the ear­ly lu­nar crust. The Moon cooled down through var­i­ous heat trans­port pro­cess­es, in­clud­ing con­vec­tion in the mag­ma ocean and in the sol­id lu­nar man­tle, but al­so through the trans­port of melts from the man­tle cu­mu­lates through mag­mat­ic heat chan­nels.
Anatomy of the early Moon
Anato­my of the ear­ly Moon
Image 4/7, Credit: DLR/Maxime Maurice

Anatomy of the early Moon

This mod­el shows the in­te­ri­or of the Moon at an ear­ly stage, when its com­po­nents be­gan to so­lid­i­fy. The float­ing crust (grey) formed a ther­mal­ly in­su­lat­ing lay­er over the re­main­ing mag­ma ocean (yel­low) and the still par­tial­ly molten lu­nar man­tle be­low. The cool­ing of the lu­nar man­tle oc­curred both by con­vec­tion, that is by the cir­cu­la­tion of ma­te­ri­al in huge cells (seen here as blue-white down­ward cur­rents), and by the ris­ing of man­tle melt, which were formed lo­cal­ly by con­vec­tion. This hot mag­ma mixed in­to the re­main­ing mag­ma ocean and slowed down its cool­ing. Due to this pro­cess and the ad­di­tion­al in­su­la­tion pro­vid­ed by the crust, it took about 200 mil­lion years for the mag­ma ocean to com­plete­ly so­lid­i­fy.
Rock and dust samples from the Moon
Rock and dust sam­ples from the Moon
Image 5/7, Credit: NASA/JSC

Rock and dust samples from the Moon

One of the most im­por­tant sci­en­tif­ic tasks of the six moon land­ings that took place be­tween 1969 and 1972 was the col­lec­tion of lu­nar rock and re­golith sam­ples. The goal was to find out how old the Moon is and how it evolved. The anal­y­sis of lu­nar sam­ples has shown that the bright com­po­nents of the lu­nar sur­face rep­re­sent the pri­ma­ry lay­er of lighter rocks such as anorthosite. These rocks are over four bil­lion years old and have been re­dis­tribut­ed mul­ti­ple times due to the im­pact of as­ter­oids. The dark ar­eas vis­i­ble to the naked eye, how­ev­er, orig­i­nat­ed from iron- and mag­ne­sium-rich vol­canic ac­tiv­i­ty that did not be­gin un­til sev­er­al hun­dred mil­lion years lat­er. The im­age shows Alan Bean, the Lu­nar Mod­ule Pi­lot of Apol­lo 12, the sec­ond mis­sion to land on the Moon, with a dust-filled sam­ple cap­sule. Com­man­der Charles Con­rad is re­flect­ed on the vi­sor.
One of the oldest Moon rocks
One of the old­est Moon rocks
Image 6/7, Credit: NASA/JSC/AACO

One of the oldest Moon rocks

For the Apol­lo 16 mis­sion, a lo­ca­tion in the cen­tral high­lands of the lu­nar near side was se­lect­ed. One goal was to bring sam­ples from the rocks found there, the old­est on the Moon, to Earth. The ob­jec­tive was to de­ter­mine when the Moon orig­i­nat­ed and when its first crust was formed. Sam­ple 60025 is an iron-rich anorthosite and one of the three old­est rocks brought to Earth by the six Apol­lo mis­sions. It is a feldspar rock with a high con­tent of alu­mini­um and cal­ci­um that so­lid­i­fied 4.36 bil­lion years ago, re­flect­ing the crys­talli­sa­tion of an ear­ly crust cov­er­ing the mag­ma ocean. As­tro­nauts John Young and Charles Duke picked up the rock just 14 me­tres from the Lu­nar Mod­ule. It is al­most 20 cen­time­tres across and weighs 1836 grams. It is coarse-grained and a small sec­tion is coat­ed by black ‘glass’, which so­lid­i­fied very quick­ly as a melt af­ter an im­pact event and did not form any crys­tals.
The Moon over Earth – extremes of planetary evolution
The Moon over Earth – ex­tremes of plan­e­tary evo­lu­tion
Image 7/7, Credit: Rolf Hempel

The Moon over Earth – extremes of planetary evolution

The Moon has been or­bit­ing Earth for al­most four-and-a-half bil­lion years. The two bod­ies rep­re­sent two ex­tremes of plan­e­tary evo­lu­tion. The Moon was formed 4.425 bil­lion years ago from ma­te­ri­al eject­ed from Earth dur­ing a mas­sive col­li­sion with a pro­to­plan­et. It sub­se­quent­ly de­vel­oped a mag­ma ocean more than 1000 kilo­me­tres deep on which a crust formed. Be­neath the crust, the man­tle rock so­lid­i­fied over the course of 200 mil­lion years. Earth al­so had a mag­ma ocean, but did not form a float­ing crust. Once the Moon had so­lid­i­fied, mag­ma formed in its in­te­ri­or and rose to the sur­face. How­ev­er, vol­canic ac­tiv­i­ty came to a halt three bil­lion years ago and the face of the Moon has hard­ly changed since then.
  • Earth's natural satellite was formed from debris created by a collision between the young Earth and a protoplanet.
  • During this process, the Moon heated up so much that a magma ocean over 1000 kilometres deep formed, which then took up to 200 million years to solidify.
  • Using a new numerical model, scientists from DLR and the University of Münster have been able to link these events with the time of the Moon's formation.
  • They found that the Moon formed 4.425 billion years ago, almost 100 million years later than previously thought.
  • Focus: Planetary research, planetary geophysics, modelling, space

The Moon formed a little later than previously assumed. When a Mars-sized protoplanet 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 at the University of Münster have used a new numerical model to reconstruct the time at which the event occurred – 4.425 billion years ago. The previous assumptions about the formation of the Moon were based on an age of 4.51 billion years – that, is 85 million years earlier than the new calculations reveal. The scientists report on this today in the scientific journal Science Advances.

Four-and-a-half billion years ago, the Solar System was still a rather chaotic place. Earth was still growing to its present size, collecting matter in the form of what are referred to as ‘planetesimals’. These had previously formed in the disc of dust and gas orbiting the early Sun. The young Earth consolidated, becoming ever hotter inside. Increasingly large parts of the rocky mantle melted and formed a magma ocean. 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 rock being ejected from the young Earth. Eventually, this debris agglomerated to form a new planetary body – the Moon.

In principle, most scientists agree about how the Moon formed, but not about the details of the process and especially not about the time at which it occurred. "The results of our latest modelling suggest that the young Earth was hit by a protoplanet some 140 million years after the birth of the Solar System 4.567 billion years ago," says Maxime Maurice, summarising the team's investigations. "According to our calculations, this happened 4.425 billion years ago – with an uncertainty of 25 million years – and the Moon was born."

At that time, Earth had just developed into a planet. During this development, the heavy, metallic components sank to its centre and formed a core of iron and nickel, which was surrounded by a thick mantle of silicate rocks. The mantle rocks became hotter and hotter due to the process of 'accretion' – the agglomeration of matter – and through heat from the decay of radioactive elements. This allowed the separation of metals and silicates to take place in Earth's interior within a few tens of millions of years.

A planetary bullseye caused the formation of the Moon

At this stage, Earth was hit by Theia, 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 bodies of this kind. Some were ejected from the Solar System, while others were destroyed by collisions with other bodies. Theia, however, hit Earth and caused the ejection of such a large amount of material from the planet’s mantle that the Moon was able to form from it. During this violent impact, a several thousand kilometre deep magma ocean of glowing hot, molten rock formed. Today, no traces of Theia remain following this collision.

Reconstructing how the formation of the Moon was triggered by this event requires a great deal of imagination and creativity. The collision of the two bodies, with its enormous energy, also vaporised a large amount of rock from Earth's early mantle. This was ejected and collected in a ring of dust around Earth before it reassembled there to form rock. "From this, the Moon was formed in a short time, probably in just a few thousand years," explains Doris Breuer, Head of the Planetary Physics Department at the DLR Institute of Planetary Research and a co-author of the study.

The oldest Moon rock is not old enough

Scientists largely agree about 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 the astronauts of the six Apollo missions and the three Soviet Luna robotic missions directly record the age of Earth's natural satellite. Researchers from DLR and the University of Münster have determined when the Moon was formed using a new, indirect method. "Our calculations show that this most likely happened at the very end of Earth's formation," says Sabrina Schwinger, another co-author of the study, describing the chronological sequence of events.

It was not only Earth that had an ocean of magma in its early youth. Energy gained from accretion also led to the formation of a magma ocean on the Moon. The Moon melted almost completely and, similarly to Earth, was covered by a magma ocean over 1000 kilometres deep. This magma ocean quickly began to solidify and formed a crust of floating, lightweight crystals at the surface – its 'interface' with the cold space. But under this insulating crust, which slowed down the further cooling and solidification of the magma ocean, the Moon remained molten for a long time. Until now, scientists were unable to determine how long it took for the magma ocean to crystallise completely, which is why they could not conclude when the Moon originally formed.

To calculate the lifetime of the Moon’s magma ocean, the scientists used a new computer model, which for the first time comprehensively considered the processes involved in the solidification of the magma. “The results from the model show that the Moon’s magma ocean was long-lived and took almost 200 million years to completely solidify into 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 advisor of Maxime Maurice’s PhD thesis, which was the base for this condensed scientific report. “Older models gave a solidification period of only 35 million years.”

Solidification models reveal the age of the Moon and Earth

To determine the age of the Moon, the scientists had to go one step further. They calculated how the composition of the magnesium- and 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 rock on the Moon to a certain stage in the evolution of its magma ocean. “By comparing the measured composition of the Moon’s rocks with the predicted composition of the magma ocean from our model, we were able to trace the evolution of the ocean back to its starting point, the time at which the Moon was formed,” explains Sabrina Schwinger.

The results of the study show that the Moon was formed 4.425±0.025 billion years ago. The Moon’s exact age is in remarkable agreement with an age previously determined for the formation of Earth’s metallic 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 very end of the Earth's formation, namely the formation of the core,” says Thorsten Kleine from the Institute of Planetology at the University of Münster.

Funding:

The work was carried out within the framework of the Transregional Collaborative Research Center TRR 170 'Late Accretion on Terrestrial Planets' and the Helmholtz Young Investigators Group 'Early dynamics of the terrestrial planets' and was funded by the German Research Foundation and the Helmholtz Association of German Research Centres.

Original publication:

M. Maurice, N. Tosi, S. Schwinger, D. Breuer, T. Kleine (2020). A long-lived magma ocean on a young Moon. Science Advances; DOI: LINK

Contact
  • Falk Dambowsky
    Ed­i­tor
    Ger­man Aerospace Cen­ter (DLR)
    Me­dia Re­la­tions
    Telephone: +49 2203 601-3959
    Fax: +49 2203 601-3249
    Linder Höhe
    51147 Cologne
    Contact
  • Maxime Maurice
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Plan­e­tary Re­search
    Plan­e­tary physics
    Telephone: +49 30 67055-9129
    Rutherfordstraße 2
    12489 Berlin
    Contact
  • Prof.Dr. Doris Breuer
    Ger­man Aerospace Cen­ter (DLR)

    In­sti­tut of Plan­e­tary Re­search, Plan­e­tary Physics
    Telephone: +49 30 67055-301
    Fax: +49 30 67055-303
    Linder Höhe
    51147 Köln
    Contact
  • Nicola Tosi
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Plan­e­tary Re­search
    Plan­e­tary physics
    Telephone: +49 30 67055-365
    Rutherfordstraße 2
    12489 Berlin
    Contact
  • Dr. rer. nat. Sabrina Schwinger
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Plan­e­tary Re­search
    Plan­e­tary physics
    Telephone: +49 30 67055-630
    Rutherfordstraße 2
    12489 Berlin
    Contact
  • Ulrich Köhler
    Pub­lic re­la­tions co­or­di­na­tor
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Plan­e­tary Re­search
    Telephone: +49 30 67055-215
    Fax: +49 30 67055-402
    Rutherfordstraße 2
    12489 Berlin
    Contact

Cookies help us to provide our services. By using our website you agree that we can use cookies. Read more about our Privacy Policy and visit the following link: Privacy Policy

Main menu