April 3, 2014
A new study based on simulations of inner solar system planets' formation suggests the Moon was created about 95 million years after the solar system formed.
Artist's impression of the Theia collision with Earth that led to the formation of the Moon. Credit: NASA/JPL-Caltech
The leading theory on the Moon formation is that the Mars-sized planet called Theia collided with young Earth early during the formation of the solar system, leaving debris in our planet's orbit that later formed the Moon. Looking at other planets in the solar system and their natural satellites, the Moon is unusually large compared to its host planet, so it is unlikely that it was a planet "captured" by Earth.
More notably, recent studies show that the Moon and Earth have very similar chemical composition, meaning they formed from the same material. However, Theia would have most likely had a different chemical configuration. In order to explain this, the final theory has to explain when exactly did this Moon-forming impact occur so that it allowed Earth and the Moon to have such chemical composition.
A new study published in the journal Nature, led by Seth Jacobson of the Côte d'Azur Observatory in Nice, France, puts the age of the Moon at about 95 million years after the formation of the solar system.
"This [recent study of the Moon's chemical composition] means that at the atomic level, the Earth and the Moon are identical," said Jacobson. "This new information challenged the giant impact theory for lunar formation."
By running 259 simulations of formation of inner, rocky planets, Jacobson and his team discovered that the impact that created the Moon happened relatively late. With each previous such impact – there were many more before the Theia – amount of certain iron-associated or "sideophile" elements such as iridium and platinum decreased in Earth's mantle by bonding with iron and sinking to the core.
Since both Earth's mantle and the Moon are poor with these elements, it means that very little material accreted onto Earth after the Moon-forming impact. If it happened early after the formation of the solar system, there would be more sideophile elements present.
"A late Moon-forming event, as suggested by our work, is very consistent with an identical Earth and Moon," Jacobson said.
This is in accord with previous studies that suggest that Theia impact had higher speed and energy than previous impacts. As the protoplanetary disk that provided material for planet formation got older, it also rotated faster and had more large bodies already clumped together by gravity.
"Older disks tend to be dynamically more active, since there are fewer bodies left in the disk to distribute energy amongst," said Jacobson.
While this new study suggests a solution to the problem of different chemical composition of Theia compared to that of Earth and the Moon, it makes us question why Mars formed nearly 100 million years earlier.
"This means that Earth and Mars formed over dramatically different timescales, with Mars forming much faster than the Earth," Jacobson said. "How can this be? Is it just a matter of size? Location? What about Mercury and Venus? Did they grow on similar timescales to the Earth or on timescales more similar to Mars? I think these are some of the really important questions that we, as a community of planetary scientists, will be addressing in the future."