Earth's other moons
A new theory says ‘moonfalls’ may have formed our planet’s first continents
The small satellites that played a big part in our existence
mini-moons may have battered Earth with debris in its formative years, shaping the young planet and maybe even building the first continent. That’s the theory put forward by a new study which turns the ‘giant-impact hypothesis’ on its head.
In the well-established giant-impact theory, a mars-sized rocky object called Theia smashed into what would become Earth around 100 million years after the solar system was formed. The giant collision spewed debris up into space forming a disc of debris, some of which gradually came together to form our moon.
This idea was initially put forward in 2012, and then four years later, researchers at the University of California, Los Angeles (UCLA) published new evidence to cement the theory. While it was previously thought that the collision, which took place around 4.5 billion years ago, was a powerful, angled side-swipe by the giant rock Theia, the new evidence confirmed it was likely to have been a violent, head-on smash. The team came to this conclusion after analysing seven rocks brought back to Earth by the Apollo 12, 15 and 17 lunar missions, along with six volcanic rocks from the Earth’s mantle. A shared chemical signature in the oxygen atoms both in the moon rocks and the Earth rocks made it likely that a head-on collision had occurred, the researchers concluded.
"Theia was thoroughly mixed into both the
Earth and the moon, and evenly dispersed between them," said Edward Young, UCLA professor and lead author of the study, speaking at the time of its publication. "This explains why we don't see a different signature of Theia in the moon versus the Earth.” The study explains that the alternative, a glancing side blow from Theia, would have meant that the vast majority of the moon would have been formed from Theia, and that the Earth and moon would have different chemical ‘fingerprints’.
However, a series of recent studies argue that the moon wasn’t formed by a single collision alone, but rather a series of impacts. The latest research, which at the time of writing had been published online and has been accepted for publication
in the Monthly Notices of the Royal Astronomical
Society, suggests that multiple impacts on Earth would have blown debris back into orbit, which then came together to form our moon, along with lots of smaller mini-moons or ‘moonlets’. Complex movement between these moonlets would have slowly changed their orbits, gradually making them more elliptical. many of them would then have crashed down on to the proto-Earth, battering the fledgling planet time after time.
These ‘moonfalls’ would have caused a build-up of material in localised spots, leading to the formation of topographical features and potentially even the Earth’s first continent. The researchers were able to demonstrate the hypothesis using a series of simulations. The study was based on smoothedparticle hydrodynamical (sPH) simulations, where a computer is used to reproduce processes such as star formation and meteor impacts.
This new theory was put forward as the researchers believe that the widely accepted idea of how the moon was formed doesn’t quite add up.
“The current [giant-impact] paradigm is intrinsically incomplete and disconnected from the wider picture of terrestrial planet formation in which the proto-Earth had experienced and grown though multiple planetary-scale impacts,” co-author of the new study Hagai Perets from the Technion Israeli Institute of Technology explains to All About
Space. “Consideration of only the last such impact in the current paradigm disregards the critical evolution taking place prior to – and possibly following – this event.”
That’s why the researchers investigated the new scenario in which Earth’s moon may be the result of a merger of lots of mini-moons, and where Earth’s geophysical and geochemical make-up was altered by the multiple-impact evolution of the moon.
“The current paradigm is challenged by several major difficulties, as mentioned in this and previous papers,” says Perets. “Generally, the multiple-impact model we suggest naturally connects Earth's moon formation with the global formation of the solar system, and potentially constrains it. These issues and challenges call for a paradigm shift, and motivate the novel conceptual framework we propose. The current paper on Earth-moon collisions is one piece in this new model, and we are working on several other implications, such as studying moon-moon collisions.”
The new theory builds on research published last year from Perets and a different inter-university team which challenges the most prevalent theory of how the moon formed. It suggests that the moon that we see is not Earth’s first moon, but the latest in a series of rocky satellites, and that it was formed by a series of impacts rather than one big smash.
While the idea of one major collision is currently the accepted theory, the researchers behind the recent studies claim that this scenario requires very specific conditions, which are rare. They claim that the idea of a series of impacts is far more feasible.
“The multiple-impact theory is a more natural way to explain the formation of the moon,” Raluca Rufu from the Weizmann Institute of science tells
All About Space. “It does not require one single and specific impact, but rather incorporates all the possible impactors Earth experienced during the late accretion stage [when Earth was formed]. Each moonlet accretes from a different debris disc and eventually merges with previous existing moonlets at low velocities, where the mixing between the two components is not efficient. This can explain some of the observed lunar heterogeneities in the moon's interior,” said Rufu, lead author of the research study, published in Nature Geoscience last year.
The authors admit that there are limitations to the new multiple-impact hypothesis, largely that it is based on a limited dataset, making it harder to model possible collisions.
“In the current model we explore the implications of moonfalls on the Earth using simulations of the impacts themselves, making use of a hydrodynamical code,” explains Perets. “We consider a grid of models for the initial properties of the impact [position of the impact, size of the impacting moon, rotation of the proto-Earth] motivated by the study of dynamics leading to the infalls. This is only one piece in the overall model of the multipleimpacts theory. The main limitations are in direct
“The current [giant-impact] paradigm is intrinsically incomplete and disconnected from the wider picture of terrestrial planet formation”
Prof Hagai Perets
“Understanding the formation of the moon can provide insights on the environment of the early solar system and help us understand satellite formation”
Raluca Rufu
In may, the China national space Administration (CnsA) launched a relay satellite which paves the way for this historic mission to the far side of the moon. The satellite will provide a way for an upcoming lunar rover to communicate with Earth. Because the moon is tidally locked to Earth we only ever see its near side, which is where all of the Apollo lunar landings were made. If successful, China’s Chang'e 4 robotic orbiter-lander-rover would be the first spacecraft to ever touch down on the moon’s far side. The mission is currently scheduled for launch in December 2018, so new insights on the moon formation theory could possibly be gleaned as early as next year.
The idea of impacts leading to the formation of topographic features on planets’ surfaces isn’t new. The latter stages of planet formation are widely thought to be characterised by extremely violent, catastrophic collisions, and features such as mountains can be seen on planets throughout the solar system. But some experts disagree that the falling mini-moons would have formed any such features on Earth.
As the Earth took around 100 million years to cool from molten magma into a solid sphere, it is questionable whether mini-moons falling during that time would have left any mark on the molten planet. However, the researchers argue that the moonlets would have struck Earth after much of the planet had solidified into a crust and that the impacts would have been gentle enough to remain on the surface, rather than smash through.
While not yet conclusive, the idea of multiple strikes on Earth producing a series of moonlets which then rained down on Earth is certainly a compelling alternative to the accepted theory. But why is it so important?
“The latter stages of accretion, named the giantimpact phase, set the final architecture of the solar system, and the composition of the final planets and their satellites,” explains Rufu. “Understanding the formation of the moon can provide insights on the environment of the early solar system and it may help us understand whether satellite formation in general is abundant or whether it requires unique impact conditions.
“moreover, due to its large size, the moon stabilises Earth’s tilt and provides a somewhat stable climate for life to evolve. If detecting exomoons [satellites that orbit planets in other satellite systems] will be possible in the near future, then it may be more beneficial to look for life around planets with large satellites. of course, this is somewhat speculative, as we know of only one planet that harbours life and it happens to have a large satellite.”
A better understanding of the impacts that shaped the evolution of Earth could give us vital insights into how life-bearing planets form. In turn, this could help us track down potentially habitable planets in future, and maybe even ensure the endurance of the human race for generations to come. so, if the new theory is correct, mini-moons may well have shaped not only our past, but also our distant future.