According to two new papers, our conventional models of the formation of planets may need to be updated.
Current planet formation theory uses the word accretion as its key ingredient. It is believed that the sun's leftover material, called the solar nebula (or the material after the Sun formed), formed the planets. This was done by accretion where smaller particles are accumulated into larger objects. These huge, boulder-sized objects, known as planetesimals were able to combine into larger entities through collisions and mergers. Through repeated collisions and mergers, four rocky planets eventually populated the inner Solar System.
New research shows that collisions occurred in a different way than previously thought. Objects collided several times with each other, in a series hit and run, before merging. This research fills in some gaps in our current understanding.
The Planetary Science Journal published the two papers. The first paper examines hit-and-runs during the late formation stages for Earth and Venus. It is called Collision Chains between the Terrestrial planets. II. II.
Researchers used 3D simulations of large impacts to create machine learning models based on these impacts. Hit-and-runs and collision chains were as common as accretion event in the latter stages of planetary formation for Earth and Venus, according to their findings. They also discovered that Earth was a sort of vanguard for Venus and helped to shepherd impactors into Venus.
The authors propose a hit-and-run-and-return model for the terrestrial planets, and have evidence to back it up. The authors claim that pre-planetary bodies would have crashed into each other, bounced off and then crashed into each other again. They would be more likely to stay together in subsequent collisions since the initial collision would have slowed their progress. The accretion model can be compared to a snowman who has each snowball stuck to him. However, it is more like billiards. Each collision occurs at a reduced velocity until the situation calms down.
Research shows that giant impacts do not result in efficient planet-forming events.
Our research shows that even slow giant impacts are often hit-and-runs. Erik Asphaug from the University of Arizona, co-author of LPL, stated that this means that in order for two planets merge, they must first slow down through a hit-and run collision. It is wrong to think of huge impacts like the formation of the Moon as one event. It was more likely that it required two collisions.
The first paper focuses on Venus, Earth and other sister planets. However, there are some interesting differences in the compositions of Earth and Venus, as well as their geology and satellite formation. Researchers believe they have the answer.
We believe that the early Earth was a vanguard for Venus during solar system formation.
The early Solar System was chaotic, with objects smashing into one another. New models show that Venus and Earth had an unusual relationship. According to them, Earth was a vanguard for Venus. Many objects that struck Earth bounced off them, sending them toward Venus at a lower speed. This allowed Venus to absorb more objects from the outer Solar System.
Asphaug stated that the Earth acts as a shield and provides a first stop to any impacting planets. Asphaug stated that it is more likely that a planet hitting Earth will hit Venus and merge with it. The Sun is closer to an object than it is so the chances of it staying there are higher. Venus is much closer to the Sun, so more objects stick to it after hitting Earth. Asphaug explained that an impactor colliding with Venus is quite happy to remain in the inner solar system and will eventually hit Venus again.
Earth, Venus and Mars. Late stage accretion theory states that Mars and Mercury (front left, right) are remnants of a population of colliding embryos. Venus and Earth were created in a series giant impacts. New research examines the preponderance hit-and run collisions in huge impacts and shows that proto Earth would have served to act as a vanguard slowing down large planet-sized bodies in hit -and-runs. It is proto-Venus that accretes them more often than not. This means it was easier for Venus and the outer solar systems to acquire bodies. Image credit: Lsmpascal Wikimedia Commons
However, Earth does not have a vanguard. Interloping objects from outer Solar System are not able to be stopped. Many objects bounced off the surface of the Sun. Because objects are attracted to the center gravity well, it is unlikely that they will ever encounter Earth again. They instead encounter Venus. This discrepancy may explain the differences between Venus, Earth and Venus. The authors explain that the runner is an identifiable remnant from a projectile in low-velocity hit and run situations (eg, a core with a mantle stripping core, sometimes just barely),
Emsenhuber stated that the dominant belief was that it didn't matter if planets collide but don't merge immediately, as they will run into each other again and merge later. This is not the case. They end up becoming more like Venus than they do returning to Earth. It is easier to travel from Earth to Venus than vice versa.
Most projectiles survive impact in hit-and-runs. However, its velocity and trajectory can be altered. Both bodies can remain gravitationally bound if the runner is slow enough. Researchers call this a "graze-and merge".
Four related conclusions were reached in the paper.
During the late stages of planetary formation, the terrestrial planets were not isolated from one another. Running runners that escape from a hit and run with one planet will likely collide with another planet. Because the projectile requires such high initial velocity, long collision chains are less probable. High velocity runners are also less likely to return. Earth was a vanguard for Venus. It slowed late-stage projectiles and sent them towards Venus. Earth only absorbed about half the projectiles that collided with them. The chances of runners from Earth colliding with Venus are approximately equal to those of returning to Earth. Venus still retains most of its runners.
The Moon
The formation of the Moon is the subject of the second paper. The title of the paper is Collision Chains among the Terrestrial Planets. III. Formation of the Moon. This article is also published in The Planetary Science Journal. Erik Asphaug of the University of Arizona's LPL is the lead author.
According to the dominant theory, Earth's young age was caused by Theia, a planet that formed about 4.5 billion year ago. Earth's core is larger than it should be for its size. This was due to Theia. The impact of the impact decimated Theia and much of its mass was put into orbit around Earth. It eventually merged into the Moon.
However, there are still some problems with this scenario. This scenario would require a very low collision velocity, as the Earth's and Moon's isotope compositions would almost be identical. One low-velocity impact would not allow the material to be mixed enough to produce similar isotope compositions.
Asphaug stated that the standard model for the Moon requires a slow collision. This creates a moon composed mostly of the proto-Earth and not the impacting planet. This is a problem because the moon's isotopic chemistry is almost identical to Earth's.
The authors explain in their paper that the graze-and merge collision causes a fraction of Theias' mantle to be thrown into orbit while Earth absorbs most of Theias momentum. O, Ti and Cr are all found in lunar rocks and Earth, so it is possible to have a Moon that derives its energy mostly from Theias.
A giant impact is believed to have caused the moon. A new theory suggests that there were two large impacts, separated by approximately 1 million years. They involved a proto-Earth and a Theia of Mars size. This image shows the hit-and run collision that was simulated in 3D. It is shown approximately one hour after impact. The iron cores are visible in a cutaway view. Theia, or most of it, barely escapes so a follow on collision is probable. Image Credit: A. Emsenhuber/University of Bern/University of Munich.
The new model by the team does not have one collision but two. Theia collides to Earth a little faster and bounces off Earth in an "hit-and-run". It returns about one million years later. It collides again with Earth in a massive impact similar to the one that occurred in the previous model.
Asphaug stated that the double impact is more than one event. This could explain the Earth-Moon isotopic similarity and how the second, slower, merging collision would have occurred.
This study shows the hit-and run and return scenarios for the formation the Moon. The left side shows the initial hit-and run impact. In about one million years Earth and Theis meet again and eventually merge into one disk. From this homogenized disk, the Earth and Moon are formed. This model shows the nearly identical isotopic compositions of the Earth's and Moon's Earth. Image Credit: Asphaug et al 2021.
This model of hit-and run impacts and chains of collisions could help to explain some of the most puzzling facts about terrestrial planets. Why are the inner planets so much different from the outer planets if the standard accretion theory is correct? Why does Venus not have its own moon? Why is Venus so weak and Earth strong in the magnetic shield?
Asphaug said that their research helped to explain why these differences could have occurred.
He stated that Earth would have accreted most its material from collisions that were either head-on hits or slower than those experienced Venus. Higher velocity and more oblique collisions into the Earth would have preferredentially led to Venus.
Common sense would suggest that Earth would be able to get more material from the outer Solar System, as it is closer than Venus. This research shows the contrary. The outer Solar System's projectiles would likely travel faster and bounce off Earth in a hit and run. Many of these projectiles would have made it to Venus, where they would be part of the planet. The greater amount of material from the outer Solar System could explain Venus' differences.
Because Earth is so close to Venus, it would seem that Earth is more composed of material from the outer solar system. Asphaug stated that Venus is more likely to accrete material from the outer solar system if Earth plays this role of vanguard.
This research may also help explain why Venus doesn't have a Moon. However, this hypothesis suggests that it is more likely for Venus to acquire one. The authors suggest that while Venus was more likely than Earth to acquire a large satellite, it could have also been more likely to lose one. The authors write that Venus is more likely than Earth to reaccrete a larger fraction of its runners than Earth. However, this could also be due to the fact that it reaccretes less of its impact debris. This means that impact debris will collide sooner with Venus. All of that debris could cause damage or even complete destruction to any satellites Venus might have acquired.
This research indicates a greater interconnectedness between the terrestrial planets. The new model could be confirmed by a better understanding of the Moon's geology, layering and solidification. Surface samples from Venus could also be used.
However, that's still a long way off.
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