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SYFY WIRE Bad Astronomy

Earth, Venus, and the Moon may all be victims of ancient hit-and-run planetary collisions

By Phil Plait

A big problem with trying to figure out how the planets formed is that it happened 4.55 billion years ago. The record’s a little blurry by now.

We have a lot of science and understanding of the planet-building processes, and can model them on computers by solving the various equations involved. But these models are only as good as the math and physics that go into them. If there’s some process involved we don’t know about, then the models that don’t include it could very well be wrong.

This has been true for decades, and as scientists learn more the models get better and our understanding grows.

And now a team of planetary scientists has thought of something new that may be an important effect that so far hasn’t been included in these models. The current thinking is that late in the process of planet formation, they grow by colliding with other big objects, other forming planets. These immense collisions produce a lot of debris that blasts away, but overall the object leftover is somewhat bigger.


The new idea is that this type of collision isn’t enough, and is unlikely to be the end of the event. The scientists looked into what’s essentially a hit-and-run: The smaller object (called the impactor) comes in, whacks the bigger one (the target) colossally at an angle, but then the bulk of it continues back out into space. Due to orbital mechanics it’s likely to return to the scene of the crime some time later and become a repeat offender, colliding with the planet again.

This may seem like nitpicking, but it can be very important, because the impactor may not hit the original target again. It might hit another growing planet, and that winds up explaining some thing about the Earth, Venus, and even the Moon.

They ran simulations using a proto-Earth at its current distance from the Sun (150 million km) and a proto-Venus at its current distance (about 105 million  km). They then slammed their sim-Earth with various sized impactors at various speeds to see what happens. In an earlier paper they found surprising results: After a grazing impact, between one- and two-thirds of impactors go on to orbit the Sun for a while before impacting Earth a second time. They also found that of the ones that don’t come back, most hit Venus. In fact Venus is about as likely to be the second target as Earth.

In a second paper just published, they look at how this affects Venus relative to Earth. Venus is very similar in size and mass to Earth, but has some profound differences. For one it doesn’t have a moon, and for another it rotates (spins on its axis) extremely slowly, once every 243 Earth days. On top of that (or on bottom I suppose) it spins backwards. This has been difficult to explain if you just go with big impacts forming Venus. A lot of ideas have been put forth (like friction with its thick atmosphere, or gravitational interactions with the Sun) and some do a decent job explaining these weird characteristics, but the authors here wondered if these hit-and-run impacts might pitch in.

The idea is that an impactor heading in from the outer solar system will tend to see Earth first, and collide with it before encountering Venus. Therefore Earth acts as “vanguard” for Venus, protecting it from the initial impact (while Earth itself has no such front-line planet). Venus then gets hit by the remnants of that impactor, merging with it. These two collisions have different impactor masses, speeds, and even directions, which could lead to the differences we see between the two planets now.

Missing media item.

This idea may also help explain weirdnesses we see in the Moon. The current model is that something Mars-sized or bigger slammed (named Theia) into Earth at an angle, blasting out a lot of debris. This debris is a mix of Earth upper layers as well Theia’s, while the core of Theia sank into Earth’s interior. This explains a lot of what we see about the Moon’s behavior, but leaves some puzzling mysteries. For example, if the crust and mantle of Theia formed a big part of the Moon, why does the composition of Earth’s mantle match the Moon’s in so many ways?

In yet another paper, the authors look into a hit-and-run scenario for the lunar formation. They use a slightly faster impact than most other research, allowing Theia to whack the Earth but continue on in its orbit around the Sun. After some hundreds of thousands up to a million years, it impacts Earth again, this time more slowly. This second impact generates a huge disk of debris that has two to three times the Moon’s current mass, in general titled at some wild angle to the Earth’s orbit.

This part I found particularly interesting: If the collision happens aligned with Earth’s spin the material interacts with Earth’s gravity (specifically through tidal interactions) and the disk reorients itself to Earth’s equator. But if the collision is retrograde (opposite Earth’s spin) a fraction of the models show the disk winds up staying tilted with respect to Earth’s equator, and the Moon inherits that tilt.

The Moon currently orbits the Earth at a significant orbital tilt, which has been difficult to explain up until now. Hmmm. Two impacts also tends to lead to better mixing of the two worlds, which could then explain why the Moon’s composition is similar to Earth’s mantle.

There are some issues. For example, it’s just as likely Theia would collide with Venus as it would with Earth after that initial Earth collision. Were we just lucky? Also, while collisions like this were not exactly common late in the Earth’s formation, it seems odd that an earlier or later one didn’t do a hit-and-run on us, dropped down to collide with Venus and help it form a moon. Luck again? They posit that it’s actually easier for a debris disk around Venus to fall down to the planet, leaving behind only enough material to form a smaller moon. In the meantime the debris still falling in might have enough velocity to erode away a smaller moon.

It’s not clear how likely that is, but what a scenario that would be. A rain of planetary collision debris so thick and rapid it would eat away an entire moon. Yegads.

This idea is pretty interesting, and I’ll keep my eyes open for papers from other astronomers supporting or refuting it. That’s how science works. But what I really like here is the inventiveness, the idea that we need to keep looking into these processes because they don’t explain everything we see in the solar system now. And even if one does explain everything that doesn’t mean it’s right.

When it comes to science, the investigation never ends. Even if you’re right, it might be for the wrong reasons, and that’s always worth a little bit more detective work.