Remains of impact that created the Moon may lie deep within Earth | Hacker News Reader

Remains of impact that created the Moon may lie deep within Earth

sciencemag.org

63 pointsamacbride17 comments

Wikipedia has a nice image: https://en.wikipedia.org/wiki/Large_low-shear-velocity_provi...

They certainly look like remnants of a gigantic impactor. Would explain why the Earth is slightly denser than Venus too.

That’s a really nice image. It looks like Earth’s heart.

To be clear, the large sphere in the diagram is not the remnant of the impactor - the distortions from being a perfect sphere are.

Why did the moon coalesce? Why is there not a ring of debris around earth? That seems like the most likely outcome of an impact that violent.

I'm not an astrophysicist. Just wondering.

Well, it was several billion years ago, so surely any debris would have had more than enough time to clear out or fall down onto Earth and the Moon?

Couldn't the same be said of Saturn's rings? Why have they been around for so long then?

Considering the timescale the asteroid belt could have formed a planet by now as well. Something doesn't add up about the established creation theory of the moon.

You missed Jupiter in your calculations.

Also, the total mass of the present-day asteroid belt is only about four percent of the Moon's mass. Ceres makes up about a third of that, so one could say that the belt did form a planet – it's just that its growth was heavily stunted by Jupiter's interference.

Last thing I heard, Saturn's rings are surprisingly young (about 100 million years old), and are indeed slowly disappearing.

At one point it was suggested it's moons "shepherd" the rings ... keep them in line.

I'm not an astrophysicist, so take this with a grain of salt, but I think it would be something about how rings only form in regions of gravitational instability. So we get the asteroid belt where a planet "should" be because Jupiter's gravity makes that area of space chaotic, and we got big rings around Saturn because Saturn has a bunch of moons.

You pick up a planet and throw it into another planet, sure there will be debris spinning around for a while which could look belt-like, but it won't be stable. Most of the little stuff will fall back towards whichever body is biggest, and any left over chunks with enough orbital momentum will converge on each other and mop up the remaining debris.

I am also not an astrophysicist, but here is what I have read. Every planetary body has an imaginary envelope called the Roche limit [1]. For Earth, this is about 18,000 km in radius. Debris inside this radius will tend to get ripped apart, while those outside tend to coalesce. This is not due to gravitational instability or chaos. This can happen even if the gravitational field is be assumed to be uniform. (Earth's gravitational field has some non-uniformities due to non-homogeneous density of the planet. But I doubt it has any role in preventing debris from coalescing. More uniform, but heavier bodies like Saturn tend to rip debris apart even more).

What really rips smaller bodies and debris inside the Roche limit is tidal forces caused by gravity-gradient. Imagine a smaller body like a satellite or a comet. The region of that body closest to the planet will be subjected to greater gravitational force and acceleration than the region farthest from the planet (gravity-gradient). If the satellite/comet's self-gravity and material strength is not strong enough to resist this differential pull/acceleration by the planet's gravity, it gets ripped apart. This is roughly the concept of Roche radius.

In case of the giant impact hypothesis, the ejecta that coalesced into the moon was certainly outside the Roche limit. Ejecta with sub-orbital velocities would have fallen back onto the planet surface shortly. I am not sure what happened to the material that was in orbit inside the Roche limit. There is another phenomenon at play here - called tidal acceleration [2]. You may be aware that the moon is slowly moving away from Earth. This is due to tidal acceleration.

Tidal acceleration causes large objects (like the moon) outside the planet's geosynchronous orbit to slowly accelerate and move away. In case of the moon, it's Earth's tides that drag moon like this. Conversely, the objects inside the geostationary orbit tend to get decelerated and pulled into the Earth's atmosphere. I am not sure how strong this effect is for small and distributed objects like planetary debris. But that may be why Earth has no ring. It may also be that those small bodies were too close and drag from Earth's thin atmosphere may have pulled it back to the planet.

The moon is believed to have been very close to Earth (a few tens of thousands of kilometers) at the time of formation. The impact is also believed to have sped up Earth's rotation a lot (reducing the length of the day). This means that the geosynchronous orbit would have been much lower and likely below where moon was back then. Tidal acceleration slowed down Earth's rotation and elongated the day, just as it raised moon's orbit.

[1]: https://en.wikipedia.org/wiki/Roche_limit

[2]: https://en.wikipedia.org/wiki/Tidal_acceleration

It's astrophysics [0].

[0] https://en.wikipedia.org/wiki/Hill_sphere

For most meteor impacts, the meteor itself completely vaporizes and doesn't deposit any solid chunks into the target. This is why moon craters are almost always circular despite the impacts occurring at all angles of incidence.

It seems the implication here is that the limit where the masses are comparable plays out very differently. Seems like it should be possible to estimate the hypothetical collion energy from the size of intact chunks.

That meteors vaporize on impact was long not known. At least that is what I learned from this story: https://en.wikipedia.org/wiki/Meteor_Crater#Daniel_M._Barrin...

Reply on news.ycombinator.com