Earth's blobs are remnants of an ancient planetary collision | Hacker News Reader

Earth's blobs are remnants of an ancient planetary collision

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110 pointsokl58 comments

Discussion yesterday: https://news.ycombinator.com/item?id=38107078

Is Earth's orbit stable enough not to be thrown away from the Sun after such a collision? Are there interactions between Earth and other large bodies which have a stabilizing effect? I know there's some interesting dynamics in the Earth-Venus interactions but I wonder if there's enough there that you could consider it stabilizing per se.

Not an expert but if the earth is in the orbit it has now, it's because that's indeed the most "stable" orbit it can assume.

Even if the earth got pulverized, the remnants would be floating as a cloud around the sun, in the shape of its orbit.

The Earth's orbit is a influenced not only by the sun's gravity, but by that of every component of the solar system (to varying degrees, of course).

Also, I'm sure the orbit we have now is not the same as the one pre-collision.

It depends entirely on the momentum of the impactor. If the impactor's momentum was much less than the Earth's then it would have little effect.

We hypothesize that Thela was like the size of Mars. Since all orbits in the solar system are due to gravity (assuming no rocket powered alien space rocks), we can calculate the maximum velocity a planetesimal would have as if it were falling from infinity into the sun. I can't be asked to do this right now, but based on the Virial Theorem and Earth's orbital velocity of 30 km/s, I bet ~60 km/s is the theoretical maximum impact speed. Mars has 10% of Earth's mass, so at best Thela would have 20% of the Earth's momentum.

And more likely the impact occurred in a grazing fashion in order to make the moon and with much less velocity, since it was probably already in an orbit around the sun. So back of the envelope, we wouldn't expect much to happen to Earth orbit other than a small disturbance that makes it more elliptical for a time. Then over the next few Gyr the solar tidal interactions circularize it.

I think the key/tricky point is that orbits tend to circularize over time. I figured it was due to other perturbations, but is it mostly tides?

Related question, how much do tidal forces inside the planet contribute vs tidal forces from the ocean?

The ocean is a tiny fraction of the planetary mass, so it's mostly the bulk mass of the planet. If you watch the moon forming simulation posted here, you can see right before impact the bulging of Theia toward Earth. This demonstrates the non-rigidity of planets pretty well. This happens to a much smaller extent between the moon and Earth and is the reason why the moon is tidally locked (one side always faces us) and is slowly moving away from us -- the tidal forces are transferring angular momentum from the Earth to the moon. Consequently as the moon moves farther away, the length of our day will increase.

Kind of a naive question, but would it be possible to move the moon back to a prior position? Would that even be a good idea?

An earth day will extend by an hour in 160 million years, it's not something to worry about.

Jupiter has some influence, but IIRC solar tides should be the largest perturbation of the Earth's orbit.

Wouldn't we see a small echo on the orbit of earth?

> the seismic wave pattern is dominated by the signatures of two continent-sized structures near the Earth's core. They are interpreted as regions with unusually high iron content, whose increased density and temperature slow down seismic waves passing through

I would have thought that increased density would cause propagation velocity to increase? With heat, density falls so that bit stacks up. What have I got wrong?

Increased density slows down propagation. It is easy to come up with false explanations of why, but here is one I think is probably true:

If you imagine a bar that has been struck by an xylophone mallet, it makes sense that increasing its inertia would slow down the periods of its alternating motion. You can build traveling waves from sums of its modes of oscillation, and that carries the effect of higher density to wave speed.

TLDR: For those waves in solids, propagation speed goes up with stiffness and down with density. The confusion happens because sometimes the thing with a higher speed is little denser... but a lot stiffer.

_________

Interesting question. Some searching yields:

> The density of a medium is the second factor [after elastic properties] that affects the speed of sound. It takes more energy to make large molecules vibrate than it does to make smaller molecules vibrate. Thus, sound will travel at a slower rate in the more dense object if they have the same elastic properties.

> If sound waves were passed through two materials with approximately the same elastic properties such as aluminum (10 psi) and gold (10.8 psi), sound will travel about twice as fast in the aluminum (0.632cm/microsecond) than in the gold (0.324cm/microsecond). This is because the aluminum has a density of 2.7gram per cubic cm which is less than the density of gold, which is about 19 grams per cubic cm. The elastic properties usually have a larger effect that the density so it is important to both material properties.

Poking around for an equation, I came across this [1] where the longitudinal/shear waves probably correspond to seismology's P-waves and S-waves. (Edited equations slightly for ASCII readability.)

> These elastic waves move through the material with a velocity determined by its elastic modulus and its density, r.

> For longitudinal waves this velocity is the sound velocity, C, and C = (E/r)^0.5, where E is Young's modulus.

> For shear waves the controlling elastic modulus is the shear modulus, G. For shear waves: C = (G/r)^0.5.

[0] https://www.nde-ed.org/Physics/Sound/speedinmaterials.xhtml

[1] https://www.princeton.edu/~maelabs/mae324/glos324/elasticwav...

Do you think humanity will seriously start to dig down at some point? For resources, energy and living space?

It already has:

  * Resources: lengthy tunnels, caverns, pipes and open pits are already bringing a wealth of subsurface minerals, salts, diamonds, hydrocarbons and other ores for the use of humans.

  * Energy: geothermal energy is used now and is often deployed in places where portions of superheated mantle are closer to the surface, like near volcanoes in Iceland and Costa Rica.

  * Living space: subsurface living space is less common due to human's aesthetic preference for "natural light." However even as long ago as the 1800s US West people built subsurface dwellings as depicted in author Laura Ingalls Wilder's book "The Banks of Plum Creek." [0]

0. https://sitesandstories.wordpress.com/tag/the-dugout-site/

I thought the Earth is rather fluid inside such that stuff gets mixed around over time. That such blobs can stay intact for billions of years seems far-fetched.

The mantle is officially "solid" (as proven, I believe, by propogating shear waves, which an actual liquid cannot do). It flows, but very slowly. Very very slowly. I'd say that big chunks of material staying intact over billions of years is, in fact, an illustration of just how slowly.

Wikipedia: "[The mantle] is predominantly solid but, on geologic time scales, it behaves as a viscous fluid, sometimes described as having the consistency of caramel."

Velikovsky would be proud.

Isn't this finding to be expected? I thought all planets were formed by accretion.

Usually not that dramatically, or that late. Both proto-Earth and Theia were already pretty well accreted. And it's still pretty wild to (potentially) find specific pieces of an impact from that long ago (if that's actually the case)

Trust in the scientific community (which is approaching crisis) gets eroded every time papers are overstated, like in the headline here. The researchers ran a simulation that seems to align with the scarce physical insights associated with the LLVP "blobs" and the hypothesized Theia body. It's cool research and invites more research attention being put in that direction. That's very exciting on its own.

And thankfully, the article body exercises honest scientific humility in its presentation, or at least strives too:

> suggests, proposes, appears that, hypothesizes, etc

But the headline? Gotta get those clicks and shares! "We've done it! We've figured out what the blobs are and it was that cinematic ancient planetary collision that created the moon too! Eureka!"

Of course, for a paper about geophysical and astronomical history, the consequences of this kind of overstatement are minor, but the editorial practice of doing so is a real problem that we should all probably pay some more attention to.

Authors of articles in publications rarely get to write the headlines. However, Hacker News people pay inordinate attention to headlines, so we see statements like this: "trust in the scientific community ... gets eroded" because of headlines. We don't need to pay more attention to the problem of clickbait headlines. We pay too much attention to that already. What matters is the substance and interested people can dive deeper, treating headlines and press releases with appropriate skepticism.

I mean, you're right that the personal adaptation everyone can start making today is to treat headlines and press releases with appropriate skepticism.

But this isn't some fringe HN phenomenon where riled up nerds focus on spurious details and make big complaints about them. Headlines matter. Before social media, headlines are what people read as they walked by the newsstand, what they heard from the crier, what they saw on the overlay on the TV news station in the lobby and at the gym. And now, in the age of social media, headlines are an even louder currency of fact. Almost all social media sites operate as headline + votes + comments. There are stories behind those headlines, a click (and maybe a paywall) away, and the minority of intellectually diligent communities like HN do a a good job of surfacing the details into the comments so that bad headlines are less toxic.

But our behavior here is already the exception and we do need to keep banging on misrepresentative headlines. While also a form of clickbait, they are a different and far more irresponsible class of it than the than harmlessly, wastefully vague click-for-the-mystery headlines that you're tired of hearing complaints about.

In some HN discussions, it seems that half of the comments are from people who only read the headline and are flaming about it, or who read the headline and the article and want to argue about whether they match, or whether the headline is overly sensational and promises something that the article doesn't deliver. Too much meta-discussion, not enough real discussion. And here I am engaging in it too, if only to push back.

It's not that HN folks pay inordinate attention to headlines in isolation. I don't think you meant it that way, but that's how it comes off. The issue is that people often don't read the articles that the headlines are for. We can see this on any social media app. For the purposes of this comment, maybe discussion?, I think that HN, Reddit, Twitter (X), Facebook, Instagram, etc., are all social media. Some folks might want to squabble over exacts but to make things simple to discuss I chose to define it as such.

For science in particular, I think it gets a bad wrap generally because people don't like it when you tell them something might be true then say it isn't. The general population really doesn't seem to like it when the scientific community says "we may have the answer for x, y, and/or z but we don't know for certain." as that's too complex of a thought to keep. We, generally, love to be told simple truths for complex problems.

We see this in politics broadly, as well. When discussing tickets as a SWE I know we tend to understand this by the time we're intermediate developers and adapt our language appropriately.

HN policy is not to modify the headlines when posting things, which I think exacerbates the issue.

> However, Hacker News people pay inordinate attention to headlines, so we see statements like this: "trust in the scientific community ... gets eroded" because of headlines.

Trust in HN's ability to think critically is eroded every time circumstances force you to remind everyone of this.

It's always interesting to see which groups of people are held responsible for other people misrepresenting them, and which aren't.

This makes me sad. Now whenever some article with ambitious claims pops up into view, my first reaction is "what's the catch?"; i.e. what elemts of the research are overstated for the sake of virality.

It's even more frustrating when done by what are considered "mature" if not "reputable" publications. "Who am I supposed to trust now?" is a question I never wanted to answer when it comes to scientific research.

> "Who am I supposed to trust now?" is a question I never wanted to answer when it comes to scientific research.

Nullius in verba (Latin for "no one's words" or "take nobody's word for it") is the motto of the Royal Society. John Evelyn and other fellows of the Royal Society chose the motto soon after the Society's founding in 1660.

https://en.wikipedia.org/wiki/Nullius_in_verba

> Nullius in verba (Latin for "no one's words" or "take nobody's word for it")

For the most literal possible translation, "on the word(s) of no one".

Well, sort of. It appears to actually say "onto the words of no one".

Sometimes it turns out that someone has already written a paper on your exact nearly pointless topic!

https://sci-hub.se/https://www.cambridge.org/core/journals/b...

So nullius in verba has bizarre grammar because it is an ungrammatical selection of a few words from a longer original Latin sentence with correct grammar. Nullius addictus iurare in verba magistri means "To swear [oneself] slave of no master", where in verba attaches grammatically to the verb, iurare, not to the master.

This interpretation isn't possible in the clipping because the only attachment point for nullius is verba. But in the original poetry, nullius is bound to magistri, which is then bound to addictus, not to verba.

It’s really not new. People have been hyping up things for centuries. Old newspapers are full of breathless coverage of the snake oil du jour or the next big thing that turns out to be nothing. Scientists have been at it for a really long time as well.

Maybe you pay more attention to it (or are more sensitive to it) now because you’re not as much of an idealist as when you were younger? More experience and having been fooled a couple of times tend to do that.

I any case, a healthy dose of skepticism is good. You just need to be prepared to change your mind when it turns out that something was actually important after all.

You always needed to do that. It’s why the scientific method even exists. One can only trust what one has tested. And when someone else makes a claim? Verify, test, evaluate, and only then trust.

It’s also why critical thinking is historically such an important part of education.

> But the headline?

If you search for Theia right now, you get the same headline and story propagated across a number of publications who syndicate their science news.

It appears that it takes one person placed in a position of power (writing headlines) to screw this all up.

The headline here is from a press release, so the culprit here is the university press office.

relevant as ever:

The Science News Cycle

https://phdcomics.com/comics/archive.php?comicid=1174

Uni PR office at 2 o'clock

They screw up fairly regularly, unfortunately. Vulgarisation and scientific news for normal people are a shit show, between incompetents journalists and hype-chasing universities.

> The researchers ran a simulation that seems to align with the scarce physical insights

Fwiw, we will probably never be able to directly observe the contents of the mantle. Computer simulation has been a godsend for geology research.

> Fwiw, we will probably never be able to directly observe the contents of the mantle.

Unless this proposal goes forward someday: https://www.nature.com/articles/423239a

Get real human beings to stop clicking on eyeball bait. Far easier said than done, when the bait is ultimately being generated not by other individual human beings, but corporations (effectively slow-AIs or Lovecraftian monstrosities) that can bring far more resources to bear on getting us to click.

Relevant not-xkcd "The Science News Cycle" https://tapas.io/episode/18523

Some 20 seconds in, got a pop-up demanding I install an app. Dismissing the pop-up caused the page to close.

OP posted a link aggregator or something, try going to the original site:

https://phdcomics.com/comics/archive.php?comicid=1174

> Trust in the scientific community (which is approaching crisis) gets eroded every time papers are overstated, like in the headline here.

I mostly agree, but the headline caught my attention and I read the article and learned some things.

This is tremendously unfair, though. The "scientific community" is not responsible for these university press releases and popular science journalism. The papers themselves almost never contain claims that aren't propertly quantified and modest, outside of grifter realms like psychologists selling self-help services.

We're not going to stop the press, and the scientists themselves are usually already doing the right thing. The only real hope I see is for the public to somehow come to better understand the difference between science and the press. When you see this, absolutely point it out, but place the blame where it belongs, on newspapers, magazines, and university PR departments, not on the researchers.

>Trust in the scientific community (which is approaching crisis) gets eroded every time papers are overstated,

While I detest clickbait with the fury of a million Betelgeuses, I welcome increased skepticism towards science and especially the "scientific community".

Science is fundamentally built upon skepticism, you never implicitly trust anyone or anything. You verify theories and findings as humanly possible and only then trust what remains only as far as you can throw them (which could still be false, depending on our ability to verify).

Science implicitly trusted is no different from blind worship in religion. I have nothing against religion, but science isn't a religion.

"Trust the science." is by far one of the most damaging mindsets that have come about for science.

I think it's more accurate to say that the trust the scientific community has for regurgitation of scientific papers erodes. I can't imagine it has much effect on the general populace's perception of the practice.

I for one support a fully free and open-source earth, with no proprietary blobs

The idea that something collided with the Earth and created the Moon is an unscientific, absurd idea. There is a zero percent chance that a collision as described could result in the Moon maintaining an almost perfect orbit around the Earth, and the Earth maintaining a near perfect orbit around the Sun.

So all the planets popped into existence with their current Mass and Orbits?

Of course not. Mass is summed as is momentum

Everything was formed by the spinning of gas clouds and consolidating into planets and such. That's why the planets haven't fallen into the Sun after 14 billion years.

indeed, and how is that proof that the moon didn't come from a collision with the earth?

Which is why nothing has ever collided in the history of the solar system, because they all started on perfectly parallel orbits? Is that about right?

Nothing large enough to cause a planet to split into two ever collided. Yes.

Really. Interesting. What do you think was the cutoff size for objects that can collide? Can you justify it quantitatively, given your "zero percent chance" line earlier?

> There is a zero percent chance that a collision as described could result in the Moon maintaining an almost perfect orbit around the Earth, and the Earth maintaining a near perfect orbit around the Sun.

I’d love to see the math you did to come to that conclusion. Do you have it in a paper somewhere?

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