Kissing moons may explain why Earth's moon is so large | Hacker News Reader

Kissing moons may explain why Earth's moon is so large

cbc.ca

99 pointspseudolus32 comments

FYI the author of the CBC article is science reporter Bob MacDonald he's like the Canadian equivalent of Bill Nye the Science Guy. Bob has been around what seems like forever.

edit: he also hosts the radio program Quirks and Quarks. https://www.cbc.ca/radio/quirks

Bob’s such a great example of forging one’s own path and finding one’s passion.

He failed grade 9. He went to university but dropped out by second year. Traditional learning models just were not for him at the time. But he showed that communicating science was much more about the human element; much more about the communication. He might be more like Canada’s Carl Sagan.

I’m sure he’s great, but that CV sounds much more like Bill Nye (Boeing engineer turned comic turned science communicator) than Carl Sagan, who was an active scientist and researcher his entire career while also doing science communication. Sagan was precocious and started college early, but after that he seems to have had a relatively traditional scientific career path until he became famous.

The article states that Earth's moon was captured by a "slap in the face" rather than by a kiss. Pluto's moon is the one captured by a kiss.

Oxygen isotope ratios on the Moon are extremely close to those on Earth, which indicates intimate sharing of material during the event.

So a very French kiss?

There was definitely sharing of bodily fluids.

Why are they expected to be different on different planetary bodies without sharing of materials? Is the early solar nebula kind of like a reverse centrifuge with denser isotopes near the sun and lighter ones further away?

It's not that they're expected to differ, it's that they do differ. If you look at oxygen isotopes from meteorites the ratios are all over the place. Also, Mars meteorites (which we know are from Mars because of noble gas isotope ratios being the same as measured by Viking landers) have different oxygen isotope ratios than Earth.

All rocks on Earth and on the Moon have ratios that lie on a single line, the Standard Mean Ocean Water (SMOW) line.

BTW, the fact that meteorites have a wide variety of ratios that don't lie on a single line indicates meteorites derived from asteroids did not come from a single parent body. There was no planet between Mars and Jupiter that exploded to form the asteroid belt, bad science fiction stories notwithstanding.

> Dwarf planet Pluto and our Earth are the only two worlds in our solar system with very large moons. These may have come about by a "kiss and capture" process, which preserves a moon's large size.

> The Earth's moon is believed to have formed from a similar process, the researchers explained in a written statement, but it was more like a slap in the face rather than a kiss.

Yup, that's just bad writing. Decide on your metaphor and stick to it. Don't say something is X, and then halfway through the article say it's not X.

NASA simulation: https://www.youtube.com/watch?v=kRlhlCWplqk

Aw, it's like the friend that's walking a quarter mile behind the group screaming "guys, wait up!", but no one ever waits.

Moon size is completely relative of course. Ganymede is larger than Mercury.

Titan as well.

By size, the Solar System has 4 Mercury-sized bodies: Mercury, Ganymede and Callisto (the 2 bigger of the 4 big satellites of Jupiter), and Titan (satellite of Saturn).

There are also 4 Moon-sized bodies: the Moon, Io and Europa (the 2 smaller of the 4 big satellites of Jupiter), and Triton (satellite of Neptune).

Pluto is smaller than Triton, but not much smaller. It is the next in size after these bodies and it could be considered to belong to the same size class as the Moon-sized bodies, because the next smaller bodies are significantly smaller. While Pluto is not a satellite of Neptune, like the satellites it does not have an independent movement, but one that is connected to the movement of Neptune through an orbital resonance.

The Neptune-Pluto gravitational resonance is pretty neat. Wikipedia has a nice listing of other orbital resonances in the solar system:

Likely gravitional:

https://en.wikipedia.org/wiki/Orbital_resonance#Mean-motion_...

Likely coincidental:

https://en.wikipedia.org/wiki/Orbital_resonance#Coincidental...

Eris is slightly smaller than Pluto in volume -- but more massive; it might also be worth considering in that same class.

You are right, Eris is close enough in size to Pluto and to the other 4 Moon-sized objects to be considered in the same class size.

In the past, for a long time the uncertainties about the sizes of Pluto and of Eris were great, so it was not clear how close in size they are to the other much better known Moon-sized bodies.

They define a large moon in terms of its size compared to the host planet.

For what it's worth (nothing, just interesting to note): Ganymede is larger, and less dense. Mercury's mass is more than twice Ganymede's.

That is a fun fact! Gave me a smile to think about our solar system’s variety. Thank you!

Interesting. Is that because of different material makeup, or is it somehow related to tidal forces?

My understanding is that the composition of each body is different. There's the obvious: More ices and liquids comprise Ganymede, possible since Ganymede is so distant from the Sun. And the not-so-obvious: Mercury is anomalously dense due in part to (likely) having a disproportionally large core of iron and nickel.

I find Wikipedia's discussion about Mercury's density in contrast to Earth's fascinating [0]:

> If the effect of gravitational compression were to be factored out from both planets, the materials of which Mercury is made would be denser than those of Earth, with an uncompressed density of 5.3 g/cm3 versus Earth's 4.4 g/cm3.

[0] https://en.wikipedia.org/wiki/Mercury_(planet)#Internal_stru...

Mercury is basically a big ball of iron.

Where would the energy come from to separate two such bodies that were already in contact, the size of Pluto and Charon?

It was not well explained in the article, but the simulation timestamps indicate that it would all have happened just over sixty hours or so. So a comparatively short kiss during which a lot of dynamic stuff was still happening.

It would also be interesting to know how the orbits became quite circular afterwards (relative to the common center of mass).

"Kissing", "contact", and photos of objects like Arrokoth are too misleading.

The Earth/Moon and Pluto/Charon situations started with extremely high speed collisions (plenty of kinetic energy) between separate bodies. Those collisions were spectacular sideswipes (vs. head-on), so there was also a huge amount of rotational inertia. That's another must-have, to end up with two bodies orbiting each other.

Presumably from tidal forces, from the differential of the gravity field in the presence of nearby massive objects (like the sun, or jupiter).

It could be largely a redistribution of rotational energy in the system. Where they contacted could not particularly stable, heat would be generated, energy dissipated. maybe ice state change helps, with luck a messy rolling contact could become a series of slingshot orbits, but I am just making stuff up here

New book idea: Does God play billiards?

How do these bodies get so spherical?

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