[HN Gopher] Kissing moons may explain why Earth's moon is so large
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Kissing moons may explain why Earth's moon is so large
Author : pseudolus
Score : 95 points
Date : 2025-03-08 22:35 UTC (3 days ago)
(HTM) web link (www.cbc.ca)
(TXT) w3m dump (www.cbc.ca)
| kristianc wrote:
| Moon size is completely relative of course. Ganymede is larger
| than Mercury.
| nkrisc wrote:
| Titan as well.
| adrian_b wrote:
| 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.
| npongratz wrote:
| 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#Coincidenta
| l...
| Figs wrote:
| Eris is slightly smaller than Pluto in volume -- but more
| massive; it might also be worth considering in that same
| class.
| adrian_b wrote:
| 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.
| nextts wrote:
| They define a large moon in terms of its size compared to the
| host planet.
| npongratz wrote:
| 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.
| aaronblohowiak wrote:
| That is a fun fact! Gave me a smile to think about our solar
| system's variety. Thank you!
| pitaj wrote:
| Interesting. Is that because of different material makeup, or
| is it somehow related to tidal forces?
| npongratz wrote:
| 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...
| randallsquared wrote:
| Where would the energy come from to separate two such bodies that
| were already in contact, the size of Pluto and Charon?
| Sharlin wrote:
| 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.
| kwertzzz wrote:
| It would also be interesting to know how the orbits became
| quite circular afterwards (relative to the common center of
| mass).
| bell-cot wrote:
| "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.
| whiw wrote:
| Presumably from tidal forces, from the differential of the
| gravity field in the presence of nearby massive objects (like
| the sun, or jupiter).
| tejtm wrote:
| 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
| nextts wrote:
| New book idea: Does God play billiards?
| _ink_ wrote:
| 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.
| pfdietz wrote:
| Oxygen isotope ratios on the Moon are extremely close to those
| on Earth, which indicates intimate sharing of material during
| the event.
| consp wrote:
| So a very French kiss?
| pfdietz wrote:
| There was definitely sharing of bodily fluids.
| crazygringo wrote:
| > _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.
| emmelaich wrote:
| NASA simulation: https://www.youtube.com/watch?v=kRlhlCWplqk
| stronglikedan wrote:
| Aw, it's like the friend that's walking a quarter mile behind
| the group screaming "guys, wait up!", but no one ever waits.
| dghughes wrote:
| 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
| interestica wrote:
| 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.
| MajimasEyepatch wrote:
| 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.
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