Prof. Sam LawlerBetween my normal meetings and writing, I'm watching a few talks at the American Astronomical Society's (AAS) Division for Dynamical Astronomy (DDA) annual meeting this week. They have this fantastic option where you pay US$10 and you can watch all the talks at the meeting. I'll try to share summaries of a few highlights using #DDA2026
Robin Canup (SWRI) is giving a prize talk on the formation of the Moon. The Moon was definitely formed by a giant impact, but the details are hard! Mars-size impactor makes most sense, but you have to shed a bunch of angular momentum. Can do this with "evection resonance" which keeps the Moon-Earth-Sun in a specific configuration and messes with the Moon's eccentricity. Big problem: matching isotopic composition. Maybe impactor was the same as Earth? #DDA2026
Talks about how tidal dissipation would change as the impact-melted Earth resolidifies.
What about co-accretion? Not for our Moon, but works for jovian planets' large moons. Shows that many generations of moons formed around jovian planets and were eaten by planets during Solar System's planet formation phase. The ones we see today are the last generation before gas disk dispersed.
She just told a story about being totally obsessed with Saturn as a middle schooler during the Voyager mission. She wrote a letter to JPL and they sent her a packet of Saturn photos and info! Comments that "I bet they had a good outreach budget back then." SIGH.
Saturn has 1 big moon, did smaller moon get Roche-shredded into the rings? Rings appear to be young, so probably not the right explanation.
Can co-accretion and giant impacts work together to explain Uranus/Neptune moons?
Peas-in-a-pod exoplanet systems (multiple similar-mass planets closely packed) maybe follow the co-accretion pattern? Simulations with gas migration show a characteristic mass for surviving planets, that doesn't depend strongly on stellar metallicity. Cool!
Ian Brunton (Caltech) shows that Io and Europa's 2:1 mean-motion resonance can be primordial, but Ganymede's 4:2:1 mean-motion resonance wouldn't have been stable in the primordial disk and would need to fall into place later
K. Dabroski (U. Idaho) How did Saturn's rings form? Uses only Chrysalis (a.k.a. proto-Hyperion), Titan, and Saturn's J2 as perturbers in REBOUND https://rebound.hanno-rein.de/ Iapetus is important for getting eccentricities high enough for a collision. More sims needed!
Guangyi Zhang (Caltech) Moon-planet tidal system is like a damped harmonic oscillator. 100 bonus points for having a cute animation of a moon on a surfboard "surfing" on the peak "gravito-inertial mode" location as it moves outwards from planet. Applies to Jupiter's and Saturn's moons
Richard 🔶UK #RejoinEU@sundogplanets I may be way off but… it isn’t a damped system, at least not in any meaningful way or the moon would be dropping out of the sky pretty soon I would think
@rpin42 It's a damped system in that the Earth's spin is slowing down due to tides from the Moon
Tim J@sundogplanets @rpin42 And what's more, pushing the Moon *further away* as a result, so in fact the opposite of making it fall out of the sky . . . And am I right in thinking that without internal friction in the earth, the effect wouldn't happen? (I'm guessing that with no friction, the earth's tidal bulge would just stay aligned with the earth–moon axis, so there'd be no sideways force exerted on the Moon and no drag exerted on the earth's rotation.)
@sundogplanets @rpin42 Thank you for confirming that my brain still works! It was quite fun to think about :-)