# What are the current theories regarding moon formation? **URL:** \ **Category:** Factual Questions\ **Created:** [November 3, 2000, 9:48pm UTC](https://boards.straightdope.com/t/what-are-the-current-theories-regarding-moon-formation/39606 "2000-11-03T21:48:29Z")\ **Posts on this page:** 19\ **Page:** 1
**Author:** ![Earl\_Snake-Hips\_Tucker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/earl_snake-hips_tucker/32/3269_2.png) [@Earl\_Snake-Hips\_Tucker](https://boards.straightdope.com/u/Earl_Snake-Hips_Tucker)\ **Post date:** [November 3, 2000, 9:48pm UTC](https://boards.straightdope.com/t/what-are-the-current-theories-regarding-moon-formation/39606/1 "2000-11-03T21:48:29Z")
Not of our own dear sweet Luna, and not the captured asteroids of Mars, but the dozens of moons orbiting the gas giants (forget about Pluto/Charon). Are the prevailing theories that most of them are similarly captured asteroids, or just solid material that coalesced in orbit early in the formation of the planets? ---
**Author:** ![Phobos](https://avatars.discourse-cdn.com/v4/letter/p/3d9bf3/32.png) [@Phobos](https://boards.straightdope.com/u/Phobos)\ **Post date:** [November 3, 2000, 10:30pm UTC](https://boards.straightdope.com/t/what-are-the-current-theories-regarding-moon-formation/39606/2 "2000-11-03T22:30:52Z")
You best bet is to try this website… [http://www.seds.org/billa/tnp/](http://www.seds.org/billa/tnp/) Offhand, I’ll bet it’s a mixed bag of captured asteroids (the moons that are only a dozen or so miles across), captured trans-neptunian objects (e.g., Triton, Titan), and material that coalesced from the original protoplanetary disk (not sure about this…maybe the 4 large Jovian moons). ---
**Author:** ![Phobos](https://avatars.discourse-cdn.com/v4/letter/p/3d9bf3/32.png) [@Phobos](https://boards.straightdope.com/u/Phobos)\ **Post date:** [November 3, 2000, 10:33pm UTC](https://boards.straightdope.com/t/what-are-the-current-theories-regarding-moon-formation/39606/3 "2000-11-03T22:33:35Z")
Sorry I just posted a link instead of providing a direct answer…but you’re asking about dozens of moons, all of which are still being researched. ---
**Author:** ![tracer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tracer/32/20578_2.png) [@tracer](https://boards.straightdope.com/u/tracer)\ **Post date:** [November 3, 2000, 10:36pm UTC](https://boards.straightdope.com/t/what-are-the-current-theories-regarding-moon-formation/39606/4 "2000-11-03T22:36:16Z")
Appropriate that the first respondent to this thread should be named “Phobos.” Even if Phobos _is_ a moon of Mars, a solid planet, rather than one of the gas giants. 😉 ---
**Author:** ![MEBuckner](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mebuckner/32/2896_2.png) [@MEBuckner](https://boards.straightdope.com/u/MEBuckner)\ **Post date:** [November 4, 2000, 12:42am UTC](https://boards.straightdope.com/t/what-are-the-current-theories-regarding-moon-formation/39606/5 "2000-11-04T00:42:46Z")
As has already been noted, it seems to be a mixed bag. Large moons, like the Galilean moons of Jupiter, probably formed from whirlpools of dust and gas along with their parent planets, like miniature solar systems: > [@](#): > > Theories on terrestrial planet formation provide us with a good model for the formation of the Galilean satellites. As mentioned previously, the two processes differ substantially only in their respective timescales. The process of planet formation may be divided into four general stages, as described by Richardson, et al. 1999. In the Initial Stage, tiny particle grains condense from a hot gaseous disk, > which gradually settles to the midplane. The Early Stage of formation involves the accretion of dust grains into planetesimals of a rather significant size (~1km), which have initially low eccentricities and inclinations due to significant gas drag in the nebulous disk. After this early stage of planetesimal accretion, we may see growth at a much faster rate in localized regions. Focused merging in this Middle Stage produces a phenomenon called “runaway growth”, the driving force behind planet and satellite formation. Large bodies with low eccentricities and inclinations are flanked by many smaller, more excited particles. Interactions between the resulting large bodies (protosatellites) causes their orbits to cross, leading to radial mixing and giant impacts. hus, the Late Stage of formation results in only a handful of surviving large bodies. In a planetary system, the effects of the Late Stage occur on a timescale of ~108 years. Progression to the same stage in the Jovian system is estimated to take place before only 103 years. – [Simulating Galilean Moon Formation](http://www-hpcc.astro.washington.edu/faculty/trq/galmoons/research.html) (University of Washington Astronomy Department) Smaller moons probably include captured asteroids and fragments of larger moons: > [@](#): > > The small moons are too small to be seen, so not much is known about their surface or composition. With no knowledge of the composition, and no clues from the surface, little can be determined about the course of their history. Nevertheless, from the location of some of the moons, as well as their shape, it can be said that some moons are pieces of a larger moon, or are captured asteroids. – [Evolution of Small Moons](http://www.windows.ucar.edu/cgi-bin/tour_def/jupiter/moons/general_evolution.small.html) (University of Michigan) Finally, [Exploring the Trans-Neptunian Solar System](http://www.nationalacademies.org/ssb/neptch2.htm) (National Academies of Science) notes that “Triton is in an inclined, retrograde orbit around Neptune, which suggests that it was captured.” ---
**Author:** ![Qadgop\_the\_Mercotan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/qadgop_the_mercotan/32/83_2.png) [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)\ **Post date:** [November 4, 2000, 3:57am UTC](https://boards.straightdope.com/t/what-are-the-current-theories-regarding-moon-formation/39606/6 "2000-11-04T03:57:45Z")
According to the Jartravartid people of Viltvodle VI, the entire universe was sneezed out of the nose of a being they call the Great Green Arkleseizure. I guess that would explain moon formation also. Apologies to DA ---
**Author:** ![Deimos](https://avatars.discourse-cdn.com/v4/letter/d/5f9b8f/32.png) [@Deimos](https://boards.straightdope.com/u/Deimos)\ **Post date:** [November 4, 2000, 4:07am UTC](https://boards.straightdope.com/t/what-are-the-current-theories-regarding-moon-formation/39606/7 "2000-11-04T04:07:23Z")
what’s the _actual_ name for our moon? is it, “moon” or is it “luna”? ---
**Author:** ![MEBuckner](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mebuckner/32/2896_2.png) [@MEBuckner](https://boards.straightdope.com/u/MEBuckner)\ **Post date:** [November 4, 2000, 4:11am UTC](https://boards.straightdope.com/t/what-are-the-current-theories-regarding-moon-formation/39606/8 "2000-11-04T04:11:50Z")
It really _ought_ to have a name–just calling it “the Moon” is hopelessly geocentric–but in fact I think only science fiction fans call it Luna; even NASA calls it “the Moon”. ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [November 4, 2000, 5:21am UTC](https://boards.straightdope.com/t/what-are-the-current-theories-regarding-moon-formation/39606/9 "2000-11-04T05:21:25Z")
**What are the current theories regarding moon formation?** Although our understanding of the precise sequence remains elusive, it is thought to result from some combination of: 1. College student 2. Ethyl Alcohol 3. Vehicle Window ---
**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\ **Post date:** [November 4, 2000, 5:24am UTC](https://boards.straightdope.com/t/what-are-the-current-theories-regarding-moon-formation/39606/10 "2000-11-04T05:24:17Z")
> [@](#): > > \*Originally posted by Deimos \* > \*\*what’s the _actual_ name for our moon? is it, “moon” or is it “luna”? \*\* Depends entirely on your native language and culture, doesn’t it? ---
**Author:** ![Diceman](https://avatars.discourse-cdn.com/v4/letter/d/22d042/32.png) [@Diceman](https://boards.straightdope.com/u/Diceman)\ **Post date:** [November 4, 2000, 5:39am UTC](https://boards.straightdope.com/t/what-are-the-current-theories-regarding-moon-formation/39606/11 "2000-11-04T05:39:20Z")
> [@](#): > > what’s the _actual_ name for our moon? is it, “moon” or is it “luna”? In English, it’s “the Moon.” Luna (and Terra for Earth) are common in sci-fi stories, but they’re not the official name of anything. In a slight hijack, where did our Moon come from? The Moon is 1/6 the size of Earth, but isn’t it’s mass far less than 1/6 of the Earth’s mass? That would mean that it has a different, lighter composition than Earth does. Is it a captured planet that formed independently of Earth? ---
**Author:** ![MattTheCroc](https://avatars.discourse-cdn.com/v4/letter/m/439d5e/32.png) [@MattTheCroc](https://boards.straightdope.com/u/MattTheCroc)\ **Post date:** [November 4, 2000, 9:48am UTC](https://boards.straightdope.com/t/what-are-the-current-theories-regarding-moon-formation/39606/12 "2000-11-04T09:48:30Z")
Ditto for “The Sun”, I suppose. No one calls it “Sol”. Us lousy Terrans (gonna get as many Sci-Fi conventions in here as possible) are terrible at this. ---
**Author:** ![scratch1300](https://avatars.discourse-cdn.com/v4/letter/s/dfb087/32.png) [@scratch1300](https://boards.straightdope.com/u/scratch1300)\ **Post date:** [November 4, 2000, 2:58pm UTC](https://boards.straightdope.com/t/what-are-the-current-theories-regarding-moon-formation/39606/13 "2000-11-04T14:58:50Z")
> [@](#): > > \*Originally posted by Diceman \* > \*\*In a slight hijack, where did our Moon come from? The Moon is 1/6 the size of Earth, but isn’t it’s mass far less than 1/6 of the Earth’s mass? That would mean that it has a different, lighter composition than Earth does. Is it a captured planet that formed independently of Earth? \*\* The current consensus is, way back in the early history of our solar system, the Earth collided with a Mars-sized planet, and the tremendous impact shattered the smaller planet completely and most of the Earth as well. The lighter planetary material swirled out into space and, over time, congealed into the Moon. That’s why the moon is less dense than the Earth: because it’s composed of this lighter material that more easily flew into space. Follow-up question: does anyone know if this famous “Mars-sized planet” has a name? ---
**Author:** ![Diceman](https://avatars.discourse-cdn.com/v4/letter/d/22d042/32.png) [@Diceman](https://boards.straightdope.com/u/Diceman)\ **Post date:** [November 4, 2000, 4:06pm UTC](https://boards.straightdope.com/t/what-are-the-current-theories-regarding-moon-formation/39606/14 "2000-11-04T16:06:48Z")
> [@](#): > > Follow-up question: does anyone know if this famous “Mars-sized planet” has a name? Isn’t it called “Nemesis”? ---
**Author:** ![scratch1300](https://avatars.discourse-cdn.com/v4/letter/s/dfb087/32.png) [@scratch1300](https://boards.straightdope.com/u/scratch1300)\ **Post date:** [November 4, 2000, 4:20pm UTC](https://boards.straightdope.com/t/what-are-the-current-theories-regarding-moon-formation/39606/15 "2000-11-04T16:20:50Z")
Thanks, **Diceman**. Can I ask your source? I know the name **Nemesis** has been applied to the putative companion of the sun that’s supposed to periodically pass through the Oort Cloud and direct comets earthward, causing extinctions. (Interesting site [here]([http://seds.lpl.arizona.edu/billa/tnp/hypo.html](http://seds.lpl.arizona.edu/billa/tnp/hypo.html). cto ECan) about it and other hypothetical bodies of the solar system). Maybe you’re thinking of this star? Or have the two things got the same name? (It’s happened before: **Vulcan** = the inframercurial planet that LeVerrier thought he saw near the Sun; and also **Vulcan** = Spock’s home world). ---
**Author:** ![MEBuckner](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mebuckner/32/2896_2.png) [@MEBuckner](https://boards.straightdope.com/u/MEBuckner)\ **Post date:** [November 4, 2000, 9:52pm UTC](https://boards.straightdope.com/t/what-are-the-current-theories-regarding-moon-formation/39606/16 "2000-11-04T21:52:16Z")
> [@](#): > > In a slight hijack, where did our Moon come from? The Moon is 1/6 the size of Earth, but isn’t it’s mass far less than 1/6 of the Earth’s mass? That would mean that it has a different, lighter composition than Earth does. Is it a captured planet that formed independently of Earth? [Some Properties of the Earth-Moon System (Encyclopedia Britannica)](http://www.britannica.com/bcom/eb/article/single_table_inline/0,5716,1035,00.html) Luna has a diameter roughly 1/4 that of Terra (27.25% to be a bit more precise). It’s mass is only 1/81 that of Terra–mass of course varies by the cube of the radius, plus Terra is about 1.65x denser than Luna. (I believe current theory explains this by noting that the collision which formed the Terra-Luna system would have ejected the relatively lighter material from Terra’s outer layers, not the dense nickel-iron core, so Luna is relatively deficient in those heavier elements. This also accounts for why Luna’s magnetic field is pretty insignificant.) Luna’s surface gravity is 1/6 g. So, what’s a good site for Luna City? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 4, 2000, 10:29pm UTC](https://boards.straightdope.com/t/what-are-the-current-theories-regarding-moon-formation/39606/17 "2000-11-04T22:29:05Z")
The main reason that Luna’s surface gravity is larger than 1/81 g is that its radius is smaller, and gravitational attraction is also proportional to the inverse square of the distance to the object. The surface of the Moon is closer to its center than the Earth’s. ---
**Author:** ![Phobos](https://avatars.discourse-cdn.com/v4/letter/p/3d9bf3/32.png) [@Phobos](https://boards.straightdope.com/u/Phobos)\ **Post date:** [November 6, 2000, 1:29pm UTC](https://boards.straightdope.com/t/what-are-the-current-theories-regarding-moon-formation/39606/18 "2000-11-06T13:29:13Z")
**tracer** - and to think that **Mjollnir** tried to exclude me! **Deimos** - I agree with **ElvisL1ve** …it’s “Moon” in English. It makes sense considering that it’s the only moon that humans knew about for most of our history. **MattTheCroc** - yeah, that would be the geocentric mindset at work. (“The Moon” and “The Sun”, as if they are the only ones or the only important ones. But perhaps they are the only important ones in our day to day lives.) **scratch1300** - I agree. Actually, there was just an article about that very subject in Sunday’s Parade magazine. AFAIK, that Mars-sized object has no name. Let’s name it! I agree that **Nemesis** is the supposed mystery star (just a speculation, no proof) that periodically grazes our solar system and brings down a rain of comets to the inner solar system causing mass extinctions on Earth. ---
**Author:** ![Diceman](https://avatars.discourse-cdn.com/v4/letter/d/22d042/32.png) [@Diceman](https://boards.straightdope.com/u/Diceman)\ **Post date:** [November 6, 2000, 4:20pm UTC](https://boards.straightdope.com/t/what-are-the-current-theories-regarding-moon-formation/39606/19 "2000-11-06T16:20:30Z")
> [@](#): > > I know the name Nemesis has been applied to the putative companion of the sun that’s supposed to periodically pass through the Oort Cloud and direct comets earthward, causing extinctions. Oops! You’re right. Nemesis is the Star O’ Doom that’s supposed to kill us all one of these days. I knew it was a big celestial thing that’s supposed to have caused alot of trouble on Earth…