The Origin of the Moon
Published on May 10, 2006 at 10:42 am.
1 Comment.
Filed under moon.
Where did the Moon come from? Once astronomers came to realize that it wasn’t always there, then the natural question was to ask how it got there. However, the Moon is far enough away that getting good data that can be used to answer the question proved difficult until the space age. True, telescopes helped, and the larger telescopes available by the middle of the Twentieth Century helped even more. Unmanned probes flying past the Moon in the early 1960’s helped as well, and unmanned missions landing on the Moon by the mid 1960’s helped even more. However, even better data came from the observations made during the Apollo missions. And the best data was from data collected on the ground during the Apollo landings of 1969 through 1972 and from analysis of the rocks brought back to Earth from the Moon. In fact, these data changed our views of the origin of the Moon.Â
Prior to Apollo, three theories competed for an explanation for the origin of the Moon. One was the coformation, or coaccretion model. Planets are believed to form from the material swirling around a star as it forms. Instabilities in this disk of material might collapse to form a planet. In fact, we think that this is exactly how Jupiter and Saturn may have formed. However, we now believe that the other planets formed from an accumulation of smaller bodies. Such an accumulation makes formation of a double system, such as Earth and Moon difficult to explain. In the manner that Jupiter and Saturn formed, the material collapsing to form the planets might also collapse to form moons. This may be how the larger moons of Jupiter and Saturn formed. But, these are modern ideas. In the 1960’s, it was thought that perhaps all planets collapsed directly out of the proplyd (the disk of material surrounding the newly formed Sun). So, the idea that the Moon may have simply formed at the same time as the Earth, out of the same material, seemed reasonable. One of the biggest problems for this model to explain is why the Moon orbits in a plane well off of the plane of Earth’s equator. The larger moons of Jupiter and Saturn orbit near those planets’ equatorial planes. In fact, that is exactly where a moon should orbit if it formed in this manner. Our Moon, though, orbits closer to the ecliptic than to the celestial equator.
A second idea for the origin of the Moon was the capture model. According to this model, the Moon formed elsewhere and passed too close to Earth, where the Earth’s gravity captured the Moon into orbit. Again, in the 1960’s astronomers were beginning to suspect that planets may have formed as smaller bodies, planetessimals, that then accreted to form the larger planets. Perhaps, they reasoned, the Moon was such a planetessimal. However, this model has a very difficult time explaining how the Moon’s orbit became so nearly circular.
>The third model for the formation of the Moon is the fission model. According to this model, Earth once spun much faster than it currently rotates. A large chunk of the Earth split off and flew into space to make the Moon. This model can be traced to a suggestion made by George Darwin (Charles Darwin’s son). George Darwin knew computed that tidal forces between Earth and the Moon should be making the Moon move farther from Earth. In fact, this was confirmed by the Apollo missions. The Moon moves, on average, just under 4cm farther from Earth each year. These tidal forces also slow the rotational rate of the Earth. Darwin overestimated the recessional rate, though, and believed that the Moon split from Earth rather recently compared with the age of the Earth. Of course, explaining how a chunk of Earth could just fly into space is difficult. Also, such a fission would certainly produce a moon with an equatorial orbit, which our Moon does not have.
At any rate, each of these models can make specific predictions of what we expect to find when we look at the rocks from the Moon. With actual moonrocks returned by the Apollo astronauts, we could test these models. For example, if the Moon formed some place else in the Solar System, it would have different isotope ratios from Earth rocks. These isotope ratios are almost like fingerprints as to how far from the Sun something forms. The isotope ratios of the moonrocks are identical to those of Earth, saying that the Moon formed at Earth’s distance from the Sun. This suggests some sort of relation between the Earth and the Moon’s formation. Secondly, if the Earth formed from the same material that was collapsing to form Earth, or if the Moon formed from a part of Earth that was flung into space, then the moonrocks would be the same as Earth rocks. However, we find the Moon has no minerals that we do not know on Earth, but it does not have all minerals that we do find on Earth. The Moon appears to be poor in volatile minerals and rich in refractory minerals. Volatile minerals are those that will vaporize at low temperatures, and refractory minerals are those that will survive high temperatures. These findings show that the Moon has gone through a different formation process than Earth. The moonrocks brought back by the astronauts are ancient, too. They seem to be nearly as old as the Solar System itself. This seems to argue against the Moon splitting from Earth recently. Also, the Moon has different proportions of materials than Earth has. For example, the Moon has a tiny core and a crust much thicker than Earth’s. The Moon, in fact, is mostly crust and mantle material. This argues against the Earth and Moon forming from the same collection of material.
So, all of the pre-Apollo Moon formation models seem to not hold up under the evidence gained by the Apollo astronauts. Thus, we need a new model for the formation of the Moon.  However, by the mid 1970’s a new model was proposed independently of one another by astronomers Alastair Cameron and William Hartmann. This model can be called the collisional ejection theory. According to this model, the newly formed Earth was struck obliquely be a very large planetessimal, perhaps something nearly the size of Mars. This collision blasts a large amount of material from Earth and the colliding body. Most of the material comes from the mantles and crust of these bodies, just like the composition of the Moon. Computer simulations of this collision show a large amount of this material coalescing into a body in Earth orbit. Tidal interactions with the Sun cause this material to coalesce into an orbit not too far from the ecliptic, just like the Moon. The material would coalesce closer to the Earth than the Moon’s distance, but as we know, the Moon is moving away from us, and would have moved to its current distance had it formed closer as proposed. The collision would be very violent and it would have experienced the material to very high heat loads, explaining why the Moon is poor in volatile minerals. As a bonus, the collision ejection model also would explain why Earth’s crust is the thinnest of all the terrestrial planets’ crusts.Â
For these, and other reasons, most astronomers now seem to go along with the collision ejection theory as the way that the Moon formed. If you want to read more about this model, then you can read Dana Mackenzie’s excellent book on the subject entitled The Big Splat. As you might guess from the name, the book is aimed at lay readers, without all of the mathematics that you get from texts aimed at astronomers. The book is a very good and very interesting read, and I highly recommend it.
-Astroprof
John on December 16, 2008 at 7:53 am: 1
Very interesting post. I was always curious why we have a moon and how it get there. Did not know that it moves every year 4 cm farther away. When reading your piece the collision ejection theory seems to be the most logical. Definitely gonna look up The Big Splat. Thanks.