Second Moon May Have Orbited Earth Billions of Years Ago


It’s a view science fiction fans could only hope for: twin moons in the night sky above Earth. But it might have been reality about 4 billion years ago. A new model suggests the lunar farside highlands could have been created from a collision with a smaller companion moon in what scientists from the University of California, Santa Cruz are calling “the big splat.”

Why the near and far sides of the Moon are so different has long puzzled planetary scientists. The near side is relatively low and flat, while the topography of the far side is high and mountainous, with a much thicker crust.

We actually have a somewhat lopsided Moon.

The new study, published in the August 4 issue of Nature, builds on the “giant impact” model for the origin of the moon, in which a Mars-sized object collided with Earth early in the history of the solar system and ejected debris that coalesced to form the moon.

According to the new computer model, the second moon around Earth would have been about 1,200 kilometers (750 miles) wide and could have formed from the same collision. Later, the smaller moon fell back onto the bigger Moon and coated one side with an extra layer of solid crust tens of kilometers thick.

“Our model works well with models of the Moon-forming giant impact, which predict there should be massive debris left in orbit about the Earth, besides the Moon itself,” said Erik Asphaug, professor of Earth and planetary sciences at UC Santa Cruz. “It agrees with what is known about the dynamical stability of such a system, the timing of the cooling of the moon, and the ages of lunar rocks.”

Other computer models have suggested a companion moon, said Asphaug, who coauthored the paper with UCSC postdoctoral researcher Martin Jutzi.

A previous collision with a smaller companion could explain why the Moon's two sides look so different. Credit: Martin Jutzi and Erik Asphaug

Asphaug and Jutzi used computer simulations to study the dynamics of the collision between the Moon and a smaller companion, which was about one-thirtieth the mass of the “main” moon. They tracked the evolution and distribution of lunar material in its aftermath.

The impact between the two bodies would have been relatively slow, at about 8,000 kph (5,000 mph) which is slow enough for rocks not to melt and no impact crater to form. Instead, the rocks and crust from the smaller moon would have spread over and around the bigger moon.

“Of course, impact modelers try to explain everything with collisions. In this case, it requires an odd collision: being slow, it does not form a crater, but splats material onto one side,” Asphaug said. “It is something new to think about.”

He and Jutzi hypothesize that the companion moon was initially trapped at one of the gravitationally stable “Trojan points” sharing the Moon’s orbit, and became destabilized after the moon’s orbit had expanded far from Earth. “The collision could have happened anywhere on the Moon,” Jutzi said. “The final body is lopsided and would reorient so that one side faces Earth.”

The model may also explain variations in the composition of the moon’s crust, which is dominated on the near side by terrain comparatively rich in potassium, rare-earth elements, and phosphorus (KREEP). These elements, as well as uranium and thorium, are believed to have been concentrated in the magma ocean that remained as molten rock solidified under the moon’s thickening crust. In the simulations, the collision squishes this KREEP-rich layer onto the opposite hemisphere, setting the stage for the geology now seen on the near side of the moon.

While the model explains many things, the jury is still out among planetary scientists as to the full history of the Moon and what really happened. Scientists say the best way to figure out the Moon’s history is to get more data from lunar orbiting spacecraft and – even better – sample return missions or human missions to study the Moon.

Sources: Nature, UC Santa Cruz

29 Replies to “Second Moon May Have Orbited Earth Billions of Years Ago”

  1. Although at first glance this appears to be a radical idea, in the end it does make a lot of sense. It explains both the lopsided nature of the moon and why there were magma outbursts on one side and not the other. This aspect of the moon’s composition had been puzzling for a long time considering it’s size. There certainly is precedent for multiple moons forming from large impacts, with Pluto being the best example I can think of, even though it is a KBO.

  2. Because the dark side of the moon is always facing away from Earth, it would make more sense to just say all the stuff over there is a result of it being bombarded by the rest of the solar system’s debris (the side facing the Earth is protected by the Earth)

    1. Right now, the “dark side of the moon” is not facing away from Earth, but rather mostly facing towards Earth. It won’t face entirely away from Earth until 9 days from now, and then every 29.5… days thereafter (if I have it right).

      Or. given the Moon’s low albedo, you could argue that the Irish doorman was correct with “There is no dark side in the moon, really. As a matter of fact it’s all dark”.

    2. The moon doesn’t act like a shield – objects from the rest of the solar system can quite easily hit the inside-face of the moon either by coming from the other direction (there’s plenty of space for them to do so) or by swinging around due to the Moon’s gravity.

    3. To clarify, Squid and Tim are making the point that the far side of the moon, which always faces away from us, is not always dark (for instance, when it is between the earth and the sun). Calling it ‘the dark side of the moon’ is a misnomer.

      1. I agree, it’s a confusing phrase but it’s clear what Matt meant. I wasn’t addressing the phrase; I was addressing Matt’s claim.

      2. “Dark in this case has never meant without light. It has meant out of view”

        Cite, please? From reputable astronomy sources, please, not just occasional misstatements.

      3. My understanding is that it was referred to as the “dark side” for the exact same reason central Africa used to be referred to as “darkest Africa” … because we knew little or nothing about that part of it. The darkness is our lack of knowledge, not the locations lack of light.

      4. I am not an astronomer. I’m willing to believe that “dark” is used in astronomy for the far side of a body if people can point out citations from reputable astronomical writings.

      5. This is just semantics; it’s the same with astronomers who describe elements heavier than helium (He) as “metals”.

      6. “This is just semantics” is saying “These are just the meanings in our communications” — in other words, the entire purpose of communication.

        And you’re begging the question: I’ve made several requests for citations of “dark” meaning “far side” in reputable astronomy writings, and the several people here upholding it haven’t given support for it.

        For any usual astronomy term, someone can point at a public information page from NASA, or a science museum, or a telescope maker, or the IAU, or someone.

      7. I’ve mentioned this elsewhere in the UT comments: there’s a difference between formal (such as a carefully edited article) writing and informal (comments, which are often spontaneous) writing.

  3. Matt, but no other moon in the solar system shares this characteristic. If planets protected the near sides of moons, then you’d certainly see this on all the jovian moons, and other moons of the solar system. Clearly something else is at work to create a near side with smooth seas, and a far side which is highly rugged.

  4. I am divided on this.

    Clearly this is a very predictive theory; lopsidedness, surface asymmetry, two surface types: magma/KREEP vs silicates, crustal thickness.

    And it is strengthened by other impact moon scenarios. I didn’t know about Pluto with moons, but Mars with two remaining moons and impact scars after orbiting bodies has been mentioned before. It is a winner.

    However, there is also a fairly predictive theory which predicts both lopsidedness, magnetic field strength and magnetic anomalies. This is the diapir theory, where the first mantle convection bulge froze instead of developing to our type of plate tectonics.

    It is also strengthened with other planet scenarios, because the diapir theory predicts the rapid onset of Earth convection and perhaps partly the development of the Tharsis bulge of Mars.

    Maybe the final theory has to be a bit of both? The main problem with the diapir theory may be that it should be centered on the backside IIRC, so the crust should perhaps be thinnest. I have to check that. Then again, this could predict precisely that feature.

    1. You mean explanatory! I didn’t see any predictions from this theory in the Nature article, although hopefully some will come out of it.

      1. Of course I mean precisely what I says, since famously theories do predictions and _not_ explanations.

        Whatever an explanation is, besides a tautology of theory perhaps? I have never seen a proper definition. Predictions is a testable definition.

    2. Speaking of moons, why do they always seem to orbit their parent planet in the plane perpendicular to the planet’s axis of rotation? I can imagine the natural process that led to our moon doing this (given that it and Earth were breifly the same body), but what natural process causes captured satellites (such as those of Mars and the gas giants) to do this?

      1. I believe it has to do with the other objects in the system pulling the moons closer to the plane of the ecliptic. The same process that keeps the planets on that plane.

      2. “Speaking of moons, why do they always seem to orbit their parent planet in the plane perpendicular to the planet’s axis of rotation? I can imagine the natural process that led to our moon doing this …”

        Most don’t, and in particular, Earth’s Moon does not.

        Earth’s Moon orbits between 18 and 29 degrees off the Earth’s equator. It orbits 5 degrees off the ecliptic. lists 64 moons of Jupiter. The closest 8 have inclinations of about a degree or less. The next 6 out have inclinations between 27 and 56 degrees. The other 50 have inclinations between 137 and 167 degrees, so I think that’s between 13 and 43 degrees retrograde.

        Saturn: much the same. The 22 closest are over Saturn’s equator, the other 39 are not, though some approach it retrograde. , which talks about captured satellites, says that retrograde satellites can have larger stable orbits than prograde satellites.

      3. The hypothesis I refer to predicts the Mars satellites (and some impact scars) from an impact. One reason is that capture can’t explain their co-rotation and some other orbital characteristics.

  5. I love pancakes, I have had them today. It looks like Vesta at some point. It’s all made of a dough. 😀

  6. The mars-like collision theory for the formation of the moon suggests that right afterwards there would have been an array of bodies in orbit around the early Earth. They coalesced into the moon eventually. Presumably this happened by a number of spats or splashes in the process. This may have just been the last big splash.


    1. I like the fractal symmetry of it all.

      Earth forms, and another planetoid forms in its (trailing?) Lagrange point. The planetoid eventually wobbles out of the point and into Earth in a colossal impact, creating a miniature accretion disc. Out of the accretion disc, the Moon forms, and so does a… let’s call it a moonoid, which eventually wobbles out of the point and into the Moon in a colossal impact messy splat.

  7. I just read the article in Nature. The authors (and no one here in the previous 25 comments) NAMED the two “pre-moons” Do I get first dibs? May I suggest SELENE for the larger pre-moon and CYNTHIA for the smaller. Both of these names have been actually used as names for the moon itself.

  8. ME AGAIN! I just had an interesting thought. Several meteorites found on earth have come from the moon. Are ANY of these from a time ERLIER than the celine-cynthia(see comment below) impact event? If so,could the composition of the meteorite indicate WHICH pre-moon it came from?

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