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The Earth and Moon as seen from Mariner 10 en route to Venus. This could be a similar view of two moons as seen from Earth. Image credit: NASA/courtesy of nasaimages.org

What if the Earth had Two Moons?

27 Dec , 2011

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The idea of an Earth with two moons has been a science fiction staple for decades. More recently, real possibilities of an Earth with two moons have popped up. The properties of the Moon’s far side has many scientists thinking that another moon used to orbit the Earth before smashing into the Moon and becoming part of its mass. Since 2006, astronomers have been tracking smaller secondary moons that our own Earth-Moon system captures; these metre-wide moons stay for a few months then leave.

But what if the Earth actually had a second permanent moon today? How different would life be? Astronomer and physicist Neil F. Comins delves into this thought experiment, and suggests some very interesting consequences. 

This shot of Io orbiting Jupiter shows the scale between other moons and their planet. Image credit:NASA/courtesy of nasaimages.org

Our Earth-Moon system is unique in the solar system. The Moon is 1/81 the mass of Earth while most moons are only about 3/10,000 the mass of their planet. The size of the Moon is a major contributing factor to complex life on Earth. It is responsible for the high tides that stirred up the primordial soup of the early Earth, it’s the reason our day is 24 hours long, it gives light for the variety of life forms that live and hunt during the night, and it keeps our planet’s axis tilted at the same angle to give us a constant cycle of seasons.

A second moon would change that.

For his two-mooned Earth thought experiment, Comins proposes that our Earth-Moon system formed as it did — he needs the same early conditions that allowed life to form — before capturing a third body. This moon, which I will call Luna, sits halfway between the Earth and the Moon.

Luna’s arrival would wreak havoc on Earth. Its gravity would tug on the planet causing absolutely massive tsunamis, earthquakes, and increased volcanic activity. The ash and chemicals raining down would cause a mass extinction on Earth.

But after a few weeks, things would start to settle.

Luna would adjust to its new position between the Earth and the Moon. The pull from both bodies would cause land tides and volcanic activity on the new moon; it would develop activity akin to Jupiter’s volcanic moon Io. The constant volcanic activity would make Luna smooth and uniform, as well as a beautiful fixture in the night sky.

New Horizons captured this image of volcanic activity on Io. The same sight could be seen of Luna from Earth. Image credit: NASA/courtesy of nasaimages.org

The Earth would also adjust to its two moons, giving life a chance to arise. But life on a two-mooned Earth would be different.

The combined light from the Moon and Luna would make for much brighter nights, and their different orbital periods will mean the Earth would have fewer fully dark nights. This will lead to different kinds of nocturnal beings; nighttime hunters would have an easier time seeing their prey, but the prey would develop better camouflage mechanisms. The need to survive could lead to more cunning and intelligent breeds of nocturnal animals.

Humans would have to adapt to the challenges of this two-mooned Earth. The higher tides created by Luna would make shoreline living almost impossible — the difference between high and low tides would be measured in thousands of feet. Proximity to the water is a necessity for sewage draining and transport of goods, but with higher tides and stronger erosion, humans would have to develop different ways of using the oceans for transfer and travel. The habitable area of Earth, then, would be much smaller.

The measurement of time would also be different. Our months would be irrelevant. Instead, a system of full and partials months would be necessary to account for the movement of two moons.

A scale comparison of the Earth, the Moon, and Jupiter's largest moons (the Jovian moons). Image credit:Image Credit: NASA/courtesy of nasaimages.org

Eventually, the Moon and Luna would collide; like the Moon is now, both moons would be receding from Earth. Their eventual collision would send debris raining through Earth’s atmosphere and lead to another mass extinction. The end result would be one moon orbiting the Earth, and life another era of life would be primed to start.

Source: Neil Comins’ What if the Earth had Two Moons? And Nine Other Thought Provoking Speculations on the Solar System.

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squidgeny
Member
squidgeny
December 27, 2011 6:20 PM
The pull from both bodies would cause land tides and volcanic activity on the new moon; it would develop activity akin to Jupiter’s volcanic moon Io. I’m sceptical. Io orbits Jupiter at roughly 400,000km and this hypothetical moon would orbit at roughly 200,000km – twice as close, but orbiting a body 300 times less massive. I doubt that the Earth (even with the Moon’s help) would exert so much tidal stress on “Luna”, even it were Moon-size. I may be wrong. The constant volcanic activity would make Luna smooth and uniform It’s worth noting that Ganymede orbits Jupiter at roughly 1,000,000km (just over 5 times as far – but again, orbiting a planet 300 times more massive) and… Read more »
magnus.nyborg
Guest
magnus.nyborg
December 28, 2011 8:36 AM
“Eventually, the Moon and Luna would collide; like the Moon is now, both moons would be receding from Earth. I’m no celestial mechanician but if they’re both receding how would they collide? Would the inner moon necessarily recede faster?” I suspect you are correct in your implication. Two important factors: 1. The ‘inner’ Luna would probably receede slower than the outer Moon. The tidal interaction that causes this, is also dependant on the differences between earth rotation and moon orbit. As an example, a moon in a geosynchronized would not receede at all. It is not that simple, but assuming faster recessions seems unwarranted. 2. Orbital interaction between Moon and Luna would require investigation, and likely they would… Read more »
HeadAroundU
Guest
HeadAroundU
December 27, 2011 6:24 PM

With 2 moons you would be Tamy Megashira Tautel. grin :d :$ Seriously, what are you? German/Japanese???, that would be a scary combination. Hard work would look like weak words. That would be a perfect robot. grin

Amyway, I liked the article even though it is kinda fictional.

HeadAroundU
Guest
HeadAroundU
December 27, 2011 6:24 PM

With 2 moons you would be Tamy Megashira Tautel. grin :d :$ Seriously, what are you? German/Japanese???, that would be a scary combination. Hard work would look like weak words. That would be a perfect robot. grin

Amyway, I liked the article even though it is kinda fictional.

The Bobs
Guest
The Bobs
December 27, 2011 7:49 PM

” what are you? German/Japanese???, that would be a scary combination.”

What are you, some kind of racist? Oh wait, you put a smiley face on it, so it must be OK.

The Bobs
Guest
The Bobs
December 27, 2011 7:49 PM

” what are you? German/Japanese???, that would be a scary combination.”

What are you, some kind of racist? Oh wait, you put a smiley face on it, so it must be OK.

HeadAroundU
Guest
HeadAroundU
December 28, 2011 1:59 AM

Well, what’s your origin sounds better. I’m sorry guys. I don’t want to sound like a racist. Otherwise, is it taboo to ask?

HeadAroundU
Guest
HeadAroundU
December 27, 2011 6:24 PM

With 2 moons you would be Tamy Megashira Tautel. grin :d :$ Seriously, what are you? German/Japanese???, that would be a scary combination. Hard work would look like weak words. That would be a perfect robot. grin

Amyway, I liked the article even though it is kinda fictional.

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
December 27, 2011 8:48 PM
I’ll pitch in with squidgeny, Comins seems severely off on particulars. Especially his biology sucks: The size of the Moon is a major contributing factor to complex life on Earth. Preposterous! We don’t know that. _Some_ theories puts tides as important for abiogenesis (simple life) and land life (complex life). But besides being in the minority, they have very non-parsimonous predictions by their very nature, having to drag in tides with them. It is responsible for the high tides that stirred up the primordial soup of the early Earth, What does that even mean? The soup, if that is the theory under consideration, wouldn’t need stirring to evolve. In fact tides would be, as it is for life… Read more »
edg
Member
December 27, 2011 9:08 PM
I’ve read Comins new book, (and its predecessor, “What if the Earth had No Moon”, and I don’t think it’s very good. Unlike the first book, his ideas are poorly considered and constrained (why didn’t he have the second moon be there at the start instead of getting captured after formation, which is very unlikely)? And he also neglects to consider resonances in the two-moon scenario, which would prevent the moons from colliding. “What if the Earth Had Two Moons” is full of incomplete ideas (the chapters just suddenly stop in the middle of things, as if he’d just ran out of ideas and stopped writing) and contains some rather poor fiction to introduce the chapters. I would… Read more »
Anonymous
Guest
Anonymous
December 27, 2011 10:42 PM

I’d be more worried about being born female. Biological rhythms would be an absolute mess, and biological / reproductive sex of all the creatures of the Earth would be completely different.

HeadAroundU
Guest
HeadAroundU
December 28, 2011 3:12 AM

Hey, you are a misogynist, but that’s ok, I’m a racist. grinDD

hdmi
Guest
December 27, 2011 11:04 PM

What if your mom was your dad?

HeadAroundU
Guest
HeadAroundU
December 28, 2011 3:13 AM

shemale hefemale

sigidunum
Guest
sigidunum
December 28, 2011 12:03 AM
No offense to Mr. Comins, but this is pretty junky. Just to point out one problem: all other things being equal, tidal forces are proportional to the inverse cube of the distance. So if Luna were the same mass as the Moon (he doesn’t say this, but let’s assume) and the same density as well, then it would produce tides 8 times as great. That would result in some huge tides, but even in the Bay of Fundy it wouldn’t mean “thousands of feet”. To be even more precise: solar tides account for about 1/3 of ocean tides, and lunar tides the other 2/3. (That’s why we have spring and neap tides — when the two are working… Read more »
Lawrence B. Crowell
Member
Lawrence B. Crowell
December 28, 2011 1:28 AM

I am not likely to read this book. It is too much of a “what if?” However, the rather extensive predictions here seem to mean this moon has a mass equal to the mass of our current moon. A rather modest body, say the mass of Enceladus or even small such as the martian moons would be far less perturbing. In fact if we could park a 10 km diameter asteroid at geosynchronous orbit we might actually stand a chance of building a space elevator. An Enceladus massed moon might be mildly perturbing. If it were in a close orbit to Earth it might actually be stable.

LC

Anonymous
Guest
Anonymous
December 28, 2011 10:51 AM
Reading through comments, I thought to add this as an, au contraire, to a few points: From Astrobiology Magazine ( 10/29/07 ) by Bernard Foing (SMART-1 principal scientist): PhD in Astrophysics & Space Techniques; Astronomer for 3 years at ESO, Chile…; Professor of Astrophysics; researcher at CNRS since ’86 — Wikipedia “The crust of the Earth is also affected. THE MOON’S TIDAL FORCING causes significant heating and dissipation of energy to take place. Part of this energy is heating the Earth, and part of it is dissipated by forcing the Moon to recede from the Earth over time. There are people who propose that the tidal effect of the Moon may have helped trigger the convection on the… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
December 29, 2011 1:58 PM
That touches many or the Rare Earth topics, including those that are raised by creationists: THE MOON’S TIDAL FORCING causes significant heating and dissipation of energy to take place. We don’t really know all the characteristics of plate tectonics, including the lower limit. It is believed that Earth is marginal, so the same argument can be made for a little less water. (The presence of water is that makes subducted oceanic plates sufficiently malleable and melting.) However, it is believed from modeling that larger terrestrials have plate tectonics with or without “rare” amounts of water, “rare” amounts of early large and close moons, et cetera. The Moon has been A STABILIZING FACTOR FOR THE AXIS OF ROTATION OF… Read more »
squidgeny
Member
squidgeny
December 29, 2011 2:15 PM

We would certainly had more volatiles for extended biosphere lifetime without the Earth-Moon impact […] We would have needed the carbon the Moon stole from us!

But wouldn’t that carbon have been in the impactor that formed the Moon anyway? If the Moon hadn’t formed, then presumably that impactor would never have existed.

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
December 29, 2011 2:20 PM

Good and quick catch! I hadn’t thought that one out.

I guess this could be argued both ways:

– If we are not given the impactor, we are worse off on volatiles.

– If we are given the impactor, we are better off without the Moon. The ideal result would have been deorbited satellites, as for Mars. Most moons from a presumed impact deorbited/will deorbit (Phobos), it will keep only Deimos indefinitely.

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
December 29, 2011 3:00 PM

Suddenly I lost edit capability. To add to r

Anonymous
Guest
Anonymous
December 29, 2011 4:32 PM
Well numerical simulations are one thing, real-world physics are another. Mars does seem to have a tale to tell in that regard. There may be a story in the retrograde motion of Venus to hear, as well, another lifeless wreck of a world, in the local neighborhood of stark contrasts to one Life-World turning in time within an array of fine-balances through space: Moon of just the right weight, size, distance (in month cycling orbit, part of our celestial time-clock). Maybe a big moon is not so critical in a Billion-year emergence-of-life Evolution frame-scenario – maybe; I, for one, would not want to bet the future of our Geo-physical stability (oceans-continents-atmosphere: tectonics motions and ocean movements (levels too);… Read more »
Chandrashekhar
Guest
December 29, 2011 9:49 AM

some fiction this. and it sure started a debate.. a good read, fictionally, but as the commentators have pointed out skilfully, lots of junk. or should we say, debris? wink

Robert Geller
Guest
Robert Geller
January 1, 2012 11:57 PM
I think some perspective might be helpful regarding the criticisms made below, because too many interesting and plausible scenarios are being disregarded by the commenters. The book by Neil Comins, “What if the Earth Had Two Moons” is a serious work by an accomplished physics and astronomy professor, and author of one of the country’s leading astronomy textbooks (Discovering the Universe). This Universe Today article summarizes some results, but can’t be as detailed with the book’s assumptions and methodology. For example, the tidal forces and orbital characteristics discussed in the book are based on high-quality simulations of the sort used elsewhere in astrophysical research, and the book explains why various masses, radii, etc, where used in simulations. By… Read more »
Robert Geller
Guest
Robert Geller
January 2, 2012 2:38 AM
I think some perspective might be helpful regarding the criticisms made below, because too many interesting and plausible scenarios are being disregarded by the commenters. The book by Neil Comins, “What if the Earth Had Two Moons” is a serious work by an accomplished physics and astronomy professor, and author of one of the country’s leading astronomy textbooks (Discovering the Universe). This Universe Today article summarizes some results, but can’t be as detailed with the book’s assumptions and methodology. For example, the tidal forces and orbital characteristics discussed in the book are based on high-quality simulations of the sort used elsewhere in astrophysical research, and the book explains why various masses, radii, etc, where used in simulations. By… Read more »
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