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View Full Version : What if Earth and Moon were same size?



CaptainToonces
2010-Jul-17, 06:22 AM
What would be different if the event that created the moon had instead created two planetoids of equal size?

WayneFrancis
2010-Jul-17, 07:56 AM
I don't know if I'd call 2 object that size planetoids. They'd both still be almost 3 times the size of Mars.

Practicality wise I'm not so sure how easy it would be to end up with that kind of configuration. A lot would depend how far apart they ended up being initially. If they started where the moon is thought to then the geological tides would keep the surface molten for a very long time. I think they'd become tidally locked while still orbiting each other because of the same effect that would have the Terra-Luna system tidally locked.

neilzero
2010-Jul-17, 01:13 PM
Assuming we lived on one of them before either tide locked, our day might be as long as a month, more likely a few days. I think they would be separating very gradually. Eclipses would occur more often, especially eclipse of the Sun. Tides would be higher, but decreasing. We might have less iron and heavy metals near the surface, as they would likely be divided approximately equally. I guessing, so please correct. Neil

swampyankee
2010-Jul-17, 04:26 PM
It's possible that neither would have long term habitability; that would depend on if plate tectonics (or something like it) exists, as without this process, some amount of carbon could be permanently locked into sedimentary rocks.

I don't think that having the two planets tidally locked to each other would be a real problem for life, as long as they don't both tidally lock to the Sun. Of course, if that happens, they'd probably crash together, which would tend to be rather damaging to any life forms.

EDG
2010-Jul-17, 04:33 PM
Assuming Earth would be smaller and the moon would be bigger (i.e. add the masses of Earth and Luna, divide by 2, divide by an assumed average density of 5500 kg/m for the combined body to find the volume, and then extract the radius from the volume), and assuming I've done the maths right... both bodies should be be about 5082 km in radius. A few differences:

- Earth would have lighter gravity, but probably would still be able to hold onto oxygen and nitrogen.
- Earth would cool down more quickly since it's less massive (less radiogenics) and smaller (radiates heat faster). However, tidal heating might counter this while the orbital evolution was happening.
- The moon would probably have an atmosphere and maybe water. Would be a very nice incentive to have a manned space program!
- If they're equal mass, then they're probably going to be sync-locked to eachother like Pluto and Charon. Initial tidal heating would have probably been enormous for both worlds, not sure how far the "moon" would have been pushed away from Earth before the rotation periods became the same as the orbital periods. I'm not sure that the bodies would be separated by their current distance - they'd probably be much closer (half the current distance at most?). Solar tides would also complicate things. The day length for both worlds is likely to be about 9.68 (current) earth days long if they're orbiting at half the current earth-moon separation (if I've done the maths right). Such a long rotational period would have implications for life on both planets, but I don't think it'd be totally uninhabitable.
- the moon would likely have a circular, non-inclined orbit. It would only be visible from one hemisphere (could cause some interesting legends in local cultures living underneath it, and in those who never see it).

Tobin Dax
2010-Jul-17, 05:14 PM
- the moon would likely have a circular, non-inclined orbit. It would only be visible from one hemisphere (could cause some interesting legends in local cultures living underneath it, and in those who never see it).

That comment made me realize that one (equatorial?) area on each body would see a solar eclipse once per day. It definitely sounds like this could make for some interesting legends.

EDG
2010-Jul-17, 05:23 PM
Imagine if you were a culture that lived beneath the perpetual gaze of the "eye of god" (i.e. the moon), then as you explored further from your hemisphere you'd watch it sink behind you and then disappear below the horizon. Would this be taken as a sign that your god doesn't approve of your journey and is abandoning you?

novaderrik
2010-Jul-17, 11:19 PM
maybe it would be a sign that your god thinks that you don't need his/her constant supervision..

EDG
2010-Jul-18, 12:02 AM
Either option would be fear-inducing. ;)

mugaliens
2010-Jul-19, 07:47 AM
Either option would be fear-inducing. ;)

I think novaderrik's postulate could be considered encouraging.

Spoons
2010-Jul-19, 08:28 AM
CaptainToonces, is that the sort of review you were looking for?

PS:
Assuming Earth would be smaller and the moon would be bigger
Bold assumption!

astromark
2010-Jul-19, 10:00 AM
That is the problem... not the answer but the question... Its so hard to know what the OP really wants to know...

Are we talking of a Earth and Moon where both are of the same size ?

Are they orbiting each other and at what distance ?

Is a assumption of tidal locking reasonable ?

Yes, yes, just as is., and Yes.

The list of unknowns is massive.

Never mind atmosphere content and temperature which would be very different if the rotational velocity were different.

or that of the continental plate thickness and movement of it. Presence of liquid water.... iron core

All of the what ifs and maybe's spoil the intent of the question. How do we fix that ?

Lets just make SOME assumptions. Two Earths as this one. Orbiting about a common Lagrange point.

Both supporting a eco systems as this one does... With stability... four billion years and counting...

rotating at velocities as presently... at a distance supportive of stability... the list is huge...

Now lets talk of the view. Yes. Interesting.

Could we expect to be at war... Oh yes. I feel ashamed... :eh:

jlhredshift
2010-Jul-19, 11:59 AM
Imagine if you were a culture that lived beneath the perpetual gaze of the "eye of god" (i.e. the moon), then as you explored further from your hemisphere you'd watch it sink behind you and then disappear below the horizon. Would this be taken as a sign that your god doesn't approve of your journey and is abandoning you?

This is getting off OP, but I have wondered similar things about NGC 3314; the galaxies not the person.

Dale Botha
2010-Jul-19, 01:24 PM
What would the gravitational consequences be if the relative sizes were such that the center of gravity was located close or at the surface of one of them? Would there be less gravity there? How much less? Zero?

EDG
2010-Jul-19, 10:39 PM
Bold assumption!

Why? It seems logical to start with a scenario where the existing mass in the system is redistributed equally between the two. Do you have an issue with that?

dgavin
2010-Jul-20, 12:21 AM
Well in reference to the OP:

Such an impact event that could produce two similar sized worlds while technically is very remotely possible.

Likely it would be a double planet (two planets orbiting a common point in space) with a very small moon or two far outside thier orbits, also orbiting that common point in space.

After 4.5 billion years though there would be some disadvantages for life.

Techtonics of a some what smaller earth 2/body system, would likely such down shortly after they became tidaly locked to each other. My guess is around 3 billion years after impact, the planets would be tidaly locked. Techtonics would totaly cease about 500 million years after that. Co2 and other gas recyling would stop, if an industrialized society developed that could counteract the loss of C02 recyling via pollution emission, it could keep the planets viable for them for a while.

Eventualy though, you would wind up with two cold planets, with thin atmospheres almost devoid of carbons. The only life would likely be kemosynetetic (extreamophiles) baterium and possible some primative life forms that feed of them, but those would also slowly die off, as the chemical deposits on the surface are not beign replenished by Techtonics either.

I would put the max life bearing age of such a system at about 3.7 billion years. The planets probably would not have enough time to develop intelligent life, much less a dinasaur age life forms. However I also think such a two body system might lead to a faster evolotionary time scale, so i wouldn't say it's imposible for such a system to spawn life forms. It's just hard to say as we really only have earth for a model. My best guess is it would be very unlike for life to evolve beyond primative things like jelly fish, or land based equivlents.

Spoons
2010-Jul-20, 12:35 AM
Why? It seems logical to start with a scenario where the existing mass in the system is redistributed equally between the two. Do you have an issue with that?
I was kidding. Your assumption seems not to really be an assumption but rather a necessity, since the event that created the Earth-Moon system here started with a certain size, and a 50% split between each would obviously make Earth smaller and the Moon larger.

It was a little fun "tack on" to the important part, which was to attempt to either get approval from the OP of the answers provided or more background info in their question.

neilzero
2010-Jul-20, 12:40 AM
I like Mark's assumptions, except war. If they were about 600,000 kilometers apart, not tide locked 4.6 billion years ago, they would have moved closer until quite recently when one of them tide locked, so we could have the same relationship as with the present earth-moon system, except the faster rotating one would have a day more than 24 hours long, but perhaps not much longer. With double the mass, they would sweep up almost twice as much stuff. Present Earth mass for both is believable. Neil

astromark
2010-Jul-20, 04:08 AM
I agree with Neil... forget the war quip.. Two Earths... at whatever distance apart that gets the stability right.

I would like to know if you all think it could be possible. Naturally. Yes we should get the book out...

I can imagine a whole culture of co-operation... I can imagine a lot of things... We could have a bi-polar society... Oh:eh:....Mark

eburacum45
2010-Jul-20, 06:53 AM
Tectonics of a some what smaller earth 2/body system, would likely such down shortly after they became tidally locked to each other. My guess is around 3 billion years after impact, the planets would be tidaly locked. Tectonics would totaly cease about 500 million years after that.

This is an intriguing possibility, but do you have a source for this? Tectonics might continue for an arbitrary time after tidal locking occurs, depending on the size of the planet, thickness of the crust, and so on. In the case of a double planet, the distance between the two planets is also very important. It is conceivable that the two planets could be close enough to form a so-called Rocheworld, where each planet fills the roche lobe of the other, like a close binary star. Planets that close together would almost certainly be geologically active.

Tobin Dax
2010-Jul-20, 07:20 PM
This is an intriguing possibility, but do you have a source for this? Tectonics might continue for an arbitrary time after tidal locking occurs, depending on the size of the planet, thickness of the crust, and so on. In the case of a double planet, the distance between the two planets is also very important. It is conceivable that the two planets could be close enough to form a so-called Rocheworld, where each planet fills the roche lobe of the other, like a close binary star. Planets that close together would almost certainly be geologically active.

By what mechanism? The small planets would cool more quickly, and there would be not tidal flexing once they are tidally locked. I'd expect that the two planets would almost certainly not be geologically active.

Trakar
2010-Jul-20, 09:07 PM
What would be different if the event that created the moon had instead created two planetoids of equal size?

with the preface that I have not yet scanned the rest of the thread:

"Equal size" or equal mass and composition?

Its very hard to get latter to happen in the type of impact event theorized in modern Lunar genesis models. There are probably a lot of senarios that would produce different results than what we see around us today, but producing two half-mass, equal composition "Earths" in orbit around each other would probably require significantly different situational circumstances.

If we are just looking at similar resultant physical diameters, we could probably obtain some of that with some tweaks to initial conditions. Probably still going to have one of them with the vast majority of the composite iron-core. We may be able to get a situation where a large amount of the mantle and crust material of the pre-impact bodies ends up coalescing into a co-orbiting body, but its probably going to be closer to resembling an almost Venus sized primary (with a slightly larger than Earth-sized core) in co-orbit with an almost Mars sized secondary (with a smaller than Lunar-sized core).

Interesting for sure, but I'm just not sure how we get to co-orbiting mini-Earths from the conditions greatly similar to those believed to have actually occurred.

EDG
2010-Jul-23, 09:10 AM
I was kidding.

Ah, I wasn't sure... this is what smileys are for! :)

EDG
2010-Jul-23, 09:11 AM
This is an intriguing possibility, but do you have a source for this? Tectonics might continue for an arbitrary time after tidal locking occurs, depending on the size of the planet, thickness of the crust, and so on. In the case of a double planet, the distance between the two planets is also very important. It is conceivable that the two planets could be close enough to form a so-called Rocheworld, where each planet fills the roche lobe of the other, like a close binary star. Planets that close together would almost certainly be geologically active.

Also, remember that our Moon's crust is thicker on the farside than the nearside. If that is something to do with its tide-locked state then this might happen in the double-planet version too, possibly more extreme as well.

Spoons
2010-Jul-23, 09:13 AM
Ah, I wasn't sure... this is what smileys are for! :)

Aw, but I'm trying to be nonchalant.

Oh, err... ;)

EDG
2010-Jul-23, 09:14 AM
By what mechanism? The small planets would cool more quickly, and there would be not tidal flexing once they are tidally locked. I'd expect that the two planets would almost certainly not be geologically active.

I don't think 5000 km radius is going to be small enough for the planets to have lost enough heat that tectonics shuts down. Particularly since they'll have had more heating from tides early on. And also, tides are ongoing, since solar tides will still be acting on the system. They should be taking angular momentum out of the double-planet system so that they get closer together (and thus the orbital period decreases over time, and the rotation periods increase to keep up).

Jens
2010-Jul-28, 07:46 AM
What would the gravitational consequences be if the relative sizes were such that the center of gravity was located close or at the surface of one of them? Would there be less gravity there? How much less? Zero?

I think that when you're standing on a planet, what is important is the gravity of that body, not the gravity of the system it is in. So the gravity would not be zero. For example, the center of the sun-Jupiter system is I think about on the surface of the sun, but that wouldn't make the gravity there zero. I don't even know if it would be less, because the two bodies are in free-fall toward one another. There is some tidal effect, I believe. Also for example, a person standing on the moon is primarily attracted to the moon, because it is so close, and is not pulled off toward the much-larger earth.

Githyanki
2010-Jul-30, 04:32 AM
Lets assume an Earth-sized world was there instead of Morpheus and a similar impact, except the iron cores didn't merge and about an equal share of the iron cores to each other? Then what's the problem? They both would have plate tectonics at 4.5BY as they both would have about an equal amount of radioactive decay heat.

Both would have similar life-forms as biological material would be exchanged between the two over billions of years of meteor impacts.

Both worlds would have about an equal amount of water and would have formed plate tectonics at about the same time; even the super-continent cycles are the same (they probably won't be), the other world could be like a mirror to them (not exactly). Also, the opposite ends would be a little drier due to tidal forces and lack of an intense atmosphere; 240 hour days will produce slow winds that won't move water vapor as much.

Spoons
2010-Jul-30, 10:17 AM
Yeah, that would be cool.

DonM435
2010-Jul-30, 01:03 PM
Hey, like, what if the Moon were, you know, bigger that the Earth? I mean, would we have to exchange their names, or what?

Spoons
2010-Jul-30, 01:41 PM
Dude, that's the sort of thing you decide over a meat lovers pizza. Waaaaaay!

Trakar
2010-Jul-31, 11:55 PM
Lets assume an Earth-sized world was there instead of Morpheus and a similar impact, except the iron cores didn't merge and about an equal share of the iron cores to each other? Then what's the problem? They both would have plate tectonics at 4.5BY as they both would have about an equal amount of radioactive decay heat.

Both would have similar life-forms as biological material would be exchanged between the two over billions of years of meteor impacts.

Both worlds would have about an equal amount of water and would have formed plate tectonics at about the same time; even the super-continent cycles are the same (they probably won't be), the other world could be like a mirror to them (not exactly). Also, the opposite ends would be a little drier due to tidal forces and lack of an intense atmosphere; 240 hour days will produce slow winds that won't move water vapor as much.

Just because you can verbally make these assumptions, doesn't make these assumptions correspond to what is actually possible or what would actually result from the intial parameters you are laying out. IOW, idle day dreams do not necessarily correspond to the actual physics involved.

Githyanki
2010-Aug-01, 08:05 PM
Then prove me wrong.

EDG
2010-Aug-01, 08:27 PM
Then prove me wrong.

Try answering these questions: How big would both the worlds be in your scenario? How do you guarantee that they have the same amount (and type) of radioactive materials? How do you guarantee that they'd have similar lifeforms? Would it be likely for meteor impacts to blast material off the surface of one body sufficiently fast to reach escape velocity and be captured by the other one? Why would they have similar amounts of water? Why would their crustal evolution be the similar? Why would their atmospheric compositions be the same? etc.

Granted, you're making assumptions, and I think it's harsh to call them "idle daydreams", but it doesn't hurt to detail the assumptions more thoroughly.

Trakar
2010-Aug-01, 11:29 PM
Then prove me wrong.

You have a peculiar take on the appropriate manner of scientific proposal and discussion.

Githyanki
2010-Aug-02, 06:31 AM
Try answering these questions: How big would both the worlds be in your scenario? How do you guarantee that they have the same amount (and type) of radioactive materials? How do you guarantee that they'd have similar lifeforms? Would it be likely for meteor impacts to blast material off the surface of one body sufficiently fast to reach escape velocity and be captured by the other one? Why would they have similar amounts of water? Why would their crustal evolution be the similar? Why would their atmospheric compositions be the same? etc.

Granted, you're making assumptions, and I think it's harsh to call them "idle daydreams", but it doesn't hurt to detail the assumptions more thoroughly.

'Bout Earth-sized: luck: by lifeforms, I mean single-celled: 4.5 billion years to; besides, we all know life came from meteors from Mars: considering water came from accreted asteroids & comets, water would be similar in amount: assuming the super-continent cycle is the same, then Si, senor: algae.

The real difference is multi-cellular life; chances are, one would would have then whereas one would only have singular, considering the time spans.

Trakar
2010-Aug-02, 08:32 AM
'Bout Earth-sized: luck:...

ah,...the correlary to "any science insufficently understood is equivilant to magic?" Before getting carried away with "lucky" imaginings, and musings it's probably a good idea to look at what current best understandings are with regards to planetary formation and in particular double planet formation. A good place to start might be with the formation of the current Earth-Moon system and one of the leading considerations for how this happened and what conditions supported and led to this event.

http://arxiv.org/PS_cache/astro-ph/pdf/0405/0405372v2.pdf

eburacum45
2010-Aug-02, 08:42 AM
If the Earth and Moon were formed from a collision, but coincidentally were the same diameter, there is no guarantee that they would both have the same fraction of core, mantle, crust and hydrosphere; in fact I would be very surprised if they did. So even if the two objects had the same 'size', they need not have the same mass, or the same gravity and surface conditions.

Conversely, even if the two objects had the same mass, once again they need not have same fraction of core, mantle, crust and hydrosphere, so they could have significantly different diameters from each other, different surface gravity, and once again different surface conditions.

On top of this the thickness of the crust fraction might be very different in the two planets, so that tectonic activity would be very different on each world. It would be a remarkable coincidence if both planets closely resembled each other in mass, diameter, crust thickness, percentage of hydrosphere coverage, surface gravity and thickness of atmosphere. Earth-like planets are likely to be very variable indeed in character, so it would be a remarkable coincidence to find two almost identical planets close together, let alone in a binary pair.

Dale Botha
2010-Aug-02, 12:14 PM
I think that when you're standing on a planet, what is important is the gravity of that body, not the gravity of the system it is in. So the gravity would not be zero. For example, the center of the sun-Jupiter system is I think about on the surface of the sun, but that wouldn't make the gravity there zero. I don't even know if it would be less, because the two bodies are in free-fall toward one another. There is some tidal effect, I believe. Also for example, a person standing on the moon is primarily attracted to the moon, because it is so close, and is not pulled off toward the much-larger earth.


Ok...makes sense...I suppose the center of gravity of the system is the balance point of gravity away from the center and not towards it.

thanks!

Shaula
2010-Aug-02, 01:21 PM
I think you are confusing centre of mass and centre of gravity with Lagrangian points. The centre of mass of the Sun-Earth system is inside the Sun but the Lagrangian points (sort of where the gravitational pulls are equal) are in space. SOHO sits in one of them. To get the centre of mass you average the positions of the objects in question weighted by their masses. Rcom = sum(MiRi)/sum(Mi). So for the Sun-Earth the CoM is (using the Sun as a zero point) Me.(1au) / (Me+Ms) or about 1au/333000 = 1.5e8/3e5 = 500 km from the centre of the Sun! The Lagrangian points on the other hand are where gravity cancels (kind of - they are the static solution to certain three body problems). The simplest one (between us and the Sun) is 1.5e6 km or so away from us on a line joining the Earth and the Sun.

EDG
2010-Aug-03, 12:37 AM
we all know life came from meteors from Mars

Um, no we don't know that at all. If you're thinking of the "life on Mars" meteorite (ALH48001) then the evidence contained within it for that is actually not very conclusive. It's tantalising, but not proof one way or another that Mars ever had life, and certainly not evidence that life on Earth started on Mars.