Can a forest moon exist?

Factual Questions
1

I was watching Return of the Jedi last night and was struck by a conundrum, could the plants of a forest exist on a moon? I deducted that even with atmosphere and good soil in place, Earth plants could not survive on our moon for two reasons:

  1. Distance. A slight tilt in the earth’s axis causes degrees from winter to summer to vary about 50 degrees. That would be much higher considering the moons difference form the sun differs much moreas it orbits.
  2. Days. With the way the moon rotates on it’s axis, days last about a month. Few if any of our plants could last that long without direct sunlight or with sunlight for that long.

So what conditions would be necessary for a forest moon to exist? I know there are many more variables possible thanthe ones I mentioned. And the real question, is a forest moon even plausible?

2

It would essentially have to be a mini Earth.

3

I am not well-versed in the global weather dynamics for all planets, whether every planet has colder regions on it’s sides that rarely see the sun. But if you will notice how delicately the earth is placed away from the sun…

It’s almost mystic. If we were any closer to the sun, we would fry like bacon. Any further away and we would freeze do death. I am getting a wierd feeling that there is “something going on”. There is an extreme delicacy that makes life possible.

4

Welcome to the board.

It would be difficult for our moon to have a forest, but the main difficulty will be the atmosphere - because of the smaller gravity, the moon cannot retain much of an atmosphere.

I think the reasons you gave are not particularly serious. The moon doesn’t have a significant tilt to its rotational axis - otherwise we’ll be able to see the north pole of the moon sometimes, and sometimes south. Also, the fact that it orbits the earth wouldn’t influence the weather that much - the distance to the sun will change, of course, but the change is insignificant because the ortit of the moon around the earth is so much smaller than the orbit of the earth around the sun.

It shouldn’t be hard for a plant to survive 15 days without sunlight. Most trees spend the entire winter without absorbing any sunlight (no leaves!).

Now if you consider a much larger moon (Earth size) orbiting an even larger planet (Jupiter size), gravity would be strong enough to retain an atmosphere. In that case I think it’ll be very possible to have a “forest moon.” Though of course it wouldn’t be all forest - the poles would be cold and the equator hot, just like the earth. Only thing I’m not sure is whether the planet’s gravity may have an undesirable effect on the moon’s atmosphere - the tidal forces will stir things up quite a bit, for sure.

5

The Earth’s seasons are not in any way caused by the distance to the Sun. They are caused by the changing angle at which the sunlight strikes the Earth. The difference in the distance to the Sun at different parts of a moon’s orbit are trivial–they would affect the moon’s climate only in very subtle ways.

There is no fundemental reason that a moon the size of the Earth would be impossible. It didn’t happen in our solar system, but under other conditions, who knows. Also, a moon don’t necessarily have to rotate synchronously, particularly if it’s pretty far from the planet (which is a good place to be, given the likelyhood that the planet has a nasty, radiation-filled magnetosphere.)

A few years ago, the easiest objection to a temperate “forest moon” was that gas giants are only found in the frigid regions of the outer solar system, where ices can freeze readily and contribute to the massive planetary core. Look at Europa, for example. Jungle paradise it ain’t. However, many of the extrasolar planets that have been discovered are “roasters”–gas giants very close to their suns. We don’t know whether they formed there (in which case our ideas of planetary formation are probably screwed!), or whether they somehow migrated inward after their formation (though there’s no good explanation of how to do that, either.) Also, many of the planets that have been discovered are too close to the Sun. We don’t know where these things form, or if or how fast they travel inward. Remember that if you want to evolve an ecosystem (much less cute furry critters), you have to have relatively stable conditions for millions to billions of years.

So I’d say on the whole there’s no obvious reason that a “jungle moon” would be impossible, but neither could one say, given what we know about solar system formation, that they’re probably a dime a dozen.

6

Earth’s Moon could hold an atmosphere for at least thousands of years, so that’s not a bar if you want to allow for terraforming a moon into a forested body. I don’t think you could have one naturally, though.

And it might be warmer than you’d think, too. I’d think the air would move around faster on a lower-gravity body, and the Moon is very small,so air heated near the equator would be spread around the whole sphere pretty quickly. Add to that a benign greenhouse effect and I think you’d have quite a warm environment.

Don’t know how well plants would take to two weeks of light/darkness. Earth plants for sure aren’t on cycles anything close to that, so they’d have to adapt. I guess some plants would be able to, others wouldn’t. You’d find the ones that work through experimentation.

Neat idea! Let’s do it.

7

I do realize that the Earth’s seasons are caused by the axis tilt and the moon does not have a tilt. I put those in the same breath so I can understand the confusion.
I was saying that the tilt in the axis causes one hemisphere to be closer to the sun thatn the other, resulting in a temperature difference of 50 degrees. The distance would be much greater for the moon when it orbits. It would be on the side of the earth closer to the son and then farthest away in the period of a month, causing great temperature differences.

Now the question was whether a forest moon was a plausible thing considering the fact that a moon orbits around a planet which is orbiting around a sun. Even if it had sufficient atmosphere, water, soil, etc, I don’t think plant from earth could survive in a moon situation where they could go half of the month without sunlight with great temperature differences form one part of a month to another.

8

[semi-hijack]
It’s been said by some that even if our moon was given an atmosphere, it would leak away in a very short time. I read (somewhere) that the presence of high molecular weight molecules in an atmosphere helps retain lighter gases. So as long as we’re postulating being able to give the moon an atmosphere, suppose we could give it an oxygen-xenon mix[sub]1[/sub]. Would that be retained for a significantly longer period than oxygen-nitrogen?

[sub]1. Yes I know, find enough xenon to supply the atmosphere for an entire planet? Maybe if we had those replicators[/sub]

9

I think I see where your confusion is. The tilt causes the seasons, but not because it brings one hemisphere closer to the sun. It’s because the tilt causes the sun to rise higher in the sky above one hemisphere. The higher the sun in the sky, the more heat is absorbed. It’s the angle between the ground and sunlight that matters - if sunlight strikes the ground at a shallow angle, there’s far less heat absorbed than if it struck the ground at a perpendicular angle.

The difference in distance is completely negligible.

10

The Xenon will remain longer, but the oxygen will escape just as quickly. They are independent - it’s not like they form a heavy compound. The atmosphere will simply consist of oxygen molecules ane Xenon atoms jumping around, and Xenon atoms will not do anything to stop oxygen molecules from flying off into space.

11

I think you havn’t considered all the possibilities in the way that a moon could rotate around the planet. Say the planet rotated around the sun on a horizontal plane. The moon need not necessarily rotate on a horizontal plane as well.

What if it rotated on a diagonal or vertical plane, so it would always have one side of it facing the sun while it too rotated. It would be clear of the planets shadow when it came around the backside. Or does this go against the established laws of planetary physics? I havn’t the foggiest idea myself.

12

I did not think of much, but threw it out there to get some ideas. As to the vertical rotation, still the same problem with half of the planet in darkness and the rotation taking a month, although faster rotations would help, but not take care of the difference in distance from the sun.

The only solution I could come to so far to get enough, and consistent heat to a moon would be the atmospfhere of the planet it orbits. Combine a planet which reflects most of the sun’s energy and a moon that rotates several times during its orbit around the planet and I think the heat problem may be taken care of.

I’ll let the rest of you fight over the issues of keeping an atmosphere and water.

Still, I keep my position that even if our moon had an atmosphere,water, etc. plants could not survive there due to the change in heat.

13

There is no “change in heat”. As has been pointed out, the seasons occur not because one hemisphere is closer to the Sun, but because one hemisphere is getting more direct sunlight, and the sunlight hits the other hemisphere at a more glancing angle. ( http://www.usatoday.com/weather/tg/wseason/wseason.htm )

And here are some numbers: The Moon is 384,000 km from the Earth. The Earth is 149,600,000 km from the Sun. The Sun’s flux (and thus the heat delivered to the Moon) falls off as r[sup]2[/sup]. Thus the heat the Moon receives when it is farthest from the Sun is 1% less than the heat recieved when it’s closest. That is not going to cause the temperature change of fifty degrees that you cite. And remember, the radius of the Earth is only 6400 km, so change in distance between the Sun and a given point on Earth is going to change much, much less.

By contrast, here you can see that the solar insolation per day, which varies due to the angle of the Sun over the course of a year, varies by about 200% (depending, of course, on latitude).

And wishbone, you’re right that if the moon is on a highly inclined orbit, it might not go through the planet’s shadow. However, that orbital plane will be (more or less) fixed in space while the planet goes around in its orbit (just like the Earth’s axis always points in the same direction, causing seasons.) So the moon will have to go through the planet’s shadow sometimes, although, depending on the moon’s orbit, it could just be for a few hours.

14

Thanks for the correction podkayne. What’s your take on plant life surviving on our moon though while going through contant cylces of being in darkness half of the month and in the light half of the month, assuming an atmosphere similar to Earth’s was there?

15

Repeat after me: Most moons are not identical to our Moon. Our Moon is too small to hold an atmosphere and has a slow (one month). There is no reason to suppose that the moon of Endor had either of these properties. A moon would be just as capable of supporting a forest ecology as a planet. It would to be massive enough to hold an atmosphere, to rotate rather quickly, and to be the right distance from its sun, but then, so would a planet.

Quoth Podkayne:

Isn’t it possible to put it in an orbit which precesses at the orbital period of the planet, so that the plane of the orbit is always perpendicular to the Sun-planet line? It seems to me that one of the solar-imaging sattelites (SOHO, I think) was in such an orbit.

wishbone, the positioning of the Earth might not be as mystic as you think. It used to be believed that the “Goldilocks zone” (neither too hot nor too cold, but just right) formed a very narrow band around the Earth’s orbit, but more recent climactic models have extended the outer boundary as far as Mars’ orbit or beyond.

16

Add also to that the anthropic principle (I think that’s right), which basically says that things are the way they are because if they weren’t, we wouldn’t be here to see them. In other words, in a universe this big, it’s no surprise that there’s a solar system like ours, with a planet with water and a breathable atmosphere at the right distance from the sun for life to come about, and there’s probably a whole lot of them. So why should we be surprised that we’re at the right spot? If the Earth weren’t, we wouldn’t be here, but there’s probably another planet out there SOMEWHERE that would do just as well.

17

Chronos, yeah, if you could pick any orbit for the moon that you wanted, you could do that. You could put it at the L1 Lagrange point, like SOHO, too. I was thinking of natural moons, though, and I know of no process that would evolve a moon into that kind of an orbit! The L1 point is unstable, too. Unlike satellites, moons last a long time–and they can’t do station-keeping. : )

nwhittamore, I don’t think that it would be a problem for trees to evolve to cope with long periods of day and night. Remember, these orbits aren’t necessarily 30 days–the orbits of the Galilean satellites are .67, 1.77, 3.55, and 7.15 earth days for Io, Europa, Ganymede, and Callisto, respectively. If the orbital period was very long, the day side would end up pretty warm, and the night side would be a bit nippy, but though I am not a biologist, that doesn’t seem like too insurmountable a problem. During the “night”, the planetward side of the moon would get some shine off the planet, which might help. And the time the moon spends eclipsed behind the planet would be relatively short.

Also, while moons tend to end up in synchronous rotation, it’s not absolutely required. If we take Chronos’ viewpoint and try to set things up just right, we can throw in a few other moons set up just so they keep our forest moon from getting tidally locked. Hyperion, a moon of Saturn, is in just such a situation. Hyperion’s rotation is chaotic, which might not be the healtiest thing for the plantlife, but if you could set things up right, you could probably give the forest moon any rotational period you want.

If you’re interested in reading more about this problem, the Farseer trilogy by Robert Sawyer is set on the inhabited moon of a gas giant. It doesn’t involve much cutting-edge science, but plays on a few interesting consequences of the situation.

18

Add also to that the anthropic principle (I think that’s right), which basically says that things are the way they are because if they weren’t, we wouldn’t be here to see them. In other words, in a universe this big, it’s no surprise that there’s a solar system like ours, with a planet with water and a breathable atmosphere at the right distance from the sun for life to come about, and there’s probably a whole lot of them. So why should we be surprised that we’re at the right spot? If the Earth weren’t, we wouldn’t be here, but there’s probably another planet out there SOMEWHERE that would do just as well.

Also, just as a side note, isn’t it true that eventually, due to tidal forces, a planet-moon system will wind up with the moon’s rotational period (the day) equal to it’s period of revolution (the year, I suppose)? And eventually, in a long long long time, Earth will have a day that’s one current month long?

19

As for holding an atmosphere, you know the Forest Moon of Endor would cut it. Just look at how things and people fall there - its gravity is comparable to Earth’s. However, if I remember correctly, in the holographic display the Rebels had, the Death Star looked to be about one-twentieth the diameter of this Moon. The specs I have give the second Death Star of having a diameter of 160 km, so that would make the Forest Moon as small or smaller than our Moon. So, it must be made of some really dense stuff. If that’s true, it’s quite possible that the matter distribution there is very close to even. The reason that our Moon is tidally locked with us is that one side of it is heavier than the other, but this wouldn’t be a problem there. And as for the days being so long, notice the passage of time in Return of the Jedi - does it seem like days are 300 hours long there? Hardly. So, if you’re going to pick a Moon to have trees growing on, the Forest Moon of Endor makes a lot of sense.

Also, Podkayne is right that the L1 and L2 Lagrangian points are potential maxima, making them, in a sense, unstable, but I believe Jupiter does have so-called Trojan asteroids orbiting in its Lagrangian points, so maybe a natural satellite there is not so far-fetched. I think, though, that if there’s something there that is large enough to have even a small effect on the planet’s orbit, the system is not going to last forever.

20

Right here in our own solar system we have a planet with moons orbiting “vertically”. Uranus pole is tilted at near 90 degrees to the ecliptic. Its moons orbit in the same plane as the equator, so they are orbiting at right angles to the orbital plane of the planets.
This site tells all about it. Unfortunately, all of Uranus moons are too small, but the phenomenon exits, right here in river city, er well, our solar system.

please, let’s not make the obvious jokes

ok, go ahead