The Earth, Moon, Tides, Orbital Paths, Life....... heck, everything.

Factual Questions
21

Related to the OP and evolution, there is a situation where the Earth-Moon system wouldn’t cause tides. If the moon was in geosynchonous orbit, there’d be no cyclical tides, just a stationary bulge of water.

Of course, geosynch orbit is pretty darn close for an object that big. Geosynch is roughly 36,000 km above the surface of the earth, the moon’s orbit is about 10x that. But the moon’s orbit has been expanding slowly due to tidal effects (Earth slows down the Moon’s velocity, causing the orbital radius to expand). Do we know if the moon was ever that close, and if so when? I suppose there’s no way to know for sure (since there’s still debate on where the Moon came from), but does taking the current orbital change rate and extrapolating back to geosynch give an obviously wrong answer (before the formation of the solar system, for example)? Or is it too hard to figure out, since the Moon’s tidal effects on the Earth has also changed the length of a day?

22

Well, my primary source is Niven, too. :smiley: Seriously, while I am not a planetary scientist nor do I play one in any distributed medium of fictionaly entertainment, my knowledge on the topic, based upon a cursory reading, is that the moon skimming hypothesis was falsified by modeling of the tidal effect of the moon on the escape velocity of atmospheric molecules in the Earth’s primordal reducing atmosphere. Even a faster moving, closer orbiting moon would not provide a sufficient impetus for outgassing to explain the difference between Venus’ runaway greenhouse effect and Earth’s oxidizing atmosphere. The moon does cause a certain amount of outgassing, of course, but we’re in a more-or-less state of equilibrium, and the effect on the atmosphere beneath the stratopause is insignificant.

The current family of theories that are most widely accepted are based upon the emergence of the carbon cycle dominated by aerobic life, which absorbed most of the organic constituants and precursors and bound them nonvolitile forms. (One estimate I’ve read is that 200 times the original carbon content of the atmosphere is now bound up in plant life and other non-atmospheric forms.) Here’s an astronomynotes.com cite on the effect of the carbon cycle on the Earth’s atmosphere (though it is pretty sparse on detail information.) Because we have essentially no fossil history on the anaerobic life that first developed on the primordial Earth we have a number of uncertainties about how the atmosphere developed and indeed what the original constituants and their proportions in the Earth’s reducing atmosphere we can only guess at the incidence and order of mechanisms and processes, and so there are many competing theories but I think the general principle is accepted by the planetary science community. Perhaps someone with experience in paleometeorology or planetology will be able to provide more details.

Interestingly, although we consider the greenhouse effect to be a dangerous, runaway process, one might imagine that anaerobic based life would think the same about the modern oxidizing atmosphere with it’s hideously caustic proportion of molecular oxygen. Niven has a short story–one of the Draco Tavern tales, though the title escapes me at the moment–about an ancient trader who had previously visited Earth over a billion years previous, and lamented the extinction of the then dominant species to the “atmospheric pollution” of the waste products created by photosynthetic blue-green alge–oxygen! One speculates that such a race might have a complementary fear of the “bluesky effect” eliminating their protective blanket of heat and moisture-protecting clouds.

Stranger

23

Well, even disposing of other considerations (gravitational influence of sun and other major planets, effect of solar wind and albedo, et cetera) this still wouldn’t be the case. One issue is that such a large object in geosynchronos orbit would exert its own significant gravitational pull back on the Earth; therefore, one could not dismiss the mass of the moon as being insignificant as compared with the Earth. Such a system would have both objects tidally locked to each other and have to orbit about a common center of mass. (The earth and moon system do, in fact, orbit about a common COM, but it is well within the surface of the planet.) At the Earth’s current rotational speed, the system would not be stable; the centrifugal effects would exceed the gravitational attraction and pull both bodies apart because the kinetic energy would always be higher than the gravitational potential energy (though I admit I haven’t run the calculation and am just basing this from the guestimate that they wouldn’t coincidentally be in balance).

Also, while the tidal forces might be constant the geologic stresses would not be; both worlds would experience significant shocks and shear effects that would alter their nominal shape and potentially tear them apart. And any irregularity in their orbit about one another–nutation due to out of plane periodic movements, or side components due to precession–would make such a system unstable even over relatively short timeframes. Theoretically, you could have any number of objects orbiting a common center of mass; in reality, we almost always have large masses as the focus (or nearly so) for a much less massive object, Pluto and Charon excepted.

Stranger

24

??

The centifugal effects would not be any greater then, for the earth, than they are now–if you’re assuming that the earth was rotating at the same speed as now.

The Roche limit for the earth is a lot lower than the geosynchronous orbit, so the moon would not be torn apart.

Of what sort? Do you mean from the irregularities in the orbit?

Three or more objects typically do not orbit a common center of mass–for instance, earth satellites do not orbit the common center of mass of the earth/moon/satellite system.

25

The geological stress of the Moon being in geosynchronous orbit wouldn’t be enough to tear either of them apart., although there would be serious volcano and earthquake activity.

You have a point on the second part though. The Earth and the Moon don’t exist in isolation. And a geosynchronous Moon’s orbit would be unstable. In fact you can make the argument that the Moon’s current orbit is unstable, when you take into account the sun, and to a lesser extent Jupiter (and to a lesser extent Venus and Saturn and to a lesser extent, everything else in the solar system) . In fact, you can make the argument that the Moon doesn’t orbit the Earth (in the sense that every other natural satellite orbits it primary), but rather the Moon and Earth orbit the sun in a complicated manner rather like the leapfrog moons of Jupiter orbit Jupiter.

Isaac Newton supposedly said that the orbit of the Moon was the only problem that gave him a headache. The complex interaction of the Earth/Moon/Sun system doesn’t lend well to simplistic Moon orbit scenarios like a geosynchronous Moon.

26

I don’t see why not. The moon is moving away from the earth because of the tidal braking of the earth–at some point, the rotation of the earth could be more than a (current) month, and match the revolution of the moon. That would take a long time, but it’s possible in theory.

27

sciguy was positing a geosynchronous orbit, and I was assuming that he meant that orbit at the Earth’s current rotational velocity. At that speed (assuming for the moment that the moon is rotating about the Earth) the Δv and ω, the rotational velocity of Luna, is enormous due to the curvature of the orbit. Of course, the two bodies are actually rotating about their common center, but for such an orbit to be stable their gravitational attraction has to equal the outward reaction, or an easier calculation is that their kinetic energy has to equal the gravitational potential energy. Such an orbit has one stable rotational speed for a given distance (hence, a single altitude for geosynchronous and geostationary orbits), and finding that stable value would require both speeding up the moon and slowing the rotation of Earth.

Point of note: sciguy used the term geosynchonous (and I unthinkingly copied it) but I believe he actually means a geostationary orbit above the equator.

I’m surprised that the Roche limit for a body the size of the Moon is so low but I’ll take your source’s answer at face value. Even if the bodies are not close enough to tear each other in half and fling the bits into bizarre orbits, I suspect the Moon, being volcanically inert and inflexible, would suffer some severe geological effects, perhaps even to the point of reducing it to disconnected chunks. The Earth, with its molten core, would remain more or less intact but would bulge significantly about its equator.

Well, no; that’s exactly my point. Theoretically, three or more objects of identical mass could form a stable orbit about their common center (assuming they’re all in plane and with the same angular velocity) but in reality this never happens; even if the conditions were just so to permit this, small perturbations in the system cause it to become rapidly unstable. This is noteworthy only for being (I think) the only degenerate, closed-form solution to the otherwise onerous three-body (or more generally, n-body) problem.

Relating back to the OP (and continuing the string of Niven-related asides) I recall that he had a race, the Glick-something-or-anothers, who evolved on a tidally locked worldlets orbiting a red dwarf star. The creatures evolved to expect an environment of high winds, dim light, and freezing temperatures. It seems an unlikely environment for abiogenesis, but then again we have but one datapoint (and our knowledge of that one massively incomplete) to reason from. Cartooniverse asks:

We really don’t know what conditions are required for “advanced life”, and indeed, I find it likely that we’ll probably overlook it when and if we ever find it, especially if it isn’t mobile, carbon-based, water soluble life that we’re accustomed to. We’re hard pressed to acknowledge even the capacity for intellect among creatures that look almost exactly like us, like other primates or ursines; people boggle at me when I describe some of the intelligent and cognitive behaviors we’ve seen in cephalopods.

Stranger

28

The problems are, the Earth isn’t a perfect oblate spheroid (neither is the Moon, for that matter), neither of them orbit in perfect circles, the Moons orbit about the Earth is in a different plane than the Earths about the sun and the Earth, Venus and Jupiter’s orbits are not co-planar. If the Moon were in geosynchronous orbit, it would quickly move out. The orbit would most likely be quasi-stable and it would follow a chaotic path with an attractor that would be centered on the geosynchronous orbit, which is to say, it would average a geosynchronous orbit but would vary.

This is like the Trojan asteroids. They are rarely actually at the Lagrange spots but move around them.

29

I accepted that assumption.

Not necessarily. See the example I gave above, where both the moon and earth slow down. That’s a natural progression that is more or less happening now.

The more synchronous the rotation and revolution, the less the effect. And the ability to break it into disconnected chunks is kinda the definition of the Roche limit.

It would be a permanent “tidal” bulge, like exists on the moon now. The current centrifugal bulge of the earth is twenty kilometers, the tidal bulge would be a lot smaller than that.

You mean, like the moon is not geosynchronous today? With libration?

If not, then I’m pretty sure that the situation is fairly stable. The other effects that you mention would cause the orbit to change, but it would remain geosynchronous, since that response is faster than the others.

Planet X could always attack…