Effect of axial tilt on a planet

[woodelf]

Careful. Tectonics is not the root cause. Tectonics are a whole bunch of things we glom together that are all caused by our having a thin crust over a hot, molten, fluid mantle.

So, the root cause is the hot rock not that far under your feet. What keeps the rock hot? Mostly radioactive decay and tidal friction from the Moon.

Yeah--i almost rewrote that to clarify that tectonics is the proximate, not ultimate, cause. But are we sure how much of an impact tidal forces have on keeping the mantle hot? Obviously it contributes, but would we really have significantly less volcanism without the Moon?

 

[S'mon]

if, however, more than 70% ish (forgive the lack of a presciese number here, it's been a long time since the geology class i learned about this stuff in) of the equator's area were covered in landmass, the land would reflect the light back into space, along with all of the heat that comes with it.

AFAIK land as such doesn't reflect that much heat - not unless it's covered in ice.

 

[Desdichado]

Very nice, until you realize that meteors are falling with alarming regularity - perhaps every other day around the equator.

No they wouldn't. The orbits of ring particles, as far as we know, are stable orbits. They don't decay with anywhere near that level of frequency.

The image of the ring as seen from the ground would not look like a gentle ring or even a mass of multitudes of mini-'moons'. Well, yes, it might seem like that latter at its most outter visible edge. But its inner edge would seem to burn. Rarely would an hour pass without a flash of light as a house size chunk of rock - or larger - falls from the sky, burning up as it does so. I imagine the entire inner edge of the ring would seem to shimmer faintly from the near continuous burn up of rock ranging from the size of your finger to the size of your body.

Every one of those statements is inaccurate. It would look like a broad silver band, not a granular band of chunks. It would not burn unless the inner edge were actually within the atmosphere, which is inconsistent with everything we know about ring systems from the four in our solar system. For the same reason, there woudn't be a constant barrage of falling stars on the surface, or in the sky -- the inner rings are too far away. Also, with the rate of ring burn you're talking about, without a constant source to feed new particles into the ring, the ring becomes a very short-lived structure. A few thousand years at most and it's been all burned into the atmosphere, and there is no more ring to speak of.

And there's no reason to assume that particles the size of a house even exist in the ring -- Jupiter's ring is made up of particles that are the size of smoke particles. And even particles the size of a house would burn in the atmosphere before reaching the surface.

 

[Umbran]

But are we sure how much of an impact tidal forces have on keeping the mantle hot? Obviously it contributes, but would we really have significantly less volcanism without the Moon?

Consider that every high and low tide is accompanied by a small flexing of the ground beneath your feet. The moon deforms the rest fo the planet, just as it deforms the oceans. Flex any solid, and you generate some heat. Flex an entire planet, and you generate a whole lot of heat.

Well, look at the other examples - Mars seems to be dead. While we've some evidence for such on Venus in recent geological past, that evidnece is still a bit sketchy. And as far as we can tell, it seems that Venus doesn't have tectonic plates as such, which suggests that this Earth-sized planet may be on it's way to becoming volcanically dead.

Earth, with a large moon, is active. There are moons of the gas giants that seem to be have similar activity (perhaps of ice rather than rock, but it is analogous) - and they have large neighbors. Everywhere we know for sure it is happening, there's a large neighbor nearby.

 

[lgburton]

AFAIK land as such doesn't reflect that much heat - not unless it's covered in ice.

actually, on the albedo scale... (highest to lowest)


clouds
ice
un-vegitated land
vegitated land
open water

...geology 101.

 

[lgburton]

It did? Point me to some sources--i was under the impression that we didn't know what happened more than ~1bya (approx. formation of Rodinia). [btw, that's info-seeking, not authority-challenging]

Anyway, how does what you're saying here interact with the idea that more polar land mass promotes the formation of larger polar icecaps, increasing albedo and causing cooling, and the idea that part of why the Cretaceous/Jurassic/Triassic periods were so warm was the high concentration of land in the temperate-to-tropical region, and almost nothing polar?

Or does that fall into the "all in a single hemisphere" exception, above? [I originally read that to mean N or S, given the context of latitude discussions surrounding it, but maybe you meant E or W, too?]

[And, yes, i'm gonna give that paper a quick read. But an executive summary would be nice. ]

it does interact with that idea, but as an opposite - the more polar landmass, the less likely are large ice-caps, since that means MORE equatorial ocean, which helps keep the world warm and wet...

well, i did mess up, i should have said a few hundred million, rather than a few billion. forgive the error, please . though later in the paper it is noted that these episodes are theorized to have occured as many as 2.3 billion years ago, then ceased until about 750 million years ago.

summary of points relavent to the conversation:
bascially, the more land you have around the equator, the more light and heat are reflected back out into space, so the LESS heat the oceans get.

now, because of the earth's axial tilt, the equator recives the most insolation (ammount of sunlight striking a square foot/meter/whatever) per square (unit of your choice). in fact, equatorial lattitudes get a LOT more insolation that temperate or arctic lattitudes. so, since the oceans on this world soak up the heat and move it around so well, a lot less heat was making it to the poles and temperate lattitudes.

you'll notice in the previous post i made i gave a relative scale for albedo - ice was quite high up there. so, as the temepratures near the poles drop, the ice caps grow, and reflect more of the sunlight that those areas were getting back into space.

it turns into a massive feedback loop - until the landmasses at the euqtor are glaciated as well, and you get a giant snowball. the more ice you have, the greater your average planetary albedo (already high because of all the land around the equator), the more light you reflect into space, the more ice you get, and so forth..


later in the paper, it discusses how a high carbon-dioxide buildup (less life to take in co2, and volcanic emissions) would also hit a critical mass, warming the planet very quickly...

anyhow, my apologies for not responding in a more timely manner, i have no net access fri-sat

 

[lgburton]

Consider that every high and low tide is accompanied by a small flexing of the ground beneath your feet. The moon deforms the rest fo the planet, just as it deforms the oceans. Flex any solid, and you generate some heat. Flex an entire planet, and you generate a whole lot of heat.

Well, look at the other examples - Mars seems to be dead. While we've some evidence for such on Venus in recent geological past, that evidnece is still a bit sketchy. And as far as we can tell, it seems that Venus doesn't have tectonic plates as such, which suggests that this Earth-sized planet may be on it's way to becoming volcanically dead.

Earth, with a large moon, is active. There are moons of the gas giants that seem to be have similar activity (perhaps of ice rather than rock, but it is analogous) - and they have large neighbors. Everywhere we know for sure it is happening, there's a large neighbor nearby.

actually, geologic evidence suggests that venus periodically re-surfaces itself in rather cataclysmic events. there is a massive ammount of volcanic activity on venus' surface... what we don't know is how the inside of venus works, becaue its atmosphere is so dense and acidic that we can't get anything down there to scan it. venus is a very odd case in the solar system, for a lot of reasons.

the problem with the thesis that you are espousing is that you are assuming mars, earth, the jovian moons, and venus to all be of similar size and chemical makeup. i'm not saying you are wrong, nor am i saying that there is evidence to disprove your theory, HOWEVER:

the other known instances of volcanism in the system, in the moons of jupiter, are invalid in this arguement. jupiter is how many times more massive than Io? yes, we are very sure that the volcanism on the jovian moons is the direct result of massive tidal forces. however, considering the mass difference between earth and Io, the differences in location, and the differences in composition (radioactive elements in earth's core, etc..) it's really not a very useful comparison.

now, would mars, if it had a large moon, still be volcanicaly active? most likely. your inital premise is very sound. however, i can throw one bone in your theory: Luna. earth's moon is not tectonicaly active, despite the presance of a larger neighbor. in fact, luna is probably more "dead" than mars is. there is evidence of past volcanic activity on luna, but nothing very recent.

so, if you are going to jump all over and say that you can't call tectonics a base cause, you certainly can't turn around and say the same thing about tidal forces.

 

[Desdichado]

Earth, with a large moon, is active. There are moons of the gas giants that seem to be have similar activity (perhaps of ice rather than rock, but it is analogous) - and they have large neighbors. Everywhere we know for sure it is happening, there's a large neighbor nearby.

But the reverse is not true, which makes your case considerably weaker. Surely there was prior volcanic activity on the moon, hence the various maria, but there is no evidence of any today. And the Moon should have much stronger tidal exertion on it than the Earth.

 

[lgburton]

But the reverse is not true, which makes your case considerably weaker. Surely there was prior volcanic activity on the moon, hence the various maria, but there is no evidence of any today. And the Moon should have much stronger tidal exertion on it than the Earth.

my point exactly ;-)

 

[Umbran]

But the reverse is not true, which makes your case considerably weaker. Surely there was prior volcanic activity on the moon, hence the various maria, but there is no evidence of any today. And the Moon should have much stronger tidal exertion on it than the Earth.

Yes, but for two things:

1)The Moon is much smaller, and notably less dense, than Earth. Those mean a much higher surface area to mass ratio, and thus much faster cooling.

2)The Moon is tidally locked. You don't get friction heating out of tidal forces unless the forces change. The heating comes from repeated/continuous flexing of the body. The forces on a tidally locked body are basically static, and cause no new flexing, and thus no heating.