There has been a lot of talk recently about the possibility that the earth was cooler much earlier than was previously believed. This article from Scientific American gives a good introduction:

http://www.sciam.com/print_version.c...7C83414B7F0000

There's something that bothers me, but I'm not sure if it's a valid question. What I'm wondering is, I believe our current assumption is that the earth was once very hot, and the heat that the earth has now is a remnant of that initial heat. So in other words, the earth is slowly cooling from that early Hadean period.

So the obvious (to me) question is, if the earth was cooler at an earlier date, what does that do to the equations about the current temperature? Wouldn't the current temperature also be lower? I'm thinking maybe that the cooling off is reverse-exponential, if that's the correct term, so a large difference at the beginning would only lead to a negligable difference today.

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As above, so below

A few points,

I think the article is talking about the temperature at the surface of the Earth, which is only a tiny fraction of the volume of the Earth. So when one is talking about the temperature of the Earth, or the rate of cooling, it makes a difference if one is considering the surface or the interior.

Second, it isn't just a linear cooling of the Earth from formation to the present. The Earth also generates heat from several sources, including radioactive decay, tidal forces, gravitational differentiation (not sure about the right word, but I mean dense materials settling toward the core). So the interior of the Earth cools slower than you would expect.

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I understand that, but isn't the temperature at the surface of the earth a function of the temperature at the interior? I would assume that most of the energy for the heat at the surface comes from below. Or are you saying that the temperature at the surface is raised by meteor strikes, for example, so that the surface could be hot even with a cold interior?

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More likely that the surface would be cool when the interior was hot.

I think the article was saying that the surface apparently cooled faster than previously thought.

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Basically the crust of the earth is an insulating layer protecting the surface/atmosphere from the temperature of the mantle. The generally held thoughts about how the crust formed and our atmosphere developed is that they kind of helped each other along. As one cooled so did the other and influenced each other until we got oceans which sped things up greatly to get to where we are now. But it was thought that as the first rocks of the crust formed, the atmosphere was still something akin to Venus. The Zircons they found appear look like really old subduction zone basalts. Well that would mean A) rocks that formed closer to the surface than would have been expected. Rocks that old are (If I remember right) all granitic, fairly deep in formation. B) to get geochemistry similar to subduction zone basalts according to all examples we know of you need liquid water to interact with the rock, and for the rock to re-melt, which should not have occurred due to the extreme heat of the atmosphere at the time and another ~few 100 million years.

I’m not sure but I’m thinking maybe the water was from a Comet, maybe a direct impact. The chances are next to zero for something like that, but liquid water on the surface, and a mechanism for re-melt is even more remote. It is still interesting that Crustal rocks seem to have formed a couple 100 million years prior to previously thought, and be maybe more widespread, But I think the jury is still out on the temperature of the atmosphere.

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Early plate tectonics: more evidence for a cooler Hadean

First water, now this: Plate-tectonics may have started over 4bn years ago
It's a good article. Here's the link again: Link

p.s. seeing as this strictly matches the title but not the OP, plus the fact that there seem to be many geologists here, I might mirror this as new thread. If no-one minds.

A brief and non comprehensive history of Earth:

Accretion (protoplanetary disc) > Big Whack (Moon formation) > differentiation (core formation) > surface cooling (retention of volatiles)

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"Quaerendo inventis"

I'm wondering if some of this has something to do with the warming sun thought to be significantly lower in output long ago compared to now. Most of our understanding has already been posted here. It's really hot deep down - like surface of the Sun hot, 6000K. Dirt/rocks are a major insulator keeping us from being evaporated by the heat. Much of this is the heat of formation of the planet. Best scenario for the moon is we were slammed by a Mars sized object quite some time back. That added a lot of energy to the Earth and evidently created the debris that combined into our moon. Thermal energy leakage is not enough to keep the surface anywhere close to warm. That's all solar radiation doing that and even this keeps us above freezing at least in some areas only because of our atmosphere.

How did the surface cool so fast, is what I'm confused about? If the interior was so warm, that is.

Is rapid cooling of just the surface normal? Is there a well-known mechanism?

Poor thermal conductivity of rock. Heat couldn't be transported from below fast enough to keep up with the heat radiated away into space. A thick blanket of rock makes an effective insulator, without the sun's input, the surface would be a lot cooler...most of our surface heat comes from the sun.

It's also not jut thermal inertia keeping the core warm. First, the same insulating effect means a relatively low amount of radioactive decay would keep the core warm for a considerable amount of time. Second, the inner core is very compact solid iron, the outer core is liquid. Removing heat doesn't cool the outer core, it just allows more of it to solidify, similar to how ice water at the surface stays at the freezing point until it has completely frozen or melted. So while the core is losing heat, it's not necessarily dropping in temperature.

Sorry cjameshuff, what you said makes perfect sense, but I worded my question badly. My fault.

I should have asked: how do we account for the faster than expected rate of surface cooling? How did the initial insulation of rock appear so fast?

As I understand it (and this is where I may be going wrong):

1. We used to think the surface of Hadean Earth was hot.
2. We now have reasonable evidence to think that it might have been cooler.

aurora and Hugh Jass referring to the OP article:
Do you know what we've changed in our models to accommodate (2), instead of (1)?

Or is this still a matter of dispute? I haven't gone and blundered into ATM have I?

This debate parallels that between Darwin and Lord Kelvin in the 19th century. Kelvin calculated from the properties of Earth rocks and estimates of the initial temperature that the Earth could be no more than 100 million years old, which was far too little time for Darwin's theory to take effect. Extraordinarily, when his theory was attacked, pricipally for ignoring convection ("Are you mad,Sir? In the solid Earth?!") in favour of conduction, he revised his estimate - downwards! To 20 million years!

See: http://www.usd.edu/esci/age/content/...n_cooling.html

John

That's an interesting link. Thanks JohnD.

That's true for small planets from Murcury up to about 2 times earth masses. With some notable exceptions such as IO thats Tectonicly active due to Gravitational Stresses.

From about about 2 to 3.5 Earth masses it's guessed you only need a large moon, or oceans, but not both to be Tectonicly active. Earth needs both.

And from about 3.5 to 8 Earth mass neither a moon nor oceans are needed for a techtonicly active planet.

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Hey, valuable tid-bits of information. Thank you May I ask where you got this? Can I read more about this somewhere?

EDIT: Wait. The website in your sig?

Kelvin is Lord! (Though he did get rather pompous in his old age).

I aam yet to be convinced that there is a problem that needs to be accounted for. The lines of evidence for anything in the Hadean are very slim and very much assumption driven. I would be extremely sceptical of any strong claaims about anything about surface conditions on the Earth before 3.5 GA (which are the oldest well preserved suprcustal rocks).

The numbers are basically guess work, but theroretical models show the heavier a planet gets the less assistance it needs for techtonics.

Here is one article about it, http://news.softpedia.com/news/Newly...cs-71669.shtml

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http://worldsofothersuns.home.comcast.net/

The interior of the earth doesn't directly impact the heat of the earth's surface. It does indirectly, by greenhouse gases being emitted volcanically. The surface of the earth is primarily controlled by solar radiation and climate/heat transfer. The important point here is that in the past the sun was less strong, and its getting stronger as time progresses. As helium builds in the core, the amount of solar radiation increases. So, early in earths history, the sun was about 6% less luminance.

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Oh ok. You mean: 1) assume for now the hot surface Hadean is correct and 2) the cold surface claim is still weak? I'm ok with that.
I would however, be interested to know how the cold surface people decide to account for this early cooling.

Thanks.

Thanks, I'm ok with what you describe. I actually, and stupidly, framed my question wrong. Post 15 is a better version! Thanks again.

I think hot or cold, the evidence is luke warm

The problem is it is all so indirect, based on isotope ratios in a few tiny mineral grains taken rocks that are incredibly metamorphosed to start with. Plus theoretical models that are vased on assumptions, reasonable perhaps, but still assumptions.

IMHO!

Strange how topics seem to come in spurts. In the ITWire: according to an Australian ancient-earth researcher, about two to three billion years ago most of the Earth was buried under six to nine miles (10 to 15 kilometers) of lava from a series of volcanic explosions. The abstract of the article they cite doesn't directly support their lead-in, and parts of the ITWire article don't inspire confidence in their scientific literacy ("the volcanoes went dormant but eventually repeated the cycle after about 100,000 to 150,000 million years" ).

It’s more then 6% for the timescales we are looking at, closer to 25%

30% according to Wikipedia.

Just curious as to your views on this paper. Other than 'they're probably scientifically illiterate'

Just asking because I found it interesting, but I don't yet have way of judging the geological feasibility/credibility of what they say.

I defer to Jon on all questions relating to geology. Saying (as the abstract does) there were dramatic burst(s) of magmatism lasting a few million years before 1.8Ga ago doesn't bother me, but claiming (as the media report does) the Earth was covered in lava that recently seems wrong. How could life have survived?

Thanks. The article says:Stopped 3bn yrs ago. That would be ok, I think?

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Dramatic outbursts of Magma have happened even in recent geological history, although I would not say these came close to ever covering the earth in Lava.

About 17 Million years ago a fissure type eruption from mid Arizona to eastern Washington occurred, the results of which are many of the high desert plains volcano fields.

Eventually it settled down into a split hot-spot system, the southern portion migrating from Arizona into Oregon to what is the Newberry Caldera region. And the Northern part migrating from Eastern Washington south then curving east then back to north east, to the Yellowstone Caldera region.

I expect that when this system was in it's fissure stage, that it would of been quite a sight, and anything alive seeing it might -think- the world was covered in Lava. All told though it only covered about 2-3% of the North American continent with lava flows. (Or about 8% of the continental united states)

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There is no problem that cannot be solved by a suitable application of high explosives - US Army Demolitions School

I just saw Hayley's comet, she waved, Said "why you always running in place? Even the man in the moon disappeared, Somewhere in the stratosphere" - Shinedown

http://worldsofothersuns.home.comcast.net/