(I tried to search if such a thread already existed, but the feature failed. Perhaps someone will "ToSeek" the thread if necessary.)
What is the current thinking on the resolution of this significant problem:
http://en.wikipedia.org/wiki/Sun#Fai...ng_sun_problem

Comments on the resolution are welcome, or on the issue of the apparent "cosmic coincidence" that stellar evolution compensates for the tendency of a terrestrial planet to lose its greenhouse gases, in such a way that the habitable zone stays fixed over timescales suitable for evolution of higher life forms.

Since geologic records shows that the Earth has remained at a fairly constant temperature most of its life I am thinking it must have something to do with changing atmospheric conditions.

The most commonly accepted resolution to the faint young sun paradox is a combination of two major factors: (1) a higher abundance of greenhouse gases in the early terrestrial atmosphere, and (2) a stronger insolation due to changes in obliquity and changes in land mass distribution due to continental drift. For example ...

• Faint young Sun and the carbon cycle: implication for the Proterozoic global glaciations
• Possible connections between the Precambrian carbonate record, the faint young sun paradox and Neoproterozoic glaciation
• Global climate model high-obliquity solutions to the ancient climate puzzles of the Faint-Young Sun Paradox and low-latitude Proterozoic Glaciation
• A general circulation model study of the effects of faster rotation rate, enhanced CO2 concentration, and reduced solar forcing: Implications for the faint young sun paradox
• The faint young sun problem

However, there is a robust minority opinion, which contends that the standard evolutionary model produces a young sun that is too faint. The alternative evolutionary models reproduce the current sun, but without going through an early "faint" phase. For example ...

• Evolution of the Solar Activity over Time and Effects on Planetary Atmospheres. I. High-Energy Irradiances (1-1700 Å)
• Our Sun. V. A Bright Young Sun Consistent with Helioseismology and Warm Temperatures on Ancient Earth and Mars
• Measured Mass-Loss Rates of Solar-like Stars as a Function of Age and Activity

There is no definitive answer as yet. Either the early Earth was rich in greenhouse gases, or the young sun was not really faint after all, or some mix of both. I don't think we really know.

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Don't try this at home - We're what you call "professionals" - MythBusters.

Thanks for that great summary Tim, and the research into those links. It certainly is an important question from the point of view of, just how special is the Earth, and how likely is advanced life even near a Sol-type star. The recent issues of water on Mars must also factor into the resolution, but since we don't know much about Mars' potential greenhouse gases, probably both answers still work.

If both the earth and Mars had roughly unchanged temperatures back then, I would find it very difficult to accept that some process both on earth and Mars happened to counteract the effect of a faint sun so well. My inclination at that point would be to accept a "bright young sun."

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As above, so below

It is my understanding that the late Hadean-early Archean (4 bya – 3.8 bya) Earth atmo was composed of a great deal more CO2 than today. I’ll have to wing it here: release all of the carbon from all of the biota (plants and animals) and what is tied up in carbonate deposition and we should have ~ > 20 bars atmo to work with. (I consider this a conservative estimate, it was possibly denser than 20 bars) All but non-existent O2 almost certainly meant no protective ozone layer to speak of. Although the overall luminosity was probably lower, the UV component may have been higher than today. In short, the early atmo may have been more efficient at trapping and holding whatever heat did penetrate into an atmo with more ‘green-house’ gasses than we have today. I can find no firm estimates on how extensive the methogenic population was but I am confident that it would have contributed to the greenhouse effect.

Less above sea level land mass (The original cratons ) should also be considered. Today’s ratio is about 25% continental crust to 75% oceanic crust. I’m pretty sure that Hadean continental crust was no more than ~ 10-12%. Perhaps the combination of denser atmo with greater ocean surface (80-90%) made for a more effective heat engine precisely when the Earth needed it.

Finally, the Late Heavy Bombardment of the Earth must have contributed to the bulk Earth heat budget. Just how much, I don’t know. I’m shooting from the hip here.

http://www.geology.wisc.edu/zircon/E.../Images/31.jpg

I am a little surprised that I have not seen something regarding surface warming from subsurface radioactive decay. I recently read where the prediction of natural U235 densities great enough to allow a fissionable chain reaction may have been verified.

IIRC, Lord Kelvin put a upper limit of the Earth's age at around 20 million years based on his respected knowledge of heat transfer (before radioactive decay was known). Assuming he was correct w/o decay, that is a lot of extra heat via radioactive decay to bump it 225x. Also, the decay would have been far greater 2 billion years ago than now, duh.

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

As far as I know, that's just in isolated spots near the surface. And the warmth that comes from such a mechanism near the surface would come at a high price, would it not?

First of all, I'm dying to ask what IIRC stands for! But as for the comment, are you sure this isn't the argument about the age of the Sun? I would have expected the cooling time of the Earth to be much longer than this, at least an order of magnitude. Note that cooling times are not affected by heating mechanisms, they are essentially defined by turning the heating off and asking how long it takes to cool. So you can stay hot longer than that if you keep the heating on, but as you say, radioactive heating doesn't stay on more than a few half-lives. So in the end, unless you can keep the decay going for longer than 20 million years, you still have a 20 million year cooling time, or so. But substances with half lives longer than 20 million years wouldn't give you much heating anyway. So I don't think that can be right, as the Earth is still hot.

Still, you raise a good point, which is that heat has been leaking out from Earth's core for a very long time, and it's not negligible even now. 3 billion years ago, it could have been an order of magnitude more heat flux. I wonder why that issue hasn't been raised?

I think that brings up a whole new question. There is a controversy going on now about how early on the earth was cool. A kind of mineral (zircons) have been found that were something like 4.4 billion years old.

http://www.geology.wisc.edu/zircon/c...arly_home.html

And zircons need liquid water to form, implying that 4.4 billion years ago, the earth was roughly the same temperature that it is now. It seems fairly suspicious to me that although the sun has brightnened and the earth has cooled, these processes have precisely balanced one another for billions of years, helped perhaps by changes in the concentration of CO2 that have also worked to keep the temperature fairly constant.

Of course, one possible interpretation of this is that it is a coincidence, and life is so rare in the universe because such coincidences are very unlikely to happen.

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As above, so below

Or intelligent life, anyway. It is the anthropic principle in spades. The rate that heat is conducting through the Earth must be falling at about the same rate that the Sun is warming, and maybe you don't even need a greenhouse component. Note this would mean the Earth's core has not cooled by more than a factor of a few since it formed, which seems reasonable to me.

I chose the wrong word. "Subsurface" was to mean all the way to the core, as opposed to minor surface or atmospheric radioactivity.

FWIW, If I Recall Correctly - it means something that gets me a little off the hook due to my bad memory.

Mainly the Earth. Obviously the Sun would be a consideration depending on when one thought it came into existance. It was a big issue for Darwin which clearly understood he needed more than 20 million years. [He called Lord Kelvin "an odious spectre", IIRC .]

I can't really say. Kelvin, I presume, was basing all his transfer on the temperatures obtained in mines. The deeper they went, the hotter the temperatures were found greater than expected. Of course, this kept lowering his age estimate, much to Darwin's dismay, no doubt.

It would be interesting to see the likely radioactive element distribution for Earth during its formation. Just how much radioactivity makes up for the heat loss currently of Earth is unknown to me. My guess is early heavy volcanic activity would release a fair amount of this additional heat; today, less volcanic activity helps slow the heat transfer. It also would tie in with the CO2 arguments above.

I would have assumed it had. Are you sure it hasn't?

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

Actually, that surprises me. Darwin's theory, IIRC (thanks!), is an explanation for the differentiation of species, not the origin of basic cellular life. 20 million years is plenty of time for speciation, is it not?

No, I just meant it hadn't been mentioned in the thread until you brought it up. And the references in the thread seem to have gone into all the possible explanations pretty deeply.

I would have guessed the same. He was, apparently, quite taken with Lyell's book - Principles of Geology (where Earth reshaped very slowly over eons). This might have involved great ages. I vaguely remember (i.e. IIRC ), that Darwin actually determined the age of the earth and published it in his first edition of Origin of the Species. IIRC, it was around 350 million years. [Someone around here will know if he did so.] It was removed in latter editions.

Ooops. [I wondered if that was the case.]

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

See Changing Views of the History of the Earth, from the Talk.Origins collection on the Age of the Earth.

Basically, in the late 1800's, and around 1900, the age of Earth was a matter of contention between physicists, who mostly followed Lord Kelvin's reasoning that Earth was "young" (no more than about 100,000,000 years), and the community of geologists & biologists, who insisted that the planet had to be much older, at least in the billions of years. Eventually, the geologists & biologists won the argument, although it was the physicists who discovered the answer in the constant output of radiogenic heat.

While it is important in keeping Earth "geologically warm", the average flux of heat from the interior of Earth, through the crust, is miniscule compared to the pounding the surface takes from the sun. But the interior heat drives volcanism & plate tectonics, both of which are substantially important in the thermal history of Earth's surface. Volcanic venting changes the constituents of the atmosphere, and migrating continents change the effect of insolation (which heats "dirt" more efficiently & quickly than water).

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Don't try this at home - We're what you call "professionals" - MythBusters.

That's a pretty interesting article, Tim. I particularly like the reference to the failed attempts by "scriptural geologists" to reconcile the geological history of Earth with Genesis. One can't help seeing the analogy with today's "scriptural biologists", the ID camp.

I've been doing some googling, and the numbers just aren't adding up. What I have so far found from seemingly reliable places (I won't bother with the links because something isn't right anyway, and I'd rather get a better answer elsewhere) that the total heat coming through the Earth is only about 1% of the solar heat flux, that the heat flux through the Earth should have been about 2-3 times greater in the Earth's early days, and that the rate of radiogenic heating has fallen by a factor of 5 since early days (Thorium 232 seems to be the main element with a half life longer than the age of the Earth). But I also saw a site that said that 90% of the heat flux through the Earth comes from outside the core, which they interpreted as saying it is mostly radiogenic. That last point is inconsistent, so let's ignore it and see what we get. The other data implies that most of the heat flux in the Earth today is not from radiogenic sources, it's the residual heat of formation still leaking out. This despite the fact that radiogenic heating tapered off billions of years ago. So I infer the cooling time (which has nothing to do with heating processes, but heat transport only) of the Earth must be like a billion years, not 20 million as Kelvin believed, or even 100 million. Was Kelvin that wrong? (and again, this is completely independent of radiogenic heating issues)

As to why the Earth had liquid water for most of its history, despite the Sun's being fainter earlier in the Earth's history, there's an interesting possible reason:

A "geological thermostat"

As the Earth's temperature increases, weathering of rocks increases, and this consumes CO2.

Less CO2 -> less greenhouse -> cooler Earth -> less weathering -> equilibrium.

And there is reason to believe that this thermostat is running out, that in around 500 million years or so, the Sun will get bright enough for weathering equilibrium to require no CO2 greenhouse effect. This will mean that the atmosphere's CO2 concentration will go far down, meaning that land plants cannot grow, and eventually also that for ocean algae/plants. And the remaining Earth life would be concentrated near volcanic / hot-spring outgassing zones, whether CO2 will continue to be released.

There are other calamities soon to come before long, like the upper atmosphere becoming warm enough to allow water vapor to reach it and become dissociated. The hydrogen leaks off into outer space, and the oceans eventually evaporate, leaving the Earth bone-dry except for hot springs and the like.

Alternatively, the Earth could become like Venus, with a super greenhouse effect.

And all this about 4 billion years before the Sun becomes a Red Giant.