New research shows that if Earth were slightly smaller and less massive it would not have plate tectonics. This may be the main reason that Venus doesn't have it.

http://www.astronomy.com/asy/default.aspx?c=a&id=6462

From that article:Hmmmm. There seem to be some unexplored assumptions in there.

Here is a one-page pdf abstract from these authors (although it doesn't look like it relates to the same meeting mention on the Astronomy page), which contains a handy number for world-builders: the continent cycle time is predicted to vary as the -0.3 power of planet mass.
And here is the arXiv page for Inevitability of Plate Tectonics on Super-Earths, by the same authors.

Grant Hutchison

If Earth is on the borderline, I'd hate to see what planets in the heart of the "sweet spot" look like!

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LOL. I suspect that barely reaching the borderline of a wide variety of factors related to Earth’s development is as sweet as it gets in this universe.

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I was just thinking of all the science fiction I've read where every life form on a planet considered every other life form (including humans) to be prime candidates for lunch!

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T. Anderson

If you think about it from the standpoint of the anthropic principle, it's quite natural that the Earth should be on the size borderline for life. That's because since presumably there are more smaller terrestrial planets than larger ones in the normal distribution of sizes, there should be more life found at the small end of the viable population of planets.

We have an example of one of typical solar systems with terrestrial planets. If any of the extra terrestial planets that we have found so far can be reasonably assumed terrestrial, they are two or more times more massive than Earth. That if, suggests Earth is a midsize terrestrial planet, or even of smallish mass. Perhaps we will know this year if we start finding Mercury, Venus and Mars mass extra terrestrial planets. Neil

So far we can't say anything, because all we're seeing are our own detection limits. This is called an "observational selection effect", and makes it impossible to say anything at all about the size distribution of terrestrial planets. However, most formation mechanisms of the type we generally imagine for terrestrial planets yield a distribution of more smaller examples than larger examples, with a lower cutoff at some "minimum size" for the described process to be in effect. Unless the Earth is already at or near that minimum size cutoff, I find it very unlikely that there are not vastly more smaller terrestrial planets than larger (notwithstanding the fact that the definition of "planet" may be pretty useless in this context).

Ken, it's great to see that you're finally coming around my way of thinking!
However, it may not be the case that small terrestrial planets are typical. I would say the size of a solar system's terrestrial planets largely depends on the metallicity of the parent star. The higher the metallicity, the easier it is for large rocky cores to form--up to a point. Too much metallicity, and the planets get too big and turn into gas giants instead. Too little metallicity, and you might not get planets at all. So the size distribution of terrestrial planets will depend on the distribution of metalicity among stars. (I have no idea if our sun is typical in that regard, though.)

(Granted, there are thousands of little asteroids compared to the handful of terrestrial planets. I take it we're limiting the discussion to "major" planets functionally defined by their ability to sweep out and dominate a particular orbital realm.)

As I recall, our debate about the anthropic principle was around whether or not it qualified as a physical explanation of anything. I say that physical explanations are held to a different standard. The idea that life is out there, occuring most places it has a good chance to do so, and that we are just a part of that larger statistical situation, does not seem scientifically controversial to me. Hence it is normal to expect that the conditions under which intelligent life appeared on Earth should be characteristic of lots of other places where intelligent life appeared. That is not the same as saying it explains any physics. Perhaps what I'm using here is what is known as the weak anthropic principle.That is all certainly possible. All I'm saying is that if you look at all the stars in our galaxy, with all their metallicities and every other relevant factor, you should find a distribution of terrestrial planet sizes that I expect to peak at smaller rather than larger sizes. That just comes from the expectation that there are generally more ways to make small things than big things, and the more variables you include as relevant factors, the truer that becomes. Possibly not in all cases, one would need more details-- I'm not suggesting a theory of planet sizes here.
That's what I meant by the "useless" aspects of the definition of "planet". All we care about to invoke the anthropic principle are rocks that are capable of sustaining intelligent life. If it was easier to get such life on the moon of gas giant, then that is where we might have expected humanity to develop. Of course, it's always possible that there are several different planet types that are roughly equally prominent in the life in our galaxy, and then Earth could be just one general possibility.

Given earth like consistency, as a planet gets more volume, it is going to have more water content per surface area. As the planet ages this water migrates out to the surface. Therefore larger worlds are likely to have more oceanic coverage as a percentage of surface area. If the tectonics of a planet are completely submerged will they be suppressed? In the case of a large water world is the surface gravity likely to higher than earth, or lower? The atmosphere much deeper?

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With earthlike composition but larger mass a planet would have higher surface gravity. The atmosphere would be more compressed, so shallower in a sense, but likely with a higher surface pressure. The atmosphere could depend drastially on irradiance - think of Venus with an earthlike mass and (probably) composition, but a very different atmosphere.

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Water is seen as promoting tectonics on Earth, and almost all of Earth's spreading and subduction zones are submerged already, so I doubt there would be a problem for tectonics. Surface gravity is going to be higher, atmospheric scale height smaller, as AndreasJ says.

Grant Hutchison

Maybe not easy worlds to develop technology on..

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That may be a good point-- if Earth is the largest you can get and still have continents, and yet if smaller sizes are not geologically active enough, then the size range for intelligent life might be pretty narrow. That would also tend to force Earth to be near to the edge-- maybe it is also near to the upper size limit. If so, that's all a new term in the Drake equation-- if a Venuslike planet could never sustain intelligent life even if it were at Earth's location, that's a potentially severe blow to intelligent life-- especially if larger planets are awash in water (in the best case scenario).

An earth without plate tentonics would still cycle carbon in and out of the crust. Volcanic activity might be fairly constant with volcanos forming over hot spots. These would release some carbon, similar to shield volcanoes on earth. The material they eject would push the crust they lay on down into the mantle, causing it to mix with the mantle. Another option is that the earth might cycle through occasional periods of extreme volcanism, but this would still cycle the crust. But either way, the average amount of CO2 in the atmosphere might be very low.

It be mentioned here that in the Januar 4 issue of Science, there was an article called "Intermittent Plate Tectonics" by Paul G. Silver and Mark D. Behn, arguing that plate tectonics has effectively shut down during some periods of the Earth's history. If true, plate tectonics presumably isn't all that necessary for life ...

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The whole point of subduction is that it recycles the carbonate sediments that get subducted. If there were only hot-spot volcanoes, carbonate sediments would just sit on the ocean floor and wouldn't get recycled.

There's another possibility-- that cessation in plate tectonics could be very temporary, because something there is beneath the surface that "wants" subduction to occur. If so, then cessations due to global topography changes might rapidly build up stresses that result in renewed subduction somewhere else. Life would only care about the integrated effect of all these "subduction stresses", not so much the details of the current topography.

Causeing the very low average levels of CO2 in the atmosphere. Of course, once CO2 drops low enough there won't be many cabonate ions for sea life to incorporate into their bodies. However, for an earth without plate tentonics, there might be a considerable amount of volcanism. What life would be like on a CO2 poor earth, I can't say, but complex life may be possible. Photosystem I, which generates energy without an uptake of CO2, might be important. Silica could become an important structural material to economise on carbon. As some amino acids are lower in carbon than others, perhaps low carbon protiens could substitude for carbohydrates in many roles. I of course don't know what is possible, it's just interesting to speculate.

That's the usual assumption; that cessation of subduction somewhere is compensated by initiation of subduction somewhere else, keeping the total "subduction flux" approximately constant. What Silver and Behn are saying is that a) there's no known mechanism for such compensation, and b) some aspects of earth history are more easily explained if we assume no or little plate tectonics in the Mesoproterozoic. They cite the temporal distribution of certain kinds of rock, and calculations suggesting that under the current plate tectonic regime the planet is losing heat "too fast", leading to impossibly high mantle temperatures in the deep past if extrapolated backwards. If plate tectonics was more sluggish in the past, heatloss would have been slower, and estimates of ancient mantle temperatures become sustainable.

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And that gibes with the reduced subduction in the Mesoproterozoic, so there's evidence that we are in a regime of greater tectonic activity now-- but we don't know that the rate we have now is what one would need to be conducive to life. It still may hold that tectonics=greenhouse gases=life, averaged over 100-million-year snippets. What's missing is the "coefficient", if you will, that converts from a certain amount of tectonics to a certain amount of H₂O and CO₂. Without that, it's hard to say how important tectonics is, even if it is not a steady process on few-million-year timescales. For example, if all tectonics ceased tomorrow, how long would it be before the Earth climate changed significantly?

Well, if plate tentonics stopped now, along with all andesitic volcanic activity, the reduction in CO2 emissions would be insignificant compared to anthropogenic emissions. Less than one percent. So the earth would keep getting warmer. If we remove humans from the equation, it will take hundreds, possibly thousands of years for CO2 levels to drop back to 1900 levels.

That suggests the CO₂ capture rate is a thousand years or so, which would mean that AndreasJ is right-- if plate tectonics cease even for a few millennia, the climate of the Earth would freeze-- if indeed plate tectonics are required for CO₂ and H₂O release. But this all raises a much larger issue-- if the greenhouse gas capture rate is a few thousand years, then the gas generation rate, by whatever means, has to be extremely time steady on geological scales-- or else unlivable climates will result. Never mind whether it is plate tectonics or something else-- what could possibly be so steady for so long? There would seem to have to be a stabilizing agent involved somewhere-- perhaps the "Gaia hypothesis" is not so far-fetched.

Umm, well, the colder it gets the less rain there is. The less rain there is, the less CO2 gets washed out of the atmosphere and into the oceans in the form of carbonate ions. You could call that Gaia if you like. I call it weather.

Basically, the less CO2 there is in the atmosphere, the less is removed. So I see no reason why complex life could not exist on a world without plate tentonics, but I do wonder what it would be like with such an important nutrient in such short supply. Of course planets could always be formed with a higher amount of original carbon than earth, which might make things easier for life on a plate tentonicless planet.

In Australia where soils are almost always phosphate poor, complex life makes do by becoming incredibly diverse, with thousands of different species of plants all using optimized strategies for their particular soil and environment to extract the optimin amount of phosphate. (North America with its rich soils seems quite boring with regards to plant diversity.) The south-west corner of Australia is proably the most diverse in the world with regards to plant species. Whether or not we'd see similar diversity happen on a carbon poor world, I can't say.

Yes, there's no need to "personify" it, really, the point is simply that life cannot exist unless the planet has built-in stabilizers that keep it within the viable range in the presence of drastically varying driving terms. It mimics a planet that "wants" life, but of course there's no need to take that part seriously.
That doesn't matter though-- if the greenhouse gases were significantly lost, the planet would freeze over, and the modified removal rate would no longer matter, not until sufficient gases were reinstated by some new driving. It is very easy to imagine a planet that is unlivable from time to time-- yet for some reason, that didn't happen. We had ice ages, but the oceans didn't freeze-- or did life have to "thaw out" periodically?
I'm wondering about Earth here-- if it is implausible that Earth had a continuously steady amount of plate tectonics over the thousand-year timescale of interest here, then either plate tectonics are not important in the greenhouse gas concentrations, or else there is some other significant stabilizing factor. It seems to me you need the latter anyway-- whether the greenhouse gases come from plate tectonics or not. Whatever the stabilizer is-- Venus apparently didn't have it.

Even if subduction ceased because all the continents collided together, there would still be volcanos. Mars has no plate tectonics, yet it also has the biggest volcano in the solar system.
Probably the oceans had something to do with it.

True enough-- but we already know that volcanoes are not consistent on the thousand-year greenhouse-gas residence timescale. A planet that needed volcanoes to maintain a consistent level of warming would be in deep doo-doo, the oceans would surely freeze every time you went 10,000 years without a major eruption.
Indeed, and Venus may not have plate tectonics at all. Yet they would seem to be the norm-- extreme cases where liquid oceans could not be consistently maintained. How consistent has Earth's oceans been, and how does it do it?
Yes, there may be some tendency for oceans to self-perpetuate, but Mars apparently couldn't keep it up and Earth could. Is it just luck for Earth, as in the "rare Earth hypothesis", or is there a niche for stable weather, or does life itself create the stabilization? Note those three possibilities suggest vastly increasing likelihoods of life.

If the earth had a drastic reduction in CO2 it would not freeze over. It would enter a severe ice age, but there would still be liquid water in the tropics. As the sun aged and gave off more energy the earth would gradually warm. And obviously the earth could have formed closer to the sun than it is now.

No, we don't know that. On an earth without plate tentonics volcanic activity may or may not be fairly continuous.

A planet need not rely on greenhouse gases to avoid low temperatures. It could simply be in a position to recieve more heat from outside or from tidal forces. As I mentioned above, the oceans on earth would not freeze with a drastic reduction in CO2. On earth CO2 production by volcanoes is fairly constant with major eruptions producing no significant uptick in CO2 levels. On an earth without plate tentonics I would expect there to be an even greater amount of volcanic activity. Whether this activity would be regular or go through cycles we don't know.