Via New Scientist: http://space.newscientist.com/articl...tectonics.html

The original paper is here: http://www.arxiv.org/abs/0710.0699v1

How then does Martian vulcanism fit into this model?

Also, as a recent paper hypothesized that vulcanism was responsible for much of the early increase in O2 in the atmosphere, would these results imply that life-forms like us are restricted to planets of ~Earth mass or above?

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You can have volcanoes without plate tentonics. Venus, Mars and Io all demonstrate this. On earth we get volcanoes at plate boundries where the ocean floor is being subducted, and we also get shield volcanoes such as the Hawaian islands in the middle of plates. Mars volcanoes are presumably of the later sort.

The article says that if the earth were a little less massive it probably wouldn't be geologically active. That seems unlikely to me. Even without plate tentonics we would still have shield volcanoes forming over hotspots. And the weight of the material they add to the surface would push the crust down into the mantle. The process could result in a cycling of materials through the crust without plate tentonics. One question I have would be are worlds without plate tentonics prone to periods of extreme volcanism as Venus might be? And would this merely be increased volcanism over tens of millions of years that in general would allow life to go on as normal or would they be rapid, Permian extinction type events?

Not only is it unlikely, it is wrong. The article mis-states what is included in the original research article, which (top of p.7) points to Venus and says "This observation indicates that the ~1 earth-mass case falls within a zone of transition between ‘hard’ stagnant lid and mobile plate regimes." The original article was talking about the distinction between plate tectonics and non-plate tectonics, not that between geologically active and non-geologically active (which even Mars certainly was at one time).

I thought the lack of Venus plate tectonics as compared to the earth was hypothesized to be due to the rotational differences between the two planets.

Venus, I thought from time to time had a period of extreme volcanic action where the entire planet is recovered with magma. I thought someone reached that conclusion based on crater pattern on Venus' surface.

From Wikipedia Venus Tectonics

A very strong argument can be made that Europa and Enceladus are experiencing plate tectonics with the "plates" being water ice. I don't know if the fact that they are moons experiencing large tidal forces would change the hypothesis but it's something to consider.

- J

Yes. The original research article addresses "rocky terrestrial planets" though.

Is there a timescale over which a planet which is just under the limit for mobile plate tectonics will become no longer geologically active? If earth had might have been slightly smaller it may have already become inactive. Now, is continuing geologic activity required to maintain the biosphere?

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You mean, active plate-tectonic-wise, right? Obviously, smaller objects can be geologically (volcanically active)Over what timescale? I would doubt that anyone would say that volcanoes are necessary to maintain the biosphere right now, for a long time into the future. Eventually, things would get worn down to a flat terrain and it would get rough

Hopefully someone can give a better answer than this, but as I understand it, without plate tentonics there is concern that most carbon could get trapped in the crust leaving little in the air for plants to use. However, shield volcanoes could recycle crust without plate tectonics, although a thicker crust may mean a greater amount of carbon could be trapped in it. But even on a geologically dead world carbon might still cycle through ground water, assuming there is liquid water on the planet. On earth life is typically limited by lack of phosphorus or other materials other than carbon. I don't know how much carbon is required for a rich biosphere.

A planet cools down sure, outgassing drives volcanoes but sooner or later it is going to freeze. Mars is already frozen. Moons that are being squeezed in gravity fields of gas giants remain active, however our own quite large moon has long since stopped being geologically active.

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Just received an email from someone who signs as the space editor for New Scientest, and they say the article has been changed to read "Our own planet seems to lie at the threshold. If it were any less massive, it would probably not have PLATE TECTONICS."

The Web Bautrol strikes again.