Saturday science subject: Alien geology

Rocky, Earth-like planets orbiting alien stars have been a hot subject of discussion recently as astronomers detect more of them. The planets' discovery has resulted in much speculation about the possibility of them harboring life, and the latest talking point seems to be their geology. As Nature reports, scientists are wondering whether rocky, alien worlds have moving tectonic plates like the Earth.

Nature says active tectonic plates, which result in moving continents and the formation of mountains, are thought to be one of the ingredients necessary for life, since they are "important for moderating a planet’s temperature and recycling materials that may nourish life." Some scientists, including Harvard University's Diana Valencia, believe tectonic plates are inevitable on large, rocky planets. However, planetary scientist Craig O'Neill from the Macquarie University in Sydney, Australia thinks Earth may be an anomaly and other similar rocky worlds may not harbor moving tectonic plates.

O'Neill and his group have come to the exact opposite conclusion to Valencia. Their model, published in Geophysical Research Letters, shows that as a planet gets bigger, the increasing force of gravity squeezes crustal rocks together into a solid lid, making it more difficult for forces from below to crack it into plates.

A large, capped-lid planet could be like Venus, with a hellish atmosphere and runaway greenhouse effect. It could be cold and dead like Earth's Moon. Or, if fed by internal heat from the core, it could vent that heat through massive bouts of volcanism, as on Io, a moon of Jupiter. That doesn't necessarily rule out life, but it might make it difficult.

A third viewpoint is exemplified by California Institute of Technology planetary scientist Dave Stevenson, who told Nature, "Science doesn't even understand plate tectonics on Earth, so it shouldn't be making predictions for other worlds."

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