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Geochemist Matthew Jackson argues that the abundance of certain elements in the Earth dictate whether plate tectonics can happen. Of the more than 1,000 verified planets found by NASA's Kepler Space Telescope, eight are less than twice Earth-size and in their stars' habitable zone.

Planet Earth is situated in what astronomers call the Goldilocks Zone — a sweet spot in a solar system where a planet’s surface temperature is neither too hot nor too cold. An ideal distance from a home star — in Earth’s case, the sun – this habitable zone, as it is also known, creates optimal conditions that prevent water from freezing and generating a global icehouse or evaporating into space and creating a runaway greenhouse.

However, a new theory by UC Santa Barbara geochemist Matthew Jackson posits that the bulk composition of a planet may also play a critical role in determining the planet’s tectonic and climatic regimes and therefore its habitability. In a paper published today in Nature Geoscience, Jackson, an associate professor in UCSB’s Department of Earth Science, and Mark Jellinek of the University of British Columbia discuss their research.

 

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According to Jackson, plate tectonics is a manifestation of the Earth trying to cool itself. Cold plates sink into the Earth and absorb heat, while volcanoes release heat where plates are spreading apart and forming. “Whether or not plate tectonics can happen actually depends on whether or not the Earth is too hot or too cold,” he said. “If it’s too hot, plate tectonics seizes up and if it’s too cold, it freezes up.”

Until a decade ago, Jackson noted, scientists based the Earth’s composition on a model tied to ancient stony meteorites called chondrites, which were considered the building blocks of the planet. Then studies analyzing the ratio of two neodymium isotopes — 142Nd and 144Nd — demonstrated that Earth’s composition may differ from that of chondrites — and differ enough to send scientists back to the drawing board.

In 2013, Jackson and Jellinek published a new compositional model of the Earth in which a large portion of the mantle was depleted to form the continental crust. The model also assumed a 30 percent reduction in the uranium, thorium and potassium content in the planet. The decay of these naturally occurring elements generates almost all of the planet’s radioactive heat.

“We argue that if the planet had as much uranium, thorium and potassium as the old model, plate tectonics might not be possible,” explained Jackson. “If this is the case, you can end up with a planet that has only one big plate and can become an extreme greenhouse like Venus. The new compositional model gives Earth a sweet spot of its own where its interior is neither too hot nor too cold — a place that allows our current mode of plate tectonics to operate.”

Jackson added that the thermal and tectonic histories of the Earth are intimately intertwined, and this latest paper explores what happens if heat production is turned down by a third, as the new compositional model suggests.

If uranium, thorium and potassium govern whether or not plate tectonics can occur, as Jackson and Jellinek propose, astronomers looking for habitable planets might have another parameter to consider. Since NASA’s Kepler Space Telescope has already found more than 1,000 planets — a small fraction of which reside in the habitable zone around their respective stars — it is important to understand how additional variables, including a planet’s composition, can narrow the field of potentially habitable extrasolar worlds.

The Daily Galaxy via http://www.news.ucsb.edu/

Image credit top of page: NASA

Comments

Given the short amount of time we have to destroy this planet (cosmological speaking) or deplete it of all it's natural resources, and the vast distances between stars and potential habitable planets, the extreme difficulties of space travel, we may never know if there is other life out there.

It fascinates me how there could be another planet just like ours out there, with people just like us who are doing the exact same research.

Don't be fascinated too long because there isn't one and they aren't. ;)

Awesome job, Thunder, you sing-handedly ruined the dreams of tens of millions of people in one comment on a lonely website.

@Brian, they'll all get over it.......

dose anyone know to temp of the sweet spot

dose anyone know to temp of the sweet spot

Habitable Zone for a Planet

Around a given star, there will be a habitable zone where the temperatures will be in a range that could support advanced life. The advanced life that we know about depends upon water, so one condition for the habitable zone is that water can exist in liquid form, so this requires a temperature range between 0°C and 100°C. That's a fairly narrow range, but this question about the habitable zone has been studied extensively, and the real required range is more like 5°C to 40°C.

Distance From a Star

To be in the habitable zone around a star a planet's orbit needs to be nearly circular and be at a distance where all three phases of water can be present on the surface of the planet.

Calculations in 1978 by Hart indicated runaway glaciation if the Earth were 1% further from the Sun and runaway greenhouse effect if it were 5% closer to the Sun. Kasting in 1993 included an additional regulatory influence called the CO2-silicate cycle, and projected a CHZ (continuously habitable zone) of 0.95 - 1.15 AU.

The article doesn't say it, but this exposes what may be the largest constraint in rocky planets as to whether or not they remain in the temperature range for liquid water to exist. Whether a planet is in the "Goldilocks Zone" or not is not sufficient at all to determine its potential for life. The planet's composition, and amount of heat it, itself, generates, is ALSON going to affect its temperature profile. Our seeming uniqueness may be because our climate--which has persistently been stable for billions of years--is itself hugely unusual, and profoundly lucky. It would seem likely that this stability may occur in an open atmosphere only very very very rarely--perhaps as often as one could balance a bowling ball on a nail.

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