Overlooking a Critical Factor in Our Quest to Find Alien Life.
How does life emerge in the universe? Just seat a rocky, Earth-like planet at the right distance from a sun — not too close and hot, not too far and cold — and liquid water and perhaps alien life could exist. Scientists call this the “Goldilocks zone:” just right for life. But astronomers have increasingly wondered what else makes Earth so unique a world.
One “killer app” of our home planet seems to be its ever-creeping plates of rocky crust and active volcanoes, a system called plate tectonics. After all, plate tectonics once saved Earth from turning into a snowball by regulating greenhouse gases. It also constantly shuttles minerals to the surface that life needs to survive. Many scientists assume that plate tectonics is a given on rocky, Earth-like worlds, but this may be rarer than anyone imagined. A new study in the journal Science Advances questions the idea that rocky worlds “self regulate” their heat after forming. The implications could be enormous, says study author Jun Korenaga, a geophysicist at Yale University. Essentially, we could be overlooking another “Goldilocks” factor in our searches for worlds habitable to aliens: a planet’s initial temperature. If a planet starts out too hot, the thick layer of rock below the crust called the mantle doesn’t create plate tectonics. If it’s too cold, plate tectonics do not exist either. The mantle is not as forgiving as scientists once assumed: you have to have the right internal temperature to begin with.
Most planets come together by sweeping up gas and dust around a star — a fairly calm and cool process, as far as building worlds is concerned. But Earth’s birth was hot and messy. Around 4.5 billion years ago, a Mars-size planet smacked into a much larger planet, spraying a cloud of debris into space that became the moon. The bigger, molten mass of rock, metal, and radioactive elements left behind coalesced into Earth, trapping the heat of its violent collision inside. Korenaga’s study argues that this starting internal temperature could be far more important than previous studies suggest. A popular idea about Earth-size rocky planets is that, within about 200 million years, the mantle — the engine of plate tectonics — starts to self-regulate the escape of a planet’s internal heat. It does this by convecting hot rock upward and pulling cold rock downward. This action can stabilize surface temperature, bury carbon, birth volcanoes that belch out complex atmospheres, and expose rare minerals that life needs to grow and survive. This worked splendidly on Earth, but not for the smaller rocky planets like Mars and Venus. Those planets have a “stagnant lid” of relatively unbroken crust, and in Venus’ case, the consequences are clear: Without the ability to bury carbon in the atmosphere, the surface turned into an 860-degree-Fahrenheit hell.
Something needs to kick-start the process required to regulate a planet’s surface temperature, that careful balancing act that traps the right amount of cold and pushes up the right amount of heat. Typically, the thinking goes, it’s radioactive elements in the rock that set this process in motion. But Korenaga recently learned that radioactive elements may be warming and stirring up Earth’s mantle a lot less than previously thought — so he decided to run advanced computer simulations to account for this new information. The new models suggest that rocky planets which can regulate their temperature, and thus develop all the geologic support systems life needs to emerge and thrive, are much rarer than we might hope. What’s more, the models also hint that “super-Earths” — rocky worlds more than two times Earth’s mass — may be more likely to have stagnant lids, and thus have a harder go at developing a surface that’s cozy enough for aliens. Korenaga noted that measuring the internal temperature of rocky worlds from afar, even with future space-based observatories, is not going to be easy. “We can’t remote-sense the internal temperature directly, so we will need to rely on the connection between the atmospheric composition and the internal temperature,” he wrote.
Tilman Spohn, a planetary scientist at the German Aerospace Center, said that while he agrees the traditional “Goldilocks” zone for habitable worlds is a bit “crude,” Korenaga’s study is “not as straightforward. We still do not fully understand how plate tectonics comes about. Does that mean that only plate tectonics planets would be habitable? It depends, I think, on the question of how far evolved life forms you request.” Vinciane Debaille, a geochemist at Université libre de Bruxelles, added to this idea, making clear that plate tectonics may not be a necessary condition for life to emerge. “For life appearing, we need liquid water, organic matter and nutriments, [and] this can be done on a planet without plate tectonics, because the heat needed for keeping the water liquid at the surface is mainly provided by the star.” And Sara Seager, a planetary scientist at MIT who has thoroughly studied exoplanet habitability, has no plans to give up looking for cozy worlds among the stars. “Rocky planets are very common. Despite the expectations of huge planet diversity and the implications, there will still be a large number of planets that are habitable no matter what we think is required.”
Credit: Dave Mosher for Business Insider (Abridged) 22 August 2016