Wednesday, September 07, 2016

Ask a Physicist: Life Without a Sun?

Gonçalo, from Portugal wants to know:

"Can a planet, theoretically, manage life without a sun?"
Your suggestion is surprisingly plausible! To understand how, we'll have to explore some of the darkest places on Earth, where life is as close to "alien" as you're ever going to find.

To rephrase an old quote: where there is a way, life will find it. That said, there are a few basic things necessary for life to arise. The first is building blocks: atoms and molecules capable of linking up in elaborate arrangements, like the carbon that forms the basis of all life here on Earth. Carbon isn't the only substance with this capability, a fact that gives scientists hope of one day discovering life based on another element, like silicon (which, having the same number of valence electrons as carbon, forms bonds in a similar manner). A water-rich environment seems to help the process along, but we're obviously biased, coming from a planet that's covered in the stuff.

The next prerequisite for life is an environment to protect those molecular arrangements—if a world is too hot, or unsheltered from the blistering gamma rays that permeate our galaxy, any bonds will be shaken apart before they can fall into a self-reproducing pattern, the fundamental basis of life. Beyond those two conditions, the main requirement is a source of energy that those self-reproducing structures can harvest to continue growing. Here on Earth, that energy most commonly comes in the form of sunlight, but—as we'll soon discover—that's far from the only option.

Earth is uniquely hospitable to life for a number of reasons, but perhaps the most significant factor is its molten interior. Convection currents, driven outward by heat from the planet's core, are constantly stirring the electrically conductive magma of the mantle, creating loops of magnetic field wherever a circular current arises. Thanks to Earth's rotation, these circular currents are produced consistently, producing a protective magnetic field that encircles the entire planet. Without this force field, called the magnetosphere, Earth's oceans and atmosphere would likely have been blasted away by the solar wind a long time ago. Mars' core cooled much more rapidly than Earth's, which is why—if there ever was life there—it's likely to be long extinct by now.

Our planet's hot core provides another kind of amenity, though: central heating. If you dig down more than a few meters, you reach a region of the crust that's roughly constant in temperature—a chilly 62° F, with seasonal variations of about 5°; the sunlight that falls on the planet's surface has little to do with Earth's internal temperature. About half of this heat is "residual", left over from the solar system's formation, when the mechanical stress of collisions among proto-planetary bodies heated them to the point of melting. The other half? Nuclear fission.

You read it right: you're sitting on top of a giant nuclear reactor, and the relatively high concentration of radioactive elements in Earth's mantle is a huge part of what's allowed it to stay liquid—and maintain the magnetosphere—longer than planets like Mars and Venus.

This information bodes quite well for the prospect of life in the universe's darker corners. Here on Earth, there are ecosystems—like those found in the vicinity of geothermal vents—that seem entirely independent of the sun's influence. In the ocean's darkest depths, microbial life forms cluster around underwater geysers, chowing down on methane and ammonia carried upward on plumes of hot water and steam, boiled by the magma that bubbles up in the rifts between tectonic plates. These microbes—perhaps descended from surface-dwellers, but just as likely to be our ancestors—form the foundation of a food web that contains everything from tube worms to octopi. If the sun disappeared tomorrow, virtually all life as we know it would follow shortly after...but the odds are that these guys wouldn't even notice.

Giant tube worms are found in the vicinity of hydrothermal vents, fed by symbiotic microbes which extract energy from the chemicals that bubble up through the vent.
Fun fact: the red color comes from hemoglobin, the same protein that makes up your red blood cells!
All this is part of why scientists are practically giddy with excitement about the prospect of sending a probe to Europa, an ice-covered moon in orbit around Jupiter. Being five times further from the sun than Earth, sunlight on Europa is only one twenty-fifth as intense, making this a very cold place—at least on the surface. But the smooth, icy shell of Europa betrays an extraordinary secret: a liquid water ocean underneath. Other planetary bodies in the solar system, like our own moon, are pocked with immense craters, the results of asteroid impacts. On Europa, no such craters have been seen. This suggests that impacts on the ice are erased by water from a subsurface reservoir, something like a stone thrown through the surface a frozen pond, which then re-freezes overnight.

Europa's surface, shown color-enhanced in this image, is criscrossed with lines that suggest
an analogue of tectonic activity among the ice sheets on its surface, further supporting the
hypothesis of a liquid water ocean underneath the ice.
Something is keeping the interior of Europa liquid, and astronomers' best guess is that there's a solid core to the moon which is subject to immense stress from the tidal forces of Jupiter. This stress deforms the core, heating it through a sort of internal friction, like a paper clip bent back and forth repeatedly—at least, that's the prevailing hypothesis; it's also possible that radioactive decay is at least partially responsible for keeping Europa's core warm. Either way, it's clear that Europa has an energy source and an environment conducive to life: that ice shell provides excellent protection against the particle radiation channeled by Jupiter's magnetic field. Now, the only questions are whether the necessary chemicals exist in Europa's interior ocean, and whether they happen to have stuck together in just the right way so as to reproduce, grow, and adapt.

If they have, Europa's interior could be an entire dark ocean world of alien life—eyeless creatures that have never had a need for sunlight. Not only is life without a sun possible, it may exist right in our own backyard.

Concept art of what complex life forms on Europa might have evolved to look like.
Image Credit: Adolf Schaller
Stephen Skolnick

No comments:

Post a Comment