If, like me, you’re a child of the 1980s, the words “There are those who believe that life here, began out there, far across the universe” may be permanently engraved on your mind. Whenever the topic of alien life comes up, it is this silly opener to a some-say-silly single season science fiction show that springs into my cerebellum. The thing is, while life may have begun out there “beyond the heavens,” it could just as easily have originated elsewhere in our solar system. From the once wet surface of Mars, to the organic rich deltas of Saturn’s Titan, to the under-surface oceans of Saturn’s Enceladus and Jupiter’s Europa, our Solar System offers a myriad of possible places for life to have originated. To modern scientists and science fiction writers, the most intriguing of all these possible ecosystems is the moon Europa.
This icy world, at first glance, is just another moon. It is about 90% the size of our Earth’s moon and shines a nondescript grey white against the background stars. Galileo Galilei first discovered it in 1610. He watched as Europa and its companions—Io, Ganymede, and Callisto—raced around their gaseous home planet. Under Galileo’s watchful gaze, it took Europa just 3.6 days to complete an orbit, as it played a game of orbital tag with its closest companion moons. The three inner Galilean moons are bound up in what is called an orbital resonance. The inner most moon Io races the fastest, orbiting in only 1.8 days. Outer moon Ganymede makes it way a bit slower, getting around giant Jupiter in 7.2 days. These numbers form a pattern—1.8*2 = 3.6, 3.6*2 = 7.2—and they define orbits that bring the moons rhythmically into alignment.
Imagine seeing all these moons lined up beside Jupiter, like a string of mythical mistresses chasing down the cheating Zeus.
Very quickly, in just 1.8 days, the middle moon Europa gets ahead of the crowds, but an even quicker Io has already cycled back to where it stated, while Ganymede finds itself neither ahead nor behind.
And they keep moving, keep rotating, keep resonating; for every complete orbit of Europa the alignments alter. After one complete orbit, Io and Europa return to the chasing position, while Ganymede races ahead of the pack.
And an orbit later, a trip back and forth of inner Io later,
they are all chasing after their lover once again.
This constant pattern of motion causes Europa to go from being gravitationally tugged on only one side, to being tugged at from all direction at varying levels. This constant change of forces causes the planet to get stretched and released, and stretched and released, over and over with every orbit. This has been known since people started thinking through the consequences of gravity. Stretch and release; that’s what happens when moons orbit in resonance, beating gravitationally against one another.
The results of this stretching and releasing didn’t become apparent until 1979 when the Voyager probes made their fly-bys of the Jovian system. All pictures of Europa up until that point had shown her as nothing more than a fuzzy blur. When the Voyager images of Europa were downloaded, they revealed a surface almost devoid of large craters and instead covered in giant cracks and strange chaotic surfaces that looked almost like the surface of a frozen and re-frozen glacial lake. The lack of craters more than anything else caused planetary scientists to gasp, and ask “how?”
Here on Earth, we don’t generally see craters. If you happen to be on just the right flight from Texas to California, you may find yourself looking down on Arizona’s meteor crater. If you hunt hard enough through Google maps, you may find a handful of craters dotting Canada, Africa, and South America. Craters here are rare because our weather erases them over time. They become lakes or get filled with sand or otherwise wear away from the landscape. On other worlds, lava may sometimes fill in craters, but in general, these holes in the ground are given a chance to stay. In the images of Europa it was clear volcanoes were not erasing craters (that was happening on Io). It was clear that weather didn’t play a role (that was happening on Saturn’s Titan). It was clear this was something new.
In the years since those images first made it to Earth, scientists have been working hard to try and define ways to resurface Europa without any of the normal physics: Without weather, without volcanoes, without even an alien Zamboni driver making his way over and over across the icy moon.
While it may seem silly to imagine some little alien Zamboni driver, the science behind this crazy cartoon notion isn’t too far off the truth. The first epiphany the scientists had was the presence of a hidden liquid ocean—think of this as the Zamboni’s tank of liquid. The orbital resonance between Io, Europa, and Ganymede has the effect of significantly heating the interiors of Europa and Io. The volcanoes of Io dramatically illustrate this point as they fling lava in arcs dozens of kilometers long. While Europa doesn’t have any features this flamboyant, it occurred to scientists that the icy surface of Europa might be caused by liquid seeping out onto the surface and recoating it like the liquid sprayed from a Zamboni machine recoats the pitted ice of a hockey rink.
The idea of a liquid ocean led quickly to the simple questions: How large, and how deep? And it led to the more fundamental question; can life exist in that under-ice sea? Here on Earth we find life clinging to deep-sea volcanic vents. This life needs no light to thrive and it demonstrates to scientists that life could, and quite possibly does, exist around deep-sea vents on Europa as well.
Today, many scientists and engineers are trying to figure out how to get through the ice on Europa to explore what lies beneath. The biggest remaining question is where to try and break through. The newest models of Europa’s structure describe a hot core, with hot vents, surrounded by a salty sea. The majority of this ocean is covered in ice perhaps tens of kilometers thick, although in certain spots, the rising hot water thins out pockets, creating places perhaps the size of Lake Superior where the water rises to within a couple kilometers of the surface. Marked by a specific type of chaotic ice pattern, these are the places to burrow down with sterile robots and cameras, and to descend into the deep to look for life that didn’t begin here on Earth, but did begin far across the solar system.
If life is found on Europa … There is really no way to scientifically finish that sentence. The discovery of life on another world will mean something different to every individual. Scientifically, it will simply put constraints on the probability of life originating under different conditions. At this moment we can make no real guess of how advanced life might be. Many scientists’ diagrams show (only half-jokingly) sea sponges and other undersea foliage on the floor of cartoon drawings of Europa. It is generally believed that any life is likely to be unintelligent and likely not at all advanced, but these are simply guesses.
At this time, none of the world’s space agencies have concrete (and funded) plans to send a mission to Europa. Various proof of concept robots are being tested in hopes that we’ll able to automatically explore in the not too distant future, but in the US at least, that future is at least fifteen years away if not longer.
For now, we scientists pour over modern detailed images of this icy moon, and try and write computer models that will allow us to reproduce the cracks and crevices and resurfacing we see. We study and we model and we dream of what awaits in the sea. Life here, most likely, did begin here, but there is nothing to say that it didn’t also begin there, where it waits for tomorrows’ robots to make first contact.
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