Here’s Europa for a shilling
No one minds, and she’s quite willing;
Spotless sheets her charms enfold,
She’ll light a fire if it be cold:
No need to have turn’d bull, dear Jove,
The day you sought Europa’s love.
— Antipater of Thessalonica, 1st century AD
Although the Greek poets of antiquity praised the unsurpassed beauty of the mythological maiden princess, they could never have imagined that hundred of million of kilometers away lay a real world of equal beauty and intrigue that would captivate the hearts and minds of astronomers, planetary scientists, and space advocates alike, up to this day. And even though scientists do not have to metamorphosise into a bull in order to cover the vast distances to go to Europa enchanted by its inviting charms, nevertheless making physical contact and landing on the distant moon presents one of the hardest engineering challenges.
Europa was one of the first things Italian astronomer Galileo Galilei saw in 1610 when he turned his telescope upward to examine the heavens for the first time, during a set of observations that would change the world forever. For the next 350 years, Europa, like most of the things that astronomers observed through a telescope, remained just a point of light in the sky. All this changed with the advent of the Space Age, when these points of light started to transform into real and exciting worlds just like our own. Still, Europa holds a special place among them, for many in the scientific community view it as the single, best place in the Solar System to hold the possibility of supporting extraterrestrial life today.
Prior to space exploration, astronomers believed for decades that the moons of the outer planets in our Solar System were frozen and boring chunks of rock and ice. After all, conventional wisdom held that so far from the Sun, every Solar System body would be a denizen of a vast icy planetary graveyard. But the Voyager probes’ fly-bys during the 1970s and ’80s revealed a very different picture. An assortment of dynamic, active, and ever-changing moons orbiting their equally fascinating planets, in setups that could best be described as mini solar systems in their own right. And exploration of Europa brought its own fascinating discoveries: a Jovian moon that, unlike Saturn’s Titan, only holds a very tenuous atmosphere of molecular oxygen, created by the breakup of the surface water ice from the radiation around Jupiter. But the real treat proved to be its interior, which is considered a promised land for astrobiology research, more so than even Titan’s surface itself.
With a diameter of 3,120 km, slightly smaller than the Earth’s Moon, Europa is believed to have a layered internal structure, much like the rest of the Solar System’s major planets and moons. Tantalizing evidence have shown that above the layers of an iron core and a rocky mantle lays a vast ocean that covers the entire moon, an ocean comprised entirely of salty water ice. And Europa’s visible icy surface that we observe from afar is this ocean’s frozen uppermost layers. The photographs beamed back by the Voyager probes showed a very smooth and young surface, mostly free of craters and impact features. That surface, though, exhibits some very interesting characteristics that have intrigued planetary scientists: it is criss-crossed by a vast network of red-colored ridges, spanning the entire globe. They look like fractured sea ice that is seen on Earth, more than anything else.
When planetary scientists started studying the photographs and data from Voyager and the subsequent Galileo mission that studied the Jovian system during the 1990s and early 2000s, they confirmed this notion: these ridges, or lineae, are fructures, or cracks, on Europa’s icy surface, caused be the intense tidal forces of the massive, nearby Jupiter and the orbital resonances with the other nearby moons.
“We see places where very clearly the ice has cracked,” says John Spencer, an astronomer working at the Southwest Research Institute’s Department of Space Studies in Boulder, Colo. “And the two sides have spread apart, and material has come up frozen in the middle, to fill the gap.” Far from being a frozen, boring moon, indeed!
“Just like the Earth’s oceans have tides, because they are pulled by the Moon’s gravity, Europa should have a tide, because it’s pulled by Jupiter’s gravity,” explains Dr. Robert Pappalardo, a senior research scientist working at NASA’s Jet Propulsion Laboratory. “And Europa’s orbit gets a little closer and a little further from Jupiter. So, when it’s closer to Jupiter, it’s streched out more, when it’s further from Jupiter it will contract more.” A dramatic representation of this effect, albeit on a larger scale, could be seen on the neighboring moon, Io, orbiting closer to Jupiter. Tidal forces acting on Io are so pronounced that the satellite’s interior is literaly molten, causing the moon to be the most volcanically active planetary body in the Solar System, sporting over 400 active volcanoes! Europa, being farther away from Jupiter, witnesses a same but gentler effect.
Yet, although tidal forces were obviously at work on Europa, this didn’t necessarily mean that a hidden ocean lay underneath. It could well be that the outer icy crust stretched all the way down to the rocky mantle.
But staying true to its seductive nature, Europa held more unrevealed charms.
Studying the long lines of lineae and other chaotic terrain features on the surface, planetary scientists realised that these surface patterns didn’t align exactly the way they should, according to theoretical predictions. Europa is tidally locked to Jupiter, just like the Moon is to the Earth, having the same hemisphere constantly facing toward Jupiter. Taking into account Europa’s synchronous rotation, if the moon’s interior was frozen solid, then the geologic formations on the surface should exchibit a certain position and pattern. But the formations seemed to be displaced, as if the surface ice was rotating faster from the rest of Europa’s interior. For some time, scientists thought that this was caused due to the moon having a “non-synchronous” rotation. But a recent study by Drs. Alyssa Rhoden and Terry Hurford, both working at NASA’s Goddard Space Flight Center, has presented evidence that the ice’s displacement is probably also caused by precession as well, with Europa rotating around a tilted axis, constantly changing direction much like the Earth does. Either way, the surface features’ displacement can best be explained by having a liquid ocean underground, with the surface ice layer sliding above it, thus rotating faster than the rest of the moon’s interior.
Intriguing as these evidence were, they still didn’t provide any proof for the existence of a water ocean. Could it be just a layer of soft ice under the surface, or a molten lava ocean just like the one inside Io?
Enter the Galileo mission’s Magnetometer. While studying the magnetic environment around Jupiter, Galileo found that Europa didn’t possess any internal magnetic field. But its findings were equally interesting. It detected an induced magnetic field close to the surface, with electric current running through the moon’s interior. Induction occurs when a moving magnetic field generates an electric current inside a conducting material. In the case of Europa, because of its orbit around Jupiter the magnetic lines from the massive planet’s magnetosphere were passing right through the moon. But what could the conducting material inside Europa be? “Ice isn’t conductive enough to create such a field,” explains Kevin Hand, a planetary scientist working at the Jet Propulsion Laboratory. “So the best explanation is a region of salty, liquid water below the frozen surface, which supplies a conductive layer. When you go through a metal detector at the airport with a conductor such as keys in your pocket, the alarm goes off. Likewise, when Galileo flew by, Europa set off the alarm.” “It’s very hard to get that pattern, without having an ocean underneath the ice,” adds Spencer.
So the picture was complete. Europa turned out to have all three basic requirements for life as we know it: liquid water, chemistry, and an energy source. But there’s a question still remaining unanswered about Europa, the most important question of all: Does it harbor life as well?
Just like the maiden in Antipater of Thessalonica’s poem, Europa proved to be a truly seductive world, revealing its charms only to those who are willing to go after them. Maybe we could let ourselves be inspired by mythology and borrow some of Zeus’ passion and desire to reach out and touch upon Europa. The rewards promise to be as unimaginable and enticing as the mythological princess’ beauty itself.