Considering that not a single instrument has landed on Europa, it's remarkable how much scientists have discovered about what lies under its icy surface. Like long-distance detectives, scientists who examine data about Europa are surveying a "crime scene" half a billion miles away and piecing a story together from clues that may appear unrelated to the untrained eye.
So far, they have no direct evidence that Europa has a vast subsurface ocean with a rocky floor (no one has actually seen it, since it's thought to lie beneath many miles of ice), but they have plenty of circumstantial evidence. Is the case convincing? You be the judge.
One very important clue came from the Galileo spacecraft's magnetometer, which detected magnetic fields around three of Jupiter's moons - Europa, Ganymede, and Callisto - that periodically flipped their north and south poles as their orientations to Jupiter's powerful magnetic field changed.
Jupiter rotates very rapidly, completing one revolution in just 10 hours, and its magnetic field rotates with the planet. Since the magnetic field is tilted with respect to the plane in which the moons orbit the giant planet, the moons find themselves crossing from the north side to the south side of the field - or the other way around - every 5 hours or so (the period varies somewhat from moon to moon because of their own orbital speeds). If the moons contain an electrically conducting layer, this motion induces a magnetic field in them that alternates north and south poles, just as Galileo detected. A substantial layer of salt water in each of the three moons seems the only reasonable explanation for the phenomenon.
Ironically, Ganymede and Callisto were thought to have layers of combined ice and water that were too substantial to make a promising case for life.
Scientists had been able to calculate the densities of the moons based on observations from Earth. Coupled with spectrometer readings which indicated that all three moons are covered in water (which, this far from the sun, would certainly be frozen into extremely cold ice at the surface), this data strongly implied that Ganymede and Callisto have layers of H2O many hundreds of kilometers thick. The pressure from such a massive amount of H2O would create high-density ice at its base. The liquid ocean suggested by Galileo's magnetometer would therefore lie sandwiched between an outer shell of regular ice and a base of high-density ice on these two moons, isolating the water from potential sources of nutrients.
The Earth-based observations of Europa, however, did not suggest such a thick layer of H2O, and neither did the flyby conducted by each of the two Voyager spacecraft that sped by the Jupiter system in 1979.
Scientists analyzed Europa's gravity field by measuring its pull on Galileo, as revealed by the Doppler shift of radio signals the spacecraft sent to Earth. This procedure enabled scientists to calculate the degree to which Europa's mass is concentrated in its center and to put limits on how thick the H2O layer could be.
The measurements indicated that Europa has an iron core and a rocky mantle, covered by a layer of H2O around 100 km (about 60 miles) thick. Like Ganymede and Callisto, the outer portion of the layer must be ultra-cold ice, but the gravity readings left open the possibility that the interior portion is liquid as suggested by the magnetometer. Unlike Ganymede and Callisto, the H2O layer is not massive enough to produce the pressure required to create a layer of high-density ice beneath the liquid, so the ocean - if it exists - lies directly over the rocky mantle, a potential source of nutrients for life. Surface features
Another important set of clues came from pictures of the surface showing a tangle of lines punctuated by domes and spots, patches of reddish material, and jumbled areas that could only be called "chaos."
The lines are fractures in the ice. In many places, water or warm ice (ice that is not far below water's freezing temperature) appears to have risen through the cracks and formed frozen ridges. In other places, cracks have evidently spread far apart and been filled in by new reddish ice to create "bands," similar to areas of the ocean floor on Earth where tectonic plates move apart and create gaps that are filled by magma which freezes into new rock.
The patterns of some of the fractures suggest that the surface has been very slowly slipping around the rest of the moon, as if a lubricating fluid lay between the crust and mantle.
The domes and spots, termed "lenticulae" (Latin for "freckles") are commonly about 10 km (6 miles) in diameter, often appear in clusters, and are thought to be places where blobs of warm ice have risen from the bottom of the floating shell and pressed against or broken through the surface in a process called "convection."
The spectral signature of the surface shows water ice as expected, but with an impurity - particularly in the dark, reddish patches - that looks like it could be magnesium sulfate, a type of salt. That could have been deposited by seawater that has risen to the surface either directly or through convection. The impurity's identity is not certain, however, and many researchers think it could be sulfuric acid, generated at the surface by radiation, or possibly a combination of the two. Neither salts nor sulfuric acid are normally red, but that color could be induced by the same radiation, which consists of a fierce and constant barrage of charged particles, trapped in Jupiter's magnetic field, that slam into Europa's surface.
The regions dubbed "chaos" are places where the surface ice has shattered into what look like massive icebergs, some of which have rotated or tilted as if a partial melting had freed them to move until they refroze into new positions.
The very small number of visible impact craters implies that the surface has been recoated within about 60 million years - a very short period of time, geologically speaking. The two largest impact craters each contain a series of concentric rings, looking like what would form if a rock fell into a pond and the resulting ripples froze. This pattern indicates that the colliding objects broke all the way through an icy shell about 20 km (12 miles) thick to a weaker layer below that could not support the normal concave shape of a crater. As subsurface water and ice flowed into the crater, they formed the frozen rings.
The source of heat
The mechanism responsible for the cracks in Europa's shell is thought to be the same one that keeps a subsurface ocean from freezing solid and powers convection in the icy crust. It's called "tidal heating."
Jupiter's gravitational pull stretches Europa, creating a tidal bulge both in Jupiter's direction and on the side that faces away from Jupiter, similar to the effect our moon has on Earth's oceans. The effect increases when Europa's orbit brings it closer to the giant planet and decreases when it is farther away, so the bulges rise and fall.
If Europa's orbit around Jupiter were perfectly circular, Europa's rotational speed (its spin around its own axis) would be synchronized with its orbital speed, so it would always present precisely the same face to its parent planet and the two bulges would always be in the same two respective places on Europa's surface. But Europa's orbit is elliptical. Its rotational speed remains constant, but like all objects in elliptical orbits, its orbital speed is greater when it is closer to Jupiter than when it is farther away. The result of this mismatch is that the part of Europa's surface that faces Jupiter varies somewhat at different points in its orbit, and the bulges - which remain in line with Jupiter - rock slowly back and forth with respect to Europa's surface.
Both the lateral movement of the bulges and the changes in the amount of stretching create friction which generates substantial amounts of heat. An even stronger version of this effect makes Io, a moon that is closer to Jupiter, the most volcanically active body in the solar system.
If there is an ocean between Europa's icy surface and its rocky mantle, calculations show that the tides would flex the ice shell by about 30 meters (100 feet). If not, the tides would be only about 1 meter (3 feet) high. It would take an orbiter to definitively measure the tides, but the arcing (or "cycloidal") shape of many of the fractures and ridges is consistent with the higher tides and the existence of an ocean.
Taking all the evidence into consideration, the argument for a subsurface ocean seems very persuasive. But we won't know for sure until an orbiter like the one envisioned in JPL's Europa Explorer concept is in operation. That would not only confirm whether an ocean exists, but also provide important insights about its habitability and identify any locations where the water may be accessible at shallow depths. With reliable knowledge of an ocean and accessible water, follow-up missions to search for life would be irresistible.
In the last few decades here on Earth, life has turned up in places once thought far too hostile to support it. And the more "extremophiles" we find on our planet, the more it seems that some of the extreme environments beyond Earth must also support life.
Does anything call Europa home? Only more exploration can tell us.
Last Updated: 3 February 2011