9. Jupiter: a giant primitive planet
The Galilean satellites
Introduction to the largest moons
When the two Voyager spacecraft flew past Jupiter in 1979, they got only a brief look at the Galilean satellites. However, it was time enough for their cameras to discover active volcanoes on Io, smooth ice plains on Europa, grooved terrain on Ganymede, and the crater-pocked surface of Callisto. The incredible complexity and rich diversity of their surfaces, which rival those of the terrestrial planets, are only visible by close-up scrutiny from nearby spacecraft. Ground-based telescopes provide only a blurred view of the tiny, distant moons.
Scientist’s have created three-layer models for the interiors of Io, Europa and Ganymede, based on the Galileo spacecraft’s gravity and magnetic data and constrained by the satellites’ surface properties and overall mass density. They all have a large metallic core, a rocky silicate mantle, and an outer layer of either water ice, for Europa and Ganymede, or rock, for Io. In contrast, Callisto is a relatively uniform mixture of ice and rock.
Io: a world turned inside out
The innermost Galilean satellite, Io, has a radius and density that are nearly identical to those of our Moon, but contrary to expectation, there are no impact craters on Io. The dramatic landscape is instead richly colored by hot flowing lava and littered with the deposits of volcanic eruptions. The active volcanoes emit a steady flow of lava that fills in and erases impact craters so fast that not a single one is left
Sulfur and sulfur dioxide give rise to Io’s colorful appearance. Its red and yellow hues are attributed to different forms of sulfur, probably formed at different temperatures. Volcanic plumes of sulfur dioxide gas fall and freeze onto the surface, forming white deposits that were first detected by ground-based infrared spectroscopy in the 1970s.
Whereas our Moon has been geologically inactive for eons, Io is the most volcanically active body in the solar system. It exhibits gigantic lava flows, fuming lava lakes, and high-temperature eruptions that make Dante’s Inferno seem like another day in paradise. Scientists estimate that Io has about 300 active volcanoes, and the hotspots of at least 100 of them have been observed.
The cameras aboard the Voyager 1 spacecraft discovered nine active volcanoes during its flyby in 1979, and the most active volcanoes, such as Prometheus, Loki and Peli, were observed from the Galileo spacecraft two decades later. Prometheus is the “Old Faithful” of Io’s many volcanoes, remaining active every time it has been observed. Loki is the most powerful volcano in the solar system, consistently putting out more heat than all of Earth’s active volcanoes combined. And Pele, the first volcano to be seen in eruption on Io, has repeated the performance for Galileo and the Hubble Space Telescope. Like other volcanic centers on Io, these active volcanoes have been named for gods of fire, the Sun, thunder and lightning.
Plumes of volcanic gas erupt from Io’s active volcanic vents, rising up to half a thousands kilometers above the surface. They spread out in graceful, fountain-like trajectories, depositing circular rings of material about a million meters in diameter. Instruments aboard Galileo have practically smelled the hot, sulfurous breath of the eruptions, monitoring the sulfur dioxide gas as it rises, cools and falls. Diatomic sulfur, consisting of two sulfur atoms joined in pairs, has also been detected gushing out of the active volcanoes by instruments on the Hubble Space Telescope.
Io’s tides of rock
What is keeping Io hot inside, warming up its interior, melting its rocks, and energizing its volcanoes? The heat released during the moon’s formation and subsequent radioactive heating of its interior should have been lost to space long ago, just as our Moon has lost the internal heat of its youth and become an inert ball of rock. Unlike the Earth, whose volcanoes are energized by heat from radioactivity and friction due to mass motion, it is the dominant tidal distortions, created by massive Jupiter and its other moons, which sustain Io’s molten state.
Just as the gravitational force of the Moon pulls on the Earth’s oceans, raising tides of water, the gravitational force of massive Jupiter creates tides in the rocks of Io. Since the pull of gravity is greatest on the closest side to Jupiter, and least on the farthest side, Io’s solid rocks are drawn into an elongated shape. But this tidal distortion does not melt the rocks by itself. If Io remained in a circular orbit, one side of the moon would always face Jupiter, its tidal bulges would not change in height, and no heat would be generated.
The three Galilean satellites Io, Europa and Ganymede resonate with each other in a unique orbital dance, known as the Laplace resonance, in which Io moves four times around Jupiter for each time Europa completes two circuits and Ganymede one. This congruence allows small forces to accumulate into larger ones. The resultant gravitational tug-of-war between Jupiter and the satellites distorts the circular orbits of all three moons into more oblong elliptical ones. The effect is greatest for Io, which revolves nearest to Jupiter, but there is a noticeable consequence for Europa and perhaps even Ganymede.
During each lap around its slightly eccentric orbit, Io moves closer to Jupiter and further away, wobbling back and forth slightly as seen from Jupiter. The strong gravitational forces of the planet squeeze and stretch Io rhythmically, as the solid body tides rise and fall. Friction caused by this flexing action heats the material in much the same way that a paper clip heats up when rapidly bent back and forth. This tidal heating melts Io’s interior rocks and produces volcanoes at its surface.
Magnetic connections with Io
Earth-based observations in the 1970s revealed a vast cloud of sodium atoms that envelops Io, forming an extended atmosphere that is nearly as big as Jupiter. The sodium cloud stretches backward and forward along Io’s orbit, until the sodium atoms become ionized and no longer emit the light that makes them visible. The neutral, or unionized, sodium atoms have probably been chipped off the surface of Io by the persistent hail of high-energy particles found near the giant planet.
The volcanoes on Io provide the raw material for the satellite’s tenuous atmosphere of sulfur dioxide, designated SO2, that gathers above the erupting vents like localized umbrellas. The volcanic plumes are like fountains, with eruptions that arch gracefully back to Io’s surface, and the gas is not propelled with sufficient velocity to escape the satellite’s gravitational pull. Nevertheless, atoms of sulfur, S, and oxygen, O, can escape from Io once they are ionized by exposure to radiation form the Sun or from the hail of energetic particles in Io’s vicinity. These ions have been detected from the Voyager and Galileo spacecraft by their ultraviolet glow.
Since charged particles cannot cross magnetic field lines, Jupiter’s spinning magnetic field confines and directs the sulfur and oxygen ions into a doughnut-shaped ring known as the plasma torus. As the giant planet rotates, it sweeps its magnetic field past Io, stripping off about a ton, or 1,000 kilograms, of sulfur and oxygen ions every second. This material is lost from Io forever, and is continuously replenished by its volcanic activity, albeit indirectly through subsequent ionization.
Europa’s bright, smooth icy complexion and young face
The smallest, and yet brightest, of the Galilean satellites, Europa, has a density comparable to that of rock, but its surface is as bright and white as ice. In fact, it is water ice! With surface temperatures of 110 degrees kelvin or less, the water ice on Europa is frozen as hard and solid as granite. Europa’s surface is nearly devoid of impact craters, and there are no mountains or valleys on its bright smooth surface. No features extend as high as 100 meters, making Europa the smoothest body in the solar systems.
Long cracks, ice rafts and dark places on Europa
A veined, spidery network of long dark streaks marks Europa’s young face, suggesting great inner turmoil. The fine lines run for millions of meters, intersecting in spider-web patterns. They give Europa a broken appearance that resembles a cracked mirror or an automobile window that has been shattered in some colossal accident. The dark lines are most likely deep fractures formed when that part of the ice cracked open, separated, and filled with darker, warm material seeping and oozing up from below. Dirty liquid water or warm dark ice has apparently welled up and frozen in the long cracks, producing the lacework of dark streaks.
As two adjacent pieces of ice pull apart slightly, warm soft ice might push up and freeze to form long ridges that parallel the cracks. Other ridges may have originated when the sides were pushed together, closing the crack and crumpling its edges to form a ridge.
The surface of Io is fragmented everywhere, as if pieces of ice have broken apart, drifted away and then frozen again in slightly different places. Large blocks of ice have floated like rafts across the moon’s surface, shifting away from one another like moving pieces of a jigsaw puzzle. Some of them are tilted; others rotated out of place, like plastic toys bobbing in a bathtub. This shows that the ice-rich crust has been or still is lubricated from below by either slushy ice or maybe even liquid water.
Explosive ice-spewing volcanoes and geysers may erupt from the buried seas, reshaping the chaotic surface of the frozen moon and leaving dark scars behind. Extended dark regions may, for example, have formed when the underground ocean melted through Europa’s icy shell, exposing darker material underneath, or when upwelling blobs of dark, warm ice broke through the colder near-surface ice.
Europa’s underground sea of melted ice
Tidal distortions could explain how water ice has melted in the frigid environment near Europa. The satellite has a slightly eccentric orbit due to gravitational interactions with Io and Ganymede, which revolve closer and farther away from Jupiter than Europa. Over the course of one trip around Europa’s elongated path, Jupiter’s strong gravity stretches and compresses the satellite, in a process called tidal flexing. Frictional heat associated with similar tidal flexing melted the rocks inside Io, and it operates on Europa as well – to a smaller extent since Europa is further from Jupiter. But the warmth generated by tidal heating may have been or may still be enough to soften or liquefy some portion of Europa’s icy covering, perhaps sustaining a subsurface ocean of liquid water.
Magnetic measurements from the Galileo spacecraft provide more evidence for an otherworldly ocean inside Europa. The satellite’s magnetism changes direction as Jupiter’s magnetic field sweeps by in different orientations to the satellite, owing to the tilt between the planet’s rotation axis and magnetic axis. This means that the magnetic field at Europa is not generated in a core, but is instead induced by the passage of Jupiter’s field in an electrically conducting liquid, such as salt water, beneath the ice. Although this evidence for a subsurface liquid ocean is indirect, it is the only indication that buried water is there now, rather than in the geological past.
Cratered, wrinkled Ganymede
Ganymede, the largest moon in the solar system, has a radius that exceeds that of the planet Mercury, but the satellite’s density is so low that it must contain substantial quantities of liquid water or water ice. Its icy surface has experienced a violent history involving crustal fractures, mountain building and volcanoes of ice.
Bright regions on Ganymede’s surface contain sets of parallel ridges and valleys, termed grooved terrain, which looks like the swath of a giant’s rake. The grooved terrain was most likely formed when the moon’s water-ice crust expanded and stretched, cracking and rifting open as it was pulled apart. The crustal expansion might have happened when the satellite’s rocks melted and moved into its interior while its water migrated to the top where they froze.
Sets of intersecting mountain ridges overlap and twist into each other. Some of the ridges cut across craters, while craters appear on other ridges. Ganymede evidently experienced several epochs of mountain building. These crustal deformations may have continued for a billion years.
Water-ice volcanism played a role in creating the bright terrain on Ganymede. Prominent depressions were apparently flooded with liquid water or icy slush, and then froze into bright smooth bands that now cover much of the moon. Craters found in these areas indicate that this also happened early in the satellite’s early history, at least a billion years ago.
Darker regions on Ganymede are older and more heavily cratered. Some of these large polygonal blocks rise about a thousand meters above the bright, grooved terrain, and look as if they have moved sideways for tens of thousand of meters along the moon’s surface.
Ganymede, a moon with its own magnetic field
One of the major surprises of the Galileo mission was the discovery that Ganymede has its own intrinsic magnetic field. The moon is generating a magnetic dipole similar to those of most planets, and roughly a thousandth of the strength of Earth’s. No other satellite now has such a magnetic field, but our Moon might have had one in the distant past.
Callisto, an ancient, battered world
Remotest of the Galilean moons, Callisto has had a much more sedate and peaceful history than the other large satellites of Jupiter, with little sign of internal activity. It is a primitive world whose surface of ice and rock is the most heavily cratered in the solar system. Unlike nearby Ganymede, the moon Callisto has no grooved terrain or lanes of bright material, and it exhibits no signs of icy volcanism. So, Callisto is a long dead world unaltered by resurfacing since it formed and ancient impacts molded its face, a fossil remnant of the origin of planets and their moons. In fact, with a surface age of about 4 billion years, Callisto has the oldest landscape in the solar system.
Yet, when seen close up by the Galileo spacecraft there are indications of subdued, youthful activity on Callisto’s surface. It is blanketed nearly everywhere by fine, mobile dark material, interrupted only where bright crater rims poke up through it. Small impact craters are mostly absent, and those that are found sometimes appear worn down and eroded. Thus, the smaller craters seem to have been filled in and degraded over time, perhaps by the dark blanket of debris that might have been thrown out by the larger impacts. Ice flows may have alternatively deformed and leveled many craters, because ice, which is rigid to sharp impact, can flow gradually over long periods of time, as glaciers do on Earth. The lack of small craters on Callisto might also be explained if the ancient population of impacting objects near the remote satellite had relatively few small objects when compared to the population near the Moon and Mercury.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University