Moons of Jupiter: Io and Europa

Posted by – August 23, 2016
Categories: Science & Cosmology

The Night SkyExcerpted from The Night Sky, by Richard Grossinger.

The Moons of Jupiter


There was nothing to prepare Earth for Io, the first Jovian world visited by Voyager in 1979. Instead of drab craters as expected, Io startled Earth with vistas of flaky sulphur-yellow sands that looked like a gigantic pizza from which volcanoes were actively shooting blue plumes as high as 200 miles into the air—unprecendented scenes of exo-geological activity unprecedented in this solar system. Frames showed splotches of dark and light material, liquid flows, bright patches of color in sharp, irregular patterns with dark crescent shapes, plus several regions in which it was impossible to tell figure from ground or which way a current was flowing:

Ten-thousand-foot mountains jut up like inverted chasms, and pools of lava heave from gashes in the surface like massive boils. Volcanic ash and gas shoot hundreds of feet into emptiness.1

The inner Galilean moon became not only the first geologically alive body but the first substantially uncratered surface other than Earth’s in the system. Continuously bombarded with Jovian radiation, Io is a moil of lava, lightning, ions, and flares. A walk across it would require a space-suited visitor to pass under sulphur geysers intermittently spewing yellow, orange, and blue snows, clouds of gas and ice steaming above. That’s excitement enough, but for the more adventuresome, there might be fire-rafting on a sulphur ocean or spelunking to the subterranean sulphur lakes that it feeds, perhaps late-evening fire-walking on springs oozing from sulphur aquifers. But then (at least by Earth standards), it’s always night-time or late dusk on Io.

Io’s position rather than its geophysical dynamics accounts for its unusual state, for its climate and geology are pretty much the outcome of its proximity to Jupiter as well as the gravitational pull of the other large satellites. The twisting back and forth of the inner moon’s core and mantle results in tidal bulges and frictional stress within a crust that rises and sinks more than 30 feet per Ionian day (or revolution of Jupiter). As hard surfaces slide against each other while resisting motion, kinetic energy is converted into heat. This is the Jovian rendition of Freud’s maxim: maybe not “anatomy is destiny” but “if you get too close to the big fella, you are pretty much going to play his tune.” In Greek mythology, Io, a nymph of Hera was seduced by Zeus and then changed by him into a heifer to disguise his deed. Unamused, Hera dispatched a gadfly to sting her. The first map of Io celebrates the global culture that charted its surface: volcanic paterae are Persian, Mongolian, Egyptian, Celtic, Iroquoian, Incan, Nicaraguan, Zambezian, Quechuan, Pawnee, Babylonian, and Hopi. One is named after the Japanese sun-goddess Amaterasu, who emerged from a cave to restore light to a darkened world. Io’s volcanic peaks, shield volcanoes, and vaporous fissures are named after multicultural fire gods and goddesses: Marduk (Sumero-Akkadian), Masubi (Japanese), Maui and Pele (Hawaiian), Prometheus (Greek), Surt (Icelandic), Loki (Norse), Amirani (Soviet Georgian), and Volund (the Germanic blacksmith god).2


EuropaThe next moon, Europa, was only distantly glimpsed by the first Voyager, but the second took spectacular close-ups. Data tapes showed an enormous glacial body streaked with ink-like wiggly lines on a hard surface smoother than that of any other known world. Europa’s dark areas have so little relief that they might be discolorations rather than geological features but are probably fissures filled by material oozing onto an icy crust from the moon’s interior. From a distance Europa looks more like DNA strands wound in a cell’s nucleus or a red-stained pearl than a geophysical chunk. Its overall relief of pits, cracks, and grooves is about that of a cue ball if one were taken off a table and blown up to the size of our Moon. Yet that sleekness is relative only to size and scale. Down on the surface, things get bumpier, as Europa bombards its visitors with gigantic ice pellets propelled from geysers—a dicey stroll in 270° weather.

In mythology, Europa was a Phoenician woman of high lineage, abducted by Zeus, who was disguised as a white bull. Under Europa’s ice is a planet-wide ocean, as deep as 100 miles in spots, heated to liquid by the gravitational plucking of her sister moons (similar to the stressing of Io). That Europan ocean is now the leading candidate for life elsewhere in our solar system, surpassing Venus and Mars, but how are we to find out? Europa’s frozen crust is at least 10 miles thick. Imagine a robotic lander trying to drill through that much shell to get at a goose’s golden egg! A hundred feet would be daunting enough, especially given payload restrictions. In 2013, without leaving Earth, astronomers Mike Brown of the California Institute of Technology and Kevin Hand of the University of Hawaii hit upon a novel method for exploring the relation between Europa’s surface and its subterranean sea. Using Hawaii’s powerful Keck II Telescope, which has an adaptive-optics system to compensate for blurring caused by the terrestrial atmosphere, they scanned Europa at a higher resolution than was previously feasible and detected an unexpected dip in its spectrum on the trailing side.

After much experimentation in the lab, Brown and Hand determined that the spectroscopic signal was caused by a magnesium sulfate salt called epsomite.

“We now have evidence that Europa’s ocean is not isolated,” study lead author Mike Brown said in a statement, “that the ocean and the surface talk to each other and exchange chemicals.… [O]cean water gets onto the surface, and stuff on the surface presumably gets into the ocean water.… That means that energy might be going into the ocean, which is important in terms of the possibilities for life there.… It also means that if you’d like to know what’s in the ocean, you can just go to the surface and scrape some off.… Magnesium should not be on the surface of Europa unless it’s coming from the ocean.”3

As warmed water from the interior pours through cracks in the rind of ice and bubbles across Europa’s surface, magnesium-sulfate and sodium-chloride salts, organic molecules deposited by comets, and other surface materials are washed back in, much as on the primitive Earth. In addition, 40 percent of Europa’s crust is made up of chaos regions, chunks of ice separated by continually refreezing water, and this uneven terrain is conducive to deep slashing chasms, hence chemical backwash and pollination from the epilimnion. “I’m not an expert on life,” Brown quipped, “but I do know that if you dip a net in the ocean here, you’re bound to pick up something.”4

Europa’s equator also delivers more heat than it models. Its source could be a reflection of sunlight by icy spikes, so-called penitents of a sort that swell to more than 16 feet in the Andes—and almost certainly longer in the frosty Europan clime. Warmth is locally contained and then spread, as the Sun’s rays get trapped in portcullises while bouncing from spike to spike. The result is a vast network of massive solar radiators. In a futuristic speculation, astronauts orbiting Europa 50 miles above the surface pan across an icepack maintained by the universe at almost 300° below zero. It is shrouded in blowing snow that nearly obliterates vision—not an amenable terrain for snowshoeing whelps. Yet the third-planeteers cull their computer-generated grids until they find what they are looking for: an open fissure in the mantle of ice. A geyser half a mile across unleashes torrents of steam thousands of feet into the sky. It is dangerous to approach because of the spout’s erratic splatter, but this will be their landing site if they are to complete their mission.


  1. Dana Wilde, Nebulae: A Backyard Cosmography (Troy, Maine: D. Wilde Press, 2012), p. 194.
  2. Jonathan Eberhart, “Io: Charting the Fire,” Science News, vol. 117, no. 16 (April 19, 1980), pp. 251–52.
  3. Mike Brown, quoted in Mike Wall, “On Jupiter’s Moon Europa, Underground Ocean Bubbles Up to Surface.”
  4. Mike Brown, quoted in Michael D. Lemonick, “Water World: A Deep Ocean on a Distant Moon May Have All the Right Ingredients for Life,” Time, vol. 181, no. 12, April 1, 2013, p. 12.
Tags: Richard Grossinger Astronomy
About the Author

Bevin is the associate comms director at North Atlantic Books.