The icy moon Europa may prove the most exciting destination in outer space that NASA could explore.
[Europa, in a photo from the Galileo spacecraft. Image reprocessed by Prof. Ted Stryk of Roane State Community College, Oak Ridge, Tennessee.]
One of Jupiter’s horde of at least 65 satellites, Europa is second out from the planet and its third-largest. Slightly smaller than our moon, it’s big enough that Galileo spotted it and three fellow satellites when he peered at Jupiter through his crude telescope in 1610.
On Nov. 16, NASA announced that images taken years ago by the Galileo spacecraft indicate that less than two miles below the surface, giant lakes may exist — one studied apparently has as much liquid water as the Great Lakes’ total volume — in addition to the enormous saltwater sea believed lurking much deeper below the ice. Warm water apparently rises through the ice like a volcano’s magma through the ground; the water forms subsurface lakes and eventually causes massive bulges of chaotic terrain on the surface.
Orbiting Jupiter 483 million miles from the sun, Europa’s surface temperature is estimated at 100 Kelvin — that is, about -280 degrees Fahrenheit. But tidal forces emanating from Jupiter cause flexing throughout the moon, and this friction warms the interior ice. Another source of warmth may be the decay of radioactive elements in Europa’s core, just as in Earth’s.
Speaking of the earlier discovery of the ocean deep under the surface, NASA officials noted that one of Galileo’s most significant discoveries, before it was sent into Jupiter’s atmosphere in 2003, “was the inference of a global salt water ocean on Europa. The ocean is deep enough to cover the whole surface and contains more liquid water than all of Earth’s oceans combined.
“However, being far from the sun, the ocean surface is completely frozen. Most scientists think this icy crust is tens of miles deep.”
The new discovery refines the picture, however, positing lakes within the ice, much closer to the surface. Rugged chaotic ice terrain seems to show that water raises slabs of ice and reaches the surface between the cracks, then freezes over.
[Schematic of one of Europa’s suspected great lakes. According to the University of Texas at Austin, “Researchers predict many more such lakes are scattered throughout the moon’s icy shell.” Credit: Britney Schmidt/Dead Pixel VFX/University of Texas at Austin.]
The space agency reported findings published online Nov. 16 by the journal Nature. An abstract of the Nature article says, “Our results suggest that ice-water interactions and freeze-out give rise to the diverse morphologies and topography of chaos terrains. The sunken topography of Thera Macula [an uplifted region of chaotic ice blocks] indicates that Europa is actively resurfacing over a lens comparable in volume to the Great Lakes in North America.”
Britney Schmidt, a postdoctoral fellow at the University of Texas, Austin, the lead author, said in a release from that university, “One opinion in the scientific community has been, ‘If the ice shell is thick, that’s bad for biology — that it might mean the surface isn’t communicating with the underlying ocean.’ Now we see evidence that even though the ice shell is thick, it can mix vigorously. That could make Europa and its ocean more habitable.”
Coauthors are Don Blankenship, also of the Austin institution; Wes Patterson of the Johns Hopkins Applied Physics Laboratory; and Paul Schenk of the Lunar and Planetary Institute, Houston. Blankenship commented in the Texas release, “This new understanding of processes on Europa would not have been possible without the foundation of the last 20 years of observations over Earth’s ice sheets and floating ice shelves.”
The researchers compared the round areas of chaotic terrain with features on Earth found in ice shelves and in glaciers that are above volcanoes, according to the Texas release.
For decades, scientists have suspected that Europa was capable of harboring life; as it was running out of fuel the Galileo spacecraft was sent into Jupiter, where it vaporized, because of concern that otherwise it might eventually crash on Europa and cause contamination.
Rising plumes of warm water might be a sign of venting on the ocean floor. Maybe these vents are comparable to the hot, sulfur-laden vents on the floors of our own oceans that support ecosystems without access to sunlight. The water’s churning would be expected to wash out nutrients and other chemicals from the mantle.
The problem with getting a better look was the impracticality of drilling through, say, 40 miles of ice.
Now, to sample a lake on Europa, a robotic probe might need only melt through a couple of miles — still a daunting task, but not one that seems impossible.
Even better, remains of Europan life might be available right on the surface. With water welling up from the interior and freezing on the surface, perhaps frozen organisms cover Europa. Or maybe they’re available in the cracks and chaotic terrain that mark the moon.
Either way, a lander laboratory might be able to find evidence.
Of course, a landing on airless Europa would be more challenging than recent landings on Mars, which used parachutes to slow the rovers. But NASA landed on our equally airless moon four decades ago.
Curiosity, the car-sized Mars Science Laboratory — set to launch next Saturday from Cape Canaveral — will utilize parachutes as well as a rocket-powered descent.
A small lander sent to Europa, something on the scale of the Rangers of the early phase of lunar exploration, would be an inexpensive investment in a search that could deliver stunning returns.