In the seminal science fiction series Space Odyssey, novelist Arthur C. Clarke called attention to the Jovian moon Europa’s special place in the Solar System. At the end of the series’ second novel, 2010: Odyssey Two, a spaceship sent to the Jupiter system receives a message from aliens: “All these worlds are yours except Europa. Attempt no landing there.” In data released publicly Monday NASA didn’t get quite such a declarative message from the intriguing moon, but the new information is nonetheless thrilling.
Astronomers using NASA’s Hubble Space Telescope have imaged what are likely water vapor plumes erupting off the surface of Jupiter’s moon Europa. If the plumes do, in fact, emerge and rain down on the surface, it will be significantly easier for scientists to study the moon’s interior ocean. “Europa is a world of great interest,” Paul Hertz, director of the Astrophysics Division at NASA Headquarters in Washington, said during a news conference Monday.
Monday’s news is significant because it comes as NASA is taking formative steps toward launching a pair missions to Europa in the 2020s—an orbiter to scout the moon, and a lander that will follow a couple of years later. The same engineers at the Jet Propulsion Laboratory in California who masterminded Curiosity’s landing on Mars have turned their attention toward how best to land a probe on Europa’s icy surface. And it is no easy feat. The moon creaks as Jupiter’s gravitation bulk rends its frozen surface in deep crevasses, pushing and pulling the ice upward and downward by tens of meters every few days. And with only a very tenuous atmosphere, it is cold: -210 degrees Celsius. The radiation from nearby Jupiter would kill a human in a matter of hours or days.
But the challenge of reaching Europa is worth it, scientists believe. Several kilometers below its icy shell lies the largest ocean in the known Universe. Encircling the entire moon, and measuring up to 100km deep, and it is warm at its depths. The same tidal forces that wrench Europa’s icy surface also tug on the core of this ocean, dissipating heat. For this reason astrobiologists rank Europa at the top, or near the top, of places in the Solar System (aside from Earth) where extant life might exist.
To date scientists have probed the icy world in only a cursory manner. NASA last visited the Jupiter system in the 1990s and early 2000s with the Galileo spacecraft, which captured images of Europa during 11 flybys. But the best of those pictures had a resolution of only about 10 meters per pixel, and Galileo’s closest approach to Europa brought the probe only to within about 200 km of the moon’s surface. Those brief glimpses of Europa left scientists hungry for more, however, and they’ve longed to return. The new findings only will increase that anticipation.
The Hubble Space Telescope has previously spied evidence of water vapor venting off the surface of Europa. Observations made in November and December, 2012, found water vapor above the south polar region of the moon, and the simplest explanation for its existence is that it erupted from plumes on the surface of Europa.
Intriguingly, active jets were only observed when the moon was farthest from Jupiter in its slightly eccentric orbit. The researchers suspected Jupiter’s tidal forces would wrench the vents open at the furthest point and then constrict the gaps closer in to the planet. Galileo never found them because the spacecraft didn’t looked for plumes—scientists hadn’t thought to look for them on the frozen world.
Until now the biggest challenge of exploring Europa was its thick, icy shell. There simply is no easy way to drill through kilometers of rock hard ice on a moon bathed in radiation some 500 million miles from Earth. Scientists longed to plumb the depths of Europa’s ocean, but had no way to reach it.
The likely existence of plumes opens the door to a new range of exploration options for Europa with the spacecraft now being designed. An orbiter in the Jupiter system might, for example, swoop down near the surface of Europa to try and fly through, and sample a plume. This spacecraft might deploy cubesats to go all the way down to the surface. And a lander might set down near a plume and both make observations, and dispatch tiny robots into the plume itself, to see what lurks below.
Note: This story will be updated with further information later today.