Hubble finds additional evidence of plumes on Europa [Updated]

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.

For the second time during the last five years, astronomers using NASA's Hubble Space Telescope have imaged what are likely water vapor plumes erupting off the surface of Jupiter's moon Europa. When they observed the moon in the far ultraviolet spectrum in 2014, they observed water vapor emissions during three out of 10 sessions. This indicates the existence of periodic jets of water emanating from the moon's interior. If the plumes do, in fact, emerge and rain down on the surface, it may 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. That would seem to be an understatement.

Why this matters

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.

Confidence in findings

The new data provide additional confidence for scientists that they are, indeed, seeing periodic water plumes. Using the same technique that scientists employ to detect atmospheres around exoplanets, planetary scientists studied Europa as it passed in front of Jupiter. In this way they were able to observe absorption features around Europa as they blocked some of the light of Jupiter. Essentially, the plumes were silhouetted.

"The chances of this occurring due to randomness is very low," said William Sparks, and astronomer with the Space Telescope Science Institute in Baltimore who led the research, during the news conference. "The problem is that there may be something we don’t understand about the instrument or what Europa looks like in ultraviolet light. These are difficult wavelengths for Hubble." Sparks added that aside from an instrument error, he could not think of any other explanation.  I'm not aware of any natural explanation for this besides water plumes," he said. "They appear to be real. The statistical significance is really good."

Scientists expect to be able to collect better data about potential plumes with the James Webb Space Telescope, launching in 2018. But the only real way to find out for sure is to go to Europa, and make in situ observations of the world and its likely jets.

Going there

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. During the news conference on Monday the program scientist for the Europa mission, Curt Niebur, said the orbiter spacecraft will contain nine instruments that are "powerful plume hunters," with the capability to "aggressively" investigate the plumes on Jupiter.

Initial estimates of the volume of these plumes indicates they are producing about 5,000 to 7,000kg of water mass per second, equivalent to an Olympic swimming pool every minute, Niebur said. Instruments on board the orbiting spacecraft will have a sensitivity to detect 1 or 2kg of outgassing per second, more than enough capability. Niebur said mission planners view carrying deployable probes on the orbital mission, such as cubesats, as too risky an option.

NASA has also been directed by Congress, as a follow-up mission to the orbiter, to develop a spacecraft capable of landing on Europa and sampling the environment. The champion of that mission, Texas Congressman John Culberson, reiterated that goal in a statement after Monday's announcement:  "These giant water plumes will make it possible to sample Europa's ocean from the surface lander which will touch down in about 10 years. The Planetary Science Decadal Survey has voted twice to make a mission to Europa a top priority because this deep ocean is the closest and best place outside Earth to look for life on another world. Today's announcement reinforces the high likelihood that the clipper orbiter and lander missions will discover life in the European ocean."

In an interview later on Monday, Culberson, who is chairman of the House subcommittee that oversees NASA's budget, told Ars that the budget remains on track to support the 2022 launch of an orbital mission, the Clipper, and a 2024 launch of the lander. "I am keeping an eagle eye on NASA Headquarters to make sure these missions fly as the scientific community intended," he said.

 

Listing image by Caltech/NASA