Evidence Mounts for Hydrothermal Vents on Enceladus, Plumes on Europa
NASA announced new findings today about two of the solar system's "ocean worlds" -- places other than Earth where global oceans may exist and, with them, the chance for life. Data from the Cassini spacecraft already have indicated that Saturn's moon Enceladus has an ocean that spews into space through cracks in its icy crust. The announcement today is that those plumes contain hydrogen, hinting that the ocean has hydrothermal vents akin to those on Earth's ocean floors where life improbably exists. Meanwhile, the Hubble Space Telescope has again detected what may be plumes emanating from an ocean on Jupiter's moon Europa, though definitive data remain elusive.
Cassini has been orbiting Saturn since 2004 and is in its last months of life. At a press conference last week, scientists at the Jet Propulsion Laboratory (JPL) reviewed some of Cassini's key findings to date, including the plumes from Enceladus, a small moon that has a liquid ocean under an icy crust. Cracks in the crust allow material from the ocean to burst up into space and Cassini was able to fly through the plumes to gather data on their constituents. Cassini project scientist Linda Spilker discussed what the spacecraft discovered when it flew through the plumes in October 2015 at a height of just 49 kilometers (30 miles) -- water vapor and organics.
Artist's illustration of Cassini flying through plumes from Saturn's moon Enceladus. Credit: NASA/JPL-Caltech
At today's press conference, Spilker revealed that hydrogen gas was also detected. Hydrogen combined with carbon dioxide in the ocean could generate the energy needed to create chemical reactions essential to life. Details of the discovery were published in the journal Science today.
Life requires three ingredients according to NASA: liquid water; an energy source for metabolism; and the correct chemical ingredients (carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur). With the findings announced today, Enceladus is known to have all of those except phosphorus and sulfur. Scientists suspect they are there, however, because the rocky core of the moon is thought to be similar to meteorites that contain them.
It will be up to future spacecraft to prove that point, however. Cassini is beginning its "grand finale" of 22 deep dives into Saturn's atmosphere later this month. The spacecraft will meet its end on the final dive on September 15. It is running out of fuel and NASA wants to deliberately destroy it to avoid accidental contamination of Enceladus or Saturn's other scientifically tantalizing moons, including Titan with its methane lakes.
Like Enceladus, Jupiter's moon Europa is thought to have an ocean under an icy crust. In 2014, data from the earth-orbiting Hubble Space Telescope suggested that it also may have plumes. Today's announcement was that observations in 2016 showed a plume at the same location as the one seen in 2014. Scientists do not have as much close up data about Europa as they do for Enceladus, however, and the Hubble data are not definitive. NASA's Galileo spacecraft orbited Jupiter for many years (1995-2003), but Europa was not a main target of its investigations and only about a dozen flybys of the moon were made.
When Hubble spotted what might be a plume in 2014, scientists looked at the Galileo data and discovered that it had indicated a hot spot right at that location. Theories are that higher temperatures under the ice might open a crack in Europa's surface and allow ocean contents to reach into space or that the contents spew into space and fall back onto the surface, making it warmer. The 2016 Hubble observations are discussed in a paper published today in The Astrophysical Journal Letters.
At today's press conference, Bill Sparks, an astronomer at the Space Telescope Science Institute, which operates Hubble, explained what is known, and not known, about the Europa plumes. He led the 2014 and 2016 plume studies. Sparks said that his team has looked for plumes 12 times and found them twice. NASA's carefully worded press release makes clear that scientists remain uncertain that the plumes are there, however, saying the 2016 images "bolster evidence" that the plumes "could be a real phenomenon." Sparks himself characterized it as "not completely unequivocal as it is with Enceladus, it's still at the limits of what Hubble can do, but we are growing in confidence" because they now have seen it twice and its location correlates with the Galileo hot spot data.
The possibility of an ocean under Europa's crust, never mind plumes, was unknown when Galileo was built, so it was not designed to look for them. NASA currently has a spacecraft orbiting Jupiter, Juno, but it is not designed to study Europa either. Close-up measurements will have to wait until the 2020s when NASA plans to launch missions whose entire purpose is investigating Europa.
A Europa orbiter and lander were added to NASA's science program by Rep. John Culberson (R-TX), chairman of the House Appropriations Commerce-Justice-Science (CJS) subcommittee that funds NASA. Sending a probe to Europa was a high priority in the last two planetary science Decadal Surveys written by the National Academies of Sciences, Engineering, and Medicine. Decadal Surveys guide NASA's science program. The most recent survey ranked Europa second, however, behind a series of missions to return a sample of Mars to Earth. NASA had enough money only to pursue the top priority, Mars sample return, so did not request funds for Europa.
Congress has the power of the purse, however, and Culberson added money for Europa. He has said in many venues that he believes life will be found on Europa and he intends to make certain NASA looks for it. He wants the orbiter, Europa Clipper, launched in 2022 and the lander in 2024. (Europa Clipper actually will orbit Jupiter, not Europa. It will make flybys of Europa.) The lander is still in the conceptual phase. As he explained in an interview with Miles O'Brien on the PBS NewsHour last night, he wrote into law that NASA must launch those missions: "The Europa orbiter and lander is the only mission it is illegal for NASA not to fly."
That being said, President Trump's budget request for FY2018 funds Europa Clipper, but specifically states it does not fund the lander. The President's request is just that, of course, a request. Only Congress decides how much money the federal government will spend and how. As chairman on the House CJS subcommittee, Culberson has considerable influence on the outcome.
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