More Clues, But Still No Certainty, About Europa Plumes

Scientists using the Hubble Space Telescope have collected and analyzed data that offer more evidence — but not certainty — that plumes of water vapor are escaping from the ocean that lies beneath miles of ice on Jupiter’s moon Europa.   Such plumes are known to exist on Saturn’s icy moon Enceladus and scientists see parallels at Europa, but the data remain inconclusive.  At the direction of Congress, notably Rep. John Culberson (R-TX), NASA is developing a spacecraft to visit Europa in the 2020s.  Since life as we know it requires water, and there is water on Europa, Culberson and others believe life will be found there.

The observations that were the subject of NASA’s teleconference today were made in 2014.  It has taken that long for the data to be crunched, which required significant software development, and verified and a paper written for publication.

William Sparks, an astronomer at the Space Telescope Science Institute (STScI) in Baltimore, discussed the findings of his team, which will be published later this week in the Astrophysical Journal.  He was very cautious, however, noting repeatedly that the observations were made “at the limits” of what can be done with Hubble’s Space Telescope Imaging Spectrograph (STIS) instrument in the far ultraviolet (UV) band of the electromagnetic spectrum. 

The findings are based on imaging observations of what may be water vapor erupting into space through cracks in Europa’s icy crust.  If true, a spacecraft orbiting Europa might be able to dip down and fly through the plumes to study the constituents of that ocean without having to drill through miles of ice.

This composite image shows suspected plumes of
water vapor erupting at the 7 o’clock position off the limb of Jupiter’s
moon Europa. The plumes, photographed by NASA’s Hubble’s Space
Telescope Imaging Spectrograph, were seen in silhouette as the moon
passed in front of Jupiter. Hubble’s ultraviolet sensitivity allowed for
the features — rising over 100 miles (160 kilometers) above Europa’s
icy surface — to be discerned. The water is believed to come from a
subsurface ocean on Europa. The Hubble data were taken on January 26,
2014. The image of Europa, superimposed on the Hubble data, is assembled
from data from the Galileo and Voyager missions.Credits: NASA/ESA/W. Sparks (STScI)/USGS Astrogeology Science Center

This is not the first time that scientists have observed what may be plumes on Europa.  In 2013, a team led by Lorenz Roth of the Southwest Research Institute in San Antonio, TX announced they thought they had detected water vapor plumes using the same instrument, STIS, but a different method, spectroscopy.

The plumes are erratic, however, and because both the Roth and Sparks observations were made at the limits of Hubble’s capabilities, skepticism remains. NASA’s press release about today’s teleconference said the topic was “evidence of surprising activity on Europa.”  As the teleconference progressed and the scientists continued to emphasize that the findings are not definitive, the question arose as to what was “surprising.”  One of the participants, Britney Schmidt of Georgia Tech, an expert on the Enceladus plumes who compared them to what was observed about Europa, replied: “I am not surprised, but I am excited, and skeptical.”

Sparks himself explained that it is not possible to say with certainty whether what was observed was plumes or an effect of the Hubble instrument itself. While the results are “statistically significant” and he did not know of any natural phenomenon other than plumes to explain the data, he could not rule out that they do not completely understand how the STIS instrument on Hubble works in this type of observation campaign.  Repeated observations using STIS would help determine the characteristics of the instrument and make his team feel more confident, he added.

Jennifer Wiseman, senior Hubble project scientist at NASA’s Goddard Space Flight Center, offered that what is exciting about the Sparks observations is that they complement those made by the Roth team.  Roth used spectroscopy, Sparks used imaging — different approaches that produced independent evidence of the plumes. 

Culberson, who chairs the House Appropriations Commerce-Justice-Science (CJS) subcommittee, which funds NASA, is the most enthusiastic politician in favor of sending a spacecraft to investigate Europa.  He has added money to NASA’s budget for the past several years to pay for such a mission even though it was not in NASA’s budget plan.   He has specified not only that NASA send a spacecraft to orbit Europa, but to land there.   In this year’s report on the FY2017 CJS appropriations bill, he did agree that the two spacecraft could be launched separately, in 2022 and 2024 respectively, and he wants NASA’s Space Launch System (SLS) to be used to launch both of them.

In a statement today, he again stressed that he wants an orbiter and a lander.  “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.”

Hubble’s STIS instrument was repaired on the last Hubble servicing mission in 2009.  John Grunsfeld, a NASA astronaut who later became NASA’s Associate Administrator for the Science Mission Directorate, was one of the space shuttle crew members who fixed the instrument.  He left NASA earlier this year.

NASA astrophysics division director Paul Hertz and Wiseman both extolled the capabilities of Hubble as a result of that servicing mission as evidenced by these findings,  Culberson agreed, saying that Grunsfeld’s “repair work has helped strengthen support” for the Europa mission.

Europa project scientist Curt Neibur, who also participated in the teleconference, stressed that the Europa mission is not being designed to detect life, but to determine habitability — is the environment conducive to the development of life.  He said there is a vigorous and ongoing debate in the science community as to what instruments would be needed to detect life itself elsewhere in the solar system.

Hertz noted that Hubble recently was approved for another two years of operations as part of NASA’s Senior Review process, adding that he expects it to continue operating as long as it has scientific value, which hopefully will be after the James Webb Space Telescope (JWST) is operating so the two telescopes can provide complementary observations.  JWST is designed to study the solar system and universe in the infrared (IR) band, rather than these UV observations using STIS.   It is scheduled for launch in 2018 and Wiseman said it also will be used to look for Europa plumes, allowing observations of transitions of water molecules in the IR band that are not visible in the UV band.