What appears to be a huge plume of water vapor
has again been spotted emanating from Jupiter's icy moon Europa, boosting
scientists' confidence that the phenomenon is real.
NASA's Hubble Space Telescope detected a
62-mile-high (100 kilometers) candidate plume near Europa's equator in February
2016, researchers and agency officials announced recently.
The newly reported candidate is in the same
location as a smaller one that Hubble saw back in March 2014. And that location
is right in the middle of an unusually warm part of Europa's surface identified
by NASA's Galileo Jupiter probe in the late 1990s.
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| Credit: NASA, ESA, |
These results are highly suggestive but don't
quite rise to the level of a definitive plume confirmation, researchers said.
"It's not completely unequivocal, but in
my mind, the pendulum has swung from caution to optimism," project team
leader William Sparks, of the Space Telescope Science Institute in Baltimore,
said during a press conference.
Hunting for elusive plumes
A huge ocean of liquid water sloshes beneath
Europa's icy shell, making the 1,900-mile-wide (3,100 km) moon one of the solar
system's best bets to host alien life. (Many astrobiologists rank Europa and
Saturn's geyser-blasting, ocean-harboring moon Enceladus as the top two such
candidates.)
A different research group first spotted an
apparent plume coming from Europa's south polar region in late 2012, also using
Hubble. Astronomers tried repeatedly to confirm the phenomenon but kept coming
up empty — until early 2014, when Sparks and his team detected one near the
moon's equator.
The March 2014 detection (which the
researchers announced in September 2016) and the newly announced one were both
made using the "transit technique." As Europa passed in front of
Jupiter from Hubble's perspective, the putative plumes blocked some of the
ultraviolet light emitted by the giant planet.
"The fact that we've got a repeat tells
you that, in a formal statistical sense, it can't happen by chance,"
Sparks said today.
"So we have to look for systematic
effects that might cause it," he added, referring to issues such as
instrument artifacts. "We don't know of any, which is why most of us, some
of us, are leaning towards thinking that this means [the plumes are] real."
Bolstering that interpretation is the thermal
imaging performed by Galileo two decades ago, which shows a "hotspot"
at the location of the 2014 and 2016 plume candidates.
If the thermal anomaly and the plumes are
indeed causally linked, there are two possible explanations, researchers said.
Water venting through cracks in Europa's ice could be warming the surface, or
water from the plume may be falling back down onto the hotspot, changing the
fine structure of the surface and allowing it to hold onto heat longer, they
said.
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| Credit: NASA, ESA, W. Sparks (STScI), and the USGS Astrogeology Science Center |
Flying through the plumes?
The detection of plume candidates is
influencing the planning of NASA's $2 billion Europa Clipper mission, which the
agency aims to launch in the early to mid-2020s.
The Europa Clipper will settle into orbit
around Jupiter, then perform 40 to 45 flybys of Europa over the course of
several years. The solar-powered probe will use a number of different
instruments to study the moon's ice shell and underlying ocean, gathering data
that should help scientists assess Europa's ability to support life as we know
it.
Clipper will also hunt for Europa's plumes
and, if possible, fly through them, as NASA's Cassini spacecraft has done with
the powerful and ever-present plume of Enceladus, said Jim Green, director of
NASA's Planetary Science Division.
Plume plunges would allow Clipper to snag
samples of Europa's vast, buried ocean without even touching down, agency
officials have said.
"If there are plumes on Europa, as we now
strongly suspect, with the Europa Clipper, we will be ready for them,"
Green said in a statement.
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