NASA’s Hubble Space Telescope has observed water vapor above the
frigid south polar region of Jupiter's moon Europa, providing the first
strong evidence of water plumes erupting off the moon's surface.
Previous scientific findings from other sources already point to the
existence of an ocean located under Europa's icy crust. Researchers are
not yet fully certain whether the detected water vapor is generated by
erupting water plumes on the surface, but they are confident this is the
most likely explanation.
Should further observations support the finding, this would make
Europa the second moon in the solar system known to have water vapor
plumes. The findings are being published in the Dec. 12 online issue of
Science Express, and reported at the meeting of the American Geophysical
Union in San Francisco.
“By far the simplest explanation for this water vapor is that it
erupted from plumes on the surface of Europa,” said lead author
Lorenz Roth of Southwest Research Institute in San Antonio. "If those
plumes are connected with the subsurface water ocean we are confident
exists under Europa's crust, then this means that future investigations
can directly investigate the chemical makeup of Europa's potentially
habitable environment without drilling through layers of ice. And that
is tremendously exciting."
In 2005, NASA’s Cassini orbiter detected jets of water vapor and dust
spewing off the surface of Saturn’s moon Enceladus. Although ice and
dust particles have subsequently been found in the Enceladus plumes,
only water vapor gases have been measured at Europa so far.
Hubble spectroscopic observations provided the evidence for Europa
plumes in December 2012. Time sampling of Europa's auroral emissions
measured by Hubble's imaging spectrograph enabled the researchers to
distinguish between features created by charged particles from Jupiter's
magnetic bubble and plumes from Europa's surface, and also to rule out
more exotic explanations such as serendipitously observing a rare
meteorite impact.
The imaging spectrograph detected faint ultraviolet light from an
aurora, powered by Jupiter’s intense magnetic field, near the moon’s
south pole. Excited atomic oxygen and hydrogen produce a variable
auroral glow and leave a telltale sign that are the products of water
molecules being broken apart by electrons along magnetic field lines.
“We pushed Hubble to its limits to see this very faint emission.
These could be stealth plumes, because they might be tenuous and
difficult to observe in the visible light,” said Joachim Saur of the
University of Cologne, Germany. Saur, who is principal investigator of
the Hubble observation campaign, co-wrote the paper with Roth.
Roth suggested that long cracks on Europa’s surface, known as lineae,
might be venting water vapor into space. Cassini has seen similar
fissures that host the Enceladus jets.
Also the Hubble team found that the intensity of the Europa plumes,
like those at Enceladus, varies with Europa’s orbital position. Active
jets have only been seen when the moon is farthest from Jupiter. The
researchers could not detect any sign of venting when Europa is closer
to Jupiter.
One explanation for the variability is that these lineae experience
more stress as gravitational tidal forces push and pull on the moon and
open vents at larger distances from Jupiter. The vents are narrowed or
closed when the moon is closest to the gas-giant planet.
"The apparent plume variability supports a key prediction that Europa
should tidally flex by a significant amount if it has a subsurface
ocean," said Kurt Retherford, also of Southwest Research Institute.
The Europa and Enceladus plumes have remarkably similar abundances of
water vapor. Because Europa has a roughly 12 times stronger
gravitational pull than Enceladus, the minus-40-degree-Fahrenheit
(minus-40-degree-Celsius) vapor for the most part doesn’t escape into
space as it does at Enceladus, but rather falls back onto the surface
after reaching an altitude of 125 miles (201 kilometers), according to
the Hubble measurements. This could leave bright surface features near
the moon’s south polar region, the researchers hypothesize.
“If confirmed, this new observation once again shows the power of the
Hubble Space Telescope to explore and opens a new chapter in our search
for potentially habitable environments in our solar system,” said John
Grunsfeld, an astronaut who participated Hubble servicing missions and
now serves as NASA's associate administrator for science in Washington.
“The effort and risk we took to upgrade and repair Hubble becomes all
the more worthwhile when we learn about exciting discoveries like this
one from Europa.”
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