This is an artist's concept of a plume of water
vapor thought to be ejected off the frigid, icy surface of the Jovian moon
Europa, located about 500 million miles (800 million kilometers) from the
sun.
Image Credit: NASA/ESA/K. Retherford/SWRI
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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
certain whether the detected water vapor is generated by water plumes erupting
on the surface, but they are confident this is the most likely explanation.
Should further observations support the finding, it would make Europa the
second moon in the solar system known to have water vapor plumes. The findings
were published in the Thursday, 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
subsequently have 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 aurora 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 to also 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 aurora glow and leave a telltale
sign that they are 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 in Germany. Saur,
who is principal investigator of the Hubble observation campaign, co-wrote the
paper with Roth.
Roth suggested long cracks on Europa's surface, known as lineae, might be
venting water vapor into space. Cassini has seen similar fissures that host
Enceladus' jets.
Also like Enceladus, the Hubble team found the intensity of the plumes varies
with Europa's orbital position. Active jets have been seen only when Europa is
farthest from Jupiter. But the researchers could not detect any sign of venting
when Europa is closer to Jupiter.
One explanation for the variability is 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.
Europa's and Enceladus' plumes have remarkably similar abundances of water
vapor. Because Europa has roughly 12 times more gravitational pull than
Enceladus, the vapor, whose temperature is measured at minus 40 degrees Celsius,
does not escape into space as it does at Enceladus. Instead, it falls back onto
the surface after reaching an altitude of 125 miles, 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 in 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."
The Hubble Space Telescope is a project of international cooperation between
NASA and the European Space Agency. NASA's Goddard Space Flight Center in
Greenbelt, Md., manages the telescope. The Space Telescope Science Institute
(STScI) conducts Hubble science operations. The Association of Universities for
Research in Astronomy Inc. in Washington operates STScI for NASA.
To view the images of the evidence for plumes visit:
For more information about the Hubble Space Telescope, visit:
NASA
Guillermo Gonzalo Sánchez Achutegui
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