Finding plumes at Europa is an exciting prospect, but scientists warn it’ll be tricky, even from up close.
In 2005, images of a brilliant watery plume erupting
from the surface of Saturn’s moon Enceladus captivated the world. The giant
column of vapor, ice particles, and organic molecules spraying from the moon's
south polar region suggested that there’s a liquid water ocean below
Enceladus’ ice shell and confirmed the moon is geologically active. The
plume also thrust Enceladus and other worlds in the outer solar system, with no
atmospheres and far from the heat of the Sun, toward the top of NASA’s list of
places to search for signs of life.
Scientists now are preparing for a mission to another
ice-covered ocean world with possible plumes: Jupiter’s moon
Europa. Scheduled to launch in 2024, NASA’s Europa Clipper spacecraft will
study the moon from its deep interior to its surface to determine whether it
has ingredients that make it a viable home for life.
Like Enceladus, Europa is geologically dynamic,
meaning both moons generate heat inside as their solid layers stretch and flex
from the gravitational tug-of-war with their host planets and neighboring
moons. This, instead of heat from the Sun, keeps subsurface water from freezing
on these ice-covered moons. The heat may also help produce or circulate life’s
chemical building blocks at the seafloors, including carbon, hydrogen, oxygen,
nitrogen, phosphorus, and sulfur.
But that’s where the similarities end.
“A lot of people think Europa is going to be Enceladus
2.0, with plumes constantly spraying from the surface,” said Lynnae Quick, a member of the science team behind Clipper’s Europa Imaging System
(EIS) cameras. “But we can’t look at it that way; Europa is a totally different
beast,” said Quick, who’s based at NASA’s Goddard Space Flight Center in
Greenbelt, Maryland.
One of the first images of Enceladus’ water jets taken by NASA’s Cassini spacecraft on Nov. 27, 2005. In this image, Enceladus is backlit by the Sun. Credits: NASA/JPL/Space Science Institute. More information here.
Evidence suggests Europa may vent water from its subsurface just like
Enceladus. For example, scientists using NASA’s
Galileo spacecraft, NASA’s
Hubble Telescope, and large Earth-based
telescopes have reported detections of faint
water plumes or their chemical components at Europa.
But no one is certain. “We’re still in the space where there’s really
intriguing evidence but none of it is a slam dunk,” said Matthew McKay Hedman, a member of Europa
Clipper’s Mapping Imaging
Spectrometer for Europa (MISE) science team and
associate professor in the Department of Physics at the University of Idaho.
Scientists are drawn to plumes for a couple of reasons. First, they’re
undeniably cool: “We’re scientists, but we’re also human,” said
Shawn Brooks, who is working with Europa Clipper’s Europa Ultraviolet
Spectrograph (Europa-UVS) science team and is based at NASA’s Jet Propulsion
Laboratory in Southern California.
But more practically, Brooks said, plumes offer scientists easier access to
Europa’s interior. “It all comes down to whether Europa is habitable, and that
comes down to having some understanding of what is happening below the surface,
which we can’t reach yet,” he said.
In other words, the magic of Europa, an archetype for a potentially
habitable world, is hidden from view deep within the moon. Compared to
Enceladus, which is the size of Texas, Europa is about a quarter of Earth’s
size, or a bit smaller than Earth’s moon. And evidence suggests Europa has a
much deeper saltwater ocean than Enceladus, possibly 40 to 100 miles (about 60
to 160 kilometers) deep, which means it could contain about twice as much water
as Earth’s oceans. Some scientists hypothesize that Europa’s ocean could be
reacting with superheated rocks below its seafloor, possibly through
hydrothermal vents. On Earth, such areas are hotbeds of chemical activity that
nourishes innumerable creatures.
Scientists say there also could be large pockets of melted water in
Europa’s ice shell, which are more likely than the ocean to be the source of
plumes. These pockets could produce cozy habitats for organisms as well.
Because of Europa’s elliptical orbit, which sometimes brings it closer and
sometimes farther away from Jupiter, plus Jupiter’s immense gravitational pull,
more heat is generated in Europa from friction as it circles its host planet.
Given that internal heat stimulates geological activity on rocky worlds,
Europa is expected to have more extensive geology than Enceladus. Some
scientists predict that Europa
has plate tectonics that shift and recycle the icy
blocks making up the moon’s surface. If so, Europa could be circulating
nutrients produced on the surface by radiation from Jupiter, such as oxygen, to
pockets of liquid in the ice shell or perhaps to the ocean itself. Through
Europa Clipper, scientists will have a chance to test some of their predictions
by analyzing the chemical makeup of plumes or the traces they may leave on the
surface.
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 Space Telescope Imaging Spectrograph, were seen in silhouette as the moon passed in front of Jupiter. The Hubble data were gathered 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. More information here.
Scientists warn that Europan plumes, even if they’re there, could be hard
to detect even from up close. They may be sporadic, and they may be small and
thin, given that Europa’s gravity, which is much stronger than Enceladus’,
likely would keep these water plumes close to the surface. That’s a drastic
departure from Enceladus’ spectacular vapor column: It’s always on and bigger
than the moon itself, spraying icy particles hundreds of miles above the
surface. “Even if they’re there, Europa’s plumes may not be that photogenic,”
Hedman said.
Though Europa Clipper scientists are devising a variety of creative
strategies to find active plumes when the spacecraft begins exploring Europa in
2031, they’re not relying on them to understand what’s going on inside the
moon. “We don’t have to catch one for a successful mission,” Quick said.
Quick added that every instrument aboard
Clipper can contribute evidence of habitable conditions
below the surface regardless of active plumes.
A few examples of how the science team will search for potential plumes
include Europa Clipper’s camera
suite, EIS. It will scout for plumes near Europa’s
surface partly by looking for their silhouettes at Europa’s limb, or edge, when
the moon is illuminated by the light of Jupiter as it passes in front of the
planet. EIS will snap photos of plumes should they appear, as well as plume
deposits that might be visible on the surface. The Europa-UVS will also strive
to detect plumes in ultraviolet light, including at the edge of the moon when
Europa passes in front of nearby stars, and it can measure the chemical makeup
of such plumes. A thermal camera, the Europa Thermal Emission
Imaging System (E-THEMIS), will look for hotspots on the surface
that may be evidence of active or recent eruptions.
The Europa Clipper team is set to succeed whether or not researchers find plumes at Europa, though many scientists hope for a spectacular water show to enrich the mission and our understanding of Europa. “I do suspect Europa is active and letting some material escape,” Hedman said. “But I expect that when we actually get to understand how it’s doing that, it’s not going to be what anyone expected.”
Life as we know it requires liquid water. Astrobiology, a field of science and engineering that describes efforts to find ingredients of life beyond Earth, is a search for planets, dwarf planets, and moons that harbor substantial liquid water. Scientists call these places “ocean worlds." Credits: NASA's Goddard Space Flight Center.
More About the Mission
Missions such as Europa Clipper contribute to the field of astrobiology, the interdisciplinary
research on the variables and conditions of distant worlds that could harbor
life as we know it. While Europa Clipper is not a life-detection mission, it
will conduct detailed reconnaissance of Europa and investigate whether the icy
moon, with its subsurface ocean, has the capability to support life.
Understanding Europa’s habitability will help scientists better understand how
life developed on Earth and the potential for finding life beyond our planet.
Managed by Caltech in Pasadena, California, JPL leads the development of
the Europa Clipper mission in partnership with the Johns Hopkins Applied
Physics Laboratory in Laurel, Maryland, for NASA’s Science Mission Directorate
in Washington. The Planetary Missions Program Office at NASA’s Marshall Space
Flight Center in Huntsville, Alabama, executes program management of the Europa
Clipper mission.
More information about Europa can be found here: europa.nasa.gov
Banner image caption: On the left is a view of Europa taken on March
2, 1979, by the Voyager 1 spacecraft. Next is a color image of Europa taken by
the Voyager 2 spacecraft during its close encounter on July 9, 1979. On the
right is a view of Europa made from images taken by the Galileo spacecraft in
the late 1990s. Image credit: NASA/JPL. Download
images of Europa here
By Lonnie Shekhtman
NASA’s Goddard Space Flight Center, Greenbelt, Md.
No comments:
Post a Comment