Based on new evidence from Jupiter's moon Europa, astronomers
hypothesize that chloride salts bubble up from the icy moon's global
liquid ocean and reach the frozen surface. Mike Brown, an astronomer at the California Institute of Technology (Caltech). Brown—known as the Pluto killer for discovering a Kuiper-belt object that led to the demotion of Pluto from planetary status—and Kevin Hand from the Jet Propulsion Laboratory (JPL)
have found the strongest evidence yet that salty water from the vast
liquid ocean beneath Europa's frozen exterior actually makes its way to
the surface.
Hand emphasizes that, from an astrobiology standpoint, Europa is
considered a premier target in the search for life beyond Earth; a
NASA-funded study team led by JPL and the Johns Hopkins University Applied Physics Laboratory
have been working with the scientific community to identify options to
explore Europa further. "If we've learned anything about life on Earth,
it's that where there's liquid water, there's generally life," Hand
says. "And of course our ocean is a nice salty ocean. Perhaps Europa's
salty ocean is also a wonderful place for life."
"We now have evidence that Europa's ocean is not isolated—that the
ocean and the surface talk to each other and exchange chemicals," says
Brown, the Richard and Barbara Rosenberg Professor and professor of
planetary astronomy at Caltech. "That means that energy might be going
into the ocean, which is important in terms of the possibilities for
life there. It also means that if you'd like to know what's in the
ocean, you can just go to the surface and scrape some off."
The finding, based on some of the first data of its kind since NASA's Galileo mission
(1989) to study Jupiter and its moons, suggests that there is a
chemical exchange between the ocean and surface, making the ocean a
richer chemical environment, and implies that learning more about the
ocean could be as simple as analyzing the moon's surface. "The surface
ice is providing us a window into that potentially habitable ocean
below," says Hand, deputy chief scientist for solar system exploration
at JPL.
Since the days of the Galileo mission, when the spacecraft showed
that Europa was covered with an icy shell, scientists have debated the
composition of Europa's surface. The infrared spectrometer aboard
Galileo was not capable of providing the detail needed to definitively
identify some of the materials present on the surface. Now, using
current technology on ground-based telescopes, Brown and Hand have
identified a spectroscopic feature on Europa's surface that indicates
the presence of a magnesium sulfate salt, a mineral called epsomite,
that could only originate from the ocean below.
"Magnesium should not be on the surface of Europa unless it's coming
from the ocean," Brown says. "So that means ocean water gets onto the
surface, and stuff on the surface presumably gets into the ocean water."
Europa's ocean is thought to be 100 kilometers deep and covers the
entire globe. The moon remains locked in relation to Jupiter, with the
same hemisphere always leading and the other trailing in its orbit. The
leading hemisphere has a yellowish appearance, while the trailing
hemisphere seems to be splattered and streaked with a red material.
The spectroscopic data from that red side has been a cause of
scientific debate for 15 years. It is thought that one of Jupiter's
largest moons, Io, spews volcanic sulfur from its atmosphere, and
Jupiter's strong magnetic field sends some of that sulfur hurtling
toward the trailing hemisphere of Europa, where it sticks. It is also
clear from Galileo's data that there is something other than pure water
ice on the trailing hemisphere's surface. The debate has focused on what
that other something is—i.e., what has caused the spectroscopic data to
deviate from the signature of pure water ice.
"From Galileo's spectra, people knew something was there besides
water. They argued for years over what it might be—sodium sulfate,
hydrogen sulfate, sodium hydrogen carbonate, all these things that look
more or less similar in this range of the spectrum," says Brown. "But
the really difficult thing was that the spectrometer on the Galileo
spacecraft was just too coarse."
Brown and Hand decided that the latest spectrometers on ground-based
telescopes could improve the data pertaining to Europa, even from a
distance of about 400 million miles. Using the Keck II telescope
on Mauna Kea—which is outfitted with adaptive optics to adjust for the
blurring effect of Earth's atmosphere—and its OH-Suppressing Infrared
Integral Field Spectrograph (OSIRIS), they first mapped the distribution
of pure water ice versus anything else on the moon. The spectra showed
that even Europa's leading hemisphere contains significant amounts of
nonwater ice. Then, at low latitudes on the trailing hemisphere—the area
with the greatest concentration of the nonwater ice material—they found
a tiny dip in the spectrum that had never been detected before.
"We now have the best spectrum of this thing in the world," Brown
says. "Nobody knew there was this little dip in the spectrum because no
one had the resolution to zoom in on it before.
The two researchers racked their brains to come up with materials
that might explain the new spectroscopic feature, and then tested
everything from sodium chloride to Drano in Hand's lab at JPL, where he
tries to simulate the environments found on various icy worlds. "We
tried to think outside the box to consider all sorts of other
possibilities, but at the end of the day, the magnesium sulfate
persisted," Hand says.
Some scientists had long suspected that magnesium sulfate was on the
surface of Europa. But, Brown says, "the interesting twist is that it
doesn't look like the magnesium sulfate is coming from the ocean." Since
the mineral he and Hand found is only on the trailing side, where the
moon is being bombarded with sulfur from Io, they believe that there is a
magnesium-bearing mineral everywhere on Europa that produces magnesium
sulfate in combination with sulfur. The pervasive magnesium-bearing
mineral might also be what makes up the nonwater ice detected on the
leading hemisphere's surface.
Brown and Hand believe that this mystery magnesium-bearing mineral is
magnesium chloride. But magnesium is not the only unexpected element on
the surface of Europa. Fifteen years ago, Brown showed that Europa is
surrounded by an atmosphere of atomic sodium and potassium, presumably
originating from the surface. The researchers reason that the sodium and
potassium chlorides are actually the dominant salts on the surface of
Europa, but that they are not detectable because they have no clear
spectral features.
The scientists combined this information with the fact that Europa's
ocean can only be one of two types—either sulfate-rich or chlorine-rich.
Having ruled out the sulfate-rich version since magnesium sulfate was
found only on the trailing side, Brown and Hand hypothesize that the
ocean is chlorine-rich and that the sodium and potassium must be present
as chlorides.
Therefore, Brown says, they believe the composition of Europa's sea
closely resembles the salty ocean of Earth. "If you could go swim down
in the ocean of Europa and taste it, it would just taste like normal old
salt," he says.
The work is described in a paper that has been accepted for publication in the Astronomical Journal.
Source: The Daily Galaxy via NASA/JPL-Caltech
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