ByKIMM FESENMAIER
Based on new evidence from Jupiter’s moon Europa,
astronomers hypothesise that chloride salts bubble up from the icy moon’s
global liquid ocean and reach the frozen surface where they are bombarded with
sulphur from volcanoes on Jupiter’s largest moon, Io. The new findings propose
answers to questions that have been debated since the days of NASA’s Voyage and
Galileo missions. This illustration of Europa (foreground), Jupiter (right) and
Io (middle) is an artist’s concept.
If you could lick the surface of Jupiter's icy
moon Europa, you would actually be sampling a bit of the ocean beneath. So says
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.
The finding, based on some of the first data of
its kind since NASA's Galileo mission (1989–2003) 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 work is described in a paper that has been accepted for
publication in the Astronomical Journal.
"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 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 sulphate 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 kilometres
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 sulphur from its atmosphere, and
Jupiter's strong magnetic field sends some of that sulphur 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 sulphate, hydrogen sulphate, 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 non-water ice.
Then, at low latitudes on the trailing hemisphere—the area with the greatest
concentration of the non-water 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 sulphate persisted," Hand says.
Some scientists had long suspected that
magnesium sulphate was on the surface of Europa. But, Brown says, "the
interesting twist is that it doesn't look like the magnesium sulphate 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 sulphur from Io, they
believe that there is a magnesium-bearing mineral everywhere on Europa that
produces magnesium sulphate in combination with sulphur. The pervasive
magnesium-bearing mineral might also be what makes up the non-water 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 sulphate-rich
or chlorine-rich. Having ruled out the sulphate-rich version since magnesium sulphate
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.
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."
The Astronomical Journal paper is titled
"Salts and radiation products on the surface of Europa." The work was
supported, in part, by the NASA Astrobiology Institute through the Astrobiology
of Icy Worlds node at JPL.
Original Source: California Institute of Technology
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