Scientists have found evidence of plate tectonics
on Jupiter’s moon Europa. This conceptual illustration of the subduction process
(where one plate is forced under another) shows how a cold, brittle, outer
portion of Europa’s 20-30 kilometer (roughly 10-20 mile) thick ice shell moved
into the warmer shell interior and was ultimately subsumed. A low-relief
subsumption band was created at the surface in the overriding plate, alongside
which cryolavas may have erupted.
Image Credit:
Noah Kroese, I.NK
Scientists have found evidence of plate tectonics on Jupiter’s moon Europa.
This indicates the first sign of this type of surface-shifting geological
activity on a world other than Earth.
Researchers have clear visual evidence of Europa’s icy crust expanding.
However, they could not find areas where the old crust was destroyed to make
room for the new. While examining Europa images taken by NASA’s Galileo orbiter
in the early 2000s, planetary geologists Simon Kattenhorn, of the University of
Idaho, Moscow, and Louise Prockter, of the Johns Hopkins University Applied
Physics Laboratory in Laurel, Maryland, discovered some unusual geological
boundaries.
“We have been puzzled for years as to how all this new terrain could be
formed, but we couldn’t figure out how it was accommodated,” said Prockter. “We
finally think we’ve found the answer.”
Plate tectonics is the scientific theory that Earth's outer layer is made up
of plates or blocks that move, which accounts for why mountain and volcanoes
form and earthquakes happen.
The surface of Europa -- one of Jupiter’s four largest moons and slightly
smaller than Earth’s moon -- is riddled with cracks and ridges. Surface blocks
are known to have shifted in the same way blocks of Earth's outer ground layer
on either side of the San Andreas fault move past each in California. Many parts
of Europa’s surface show evidence of extension, where wide bands miles wide
formed as the surface ripped apart and fresh icy material from the underlying
shell moved into the newly created gap -- a process akin to seafloor spreading
on Earth.
On Earth, as new surface material forms at mid-ocean ridges, old material is
destroyed at subduction zones, which are regions where two tectonic plates
converge and overlap as one is forced under the other. However, despite the
degree of extension evident on Europa’s surface, researchers had not been able
to determine how the surface could accommodate all the new material.
Scientists studying Europa often reconstruct the moon’s surface blocks into
their original configuration -- as with a jigsaw puzzle -- to get a picture of
what the surface looked like before the disruption occurred. When Kattenhorn and
Prockter rearranged the icy terrain in the images, they discovered about 7,700
square miles (about 20,000 square kilometers) of the surface were missing in the
moon’s high northern latitudes.
Further evidence suggested the missing terrain moved under a second surface
plate -- a scenario commonly seen on Earth at plate-tectonic boundaries.
Kattenhorn and Prockter saw ice volcanoes on the overriding plate, possibly
formed through melting and absorption of the slab as it dove below the surface,
and a lack of mountains at the subduction zone, implying material was pushed
into the interior rather than crumpled up as the two plates mashed against each
other.
The scientists believe the subducted area was absorbed into Europa's ice
shell, which may be up to 20 miles (about 30 kilometers) thick, rather than
breaking through it into Europa's underlying ocean. On Europa’s relatively young
surface -- about 40-90 million years old, on average -- scientists have seen
evidence of material moving up from under the shell but, until now, no mechanism
had been found for moving material back into the shell, and possibly into the
large ocean below the ice.
“Europa may be more Earth-like than we imagined, if it has a global plate
tectonic system,” Kattenhorn says. “Not only does this discovery make it one of
the most geologically interesting bodies in the solar system, it also implies
two-way communication between the exterior and interior -- a way to move
material from the surface into the ocean -- a process which has significant
implications for Europa’s potential as a habitable world.”
The team’s results appear in the Sunday online edition of the journal Nature
Geoscience.
In July, NASA issued an Announcement of Opportunity (AO) for proposals for
science instruments that could be carried aboard a future mission to Europa.
“Europa continues to reveal itself as a dynamic world with compelling
similarities to our own planet Earth,” said Curt Niebur, Outer Planets program
scientist at NASA Headquarters in Washington. “Studying Europa addresses
fundamental questions about this potentially habitable icy moon and the search
for life beyond Earth.”
Previous scientific findings point to the existence of a liquid water ocean
located under the moon’s icy crust. This ocean covers Europa entirely and
contains more liquid water than all of Earth's oceans combined.
NASA’s Galileo spacecraft, launched in 1989, was the only space mission to
make repeated visits to Europa, passing close by the moon about a dozen
times.
Galileo’s many firsts include discovering evidence for the existence of a
saltwater ocean beneath Europa's icy surface. The mission officially was ended
when Galileo plunged into Jupiter's atmosphere in September 2003 to prevent an
impact with Europa. The mission was managed by NASA’s Jet Propulsion Laboratory
in Pasadena, California, for the agency’s Science Mission Directorate in
Washington.
For more information about Europa and images of the plate tectonics,
visit:
Information is available online about the Galileo Mission at:
NASA
Guillermo Gonzalo Sánchez Achutegui
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