Image: Europa’s enigmatic ridged surface is peppered by pits and spots termed lenticulae, which is Latin for freckles. In this area, the lenticulae are all about 6 miles in diameter. Their similar sizes and spacing suggest that Europa’s icy shell is churning away like a lava lamp: warmer ice moves upward from the bottom of the ice shell, while colder near-surface ice sinks downward. Reddish ice that erupts onto the surface may hold clues about the composition of Europa’s subsurface ocean, and whether that ocean supports life. Photo courtesy Jet Prolpulsion Laboratory
The oozing of glacial material in the floating ice shell on Jupiter’s moon
Europa has important implications for future exploration of the enigmatic
moon and prospects of life in its ice-covered ocean, according to a
University of Colorado at Boulder professor.
Robert Pappalardo, an assistant professor in the astrophysical and planetary
sciences department and one of the world’s foremost Europa experts, said the
icy moon is believed to contain an ocean some 13 miles under its icy
surface. Satellite images appear to indicate surface warping — including
domes and reddish spots — showing that “elevators” of sorts transport
material up and down from the ocean to the surface, said the planetary
scientist.
“Europa acts like a planetary lava lamp, carrying material from near the
surface down to the ocean, and, if they exist, potentially transporting
organisms from the ocean up toward the surface,” he said. “Just a mile or
two beneath the surface, the conditions may be warm enough to allow
organisms to survive the journey.”
The “thick shell” model of Europa has implications for the future
exploration of the moon and whether the existence of life is possible in the
lightless depths beneath the planet’s surface, said Pappalardo. “It would
be very difficult for a future spacecraft to drill all the way through a
13-mile-deep ice shell to search for life in the underlying ocean. But the
motions of glacial ice may transport ocean material, and any life it might
contain, to the surface.”
Pappalardo and his research group at CU-Boulder’s Laboratory for Atmospheric
and Space Physics are attempting to tie together pieces of an elaborate
puzzle to assemble a comprehensive model of how Europa functions. The
results are being reported at the Geological Society of America meeting in
Denver Oct. 27 to Nov. 1.
Under similar conditions in Arctic ice on Earth, organisms can remain in a
state of hibernation until exposed to warmer and wetter conditions, he said.
“If life exists in Europa’s ocean, organisms might be carried on a slow ride
from the bottom to the top of Europa’s icy crust. Sampling the surface
composition may provide direct insights into the nature of the ocean deep
below, and could plausibly reveal dormant organisms if they exist within
Europa.”
CU-Boulder graduate student Amy Barr is developing a computer model to
illustrate the Europa ice motions, said Pappalardo. She is modifying a
computer model that has been used to understand Earth’s plate tectonics and
to better understand Europa’s geology, including how nutrients created by
ice irradiation at Europa’s surface might be transported down to the moon’s
oceans.
Barr’s ice-convection model, the most sophisticated yet applied to Europa,
may show that organisms could thrive below the thick cap of ice, Pappalardo
said. It incorporates information on how the satellite’s thick ice shell is
heated and how it flows as it is squeezed by the gravity of Jupiter, which
raises huge tides on Europa.
CU undergraduate Michelle Stempel is analyzing Europa’s pattern of cracks
and ridges to understand how the Jupiter tides have fractured the surface,
and over what time scales the cracking has occurred. By matching stress
patterns to surface geological features, she is studying where and how the
surface cracks are created in response to short- and long-term deformation
of the thick icy shell overlying an ocean.
Pappalardo also has teamed with Francis Nimmo of University College, London,
to understand the similarities and differences between Europa and its
sibling Jovian moon, Ganymede. Ganymede may hide an ocean beneath its icy
crust much deeper than Europa’s, although Ganymede’s era of geological
activity has likely long ceased. By analyzing the topography of fractures
on Ganymede, the two scientists have determined that Ganymede was once
nearly as warm inside as Europa is today.
“This has important implications for the history of Ganymede, and also for
how Europa’s surface is shaped today,” Pappalardo said. “Ganymede may be a
fossil version of Europa.” The two scientists found similar internal and
external forces that probably have influenced the two moons, but with
different geological expressions.
In addition, Pappalardo is working with Nick Makris of the Massachusetts
Institute of Technology to study how a future Europa lander could precisely
determine the depth and thickness of Europa’s ocean, using the same
techniques routinely used by the Navy to measure the depth and composition
of Earth’s oceans. The two are presenting back-to-back talks at Denver’s GSA
meeting to illustrate how the proven terrestrial technique can apply to the
exotic environment of Europa.
Pappalardo recently served on a National Research Council panel that
reaffirmed a spacecraft should be launched in the coming decade with the
goal of orbiting Europa. The Europa Geophysical Explorer would have
scientific objectives that include confirming the presence of an ocean,
remotely measuring the composition of the surface and scouting out potential
landing sites for a follow-on lander mission.
