A new University of Colorado at Boulder study of Jupiter’s moon Europa
may help explain the origin of the giant ice domes peppering its surface
and the implications for discovering evidence of past or present life
forms there.
Assistant Professor Robert Pappalardo and doctoral student Amy Barr
previously believed the mysterious domes may be formed by blobs of ice
from the interior of the frozen shell that were being pushed upward by
thermal upwelling from warmer ice underneath. Europa is believed to
harbor an ocean beneath its icy surface.
But the scientists now think the dome creation also requires small
amounts of impurities, such as sodium chloride or sulfuric acid.
Basically the equivalent of table salt or battery acid, these compounds
melt ice at low temperatures, allowing warmer, more pristine blobs of
ice to force the icy surface up in places, creating the domes.
“We have been trying for some time to understand how these ice blobs can
push up through the frozen shell of Europa, which is likely about 13
miles thick,” said Pappalardo of the astrophysical and planetary
sciences department. “Our models now show that a combination of
upwelling warm ice in the frozen shell’s interior, combined with small
amounts of impurities such as sodium chloride or sulfuric acid, would
provide enough of a force to form these domes.”
A paper on the subject co-authored by Pappalardo and Barr was presented
at the annual Division of Planetary Sciences Meeting held Sept. 2
through Sept. 6 in Monterey, Calif. DPS is an arm of the American
Astronomical Society. The meeting schedule is available at
http://dps03.arc.nasa.gov/administrative/schedule/index.html.
Europa appears to have strong tidal action as it elliptically orbits
Jupiter – strong
enough “to squeeze the moon” and heat its interior, said Pappalardo.
“Warm ice blobs rise upward through the ice shell toward the colder
surface, melting out saltier regions in their path. The less dense
blobs can continue rising all the way to the surface to create the
observed domes.”
The domes are huge – some more than four miles in diameter and 300 feet
high – and are found in clusters on Europa’s surface, said Barr, who did
much of the modeling. “We are excited about our research, because we
think it now is possible that any present or past life or even just the
chemistry of the ocean may be lifted to the surface, forming these
domes. It essentially would be like an elevator ride for microbes.”
Barr likened the upwelling of warmer ice from the inner ice shell to its
surface to a pot of boiling spaghetti sauce. “The burner under the pan
sends the hottest sauce to the top, creating the bubbles at the
surface,” she said. “The trouble is Europa’s icy skin is as cold and as
hard as a rock.”
The idea that either small amounts of salt or sulfuric acid might help
to create Europa’s domes was Pappalardo’s, who knew about similar domes
on Earth that form in clumps in arid regions. On Earth, it is salt that
is buoyant enough to move up through cracks and fissures in rock
formations to form dome clusters at the surface.
“In addition, infrared and color images taken of Europa by NASA’s
Galileo spacecraft seem to indicate some of the ice on the surface of
these domes is contaminated. Impurities seen at the surface are clues
to the internal composition of the Jovian moon, telling of a salty ice
shell,” he said.
“The surface of Europa is constantly being blasted by radiation from
Jupiter, which likely precludes any life on the moon’s surface,” said
Barr. “But a spacecraft might be able to detect signs of microbes just
under the surface.”
Both Pappalardo and Barr also are affiliated with CU-Boulder’s
Laboratory for Atmospheric and Space Physics. The project was funded by
NASA’s Exobiology Program and Graduate Student Research Program.
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. He currently is part of a NASA team developing
goals for the Jupiter Icy Moons Orbiter mission.
The scientific objectives of the mission probably will include
confirming the presence of an ocean at Europa, remotely measuring the
composition of the surface and scouting out potential landing sites for
a follow-on lander mission.
