Дата: 03 марта 1998 (1998-03-03)
От: Alexander Bondugin
Тема: Detailed Images From Europa Point To Slush Below Surface
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The Brown University News Bureau
Distributed March 2, 1998
Contact: Scott Turner
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Detailed images from Jupiter moon Europa point to slush
below surface
The latest, most detailed pictures of the Jupiter moon
Europa lend more support to the theory that slush or
even liquid water lurks beneath the moon's surface.
Those pictures were presented and discussed by
scientists from Brown University and NASA during a press
briefing today on the Brown campus.
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PROVIDENCE, R.I. -- The most detailed images ever taken of the
Jupiter moon Europa show more evidence for slush beneath the
bright moon's icy surface, say planetary scientists from Brown
University and NASA who have analyzed data recently transmitted
from the Galileo spacecraft.
Slightly smaller than Earth's moon but many times brighter,
Europa's icy surface has intrigued scientists ever since the
Voyager spacecraft missions flew through the Jupiter system in
1979. At -260° F, the moon's surface temperature could deep-freeze
an ocean over several million years, but some scientists are
beginning to think that warmth from a tidal tug of war with
Jupiter and neighboring moons could be keeping large parts of
Europa's ocean liquid.
The latest images released today were taken in December 1997 by
the Galileo spacecraft and just received on Earth. The new images
provide three key pieces of evidence showing that Europa may be
slushy just beneath the icy crust and possibly even warmer at
greater depths. The evidence includes a strangely shallow impact
crater, chunky textured surfaces like icebergs, and gaps where new
icy crust seems to have formed between continent-sized plates of
ice.
Some of the new images focus on the shallow center of the impact
crater known as Pwyll. Impact rays and debris scattered over a
large part of the moon show that a meteorite slammed into Europa
relatively recently, about 10-100 million years ago. The darker
debris around the crater suggests the impact excavated deeply
buried material. But the crater's shallow basin and high set of
mountain peaks may mean that subsurface ice was warm enough to
collapse and fill in the deep hole, says Brown graduate student
Geoffrey Collins, a member of the Galileo research team.
A subsurface ocean warm enough to be slushy also may explain the
origins of an area littered with fractured and rotated blocks of
crust the size of several city blocks, called "chaos" terrain. The
new images show rough and swirly material between the fractured
chunks, which may have been suspended in slush that froze at the
very low surface temperatures, says Robert Pappalardo, a
postdoctoral research scientist at Brown and a member of the
Galileo research team.
On a larger scale, large plates of ice seem to be sliding over a
warm interior on Europa, much like Earth's continental plates move
around on our planet's partly molten interior.
The new images of Europa show that the darker wedge-shaped gaps
between the plates of ice have many similarities to new crust
formed at mid-ocean ridges on the Earth's sea floor, says Brown
graduate student Louise Prockter, a member of the Galileo research
team who has studied high-resolution sonar images of the
Mid-Atlantic Ridge and has visited the Pacific Ocean floor in the
research submersible vehicle Alvin. The new crust welling up
between the separating plates on Europa was likely initially
slushy ice or possibly liquid water that has frozen and fractured,
Prockter says.
"Together, the evidence supports the hypothesis that in Europa's
most recent history, liquid or at least partially liquid water
existed at shallow depths below the surface of Europa in several
different places," says James Head, Brown University professor of
geological sciences and a group leader of the Galileo research
team.
"The combination of interior heat, liquid water, and infall of
organic material from comets and meteorites means that Europa has
the key ingredients for life," Head says. "Europa, like Mars and
the Saturn moon Titan, is a laboratory for the study of conditions
that might have led to the formation of life in the solar system."
Images are available at http://www.jpl.nasa.gov/galileo and
http://photojournal.jpl.nasa.gov.
######
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The Brown University News Bureau
Distributed March 2, 1998
Contact: Carol Cruzan Morton
--------------------------------------------------------
Background on Europa data from the Galileo Mission to
Jupiter
At today's press briefing, Brown and NASA scientists
will show the most detailed images ever seen of the
Jupiter moon Europa. Recently transmitted from the
Galileo spacecraft, the images provide three key pieces
of evidence supporting the idea that water may lurk
beneath Europa's surface. (See also news advisory.)
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Background
Water or ice? Liquid or slushy or frozen solid? Ever since the
Voyager spacecraft missions flew through the Jupiter system in
1979, planetary scientists have wondered about the layer of ice
surrounding the planet. Europa's blindingly bright ice surface
makes it one of the brightest objects in our solar system. Recent
Galileo spacecraft images have provided evidence that Europa had a
liquid ocean underneath the frozen crust sometime in its history,
but it is not clear if this ocean still exists. Of the various
explanations proposed by scientists, most scenarios of Europa's
evolution have the water layer freezing solid earlier in its
history. The moon's surface is -260° F, which could freeze an
ocean over several million years. But some scientists are
beginning to think that the warming caused by a tidal tug of war
with Jupiter and neighboring moons could be keeping large parts of
the ocean liquid.
Key images
New stereo and very high resolution images of Europa just
transmitted to Earth from the Galileo Europa Mission fly-by in
December 1997 may help support the theory that water or slush may
slosh beneath Europa's frozen crust. Detailed enough to see a
truck-sized object on the surface, the new images are hundreds of
times higher resolution than the best Voyager images and three to
20 times higher than earlier Galileo pictures. The Brown and NASA
scientists point to three key pieces of evidence from the detailed
images:
* The subdued topography of the young impact crater Pwyll,
whose rays cover a significant part of the surface of Europa;
* Large plates of ice and iceberg-like structures called "chaos
terrain"; and
* Gaps between plates of ice known as "wedges" where new crust
appears to have formed recently.
Oceans and life
"Together, the craters, chaos and wedges support the hypothesis
that in Europa's most recent history, liquid or at least partially
liquid water existed at shallow depths below the surface of Europa
in several different places," says James Head, Brown University
professor of geological sciences. "These and other data lend
support to the hypothesis that Europa is warm and active today and
potentially characterized by a global subsurface water layer or
ocean. Europa, like Mars and the Saturn moon Titan, is a
laboratory for the study of conditions that might have led to the
formation and evolution of life. The combination of interior heat,
liquid water, and infall of organic material from comets and
meteorites means that Europa has the key ingredients for life, and
it represents an exciting environment that is worthy of further
detailed exploration."
Crater evidence
Rays and debris from the impact that formed Pwyll Crater radiate
over a large part of the moon's surface. Galileo took pictures of
the impact crater from two perspectives to determine the
three-dimensional shape of the crater. Colleagues at the DLR
(German Aerospace Research Establishment) converted these images
into a colored map showing the depth of the crater and the height
of its peaks. Unlike most young, deep impact craters, the floor of
Pwyll is at the same level as the exterior, says Brown graduate
student Geoffrey Collins. The central peaks of the crater are more
than 2,000 feet high - four times higher than the Washington
Monument - and higher than the crater rim. This means that this
young crater was warm and weak and collapsed during or very
shortly after the meteorite impact, in contrast to craters formed
in cold, stiff material. Debris that flowed from the violent
impact is dark, suggesting excavation of different material from
below the surface. All this suggests that water just beneath the
surface was warm enough to be slushy in the moon's recent history.
Chaos evidence
The new images from Galileo help answer some questions about other
areas of Europa that are littered with fractured and rotated
blocks of crust the size of several city blocks (dubbed chaos
terrain). These fractured ice chunks appeared to be either sliding
on soft glacier-like ice below the surface or floating like
icebergs in a more fluid material. The new images show that the
material between the cracked and separated plates of crust is
rough and swirly, says Robert Pappalardo, a postdoctoral research
scientist at Brown. The pieces are immersed in what appears to be
a slush that is now frozen solid. The very low temperatures at the
surface of Europa (-260° F) mean that any water exposed at the
surface would freeze immediately and might create this kind of
texture. The rough chaos terrain, as well as the movement and
rotation of the blocks, suggest that the crust was at least
partially liquid at shallow depths.
Wedges Evidence
Other images are helping unravel more mysteries. Pieces of the
moon's glaringly white crust are separated by wedged-shaped pieces
of darker, newer crust, welling up from below, freezing and
cracking. The separated pieces of white crust would fit back
together like a jig-saw puzzle, suggesting that plate
tectonic-like activity might be occurring on Europa to form the
wedges. Composed of a set of narrow linear ridges and parallel
grooves, the dark wedge has many similarities to new crust formed
at mid-ocean ridges on the Earth's sea floor, says Brown graduate
student Louise Prockter, who has studied high-resolution sonar
images of the Mid-Atlantic Ridge and has visited the Pacific Ocean
floor in the research submersible vehicle Alvin. Like Earth, new
crust seems to be welling up, separating, and replacing older
crust. On Europa, the molten material solidifying on the surface
was likely slushy ice or liquid water.
Next Step
To confirm the existence of such a layer, determine its depth and
investigate its nature and global extent, further observations are
planned for the Galileo Europa Mission, and other experiments are
planned for a Europa Orbiter Mission to be launched in 2003, says
Michael J. S. Belton of the National Optical Astronomy Observatory
in Tucson, Ariz., and team leader for the solid state imaging
system.
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