Above: Fragmented chunks of ice
on Europa, similar in appearance to those seen in Earth polar seas during a
springtime thaw. [more
information] |
September 9, 1999: A future guide book for Solar
System vacationers might describe Jupiter's moon Europa this
way: "Europa is cold -- really cold. The surface
temperature is a chilly -260 deg F, so bring your space parka.
The entire planet is covered with a relatively smooth layer of
frozen water and ice skating is allowed in most regions. Travelers
should bring their own air, as Europa's oxygen atmosphere is
a million times thinner than Earth's. Other items of note include
cold water volcanoes, gigantic ice rafts, and an underground
ocean. Ice fishing is not encouraged."
It may sound like science fiction, but most of this fanciful
description is true. NASA's Voyager and Galileo spacecraft have
shown Europa to be a frigid world dotted with remnants of "cryo-volcanoes"
and rafts of ice similar in appearance
to those seen on Earth's polar seas during
springtime thaws. As astonishing as the surface sounds, Europa
may be even more interesting underground. Many scientists think
that tidal friction from nearby Jupiter heats the interior of
the moon to temperatures where liquid water is possible.
Beneath
its icy crust, Europa could harbor the solar system's
largest ocean!
Aside from the novelty of oceans on another planet, scientists
are fascinated by the possibility of Europan seas because they
could be sites for extraterrestrial life. Here on Earth, undersea
volcanoes and hydrothermal vents create environments that sustain
rich colonies of microbes. If similar systems are active on Europa,
scientists reason, life might be present there too. If there
is an ocean on Europa, will it contain life? Chris Chyba of the
SETI Institute responds, "Can an ocean of liquid water persist
for 4.5 billion years and not have life in it?" [ref].
We simply don't know.
Mounting evidence...but still not enough
Circumstantial evidence for subterranean seas continues to
mount. New Galileo images released on August 27, 1999, show large
chaotic areas on Europa's surface where a liquid ocean or warm
ice may have welled up and disrupted the moon's icy shell. Thera and Thrace
(pictured below) are two rust-colored
areas, each over 50 miles wide, consisting of remarkably
jumbled terrain.
The latest images show that Thera lies slightly
below the level of the surrounding plains. Curved fractures along
its boundaries suggest that the whole region collapsed some time
in the past.
Cracking,
shifting ice rafts. Cold water volcanoes. Gigantic surface melts.
How long ago did these events take place? No one knows for sure.
Some planetary scientists think less than 50 million years ago
based on Europan cratering statistics. That may seem like a long
time, but to a geologist, it's a brief interval.
Right: A color-enhanced close-up
of Europa's surface. Areas of blue are thought to be pure water
ice. The "cryo-volcanic" ridges have a brown color
showing where mineral-laden water from underground appears to
have percolated to the surface through cracks in the crust.
"Fifty million years is only 1% of the age of Europa,"
says Dr. Bob Pappalardo, a planetary geologist at Brown University.
"Geologically speaking, if something was happening there
50,000,000 years ago then it's probably still happening today.
Still, we can't rule out the possibility that Europa's oceans
have 'recently' frozen. For example, there might be some cyclic
tidal effects associated with Jupiter and the other Galilean
satellites that cause Europa to warm up and then cool off with
a period of a hundred million years. At this point it's speculative."
One way to get to the bottom of the mystery
without actually drilling through the ice is to watch for changes
on Europa's surface. Photos of, say, an active cryo-volcanic
geyser would prove that liquid water exists. So far, after numerous
encounters with Europa Galileo has found no direct evidence for
liquid water -- just tantalizing hints.
"Cynthia Phillips, a University of Arizona graduate student
working with Dr. Alfred McEwen, is also looking for changes on
Europa by comparing Voyager and Galileo images," adds Pappalardo.
"It's difficult work because many of the images are low
resolution, there are lighting differences to contend with, etc.
So far they haven't found any definitive evidence for changes
across a 20 year interval."
Above: Thera (left) and Thrace (right)
are two dark, reddish regions of enigmatic terrain that disrupt
the older icy ridged plains on Jupiter's moon Europa. One model
for the formation of these and other chaos regions on Europa
is complete melt-through of Europa's icy shell from an ocean
below. [more
information]
Now that Galileo has completed its scheduled flybys of Europa,
researchers are poring over the data for additional clues about
what might lie below the moon's surface.
"There's lots of research going on," says Pappalardo.
"What I and others are doing is refining and testing models
of how various surface features form. When the Galileo images
first came out we developed models based on warm ice to explain
features like we see around Thera and Thrace. Combined with NIMS [Near Infrared
Mapping Spectrometer] measurements, these models support the
idea that the surface may be salty in places. The best spectral
match is to magnesium sulfate, better known as Epsom salts. When
warm convecting ice rises up and hits a salty area it melts at
a lower temperature than the surrounding plains. This sort of
process might explain regions like Thrace where we see what appears
to be melting and liquid-like flows across the surface."
Researchers aren't certain what causes the brown discoloration
of regions like Thera and Thrace and around cryo-volcanic ridges.
NIMS data indicate a correlation between brown areas and salty
spots.
"The problem," continues Pappalardo, "is that
Epsom salt isn't brown. The best candidates for the brown material
are iron compounds or some sort of sulfur compound. Both iron
and sulfur are relatively abundant in the solar system and have
a red appearance visually. Plus, we know that there's SO2 on Europa."
"We may not be able to answer all the questions now,"
says Pappalardo, "but framing the problems is important
because of the planned Europa Orbiter mission."
Still under development, the Europa
Orbiter will use a radar sounder to measure the thickness
of Europa's icy crust and possibly determine whether liquid water
exists below the ice. An on board altimeter will gather topography
data and characterize the tidal response of the surface. Tidal
distortions of Europa's crust will be large if there is an underlying
layer of liquid water and smaller if the water is in the form
of ice. The mission could launch in 2003 and would serve as a
precursor to spacecraft that might send undersea explorers into
the Europan oceans.
A Ring Around The Moon
Unwilling to wait years for an answer, Dr. Robert Carlson
and colleagues at JPL recently tried an ingenious experiment
to solve the mystery now. According to Carlson, a crucial clue
to the state of affairs below Europa's crust lies in the crystalline
form of ice on the surface.
In theory, three different forms of ice can exist at the low
temperatures and pressures on Europa's surface: amorphous ice,
cubic crystals and hexagonal crystals. High pressure ices can
take on other forms, but they are not relevant to Europa.
"Ice that forms from water vapor at 100 [Kelvin] is amorphous,"
explains Carlson. "At 140K, ice formed from water vapor
has a cubic crystalline form. At temperatures above 170K, ice
formed from liquid water or from 'warm' water vapor has a hexagonal
crystal, just like ice on Earth. If we could find evidence for
hexagonal ice crystals on Europa, it would mean that the ice
formed fairly recently from liquid water or warm vapor."
On Earth, hexagonal ice crystals are responsible for a sight
well-known to stargazers: a 22 degree halo around the Sun or
Moon when it is viewed through humid cirrus clouds. Similar halos
can be seen from sunlight glinting off snowy surfaces, especially
from freshly fallen snow.
Right: Two models consistent with
images of Europa's surface include a subsurface layer of liquid
water or perhaps warmer, convecting ice. Image credit: JPL and
the SETI Institute.
Carlson and colleagues reasoned that if hexagonal ice crystals
are present on Europa, they might be able to see a similar effect
using Galileo's Near Infrared Mapping Spectrometer. When sunlight
hits the surface of Europa at near grazing incidence it penetrates
the surface and refracts through the ice crystals. Hexagonal
crystals will produce a ring-like glow in the infrared much like
the familiar "ring around the Moon."
"NIMS is
a spectrometer, but we're not using its spectral capability for
this work," says Carlson. "The search for hexagonal
ice on Europa involves a search for light intensity variations
as a function of phase angle."
"Geometry was crucial for this experiment," he continued,
"There was only one orbit when we could do it when the Sun
was only 22 degrees above the grazing incidence reflection point.
The spot we observed doesn't have a particular name, it was a
small patch of ice surrounded by darker material at about 48
degrees longitude on Europa. The patch was about 100 km across,
and we could detect hexagonal crystals even if they make up as
little as 10% of the ice."
The observations were played back earlier this year and Carlson
et al are busy analyzing the data.
"If we don't see the halo, it doesn't necessarily mean anything,"
says Carlson. "That's because crystalline ice exposed to
energetic particle radiation over a period of time becomes amorphous,
and there is a strong source of radiation in the neighborhood
-- Jupiter's magnetosphere. No one knows what's the time scale
for transformation. There has been some lab work at temperatures
less than 80K, but Europa's surface is warmer than that and the
time scales are sure to be longer at higher temperatures."
If hexagonal ice is discovered, it will certainly spur new lab
experiments that measure the transformation time, which will
in turn set limits on the age of the hexagonal ice. In that way
scientists will be able to "date" cryo-volcanic activity
on Europa more precisely than ever before, and possibly draw
definitive conclusions about modern-day Europan oceans.
Meanwhile, back on Earth....
Many scientists would love to travel to Europa to study conditions
there first-hand, but regular flights to Jupiter probably won't
begin for some time. Meanwhile, there is an environment right
here on Earth with significant similarities to Europa, a place
called Lake Vostok.
In 1974, a team of scientists conducting
airborne research passed over the Soviet research station Vostok
in Antarctica. Their sounding instruments detected an expanse
of water beneath the ice roughly the size of Lake Ontario. Although
Antarctica records some of the coldest temperatures on Earth,
Lake Vostok is buried under four kilometers of ice. The ice sheet
acts as a blanket, shielding the lake from cold temperatures
on the surface. It is also thought that geothermal heat helps
keep the water liquid.
Above: In 1993, altimetric and radar
data were used to trace this outline of Lake Vostok, located
about 1000 miles from the South Pole. Image used is copyrighted
by the Canadian Space Agency. More
Information from Caltech-JPL.
Last year, Richard Hoover of NASA's Marshall Space Sciences Lab
and Dr. S.S. Abyzov of the Russian Academy of Sciences used an
Environmental Scanning Electron Microscope to examine ice cores
from above Lake Vostok for evidence of microbiotic life. What
they found surprised them.
"We've found some really bizarre things - things that we've
never seen before," said Hoover. "There are all sorts
of microorganisms in the ice. Some are readily recognizable as
cyanobacteria, bacteria, fungi, spores, pollen grains, and diatoms,
but some are not recognizable as anything we've ever seen before."
Hoover
is about to begin new studies of the deep ice microorganisms
in collaboration with Academician Mikhail V. Ivanov, Director
of the Institute of Microbiology of the Russian Academy of Sciences
to search for microorganisms in very deep samples of Vostok ice.
The extremely deep ice samples from approximately 100 meters
above the surface of the lake (about 400,000 years old) were
obtained by an international team of US, Russian, and French
scientists.
Right: One of the more exotic forms
Hoover and Abyzov found in the deep ice above Lake Vostok. Many
of these microbes will undoubtedly fall into known categories
when identifications are made.
It's possible that Europa and Lake Vostok share a number of remarkable
properties, including a kilometers-thick covering of ice over
liquid water, an environment where life may have developed along
unique evolutionary paths and subterranean waterways accessible
to remote observation via radio sounding techniques and in
situ observations by means of melting probes. A group of
scientists and engineers from JPL, The University of Nebraska,
and the Woods Hole Oceanographic Institute have begun
a research program to identify technologies needed for
the exploration of both Lake Vostok and Europa.
This program benefits researchers interested in both
areas - Vostok is very difficult to explore with
current technology, but with the infusion of NASA technology a
safe entry may be possible for the first time. At the same time,
Europa explorers will benefit from experience obtained at
Vostok and similar places. The program is studying designs for
a cryobot vehicle that would penetrate a thick icy crust and
a companion hydrobot vehicle that would explore subterranean
waterways looking for signs of microbial life.
Left:
Artist's concept of an undersea explorer on Europa. The hydrobot
would map the undersea terrain and look for life near volcanic
vents. Image credit: JPL
One of the highest mission priorities for any exploration of
Lake Vostok or Europa is to avoid environmental contamination.
"Lake Vostok is an incredibly precious resource," Richard
Hoover said in a Feb. 1999, interview, "and it would be
a colossal mistake to take samples before it can be done without
contaminating the waters with chemicals or surface microorganisms."
Lake Vostok is clearly a valuable new laboratory for astrobiologists,
and they intend to proceed very carefully. Whatever we learn
from this mysterious Antarctic lake will undoubtedly influence
future missions to search for and explore the waterways of Europa.
Galileo has been orbiting Jupiter and its moons since December
1995. Its primary mission ended in December 1997. The spacecraft
is currently near the end of a two-year extended mission that
will culminate in two daring flybys of volcanoes on Io later
this year. More information about the Galileo mission is available
at: http://www.jpl.nasa.gov/galileo/
JPL manages Galileo for NASA' s Office of Space Science, Washington,
D.C. JPL is a division of the California Institute of Technology,
Pasadena, CA. |