杏吧原创

WHERE there鈥檚 water, there鈥檚 life. Among astrobiologists it is almost a
mantra. On Earth, they point out, we have found life flourishing in searing heat
and shattering cold, bright light and pitch darkness. As long as there鈥檚 liquid
water, living things will find a way. And what鈥檚 true here could very well be
true elsewhere in the Universe. You want alien life? Then look for alien
water.

That鈥檚 why astronomers around the world are straining to pick up even the
faintest cosmic traces of the precious liquid. Perhaps there are miniature oases
in Mars鈥檚 frozen desert, warmed from below. Perhaps we鈥檒l find Earth-like
planets orbiting far-off stars at just the right distance to allow water to flow
(see 鈥淎lien haven鈥�).

And perhaps there is a perfectly good alien ocean right on our doorstep. One,
moreover, that contains enough water to fill all of the Earth鈥檚 ocean basins and
then some. Sounds implausible? It isn鈥檛. Beneath the ice crust of one of
Jupiter鈥檚 moons, Europa, evidence is mounting for enough water to fulfil
astrobiologists鈥� wildest dreams. 鈥淎ll the observations hang together if you鈥檝e
got a deep ocean under the ice,鈥� says Mike Belton of Kitt Peak Observatory in
Tucson, who heads the imaging team for Galileo鈥攖he spacecraft studying
Jupiter and its moons.

NASA is now putting together a new mission that will follow up Galileo鈥檚
observations and determine whether the ocean really exists. If it does, they
will send a spacecraft down to the surface to test the chemistry of the ice. And
if that is encouraging, another mission will be sent to drill through the ice
crust and reveal whatever alien creatures might lurk below.

Slightly smaller than our own Moon, Europa is five times as far from the Sun.
Its surface temperature is a chilly 鈭�145 掳C, but there has always the
been the possibility that the frigid crust could hide something altogether more
lush. The Sun鈥檚 rays may be feeble on Europa, but Jupiter鈥檚 gravity is not: with
its mighty tidal tugs, Jupiter kneads away at the moon鈥檚 interior, warming it
with squeezing and wrenching.

Hints that this could be enough to melt ice below the surface came in the
1970s, when Voyager flew by. The spacecraft discovered a surface that was
scarred all over with mysterious lines, but bore almost no craters. That was
odd, because space debris would be raining down constantly. The lack of craters
implied a surface that was continually renewing itself from below, and the
lines, too, suggested that something mobile lay beneath the crust. But the
images weren鈥檛 detailed enough for scientists to be sure: perhaps there was a
host of craters just smaller than Voyager鈥檚 eye could see.

Then, nearly two decades later, Galileo arrived. With its keener eyesight, it
confirmed from the sparseness of the craters that parts of Europa鈥檚 surface are
a mere 10 million years old. Soon other evidence began to pour in.

First came the 鈥渋cebergs鈥�, announced with fanfare in 1997. Europa, it seems,
is scattered with regions of crazy paving, where the ice has broken into flat
blocks that look for all the world like the great tabular icebergs of
Antarctica. 鈥淭hey鈥檝e floated away from where they started,鈥� says Belton. 鈥淏ut
they fit together like jigsaw pieces.鈥� Between the bergs is a matrix of
mysterious dark red ice, frozen now, but surely warm and soft when the blocks of
ice broke free and wandered off.

Then there鈥檚 the magnetism. Jupiter鈥檚 intense magnetic field continually
sweeps through Europa, and Galileo spotted signs that the moon is answering back
with a weak magnetic echo that points the opposite way to Jupiter鈥檚 field. For
that to happen, says Margaret Kivelson from UCLA, another member of the Galileo
team, Jupiter鈥檚 magnetic field must be generating electric currents inside
Europa, which in turn create a magnetic field of their own. Last year she and
her colleagues published a paper in Nature(vol 395, p 777) showing that
the field makes perfect sense if a conducting shell lies just a few kilometres
below Europa鈥檚 surface. And measurements of the moon鈥檚 gravitational field show
that ice鈥攐r something about as lightweight鈥攅xtends down 100
kilometres before rock takes over. So Kivelson and her colleagues concluded that
they had seen the electrical effects of a briny liquid ocean.

Meanwhile, something stranger began to emerge. Poring over the images from
Galileo, researchers spotted signs that Europa鈥檚 crust is spinning at a
different rate from the interior. The vast global lines first seen by Voyager
turn out to be ridges that extend for thousands of kilometres. At first, they
looked like a complex jumble. But it quickly became clear that the youngest
ones鈥攖hose that cut through other formations鈥攖end to be a dull dark
red, the same colour as the ice around the bergs, while the older ones are
progressively brighter. When the Galileo team used this to disentangle the
cracks and follow their progression with age, they found exactly the pattern of
twists and curves that you鈥檇 expect if an outer skin of ice was creeping over
Europa鈥檚 interior.

The skin couldn鈥檛 be moving over hard rock. It would have to be sliding on
something much softer鈥攕omething like water. Excited, the team looked for
more tell-tale signs. Like our own Moon and most others in the Solar System,
Europa is tidally locked: it spins and orbits at the same rate, always showing
the same face to Jupiter. And also like our own Moon, Europa bears two tidal
bulges, one perpetually facing Jupiter and one diametrically opposite. If the
surface crust really is slipping over the interior, the ice should stretch out
before it mounts these bulges.

Sure enough, the researchers found a region 45 degrees west of the outer
bulge where the surface had clearly been torn apart. Jagged edges that once
fitted together are now separated by bands of the familiar dark red ice. 鈥淚t鈥檚
pretty obvious from the images that the crust has pulled apart and filled in
from below,鈥� says Belton.

In November, Galileo will take pictures of the inner tidal bulge, facing
Jupiter. If that, too, shows stretch marks to the west, the case for a sliding
skin will be very strong. Still, it wouldn鈥檛 quite answer the question of what
lies below. 鈥淚t鈥檚 easy to slide if you have an ocean,鈥� says Bob Pappalardo from
Brown University in Rhode Island. The problem, he says, is that soft, warm ice
could also do the trick and might explain the movement of the icebergs too. But
it would be most unlikely to host alien life.

So what is it, water or soft ice? There are encouraging hints for the life
brigade in the various dark red pits, spots and domes that freckle Europa鈥檚
surface. In a paper published in Naturelast year (vol 391, p 365),
Pappalardo and his colleagues suggested that these are signs of convection
beneath the ice. Heating deep in the interior, they said, could be causing
packets of warm material to rise just as soup does on the stove. Sometimes this
warmer material breaks into the cold brittle ice skin to create a dark spot on
the surface, and sometimes it stops when it reaches the skin, forcing the ice
into a dome.

Could that convecting material be water? It鈥檚 tempting to think so says
Pappalardo, but the answer is probably no. Convection has to be driven by heat
from below, and although such heat could be pouring out of the moon鈥檚 rock, from
vents like the black smokers on Earth鈥檚 ocean floor, the rock is a very long way
down. 鈥淚t鈥檚 attractive to think a chimney like that could make a hot plume of
water that will reach all the way up through a Europan ocean and melt the ice
shell up at the surface,鈥� says Pappalardo. 鈥淏ut the calculations we鈥檝e done say
the water would cool much too quickly. It would never make it to the top.鈥� So
could soft convecting ice extend all the way down to the rock, leaving no room
for a liquid ocean? No, says Pappalardo, for just the same reason as the water.
There鈥檚 no way that heat from the rock can drive plumes of convecting ice all
the way to the surface.

He believes that the likeliest arrangement is a combination of ice and water.
At the surface, there is a thin layer of cold brittle ice. Below is soft, warm
ice, and below that lies nearly 100 kilometres of warm liquid ocean. In this
scenario, heat from the water drives convection in the soft ice layer above
(see diagram).FIG-22045001.jpg

When you put all Galileo鈥檚 findings together, then, the case for a Europan
ocean is persuasive鈥攂ut not conclusive. 鈥淚鈥檓 not going to say `Yes, 100
per cent, we know there鈥檚 an ocean'鈥� says Pappalardo. 鈥淏ut I鈥檇 give it a 60 or
70 per cent chance.鈥�

We won鈥檛 have to be in suspense for long. Stirred by the prospect of finding
alien life so close to home, NASA is now funding a new mission, scheduled for
blast-off in 2003. The Europa Orbiter鈥檚 main aim will be to look for water.

If all goes as planned, the craft will be equipped with radar for peering
through the ice: there should be a distinctive signal where the radar bounces
off the interface between ice and ocean. But it won鈥檛 be easy. With 20 years鈥�
experience of using radar to see through the Antarctic ice sheet, Don
Blankenship from the University of Texas at Austin headed the committee
considering whether radar could solve the ocean question on Europa. The
researchers went through the likely options. If the ice formed quickly, like sea
ice on Earth, it could be full of impurities. Would they absorb the radar, even
at such low temperatures? What if鈥攁s Pappalardo suggests鈥攖he
outermost skin of cold ice conceals a layer of warm convecting ice, before you
reach the water?

The team concluded that radar could see through around 10 kilometres of cold
brittle ice, but would be absorbed as soon as it encountered warm convecting
ice. Will 10 kilometres be enough? Very probably, says Clark Chapman from the
Southwest Research Institute in Boulder, Colorado. He points out a curious
pattern in the few craters that have survived on Europa鈥檚 surface. The smaller
ones鈥攍ess than 20 kilometres across鈥攁re neat bowl shapes just like
the craters on our own Moon. But those bigger than 30 kilometres are much
flatter, suggesting that whatever hit the surface broke through into something
softer. According to Chapman, that implies an uppermost layer of brittle ice
less than 10 kilometres thick.

Radar might even be able to see through warmer ice to a water layer below.
Bill McKinnon of Washington University, St Louis, calculates that the down
welling regions in convecting ice would be cooler than ice that is not
convecting, and the radar might be able to use these cooler parts as a kind of
window.

And there will be other instruments to help. An altimeter will measure the
rise and fall of Europa鈥檚 surface as the tides raised by Jupiter ebb and flow. A
water ocean would give changes of the order of 30 metres, whereas ice would be
more like 1 metre. There will also be more detailed measurements of Europa鈥檚
gravitational field to help the team choose between the different models.

If there is an ocean, is it likely to be teeming with aquatic aliens? Water
isn鈥檛 enough; they will presumably need minerals and organic substances to feed
on. Again, the portents are good. All the regions that have recently welled up
to the surface tend to be reddish rather than white, a clear sign of impurities.
鈥淭he picture you have in your head is that comets and other things have been
raining down on Europa, creating a slush that includes a lot of other things
besides just water,鈥� says Chapman. The likeliest impurities, he says, are salts
and organic molecules. And there鈥檚 heat there too. 鈥淵ou have to have energy
there to keep it liquid,鈥� says Belton. 鈥淭hose are all the ingredients you need
to start a discussion about life.鈥�

Amid all the excitement, some researchers strike a cautionary note. Joseph
Kirschvink from Caltech in Pasadena and two colleagues pointed out in
Science earlier this year (vol 284, p 1631) that a gloomy Europan ocean
might not be particularly hospitable. Although creatures clinging to deep sea
vents here on Earth cope contentedly with darkness, they have a constant rain of
oxygenated chemicals from the surface to feed their metabolic fervour. Capped in
ice, and with only a feeble atmosphere of oxygen, Europa would be much more
hostile. Even so, the researchers conclude that there are some bacteria on Earth
that might be able to survive the Europan strictures.

And if we eventually succeed in finding alien creatures in this alien ocean
it will change everything. 鈥淚t was the satellites of Jupiter that Galileo first
saw that suddenly changed the place of Earth in the Solar System,鈥� says
Pappalardo. 鈥淲e were no longer the centre of everything鈥攋ust a planet
going round the Sun.鈥� If we find life in a Europan ocean, he points out, the
revolution will have come full circle. 鈥淲e think of Earth as the centre of life,
the only place that life exists. If we find evidence for life somewhere
else鈥攑erhaps the same place that brought on the Copernican
revolution鈥攊t would change our way of thinking about our place in the
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