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history of fracturing and healing. Europa's patchwork ice cover is able
to flex and shatter in this particular fashion because beneath it lies an
ocean estimated to be of the order of 100 kilometres deep, detected
by magnetic patterns—which gives it roughly twice the bulk of the
Earth's oceans. And beneath that is the main part of the moon, which
is rocky with an iron core.
Powering the whole thing, as with Io, is the energy generated by the
tides raised by Jupiter. This will be driving some kind of geological
activity in the rocky heart of the planet. The tidal forces are thought
to generate long low waves, termed Rossby waves, that propagate
through the ocean of Europa, the kinetic energy produced being
enough to keep the oceans liquid—and perhaps even lukewarm
(although, with a surface temperature below −160 degrees Celsius, the
external ice is rock-hard). The flexing produced as the moon is
stretched and squeezed generates the fractures, the pattern of which
shows that the entire icy carapace, with no attachment anywhere to
the rock far below, is literally slipping around the moon's rocky inte-
rior, making a full revolution about every 12,000 years. The chaotic
areas seem to be due to eruptions of warm, softer ice from below,
perhaps associated with lakes of water entirely enclosed in the icy
carapace. The water is salty, and the fracture lines seem to be enriched
in crystallized salts—perhaps magnesium sulphate (Epsom salts) and
sulphur compounds, to give them their dark reddish colour.
Among Earthbound scientists Europa has generated fierce debate.
For instance, one question is whether it has a 'thick' surface ice cara-
pace of perhaps 30 kilometres—which would mean that the water
below never reached the surface—or a 'thin' one of only a few kilo-
metres, where water could now and then break through, perhaps
to form the 'chaotic terrain'.
There is, of course, the hot question of whether life might exist on
Europa. Of water, there is no shortage. There may be more or less
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