On a cold clear night in 1610 Galileo Galilei
made a startling discovery of moons swirling around the planet Jupiter.
Of particular interest is the second largest satellite outward from
Jupiter...the moon we know to be Europa.
Europa is one of the brightest objects in
the solar system, and the way it reflects sunlight shows that water
ice is a dominant part of the surface. Europa is about the size
of our moon, but has a slightly lower overall density. Such characteristics
would suggest that the object is largely rocky with an ice rich
outer layer. However this presents us with so many questions...How
thick is the ice? Is some of the ice melted below the surface? Could
there be a sub surface global ocean of liquid water?
The clues as to the history and make up of
Europa came in the 1970s/80s from data returned from the fly bye's
of NASA's Pioneer and Voyager spacecraft. Pictures have shown us
a bright orange splotchy surface with very few impact craters, suggesting
geologically recent activity. Whilst there is evidence of
active volcanism on Europa's neighbouring moon Io, yet given that
fact that Europa is further away from Jupiter, the tidal heating
is less than on Io, and therefore it was not clear as to whether
there was sufficient energy present to drive active volcanoes. However
the lack of impact craters would offer the view that some process
had resurfaced Europa, and volcanism seemed the logical candidate.
with regard to volcanism on Europa would involve water rich magmas/geysers,
and if geysers were erupting then there could be enough heat to
maintain a water ocean beneath the icy crust.
The best evidence for liquid water beneath
the crust come from high resolution images produced by the Galileo
team. They show a surface that has been disrupted, leaving blocks
of ice as small as a few km across that have been rafted into new
positions. Many of the blocks fit back to back like a jigsaw, but
with space left over...suggesting that some surface materials have
been consumed, melted or that Europa has experienced global expansion.
In either of the aforementioned scenarios, these crustal blocks
were either floating in water or suspended in warm mobile ice or
A Theory of Europa – Richard Hoagland (1979)
Hoagland proposed that a planet wide ocean
still exists under the tens of miles thick sulphur tinged ice now
completely covering the satellite. Furthermore in that extremely
ancient ocean that exists, life may have once originated there because
of the present uniqueness of Europa in the entire solar system..and
that such life might still exist.
At the time of forwarding this theory Hoagland
encountered overwhelming opposition from almost everyone at NASA,
except for two significant exceptions. The first was that of Arthur
C. Clark, the inventor of the communications satellite and famous
science fiction writer. The second, Dr Robert Jastrow, one of the
founders of NASA. In the acknowledgements to his sequel to the film
2001 – A space odyssey Clark wrote 'The fascinating idea that there
might be life on Europa, beneath ice covered oceans kept liquid
by the same Jovian tidal force that heat Io.' NASA's Galileo mission,
which set out to explore Jupiter and its satellites, finally arrived
at Jupiter on December 7th 1995. The probe successfully streaked
into Jupiter's crushing atmosphere that afternoon, sending back
54 minutes of invaluable data as it fell. Meanwhile the Orbiter
successfully fired its own retro rockets – becoming the second man
made object to be forever captured by the planet Jupiter.
If the result from such a mission turns out
to be positive, then future missions may look towards sending some
form of robotic submarine to melt through the ice and explore the