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Planetary Science 2014 – Part 3
We have been fascinated with Jupiter’s moon Europa since the Voyagers observed them in
the late 70’s and 80’s. This first image from Europa was surprising as it has such
a bizarre surface with extremely exotic terrain. Europa has lots of straight lines, and almost
no cratering which indicates a very young surface which is constantly being renewed.
These features could be the result of water underneath the surface, and some are similar
to the patterns produced by ice sheets in Antarctica bumping and grinding against each
other during seasonal changes. The Thera and Thrace Macula have both similarities
and differences. The Thera Macula is lower than the surrounding terrain. It has collapsed
somewhat into the surface. The Thrace Macula however is elevated above the surface. This
indicates that there is a lot of water mobility fairly close to the surface that would allow
melting to take place for the Thera Macula to sink, and some icy warmer water /slush
to rise up and elevate the Thrace Macula above the surface of Europa.
The Conamara Chaos is a section that looks like the surface has partially melted and
large blocks of ice became tilted, rotated and half-submerged and then refroze to produce
the chaotic terrain on the surface in this area.
Embedded within this exotic terrain are ridges – double ridges even – with very straight
lines. Europa is a moon of Jupiter, and Jupiter has several other moons. The interaction of
tidal forces and stresses among Jupiter and the other moons on Europa on Europa cause
sections of the surface to be pulled apart, pushed together, or to slide against each
other. Then there are the cycloids. What could cause
such a bizarre feature? It was first thought that Europa was locked into synchronous rotation
with Jupiter but it is not. It orbits as it rotates. This motion produces rotating diurnal
stress fields as Europa rotates and orbits within the tidal forces of Jupiter and its
moons. The cycle of changing forces causes a change in the direction of the ridges. The
ridges sometimes are cycloidal and then become straight and then change again. It is not
known if this is a repeating cycle. There are almost no plains on Europa and very
little cratering which is additional evidence that a large volume of water is close to the
surface of Europa. It has been calculated that about 70% of the volume of Europa is
water. The schematic shows the volume comparison of the amount of water on Europa compared
to the volume of water on planet Earth. There is a planned mission – the JUICE mission
– to Europa planned by the European Space Agency with other organizations in the United
States to study the subsurface conditions in 2022. Enceladus is a bizarre looking moon of Saturn.
It has cratered areas, uncratered areas, fissures, plains, and corrugated, wrinkled areas. This
might be a similar situation to Jupiter and Io where the tidal stresses from Jupiter and
its moons are pulling Io apart and producing volcanism. Parts of Enceladus exhibit similar
stresses. In the south polar regions about 98 geysers have been observed that have plumes and jets of
icy particles and/or organic materials being ejected into the atmosphere. The geysers are
squeezing the materials from under the surface of Enceladus due to the tidal forces stressing
the surface. This is an indication of a large volume of water below the surface of Enceladus.