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>> One planetary body that has reallyfascinated
astrobiologists is Enceladus, atiny moon of the giant gas planet,
Saturn.Here are some basic facts about Enceladus.It was discovered
in 1789 by astronomerWilliam Herschel.It's over a billion
kilometers away fromthe Earth.It seems to have a water-ice surface,
alittle bit like Europa, hence the interestfrom
astro-biologists.And it orbits very close to Saturn'srings, which
makes it very difficult toview from the earth.It's a tiny little
moon, about 500kilometers in diameter.Much smaller them the earth,
much smallerthem even our own moon.Like Europa, much of our
knowledge ofEnceladus is being gathered from flybymissions.In 1981,
the Voyager 1 and 2 spacecraftgave us the first images and
information
on the nature of this tiny moon.What did it tell us?Well,
Voyager saw a highly reflectivesurface with few impact craters,
that toldus that Enceladus probably had an icysurface that's also
very young and active.It discovered a very heavily createdmid-high
northern latitude and lightlycreated region close to the equator.It
gives us some idea of the differencesin the composition of the ice
and it's ageacross the surface of the moon.It also found that
Enceladus is embedded
in the densest part of one of Saturn'srings, the E ring.And it's
thought that Enceladus may becontributing material to the
production ofthis ring, as we'll see in a bit.The chief conclusion
from what we observedwith Voyager is that Enceladus is a
young,geologically active moon possibly feedingthe e ring of
Saturn.Many questions about Enceladus began to beanswered with the
flybys by the Cassinispacecraft beginning in 2004.It returned some
of the best resolution
images we have of Enceladus.But, there's little doubt in saying
thatit's a most remarkable discovery.With it, there are clues,
containinghydrocarbons and water, being ejected fromthe south pole
of Enceladus, and, we'llcome back to those in a little while.Let's
have a look at some of theinteresting features that have
beenobserved on the surface of Enceladus.
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First of all the Cassini spacecraftconfirmed this is a moon with
a very youngsurface, a very active surface.Different levels of
cratering were foundin different areas, but all craters shownsigns
of deformation and degradation,suggesting an icy surface where
thecraters are collapsing and beginning tosubside and dissipate in
this activesurface.Near the south pole of the moon arefeatures that
are being called tigerstripes, which maybe as young as half
amillion years old, which is very young isgeological terms.They
seem to have a different compositionfrom the rest of Enceladus.They
contain coarse ice grains instead offine grains, and lots of
organics seem tobe present in these tiger stripes.There's little
doubt in saying that themost remarkable discovery was these
vaporplumes being ejected from the tigerstripes from the south pole
of Enceladus.
The Cassini spacecraft had the opportunityto fly though these
plumes and measuretheir composition, and it found someextraordinary
things.It found that these plumes containedwater, water ice.It also
found methane, carbon dioxide, andorganic compounds.Compounds such
as propane, ethane,acetylene, and also more complex
organiccompounds.In fact the instrument, the GCMS on thespacecraft
was not adequate to identify
these very complex organic compoundsbecause it wasn't designed
for this task.They weren't expected to find hydrocarbonsin these
plumes being ejected fromEnceladus.But these complex materials,
these complexorganics, suggest that the interior ofEnceladus
contains liquid water, and mightbe acting as a huge chemical
reactor,producing organic compounds that are beingejected into
space in these plumes.And one obvious questions forastrobiologists
is could these complex
organic molecules indicate life within themoon.And even if there
is no life on Enceladus,perhaps these complex organic compoundscan
tell us something about the reactions,the chemical reactions that
are necessaryto build the building blocks of life, tocreate the
building blocks of life thatare necessary for the evolution of life
onplanetary surfaces or in the interior of
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planets.All of these data come together to give usclues to the
interior structure ofEnceladus.We think that Enceladus contains a
rockycore surrounded by a water ice mantle.But the tidal heat
generated from thegravitational force of Saturn, causes thebuckling
of this moon that generates heat.This heat might melt the water ice
mantlebeneath the surface, creating pressurizedbodies of liquid
water.These pressurized bodies of liquid waterthen reach the
surface where they breakout through the tiger stripes and
createthese plumes, ejecting these complexorganic materials into
space.A forthcoming challenge of astrobiology isto determine the
composition of possibleliquid water beneath the surface ofEnceladus
and what these organics reallymean.Could this be a location
forextraterrestrial life or just an
interesting factory for organic compounds?Future missions to
Enceladus are alreadybeing planned.The Titan Saturn system mission
is beingproposed by NASA and the European spaceagency, as proposed
mission to studySaturn's moons, including Enceladus.It lost out for
funding in a mission toEuropa, but nevertheless in the futurethere
will be missions to Enceladus toexplore those plumes, to see what
they cantake and to see whether they might revealsomething about
the interior of Enceladus,
whether it can even be a location forlife.So what we learned in
this lecture?We've learned that Enceladus is a tinymoon of
Saturn.It has a young, actively deformingsurface.Most importantly
for an astrobiologist,we've learned that it's spewing out jetsof
water vapor and organics into space,and it's thought that Enceladus
might havesalty water beneath the surface that'sproviding the
source of water for these
plumes.It could even be a location to search forlife.
