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One planetary body that has really fascinated astrobiologists is

Enceladus, the tiny moon of the giant gas planet, Saturn.

Here's some basic facts about Enceladus.

It was discovered in 1789 by astronomer William Hershel.

It's over a billion kilometers away from the Earth.

It seems to have a water-ice surface, a

little bit like Europa, hence the interest from astrobiologists.

And it orbits very

close to Saturn and its rings, which makes it very difficult to view from the Earth.

It's a tiny little moon, about 500 kilometers in diameter, much

smaller than the Earth, much smaller than even our own moon.

Like Europa, much of our knowledge of

Enceladus has been gathered from fly-by missions.

In 1980 to 81 the Voyager one and two spacecraft gave us

the first images and information on the nature of this tiny moon.

And what did it tell us?

Well Voyager saw a highly reflective surface with few impact craters and told

us that Enceladus probably had an icy surface that's also very young and active.

It discovered a very heavily cratered mid-high northern latitude and

lightly cratered region close to the equator, that gives us

some idea of the differences in the composition of the

ice and its age across the surface of the moon.

It also found that Enceladus

is embedded in the densest part of one of Saturn's rings, the E ring.

And it's thought that Enceladus may be contributing material to

the production of this ring as we'll see in a bit.

The chief conclusion from what we observe with Voyager is that Enceladus is a young,

geologically-active moon possiby feeding the E-Ring of Saturn.

Many questions about Enceladus began to

be answered with the fly-bys by the Cassini spacecraft beginning in 2004.

It returned some of the best resolution images we have of Enceladus.

But there's little doubt in saying this most remarkable

discovery was that there are plumes containing hydro-carbons and

water being injected from the South Pole of Enceladus

and we'll come back to those in a little while.

Let's have a look at some of the

interesting features that have been observed on the

surface of Enceladus.

First of all, Cassini spacecraft confirmed this is a

moon with a very young surface, a very active surface.

Different levels of cratering were found in different areas, but all craters show

signs of deformation and degradation suggesting an

icy surface where the craters are collapsing.

And beginning to subside and dissipate in this active surface.

Near the south pole of the moon are features that have been called

tiger stripes, which may be as young as a half a million years old.

Which is very young in geological terms.

They seem to have a different composition from the rest of Enceladus.

They contain coarse ice grains instead of fine grains and

lots of organics seem to be present in these tiger stripes.

There's little doubt in saying that the most remarkable discovery was these vapor

plumes being ejected from the tiger stripes from the south pole of Enceladus.

The Cassini spacecraft had the opportunity to

fly through these plumes and measure their composition.

And it found some extraordinary things.

It found that these plumes contain water, water ice.

It also found methane, carbon dioxide, and organic compounds.

Compounds such as propane, ethane, acetylene

and also more complex organic compounds.

In fact, the instrument, the GCMS on this spacecraft, was

not adequate to identify these very complex organic compounds because

it wasn't designed for this task, they weren't expecting to

find hydro carbons in these plumes being ejected from Enceladus.

But these complex materials,

these complex organics, suggest that the interior of

Encedalus contains liquid water, and might be acting

as a huge chemical reactor, producing organic compounds

that are being ejected into space in these plumes.

And one obvious question for astrobiologists is could

these complex organic molecules indicate life within the moon?

And even if there is no life in Enceladus,

perhaps these complex organic compounds can tell us something

about the reactions, the chemical reactions that are necessary to

build the building blocks of life, to create the building

blocks of life that are necessary for the evolution of

life on planetary surfaces or in the interior of planets.

All of these data come together to give

us clues to the interior structure of Enceladus.

We think that Enceladus contains a rocky core

surrounded by a water ice mantle but the

tidal heat generated the gravitational force of Saturn

causes the buckling of this moon that generates heat.

This heat might melt the water ice mantle beneath the surface creating pressurized

bodies of liquid water.

These pressurized bodies of liquid water then

reach the surface where they break out

through the tiger stripes and create these

plumes ejecting these complex organic materials into space.

A forthcoming challenge of astrobiology is to determine the composition of possible

liquid water beneath the surface of

Enceladus, and what these organics really mean.

Could this be a location for extraterrestrial life or

just an interesting factory for organic compounds.

Future missions to Enceladus are already being planned,

the Titan-Saturn system mission has been proposed by

NASA and the European Space Agency, and it's

a proposed mission to study Saturn's moons, including Enceladus.

It lost out on funding in a mission to Europa, but nevertheless,

there will be missions to Enceladus to explore those plumes to see

what they contain and to see whether they might reveal something about

the interior of Enceladus whether it could even be a location for life.

So what have we learned in this lecture?

We've learned that Enceladus is a tiny moon of Saturn.

It has a young, actively deforming surface.

Most importantly for astrobiologists, we've learned that it's spewing

out jets of water vapor and organics into space.

And it's thought that Enceladus might

have salty water beneath the surface, that's providing the source of water

for these plumes, it could even be a location to search for life.

Enceladus

Astrobiology and the Search for Extraterrestrial Life

Meet the Instructors