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There are many worlds within
our solar system. Most reside

beyond the asteroid belt.
These are the moons of the gas

giants of the solar system.
Each a unique and mysterious

world of its own. Some have
oceans of water, geysers of

sulfur, or atmospheres of
plastic. Some are just now being

seen at the outer rim of our
solar system. All are worthy of

much more scrutiny.

What is Jupiter? Currently
en route to the Jupiter system

is the scientific probe
Juno. It will be the first orbiter

mission to Jupiter since the
troubled Galileo spacecraft in

2003. Its task? To establish
a highly elliptical polar orbit

and study Jupiter in
the greatest detail so far.

Several other probes have
made flybys of the system en route

to other destinations. Some
of these return fascinating data

on the Jovian moons.

Jupiter, the largest planet in
our solar system, has, as far

as can be ascertained, 67
orbiting satellites. Most of them

are odd-shaped rocky
masses. Probably asteroids like

Amalthea trapped in a
massive gravitational field of the

planet.

However, there are four
major satellites of Jupiter. Each

large and dense enough to
form spherical bodies. These are

called the Galilean moons.
Named after the great Italian

scientist and astronomer
Galileo Galilei, who discovered

them in 1610. These are called
the Galilean moons. Named

after the great Italian scientist
and astronomer Galileo

Galilei, who discovered them in 1610.

Io is the smallest of the
Jovian moons, though somewhat

larger than our own moon
and closest to orbit Jupiter.

Io is the smallest of the
Jovian moons, though somewhat

larger than our own moon
and closest to orbit Jupiter. The

intense gravitational effect
causes the violent and active

nature of the moon. It has
over 400 volcanoes, lava flows,

and plumes of sulfur, 300
kilometers above the surface.

It has over 400 volcanoes,
lava flows, and plumes of sulfur,

300 kilometers above the
surface. The most dense of the four

moons, Io is also the driest.
Io is thought to be composed

of mainly silicate rock with
a molten core of iron or iron

sulfide. Most of the surface
is composed of extensive

planes, coated with sulfur
and sulfur dioxide frost. Most of

the surface is composed of
extensive planes, coated with

sulfur and sulfur dioxide
frost. The surface is geologically

young, which accounts for
the lack of impact cratering.

Unlike all the other planetary
bodies, these craters have

been covered over by
volcanic activities. Unlike all the

other planetary bodies, these
craters have been covered over

by volcanic activities.

Unlike all the other
planetary bodies, these craters

have been covered over by
volcanic activities. also eject

material high above the Moon
and into orbit around Jupiter.

The internal energy for this
overactive Moon is due to

gravitational tidal forces
between Jupiter and

the other Moons orbiting
farther out from Io.

Just slightly smaller than our
own Moon, Europa has an icy

crust covering what is believed
to be a salty global ocean

capable of sustaining
indigenous life. It also

has active geysers
ejecting material into space.

So how do we think we know
that Europa's ocean exists? Well

it's a combination of using
telescopes on the ground and

having spacecraft that have
flown by Europa and collected

data about the surface,
about the interior structure and

about the magnetic field around
Europa and the combination

of those data sets leads us
to a high degree of confidence

that this global liquid water
H2O ocean is there today and

it's been there for much of
the history of the solar system.

And here's where Europa is
a real game changer. It is far,

far out from the Sun and
yet it's got this liquid water

ocean and the reason that
Europa has liquid water is because

it's orbiting Jupiter and
the tidal tug and pull causes

Europa to flex up and down
and all that tidal energy turns

into mechanical energy
which turns into friction and heat

that helps maintain this liquid
water ocean beneath an icy

shell. Along with helping
maintain liquid water, we think

that tidal energy may also
allow that ocean to interact

with rocks on Europa's seafloor
and it may even give rise to

things like hydrothermal vents
which could help provide not

just the building blocks for
life but also the energy for

life. Europa is the most
likely place to find life in our

solar system today because
we think there's a liquid water

ocean beneath its surface
and we know that on Earth

everywhere that there's water
we find life. So could Europa

have the ingredients to support
life? We might be actually

looking at a body that is
presently alive, presently active

and presently undergoing
its geology. There is too much

evidence right now lying
around on the surface, the red

stuff that suggests that
something's going on there. Is that

an environment that is
habitable for any sort of life form?

By golly we really have got
to go back and figure that out.

We have designed the Europa
mission to take a spacecraft and

its set of instruments all
the way from planet Earth to

Jupiter. Previous mission
concepts were for a spacecraft

that would orbit Europa but
Europa is bathed in radiation

from Jupiter. Any mission
that goes in the vicinity of

Europa is cooked pretty
quickly. Instead we're looking at a

mission that will orbit Jupiter,
make close flybys of Europa

and then zip out of the high
radiation region. This allows

us to have a mission that's
many years long and to collect

and transmit lots and lots of
data. As Europa orbits Jupiter

it flexes and we could measure
the gravitational change of

Europa by encountering
Europa at different points in its

orbit. On a typical flyby
we would turn on our remote

sensing instruments, we would
image the surface, we would

interrogate the surface with
spectroscopy and we would do

the same thing on the way out
and we would essentially rinse

and repeat and do this many,
many times until we understand

Europa globally.

Images from the Hubble Space
Telescope tell us that Europa

might be emitting of water
high into space. If so, a

spacecraft could fly through
those plumes and sample it

directly to understand the
composition of Europa's interior.

If it does have the ability
to harbor life, how does that

work exactly? We'll have
enough instrumentation to really

pinpoint exactly how the
mechanisms would work for

replenishing the nutrients in
a subsurface ocean. Europa is

so important because we want
to understand are we alone in

the cosmos? If there is life
in Europa it almost certainly

was completely independent
from the origin of life on Earth.

And for the first time in the
history of humanity we have

the tools and technology
and capability to potentially

answer this question and we
have nowhere to go to find it.

Jupiter's ocean world, Europa.

The Europa Clipper mission
has passed preliminary

development and strategy
proposals and acquired further

funding. The European Space
Agency has been invited to

develop an additional probe
to ride along with the Clipper,

as with the Cassini-Huygens
mission, and either land or

impact on the moon's surface.
The nominal Europa Clipper

mission would perform 45
flybys of Europa at altitudes

varying from 2,700 to 25
kilometers. A proposed launch

window would be in 2025. If
launched with NASA's new SLS

heavy lift rocket system, the
probe would take less than two

years to reach Jupiter. Otherwise it
would be a six and a half year flight.

The largest moon in the solar
system and another icy world

is Ganymede. Composed of
silicate rock and water ice, the

surface is heavily pockmarked
by impact craters and regions

of tectonic movement. The
moon has a thin atmosphere of

oxygen and possibly ozone and
atomic hydrogen. The moon has

a liquid iron rich outer core
and an internal ocean with

possibly more water than
Earth's oceans. Most recent

modeling suggests the interior
may be layers of water and

ice like a club sandwich.
Ganymede is also the only moon

known to have a magnetic
field. However, it is embedded

within the powerful Jupiter
magnetic field and overwhelmed.

But there are indications of
auroral activity on the moon.

Ganymede is also the final
target for the Jupiter IC Moon

Explorer, or JUS, the ESA
designated mission to three of the

Jovian moons. It will be
launched in 2022 from Europe's

spaceport in Kourou, French
Guiana on an Ariane 5, arriving

at Jupiter in 2030 to spend at least
three years making detailed observations.

It will visit Callisto, the
most heavily cratered object

in the solar system, and
will twice fly by Europa.

Callisto is the fourth and most
distant of the Jovian moons

from Jupiter and outside
the main radiation belt of that

planet. Unlike its sister
moons, Callisto has a very thin

atmosphere of carbon dioxide
and molecular oxygen and low

radiation exposure.

Saturn has 62 confirmed orbital
satellites, many less than 50 kilometers in

the solar system. In addition
to Saturn, Saturn is also

known to have a small amount
of rock. They include Mimas,

Enceladas, Thetis, Dione, Andrea.

Enceladas is covered in ice
with a subsurface ocean at the

southern pole. Geologically
active in the southern region,

geysers have been observed.
This would be from tidal heating

and orbital resonance with
Dione and Andrea and could

contain a liquid ocean heated
by internal radioactive decay.

However, the prize of the Saturnian
system is undoubtedly Titan.

Titan is the only moon with a
dense atmosphere and other

than Earth, the only body to
have stable bodies of surface

liquid. It is larger than both
the planet Mercury and our

own moon. Cassini deposited
the probe Huygens on its surface

in 2005.

Titan is Saturn's largest
moon. It's actually the second

largest moon in the solar
system, and it's the only moon in

the solar system that
has a large and substantial

atmosphere. And that atmosphere
in some respects is really

similar to that of the Earth,
being composed mainly of

nitrogen, but in other
respects it's really different. It

has methane as its second
most abundant gas, and that takes

the same role as water vapor
in the Earth's atmosphere. It

evaporates from the surface,
it forms clouds, and then rains

down again, and in fact forms
lakes that we see at Titan's

north pole, including ethane
and propane and all sorts of

complex chemicals. We also
see these vast dune fields at the

equator, which are not made
of silicates as they are on the

Earth, but actually made of
organic substances, essentially

plastics, which have actually
sedimented from the atmosphere

and are being blown around
into dune fields, the same as

we'd see on a desert on the Earth.

And through this we can
detect which molecules are in the

atmosphere. We see all the
molecules that were previously

discovered by Voyager. But
we're also able to look for new

molecules, and in fact buried
within the signatures of these

more abundant molecular
species, we saw a very small spike,

which was due to a new
species which had not been seen

before. In fact, this was
propylene. So the discovery of

propylene on Titan is
really exciting. First of all, it

completes this chemical family
where we have this missing

link dating 32 years back to
Voyager. But also it shows that

there's much more there
still in Titan's atmosphere to be

discovered. Some people
think that Titan is similar to the

prebiotic Earth long ago when
the molecules were forming the

basis of life. And we don't
know what we're going to find on

Titan if we send back
further spacecraft with new

instruments, more sensitive
instruments, if some of the

molecules on Titan could be
similar to the basis of life on

Earth. NASA is preparing a
new probe to follow in Cassini's

footsteps. It is called the
Titan-Saturn System Mission.

Cassini was able to look at
the lakes, get a sense of the

coarse composition of the
lakes, but nothing about the

organic molecules that are
dissolved in the lakes. The Titan

-Saturn System Mission is a
three-in-one mission with an

orbiter for Titan, a balloon
that will float through Titan's

atmosphere, and a lander that
will splash down on one of the

northern lakes of Titan. This
mission will actually go into

a lake, sample the liquid
directly, see what the organic

molecules are that are
present. The Titan-Saturn System

Mission also will go to
Enceladus, the tinier moon, a

thousand times smaller than
Titan, which has volcanoes,

geysers essentially, that
are spewing material from the

inside of this moon outward.
And it's a chance to see

whether there might be
molecules that would indicate that

life has actually formed within
the source region of these

geysers. These geysers
have water ice and we strongly

suspect that there's liquid
water in the region that these

geysers are coming from. We
know there are organic molecules

there because they've been
measured by Cassini. The ability

to follow this up quickly is
essential because with Cassini

-Huygens we have now trained
a generation of scientists who

are ready to take a new
generation of instruments and

capabilities back to Titan and
Enceladus and really answer

the questions that Cassini-Huygens
has left for us. And that

continuity of knowledge and
of enthusiasm is essential and

very difficult to maintain in
the outer solar system because

trip times are so long. The
Titan-Saturn System Mission

really, as Jules Verne
realized, it's a kind of planetary

exploration that we have never
ever done before anywhere

else in

the human heart. In terms
of the way it's exploring this

fascinating world, it will be
floating on the surface of a

lake, it will be floating through
the atmosphere, it will be

revealing the entire surface
from orbit at the same time. As

we think of exploration,
of unveiling a new world,

it's exploration in the
true sense of the word.

The planet Uranus has 27
known moons grouped in three

categories, 13 inner moons,
five major moons and nine

irregulars. Only Voyager 2 has
passed by these worlds, there

remains much to learn. The
major moons in order from the

planet are Miranda, Ariel,
Umbriel, Titania and Oberon.

Four of these moons have
known internal processes such as

vulcanism and surface
canyon formation. Miranda is the

smallest of Uranus's round
moons and one of the smallest

objects in the solar system
to be spherical under its own

gravity. Strangely, it also
has the tallest cliffs in the

solar system. Thought to
consist of equal parts of rock and

ice, Ariel's surface terrain,
probably of ice, is cross-cut

with canyons, scarps and ridges.

Umbriel is again thought
to consist of rock and ice. Its

surface is the darkest of the
moons and heavily cratered.

The surface seems to have
gone through some form of surface

heating that destroyed its
very early features. Titania is

the largest of Uranus's
moons. Similar in composition,

heavy cratering has been
obscured by changes to the surface

through some heating event
like its sister Umbriel. Titania

may have a tenuous atmosphere
as carbon dioxide and water

ice have been detected on its surface.

Oberon, the outermost of the
spherical moons, seems similar

to the others. Ice and rock
and heavily cratered. The

surface has numerous
scarps and graben from crustal

movement. There are
currently no plans to revisit these

worlds.

Neptune has 14 known moons
categorized into two groups, the

regulars and the irregulars.
The inner seven moons orbit

normally. The remaining
half, including its largest moon

Triton, orbit in either an
eccentric, inclined or retrograde

motion. Triton orbits in
direction to Neptune's spin.

Scientists believe it was
probably captured by Neptune's

gravity in the early days of
the solar system. Triton has an

atmosphere that forms clouds
and haze and is the only moon

closely observed by Voyager
on its flyby of the system.

However, Neptune plays a
very important role at the edge

of the solar system. The
planets formed from a disk of dust

surrounding our sun billions
of years ago. Remnants of this

disk still remain. The rocky
asteroid belt influenced by

Jupiter and the icy debris
cloud beyond Neptune. Neptune

creates a ring structure in
the dust cloud which features a

gap where the planet itself
resides. And this gap should

make it fairly easy to tell
where Neptune is from afar, even

at distances where the planet
is too dim to detect directly.

The supercomputer simulations
that Mark Kushner and I

performed also allow us to
see what the dust in the solar

system may have looked like
when the solar system was much

younger. In effect, we can go
back in time and see how the

distant view of the solar system
may have changed. When we

included collisions between
dust particles, we were

really changed Neptune's
gravitational imprint.

The gap in the ring
structure disappears.

Over billions of years, Neptune
shepherds the dust cloud

into an outer ring to what
is now called the Kuiper belt.

The New Horizons spacecraft
is exploring this region with

its first flyby of the enigmatic
Pluto and its moon Charon.

Charon is

the largest of Pluto's five
moons. The other four orbit an

erratic motion around the
Pluto-Charon pair. Nix and Hydra

are both odd-shaped,
contributing to their erratic orbital

motions. This is Hydra,
taken by New Horizons from a

distance of nearly 650,000
kilometers, revealing its

irregular shape. This is the
largest of Pluto's five moons.

Pluto was the first of these
trans-Neptunian objects

detected and first thought
to be a ninth planet.

And then Pluto was this kind
of, you know, odd guy out. It

was this little object at the
edge of the solar system. And

then when we found all these
other Kuiper belt objects, this

is kind of almost a third
type of object. So for the first

time ever, we'll be able to
fly by a brand new object,

an object that's been
forming for billions of years and

understand what outer parts
of the solar system are all

about. Pluto is the first of
the Kuiper belt objects, or

KBOs, to be seen up close.
There are many other KBOs or

dwarf planets awaiting
detailed scrutiny, such as Eris,

almost the size of Mercury,
and Quahua, the first KBO

discovered. The most eccentric
orbit belongs to Senda, which

has an elliptical orbit of
11,000 years, taking it to

the icy Oort cloud at the
edge of the solar system.