Launched three years before the new century... a spacecraft wound its way through the empty
reaches of the solar system.
On Earth, its progress was little noted, as it swung twice by the planet Venus, then our
moon. And Earth. The asteroid belt. And Jupiter.
Almost seven years later, on the first of July 2004, the Cassini probe entered the orbit
of Saturn. It then began to compile what has become one of the greatest photographic collections
of all time, of a giant gas planet, surrounded by colorful rings, guarded by a diverse collection
of moons, and millions of tiny moonlets.
Within this record, is a trail of clues... pointing to the energy sources and complex
chemistry needed to spawn life. What are these mysterious worlds telling us about the universe,
and Earth?
In the outer reaches of the solar system, a billion and a half kilometers from the Sun...
there is a little world known as Enceladus. Nearly all of the sunlight that strikes its
icy surface is reflected back into space, making it one of the brightest objects in
the solar system.
At its equator, the average temperature is minus 198 degrees Celsius. It can rise about
70 degrees higher in grooves that stretch across the south pole like tiger stripes.
Looming over it is the giant planet Saturn.
In myth, Saturn - the Roman name for the primal Greek God Chronos - was the youngest son of
Gaia, or Earth, and Uranus, sky.
Wielding a scythe provided by his mother, the story goes, Saturn confronted his abusive
father, castrating him. The blood of Uranus flowed into the seas, fertilizing the Earth
and giving rise to Enceladus and other giant offspring.
Saturn's moon Enceladus has its own tangled story. In 2005, the Cassini spacecraft spotted
plumes of water vapor shooting out into space from several locations near its south pole.
More recent close encounters have revealed jets of water, flavored by slightly salty
chemical compounds, spewing out from vents in the rough, cracked polar terrain. That
may mean that Enceladus harbors a remarkable secret below its frigid surface: A liquid
ocean, and perhaps, a chemical environment that could spawn simple life forms.
It's not the only promising stop in the realm of Saturn. The moon Titan is often said to
resemble Earth in its early days. It is lined with volcanoes and a hazy atmosphere rich
in organic compounds.
While Enceladus is the size of Great Britain, Titan is ten times larger, 50% larger than
our moon, and the second largest moon in our solar system.
We've known about Titan since the astronomer Christian Huygens discovered it in 1655, and
Enceladus since William Herschel spotted it in August 1789, just after the start of the
French Revolution.
Scientists began to investigate these moons in earnest with the launch of the two Voyager
spacecraft in 1977. The lineup of outer planets in the solar system allowed the spacecraft
to fly past each of them.
Along the way, they sent back tens of thousands of images, of planetary realms more diverse
than anyone had imagined. These long-distance marathon flyers - both now headed out toward
interstellar space - made discoveries about the chemical environments that have turned
these planets into gigantic works of abstract art.
They also disclosed new details about their magnetic fields, atmospheres, ring systems,
and inner cores. But what really turned heads were the varied shapes and surfaces of their
moons.
They've all been pummeled over the millennia by wayward asteroids and comets. A few appear
to be sculpted by forces below their surfaces. Neptune's largest moon Triton has few craters.
It's marked with circular depressions bounded by rugged ridges. There are also grooves and
folds that stretch for dozens of miles, a sign of fracturing and deforming.
Triton has geysers too, shooting some five miles above the surface. But on this frigid
moon - so far from the Sun - the liquid that spouts is not water but nitrogen.
Tiny Miranda, one of 27 known moons that orbit Uranus, wears a jumbled skin that's been shaped
and reshaped by forces within.
Jupiter's moon Io - orbiting perilously close to the giant planet is literally turning itself
inside out. Rivers of lava roll down from open craters that erupt like fountains.
What's causing these tiny moons to come to life? The surface of Jupiter's moon Europa
offers a window into the churning activity at its core.
Flying by Europa, Voyager documented a complex network of criss-crossing grooves and ridges.
In the 1990s, the Galileo spacecraft went back to get a closer look. It found that Europa's
surface is a crazy quilt of fractured plates, cliff faces and gullies... amid long grooves
like a network of superhighways. How did it get like this?
Europa orbits Jupiter in a slight ellipse, diving in close before swinging wide. That
allows the massive planet's gravity to constantly tug at its rocky center. The friction of rock
rubbing on rock causes that core to heat up.
Then, heat rising up through a subsurface ocean of liquid water cracks, and shifts,
and spreads the icy surface in a thousand different ways. Europa's neighbors, Callisto
and Ganymede, show similar features, suggesting they too may have liquid oceans below their
surfaces.
Crossing outward to Saturn, Voyager found a similar surface on the moon Enceladus. So
when the Cassini spacecraft arrived in 2004, it came looking for answers to a range of
burning questions: if this moon and others have subsurface oceans? Do they also have
the ability to cook up and support life? And what could they tell us about the origin of
life throughout the galaxy?
Cassini came equipped with 12 separate instruments to analyze light waves, to figure out the
composition of dust, measure magnetic fields, and more.
After a year in orbit, Cassini let go of its traveling companion, the Huygens probe...
and sent it parachuting down to the surface of Titan.
Unexpected crosswinds buffeted Huygens' parachute, but the probe was able to snatch a wealth
of revealing information.
Its on-board radar showed a complex topography with ridges, rivers, and lakes, and even rocks
on the ground.
Titan's atmosphere is thick and cold. Like Earth, the air here is mostly nitrogen gas.
What's more, Titan's muddy, wet terrain is laced with organic compounds. On our planet,
it's water that cycles from ice to steam. On Titan, it's methane.
Methane gurgles up from the ground in liquid form and flows into lakes. It freezes in spots,
or floats off as gas in orange clouds. This image shows a flash of sunlight reflecting
off a sprawling 400,000 square kilometer lake known as Kraken Mare.
Scientists consider Titan to be a possible mirror of Earth in its early days with the
chemistry of life still in its earliest stages. That's why they are so eager to explore its
varied landscapes. So far, radar passes by Cassini have revealed expansive dune fields,
wetlands, hilly terrain, and the occasional crater.
Scientists mapped an area near Titan's south pole called Sikun Labyrinthus. The image shows
a network of canyons and cliffs sculpted by liquid methane.
Nearby, methane flows into a lake called Ontario Lacus. A little smaller than Lake Michigan,
it's bounded by bays, river deltas, beaches, and other familiar features.
And here, in a region called Sotra Facula. Cassini mapped what looks like a volcano.
Thousand meter high peaks tower over craters that are 1500 meters deep.
Are these features the result of an internal heat source, like the grooves that line Enceladus
and Europa? Or is Titan's story unique?
Cassini has been sending a stream of images and data across nearly a billion and a half
kilometers to Earth. It has been able to draw an impressive portrait of a ringed planet,
a turbulent gas giant.
Cassini's sensors captured a storm that raged across Saturn's southern hemisphere for months
- generating lightning strikes thousands of times more powerful than those on Earth. They
documented a giant cyclone spinning around at Saturn's south pole. Its eye is so big
you could drop the entire continental United States into it without touching the cloud
walls.
Cassini radioed back unprecedented new details of Saturn's most famous feature, its rings.
In the year 1610, the Italian astronomer Galileo Galilei discovered these broad, flat concentric
bands. But he didn't realize they were actually rings, nor had he any clue how intricate they
are, or where they came from.
Until recently, the reigning theory held that the rings were made up of debris from a small
moon destroyed in a collision. But they are now known to be 99.9% water ice. How it got
there seems to reach back again to Greek myth.
Fearing vengeance from the offspring of Uranus, Saturn began to savagely devour the young
titan offspring. The youngest, Zeus, escaped unharmed and later on would lead an insurrection
by the Olympian Gods.
A recent study suggests that Saturn's rings formed when the planet swallowed one of its
moons. In the process, its gravity stripped the moon of its icy surface, which remained
in orbit and spread out to form the rings. And from the rings, smaller, ice moons like
Enceladus took shape.
The Hubble Space Telescope showed that Saturn's rings stretch more than 240,000 kilometers
across. Yet in some places they are as little as 10 meters thick. If you were to shrink
Saturn down to the size of a basketball, the rings would be about 1/250th the thickness
of a human hair.
Around Saturn everything acts upon everything else. Moons and countless tiny moonlets pull
and tug on ring particles - shaping and twisting and clearing lanes, in a dance of twilight
moons. The medium-sized moon Mimas, orbiting close in to Saturn, has hollowed out a large
gap called the Cassini Division.
Some tiny moons ride within the rings. Prometheus and Pandora actually hold the F-Ring in line.
Other moons continually sculpt the rings, gently shepherding the granules with their
gravity.
And across the ring plane, spokes of different colored dust occasionally cling like strands
of hair on a cold dry day. Like the rings, Saturn's moons each tell a different story,
drawing us in for a closer look. There's dense little Dione, hard rock covered with ice,
pummeled on one side by asteroids that left a system of craggy cliff faces.
Hyperion: looks like a sponge. It's an oddly-shaped world that tumbles chaotically.
Iapetus: with one hemisphere brighter than snow and the other darker than tar. A strange
ridge, like the spine of a rhino, runs around its equator. It's the moons Titan, and Enceladus
that continue to steal the show.
Compared to many of its sister moons, Enceladus has fewer craters. Even the largest is relatively
small, only 35 kilometers across. Like Europa, its landscape is fractured and wrinkled, a
sign that it's constantly being reshaped by geologic activity.
Enceladus lies outside of Saturn's bright inner rings, in the wide and diffuse E Ring.
With Cassini's camera pointed at just the right angle to the sun, the E Ring lit up.
Enceladus is the bright object in the center of the frame. What is all this hazy material
made of? Could it be coming from the moon itself?
The Cassini science team suspected that they were onto something big. So they went to work:
plotting new orbital paths, building a new target list, and preparing to pay Enceladus
a visit.
They set the spacecraft on course for a series of close flybys of its south pole, down to
about 25 kilometers above its surface. This is what Cassini saw: plumes of vapor rising
out of the ice.
Scientists began to think of them as geysers, much like those in geologic hot spots on earth.
If the team could find out what these jets are made of, they might just have some clues
to what's going on below the surface.
Flying through the E Ring, Cassini sampled particles within it. It found crystals of
water, and within them, it detected the presence of a compound well known on Earth: salt. Salt
dissolved in water is evidence that friction from a rocky core, jostled by Saturn's gravity,
warms a reservoir of liquid water below the icy surface of Enceladus.
Another flythrough found water molecules that are negatively charged. On Earth, that's a
product of waves smashing a shoreline or water flowing over a waterfall. And there's even
more to get excited about. The plumes were found to contain nitrogen, likely given off
by ammonia heated to nearly 600 degrees Celsius.
Here's a diagram illustrating the possible set up. Powering the geysers is an underground
reservoir heated and pressurized by an internal heat source. Close in observations of Enceladus
show the disfiguring effect of all this activity on its south pole.
How amazing that even out here, about nine and a half times the distance between Earth
and the sun, this frozen little moon might just harbor the conditions needed to turn
chemistry into biology.
If Enceladus holds its secrets in, those of Titan are written on its surface. Scientists
fed data from Cassini's radar instrument into computer models designed to explore the relationship
between its surface and its interior. Their findings suggested that Titan's interior has
been steadily cooling. That has caused the moon to shrink, and its surface to wrinkle
into a pattern of ridges and valleys.
In addition, a recent NASA study found little evidence that an internal heat source is currently
shaping Titan's surface. The study found that most of its features can be explained by impacts,
or by this moon's surprisingly active weather.
For eight years, astronomers at the Gemini Telescope in Hawaii had been monitoring infrared
light streaming in from the Saturn system. For the first time, on Titan, they saw evidence
of a massive storm erupting over what's thought to be a vast equatorial desert. Cassini has
picked up clouds swirling above large methane lakes and seas on the North Pole.
And now along the equator, in the large arrow-shaped cloud formation to the left. Notice the dark
regions in the center of the moon. These are thought to be from methane rain drenching
the landscape. The clouds likely formed in the recent change of seasons as the sun began
to shine directly down on Titan's equator.
It may not be completely crazy to imagine primitive forms of life arising out of this
cold carbon-rich chemistry. One theory is that ultra-violet light from the sun could
zap nitrogen molecules in the atmosphere, creating a kind of organic smog that could
settle into volcanically active places along the surface, and develop from there.
Meanwhile, Cassini keeps adding to its historic collection of data and images. With Saturn
moving into its farthest point from the Sun, its rings lay flat on the plane of the solar
system.
The angle of the light allowed objects to cast long shadows across the rings, like this
little moon, never before seen. Or these moons that seem to have punched through the F Ring.
Cassini completed its primary mission back in 2008. Based on all its successes, it's
now set to keep padding its amazing record until at least the year 2017.
At this point, no one can say whether Saturn's moons have ignited the spark of life. They
have certainly ignited our imaginations.
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