Extract from ABC News
Updated
Photo:
Cassini has been observing Saturn (pictured here in February 2016) for almost 20 years. (Supplied: NASA/JPL/Space Science Institute)
For more than a decade, the Cassini spacecraft has
been exploring the system of Saturn, some 1.2 billion kilometres from
Earth. As the first phase of its final descent begins, we look at what
it has discovered about the ringed planet and its moons, and what
happens next.
Launched in October 1997, the seven-metre-long
spacecraft has spent its time in the outer solar system measuring and
observing the mysterious gas giant Saturn, its rings, and the moons that
surround it.
On December 1 at 10:30am (AEDT), it will begin a series of orbits that will bring it within 10,000 kilometres of the outer edge of Saturn's main rings to observe them and a moon close by.
That is the prelude to Cassini's 'grand finale': a dramatic orbit beginning in April next year that takes the spacecraft high above Saturn, before plunging it between the planet and its innermost ring 22 times.
After that, it will kamikaze dive into Saturn's upper atmosphere on September 15, 2017, collecting data before eventually breaking apart and ending the 20-year mission.
Deputy project scientist on the Cassini mission, Scott Edgington, says the discoveries made by Cassini over the 12 years it has been in Saturn's system are of huge significance to the future of space exploration.
"This is data that PhD students will be looking at for decades to come. They're going to be rifling through the data, who knows what discoveries they're going to make over the coming years," Dr Edgington said."If you look at the direction NASA was headed when it came to exploring habitable worlds, over the last decade that's changed due to the discoveries that Cassini and [the Huygens probe] have made."
Among those discoveries are liquid methane oceans on the moon of Titan, a salty ocean beneath the crust of another moon, Enceladus, and new details about the bizarre weather systems of Saturn itself.
Titan's rain, lakes, and seas
Photo:
A near-infrared, colour mosaic from Cassini shows the sun glinting off of Titan's north polar seas. (Supplied: NASA/JPL-Caltech/Univ. Arizona/Univ. Idaho)
We know so much about Saturn's largest moon Titan because of the work of Huygens, the atmospheric entry probe carried by Cassini until the two separated in 2004.
As it travelled through Titan's atmosphere, Huygens captured the first glimpses of the moon's surface, revealing what appeared to be oceans and rivers below.
"Those pictures were just amazing," Dr Edgington said."It was our first glimpse of the surface of Titan and in those images you could see what looked like lakebeds or oceans, you could see rivers and streams flowing into those areas that looked like they once contained some sort of liquid."
Later, imaging with radar would reveal that the geologic structures on Titan were indeed channels and seas, some of which contain liquid methane. The presence of these structures revealed Titan to be similar to an early Earth, with erosion, dunes, and dry riverbeds all present on the distant moon.
Dr Edgington said the discovery of these geologic structures and the liquid methane on the surface of the moon could allow us to better understand the early processes that formed the Earth.
"By looking at how this liquid methane carves through that ice you get insights … Is the physics the same [as Earth]? How are these properties of ice there different or similar to rock here?"
"It basically allows you to ask the question, how are the geologic processes different, and if you find similarities you can refine your theories as to why something might form here on Earth or Mars or Titan."
Icy plumes on the surface of Enceladus and an ocean below
Photo:
Plumes of water ice spraying out of one of Saturn's moons, Enceladus. (Supplied: NASA/JPL-Caltech/Space Science Institute)
Enceladus is Saturn's sixth-largest moon, measuring about 500 kilometres across. Its smooth surface — made up mostly of water ice — makes it the most reflective body in the solar system.
When Cassini flew past Enceladus for the first time in 2005, scientists noticed that the gravitational field of Saturn nearby was not quite as they had predicted — suggesting the tiny moon may have been influencing the gravity of the planet in an unexpected way.
Closer inspection by Cassini's cameras spotted the culprit behind this unusual gravity — plumes of water ice spouting from cracks in the surface of the south pole of Enceladus that were pushing on Saturn's gravitational field.
The jets also contain water, organic chemicals, and particles of silica. Dr Edgington said silica could only be found in the icy plumes if the rocky interior of the moon was mixing with boiling water, creating a chemical reaction that would produce the compound.
Later geophysical measurements of Enceladus' internal structure done by Cassini confirmed what the presence of icy jets suggested: a global salty ocean beneath the ice shell of the moon.
"If you think of the Earth here, at the bottom of the oceans, there's these vents — very hot and so deep they don't receive sunlight at all — and yet there's all this activity going on down there, biologic activity," Dr Edgington said.
"That makes you wonder what's going on inside of this tiny little moon. It's definitely changed our understanding of where life might be found."
The active, dynamic rings of Saturn
Photo:
Cassini has been able to image Saturn's rings in unprecedented detail. (Supplied: NASA/JPL-Caltech/Space Science Institute)
Scientists have been fascinated by the famous rings that encircle Saturn for centuries. Cassini has now captured the makeup of these rings in unprecedented detail — and even revealed a new, never-before-seen ring.
Saturn's rings are made up of countless particles, some as small as a grain of sand and others as large as boulders, that together form the distinctive 'disk' around the planet. The major rings are labelled A to G in order of their discovery.
The ring closest to Saturn is the D ring, while the outermost of the planet's major rings is the E ring. And while they measure hundreds of thousands of kilometres across, the rings are no more than 30 metres thick in most parts.
In 2006, Cassini detected a faint new ring between the F and E rings, likely made of the dust produced when meteoroids hit Saturn's moons.
It also discovered that Saturn's rings are not static. The gravitational forces of the moons orbiting Saturn mean the particles that make them up are pushed and pulled, colliding and bouncing off one another in the process.
Dr Edgington explains that as the particles in the rings move around, some clump together to form objects. Indeed, Cassini has spotted what may be a moon forming — a clump of material at the edge of Saturn's A-ring that has not dissipated since it was first observed and that scientists have christened 'Peggy'.
Given a number of Saturn's moons are made predominantly of ice, Cassini scientists believe their origins may lie in the particle collisions of the rings.
"Someday this clump of material might stick together and form its own moon and be ejected from the rings," Dr Edgington said.
"If you imagine the ring as a mini solar system that's forming, you could imagine that planets might form in the same way."From next month, Cassini will fly close by the rings to make further observations about the way the particles within them interact and are affected by Saturn's moons.
A hexagon at Saturn's north pole
Measuring 30,000 kilometres across, the peculiar hexagon at Saturn's north pole is unique in the solar system. First spotted by Voyager in the 1980s, the weather pattern has persisted for decades and continues to captivate the scientists on the Cassini mission.
"We're still trying to figure out why it's six-sided, why isn't it seven, or eight, or 20-sided," Dr Edgington said.
Cassini has captured high-resolution images — and even video — of the hexagonal jet stream. A jet stream is made up of flowing air currents produced when solar radiation heats the atmosphere. Jet streams are also found on Earth — but usually form wavy, meandering lines, not straight ones.
"If there was no land or changes in temperature on the ocean our jet streams would look like geometric figures. We're seeing this jet stream in its purest form, land and sea can't interrupt its six-sidedness," he said.
At the centre of the hexagon is a swirling vortex. The eye of the vortex is about 2,000 kilometres across — 50 times larger than the average hurricane eye on Earth — with clouds within it travelling at speeds of more than 500 kilometres per hour.
Scientists have been studying the vortex in the hope of gaining greater insight into how hurricanes on Earth are generated.
The grand finale ahead
In the months before its final orbit, Cassini will perform a series of manoeuvres taking it just outside the rings of Saturn to get a better view of the activity within them. It will also take thermal measurements of Enceladus to try to determine the heat of the interior of the moon and the properties of its ocean.
The probe will then thread the needle between Saturn and its innermost ring, conducting 22 loops along that path to measure the gravity of Saturn.
Once scientists work out Saturn's gravitational field and its mass, they can use that information to determine the mass of its rings too. Dr Edgington said the more massive the rings, the older they are likely to be, allowing scientists to place the age of Saturn's ring system into the millions or billions of years old.
"Getting that mass will help us refine our theory of how the rings formed at Saturn," he said.
But a dwindling fuel supply means the mission team has had to devise a method of safely disposing of the craft in a way that does not interfere with future missions in the system.
Because scientists believe there is a possibility of life existing on Enceladus, Titan, and other moons in the Saturn system, they are going to drop it into Saturn's atmosphere, destroying the probe in the process.
Photo:
An artist's concept of the final orbits of Cassini as it
repeatedly dives between the innermost ring of the planet and Saturn
itself. (Supplied: NASA Jet Propulsion Laboratory)
"We really don't want to contaminate [these moons] with anything we might have brought along. So the safest thing to do is to dump it into Saturn's atmosphere," Dr Edgington said.Because there is no oxygen in Saturn's atmosphere, Cassini will not burn up as it comes down. Rather, the heat of the entry will split the craft apart and melt its components.
The probe's antenna will send data about the composition of Saturn's atmosphere and temperature back to Earth right up until the moment it fails.
"That'll be our first in-situ measurement of Saturn's atmosphere," Dr Edgington said.
He said Cassini had reshaped what scientists thought about where life might exist in the solar system, something that would influence where astronomers decided to visit in future.
"It used to be that we thought you had to have the right planet, at the right location from the Sun, and if you didn't find that there's no chance of life. But now there's this opportunity that we have to visit these icy worlds out there that may have oceans," he said.
"That's going to be the lasting legacy of Cassini, that it's changed our understanding of where you might find life."
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