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  • 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.

  • 6

Launched three years before the new century... a spacecraft wound its way through the empty

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土星の不思議な月 (Saturn's Mysterious Moons)

  • 62 17
    Wonderful に公開 2021 年 01 月 14 日
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