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  • Some 900 million miles from the Sun... in the outer regions of our Solar System... orbiting

  • the planet Saturn....

  • ...lies a mysterious world.

  • Enceladus is enveloped in ice. Because nearly all of the sunlight that manages to hit its

  • surface is reflected back into space, it's one of the brightest objects in the solar

  • system.

  • At its equator, the temperature is -315 degrees Fahrenheit.

  • But, at the poles, the temperature is at least 15 degrees warmer... and as much as 65 degrees

  • warmer in grooves that stretch across the south like tiger stripes.

  • In 2005, the Cassini spacecraft spotted a complex plume of water vapor shooting out

  • into space from several locations near the south pole.

  • That may mean that Enceladus harbors a remarkable secret below its frigid surface:

  • A liquid ocean... and maybe... some forms of life.

  • This discovery was the culmination of a search that began over three decades ago. .

  • Back in 1979, the outer planets of the solar system lined up in such a way that mission

  • planners were able to dispatch the Voyager spacecraft to fly past each of them ...

  • The two Voyagers 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 towards interstellar space

  • - made discoveries about the planetary chemistry that make these gas giants appear to us as

  • gigantic works of abstract art.

  • The Voyagers disclosed new details about their magnetic fields, atmospheres, ring systems,

  • and even the nature of their inner cores.

  • Voyager turned up some surprising new mysteries too:

  • a huge dark spot - a storm in fact - on Neptune...

  • They found that Uranus is tipped 90 degrees to one side...

  • That Saturn is less dense than water; if you had a bathtub big enough, Saturn would float!

  • ...And that you'd need the mass of three Saturns to make just one Jupiter!

  • But what really knocked the scientists' socks off were the moons that orbit these gas giants.

  • All of them have been pummeled over the millennia by wayward asteroids and comets...

  • But a few appear to also be sculpted by forces below their icy surfaces.

  • Neptune's largest moon Triton has few craters. It's marked with circular depressions bounded

  • by rugged ridges that may mean the icy surface is collapsing.

  • There are also grooves and folds in the land that stretch for dozens of miles, a sign of

  • fracturing and deforming.

  • Triton has geysers too. But these are not spurts of water. On frigid Triton - so far

  • from the Sun - the liquid that spouts some five miles above the planet is nitrogen. No

  • one yet knows exactly what drives them.

  • Tiny Miranda... one of 27 known moons that orbit Uranus... wears a jumbled skin that's

  • been shaped and reshaped. Most likely, its outer crust is slipping and sliding on a molten

  • core.

  • The moon called Io - orbiting perilously close to giant Jupiter is literally turning itself

  • inside out! Rivers of sulfurous lava roll down from open craters that are constantly

  • erupting.

  • What was causing these tiny moons to generate so much energy from within? The answer may

  • well be here... on Jupiter's Europa - just slightly smaller than Earth's Moon.

  • Voyager saw no signs of volcanic activity, but -but instead documented a complex network

  • of criss-crossing grooves and ridges.

  • In the 1990s, the Galileo spacecraft was sent back to get a closer look at Europa and its

  • sister moons. .

  • 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?

  • Well, as it orbits around Jupiter in a nearly circular ellipse, the massive planet's gravity

  • constantly tugs at Europa's rocky core.

  • The friction of rock rubbing on rock causes that core to heat up.

  • That heat rises up through an ocean of liquid water... then cracks and spreads the icy surface

  • in a thousand different ways.

  • Callisto and Ganymede also show such features... suggesting they have - or perhaps once had

  • -liquid oceans below their surfaces too!

  • Crossing outward to the Saturn system, Voyager's images from the late 1970's showed that the

  • moon Enceladus had a similar surface...

  • The same was presumed of Saturn's by far largest moon, Titan... enshrouded in heavy clouds.

  • So when the Cassini spacecraft arrived in 2004 to scrutinize the kingdom of Saturn,

  • it came ready to answer a range of burning questions...

  • Can such moons really have liquid oceans beneath their surfaces... and do those oceans have

  • the ability to cook up and then support life?

  • After a year in orbit, Cassini let go of its traveling companion, the Huygens probe...

  • and sent it parachuting down towards the surface of Titan.

  • It's the largest moon in the solar system, larger in size - in fact - than the planet

  • Mercury... and massive enough to hold an atmosphere.

  • Unexpected crosswinds buffeted Huygens' parachute... but the probe was able to snatch a wealth

  • of revealing information.

  • During Huygens' descent, its on-board radar showed a complex topography... with ridges,

  • rivers, and lakes... and even individual rocks on the ground.

  • Titan's atmosphere is thick and cold, the thermometer hovering just above minus 300

  • Fahrenheit. Like Earth, the air here is mostly nitrogen gas.

  • And like Earth in its very early years, Titan's muddy, wet terrain is a stew of organic compounds.

  • But while the liquid that cycles from ice to steam here on Earth is water, Titan's ever-changing

  • fluid is methane.

  • On Titan, methane gurgles up from the ground in liquid form and flows into lakes...freezes

  • into hydrocarbon icebergs ... and puffs off as gas in orange clouds and pinkish fog.

  • It may not be completely crazy to imagine a primitive form of life arising out of this

  • chaotic, cold carbon-rich chemistry.

  • Once again, the evidence points toward a moon with a rocky core, heated by the tidal gravity

  • of a massive planet... and layers of liquid and ice near the surface that burst forth

  • in cold volcanoes and icy geysers.

  • All this activity has left a profound imprint upon the landscape.

  • Repeated radar passes by Cassini reveal expansive dune fields of methane mud... wetlands...

  • structures that look like volcanic vents... and the occasional crater.

  • And there's at least one continent... now known as Xanadu.

  • To reach us, these images had to travel nearly a billion miles. At the end of the journey...

  • ...they were picked up by the Deep Space Network... think of the DSN as the early prototype for

  • an interplanetary internet.

  • Within just a few years, Cassini had transmitted one of the most impressive photographic records

  • of all time...

  • Of a fascinating ringed planet... a turbulent gas giant.

  • It saw a storm that raged across Saturn's southern hemisphere for months - generating

  • lightning strikes thousands of times more powerful than those on Earth.

  • Cassini witnessed a giant cyclone larger than Earth ... spinning around at Saturn's south

  • pole, - a huge hurricane with a eye so big you could drop the entire continental United

  • States into it without touching the cloud walls...

  • And Cassini radioed back unprecedented new details of Saturn's most famous feature...

  • its rings.

  • The Italian astronomer Galileo Galilei discovered these broad, flat concentric bands of ice

  • and dust, four hundred years ago, in 1610.

  • But he didn't realize they were actually rings... And he couldn't have had any clue how intricate

  • they truly are.

  • Even today, we don't yet know just where they came from.

  • Nor how old - or young - they actually are.

  • The chunks and particles range from boulders the size of houses down to tiny granules,

  • finer than baby powder.

  • They may contain the remains of two moons that collided and shattered. But something

  • may also be continually adding material.

  • The rings stretch more than 150,000 miles across - but, in any one place, they are less

  • than a mile from top to bottom.

  • Saturn is almost like a solar system unto itself:

  • Everything acts upon everything else.

  • The planet's moons - more than 60 have been named so far - and countless small moonlets

  • pull and tug on the Ring particles - shaping and twisting and clearing lanes - making them

  • dance to the combined music of their masses.

  • The medium-sized moon Mimas - orbiting close in to Saturn - has hollowed out a large gap

  • called the Cassini Division.

  • Some tiny moons actually ride within the rings.

  • Other moons continually sculpt the rings... gently shepharding 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, each of Saturn's sixty-plus moons tells a different story... enticing

  • us in for a closer look.

  • There's dense little Dione - hard rock covered by ice - pummeled on one side by asteroids...

  • causing craggy cliffs to spring up on the other.

  • Hyperion... looks like a sponge... an odd-shaped world pocked with odd-shaped craters...it

  • tumbles around so chaotically, it's really tough to predict its exact orbit.

  • Iapetus... looks like a cosmic Yin-Yang symbol - with one hemisphere brighter than snow

  • and the other darker than tar.

  • A strange ridge system of mountains - like the spine of a rhino - runs around its equator,

  • in places more than 12 miles tall.

  • But it's one of Saturn's smallest moons, Enceladus, that really steals the show. That's Saturn's

  • second largest moon, Rhea, passing behind it.

  • Compared to many of its sister moons, Enceladus has fewer craters. The largest is relatively

  • small, only 22 miles across.

  • Like Europa... Enceladus has a finely textured landscape - with at least five different kinds

  • of terrain... a sign that it's frequently remade by geologic activity.

  • It's fractured and wrinkled surface is most obvious around its south pole. But there is

  • something else going on down here among these ridges.

  • 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, Saturn's E Ring lit

  • up. Enceladus is the bright object in the center of the frame.

  • What is all this hazy stuff made of? Could it be coming from the moon itself?

  • The plan had been to let Cassini tour the Saturn system until the middle of 2008.

  • But the mission was clearly too successful - and too intriguing - to let it end... And

  • 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... preparing

  • to have another go at Enceladus.

  • They set the spacecraft on course for a set of close flybys of its south pole... down

  • to as close as 25 miles 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 major

  • clues to what's going on deep inside Enceladus.

  • And if there truly are liquid oceans, what's in them?

  • To get some answers, Cassini sampled particles that make up the E Ring. It found crystals

  • of water, and within them, it detected the presence of a simple compound well known on

  • Earth... salt.

  • Salt dissolved in water tastes like evidence to a scientist... evidence that friction from

  • a rocky core jostled by Saturn's gravity warms a reservoir of liquid water below the icy

  • surface of Enceladus... and sends it bursting through!

  • Now, to be accurate, Enceladus' jets could possibly form without liquid water.

  • And even if they begin as liquid, they may boil up out of a few water-filled caverns,

  • rather than a huge ocean.

  • But if this tiny moon's waterspouts do originate in a sizeable subterranean sea, the possibilities

  • for life throughout the galaxy may be that much greater.

  • How amazing that, even out here... almost 900 million miles from the Sun...

  • ...far beyond the orbit of Earth...

  • and far outside what many call the "Goldilocks Zone" ... this distant little moon might just

  • harbor the conditions needed for chemistry to become biology.

  • So we continue to mine the kingdom of Saturn for knowledge and images.

  • And not just from space. For eight years, astronomers at the Gemini Telescope in Hawaii

  • had been monitoring...

  • ...The infrared light streaming in from the Saturn system.

  • For the first time, on Titan, they saw evidence of a massive and unlikely storm erupting over

  • what's thought to be a vast equatorial desert. If nothing else, it shows how much we still

  • have to learn about these mystery worlds.

  • Meanwhile, Cassini keeps adding to its unprecedented photographic collection.

  • With Saturn now at its farthest point from the Sun, the angle of the light allows objects

  • to cast long shadows across the rings... like this little moon, never before seen.

  • Or these ones that seem to have punched through the F Ring.

  • Who knows whether Saturn's moons have ignited the spark of life.

  • Since Galileo, they have certainly ignited our imaginations.

  • 6

Some 900 million miles from the Sun... in the outer regions of our Solar System... orbiting

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土星的神祕月亮(V1 (Saturn's Mysterious Moons (V1))

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    Wonderful 發佈於 2021 年 01 月 14 日
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