杏吧原创

TITAN鈥橲 methane lakes. Icy Enceladus spouting geysers of hot water. Sponge-like Hyperion. Ravioli-shaped Pan and Atlas. Iapetus with its equatorial ridge battered by ancient craters. Close-ups of those iconic rings engirdling the gas-giant planet itself, and gigantic hurricanes around its poles.

The Cassini probe, launched in 1997, has orbited the Saturn system for 13 years. What it has revealed is astounding 鈥� and challenges our understanding of planets and their satellites everywhere, says planetary scientist of the Southwest Research Institute in Boulder, Colorado. 鈥淚f everything comes from the same processes, should we get so much diversity?鈥� he asks. 鈥淚t seems like every moon has a part of the story to tell.鈥�

Right up to this weekend鈥檚 鈥淕rand Finale鈥�, Cassini has been collecting more data and snapping fresh images with a series of low swoops over Saturn鈥檚 moons and through its rings. But what it has already seen constitutes, arguably, the richest haul of discoveries from any mission yet mounted to another planet. 鈥淚t鈥檚 going to be tough to say goodbye to Cassini,鈥� says Salmon.

The rings

After a six-and-a-half-year journey, Cassini entered orbit around Saturn on 1 July 2004 鈥� and immediately encountered the planet鈥檚 signature feature, its rings. Pictures collected by the Voyager probes when they flew past Saturn in 1980 and 1981 suggested the planet was girdled by about 10,000 rings, each a cloud of particles tightly confined to a narrow orbit. 鈥淣ow the number is in the millions,鈥� says , principal investigator for Cassini鈥檚 ultraviolet imaging spectrograph.

The rings are also complex: Cassini鈥檚 images have revealed clumps, holes, gaps and other structures. Some wave-like features are due to gravitational interactions with the moons embedded in the rings, but the origin of others is unclear.

The rings probably formed initially when a large moon came too close to Saturn and was ripped apart by gravitational forces. Esposito thinks this was early in the solar system鈥檚 history, and that the rings have gradually spread since then, perhaps forming moons in the process. Others think the rings go through cycles: moons collide forming new rings that coalesce into new moons which eventually collide again, with the current rings as little as 100 million years old.

Old rings are expected to be much more massive than young ones. In its past final weeks, Cassini has been repeatedly passing between the rings and the planet, accurately measuring the rings鈥� mass to perhaps resolve the question.

Enceladus

Before Cassini, researchers had expected this icy, 500-kilometre-diameter moon to be frozen solid. But on an early fly-by in February 2005, the spacecraft鈥檚 magnetometer 鈥渟ensed something unusual going on with its magnetic field鈥�, says Cassini project scientist of the Jet Propulsion Laboratory (JPL) in Pasadena, California.

A later pass showed that the south pole was much warmer than expected, and was spouting geysers of salty water into space. Enceladus circles Saturn twice for every orbit of the larger moon Dione, inducing a gravitational interaction that melts ice inside both moons. The process squeezes Enceladus, ejecting jets of water from large fracture zones near its south pole (pictured below). Cassini measured the composition of these jets, detecting raw materials for life including salt, water, carbon dioxide, methane, other organic molecules and, most recently, hydrogen, an ideal energy source for life.

Silica found in the jets can be produced only in water close to boiling point, indicating that hydrothermal vents are also present in the subsurface ocean 鈥� making the icy moon a hot target in the search for life.

Hyperion

Trapped in a gravitational resonance with Titan, Hyperion (right) tumbles chaotically in orbit. Subject of an early fly-by in September 2005, its light, porous-looking surface resembles a battered sponge, but no one quite knows why. One possibility is that it is a fragment of a larger object shattered in a past collision. The dark zones look lower than the light-coloured ridges, perhaps because they absorbed more sunlight, causing ices below them to evaporate and the dark layer to sink down.

Iapetus

At first glance, an equatorial ridge girdling Iapetus looks like a moulding mark on a factory-fresh rubber ball. A Cassini fly-by in 2007 revealed that the ridge is as heavily cratered as the rest of the 1500-kilometre-diameter moon鈥檚 surface, so it must have formed long ago. Iapetus鈥檚 surface is also oddly two-toned, with a darker leading edge. This is caused by gravitational forces that lock the moon into position around Saturn, causing its front face to sweep up dust.

Titan

When Voyager 1 passed Titan in 1980, it couldn鈥檛 see the surface of Saturn鈥檚 largest moon: solar ultraviolet radiation drives reactions between nitrogen and methane molecules in its atmosphere that yield a thick, orange-brown gunk. The purpose of Cassini鈥檚 Huygens lander, built by the European Space Agency, was to find out what lay beneath. Voyager had discovered that the temperature and pressure on Titan鈥檚 surface would allow liquid methane. Huygens, released on 14 January 2005, was made to withstand a wet or dry landing.

Photos taken during the lander鈥檚 150-minute descent showed networks of branching streams possibly carved by liquid methane. But the touchdown was hard, on a cobblestone-strewn flood plain near Titan鈥檚 equator like 鈥渟omething you might see in Death Valley鈥�, says of Cornell University in Ithaca, New York. But at around -180掳C it was much colder, with a surface covered in plastic shavings and foam beads.

Huygens transmitted data from the surface for 72 minutes until its battery failed. In the years since, Cassini has probed Titan鈥檚 atmosphere and mapped its surface on successive fly-bys, confirming the presence of liquid methane. In radar observations a few weeks apart it found evidence that methane showers had soaked the soil, then evaporated 鈥� the first proof of precipitation beyond Earth.

Titan鈥檚 landscape is eerily calm, with methane seas and lakes that are 鈥渇antastically flat鈥�, says Hayes. They are more transparent than water lakes: a radar echo from one was reflected from its bottom, 160 metres down. Bright 鈥渕agic islands鈥�, which appear briefly in the dark lakes before disappearing, are thought to be nitrogen bubbling out of solution.

Perhaps oddest of all, Titan has two ocean levels. Beneath the hydrocarbon seas on the surface, under a shell of water ice, lies salty liquid water. This hidden ocean is, says Hayes, 鈥渢he most accessible laboratory for prebiotic chemistry in the solar system鈥� 鈥� a potential habitat for life.

Mimas

At 396 kilometres in diameter, Mimas is the smallest known rounded body in the solar system. Seen closest by Cassini in February 2010, it鈥檚 not completely round, however: one side is dominated by the 130-kilometre Herschel crater with walls 5 kilometres high. The giant pit makes Mimas, right, look eerily like the planet-destroying Death Star in the 鈥淪tar Wars鈥� movies. It is, however, extremely vulnerable: made principally of water ice, cracks on its opposite side show that a past impact came close to shattering it.

Hurricanes on Saturn

Hurricanes on Earth tend to go towards the poles, but those on Saturn, pictured right, are fixed there. They have central eyes and eye-wall clouds like terrestrial hurricanes, and spin in the same way, but at 4000 kilometres across, three of them side by side would span Earth鈥檚 diameter. Terrestrial hurricanes are powered by heat released from warm ocean surfaces. There鈥檚 nothing like that on Saturn, so what powers its storms remains a mystery.

Pan

Fat, round, ravioli-shaped Pan orbits in a gap in Saturn鈥檚 A-ring, the outermost of the large, bright rings. Its central core is icy, but ring particles accumulate on a strip around its circumference, fattening Pan out to a 35-kilometre diameter. Revealed in great detail in images taken in March 2017, this belt is cratered, with signs of a small landslide pulled downhill by the moon鈥檚 gravity. Atlas, another moon in the A-ring, is similar, but its skirt shows no craters and looks fluffier. The moons鈥� growth may be limited by a gravitational tug of war between them and Saturn: if ring particles pile too high on Pan鈥檚 equator, the planet鈥檚 gravity tugs them off again.

Why the grand finale?

Almost 20 years after leaving Earth, Cassini鈥檚 plutonium-powered generators are running out of fuel needed to adjust its course. Left to drift, it might collide with one of Saturn鈥檚 moons, perhaps contaminating an environment that might contain or provide the conditions for life. To avoid that possibility, it will be steered into the atmosphere of Saturn itself, burning up like a meteor as it becomes part of the planet whose environment it has spent so long exploring.

This article appeared in print under the headline 鈥淲hole new worlds鈥�