Cassini’s discoveries about Saturn’s rings are undeniably sensational, but I’d venture to suggest that its findings concerning the planet itself and its extensive systems of moons are even more amazing. During the spacecraft’s 13-year sojourn, it completed no fewer than 294 orbits of Saturn in four distinct phases. First came the Prime Mission (2004–2008), which gave scientists their first up-close-and-personal view of the Saturnian system. That was extended to become the Equinox Mission (2008–2010) covering Saturn’s equinox of 11 August 2009, when the Sun illuminated the rings exactly edge-on. (What does edge-on illumination do? It throws any ‘vertical’ structures in the rings into relief by virtue of the shadows they cast, revealing, for example, graceful waves in the ring system produced by the gravity of small moons orbiting within it.)
And, with the spacecraft still operating flawlessly, the project was extended again to cover the northern summer solstice on 25 May 2017. This became the Solstice Mission (2010–2017), during which Cassini gained spectacular views of the northern polar regions of the planet and some of its moons bathed in summer sunlight. Finally, with Cassini running out of fuel for orbital manoeuvring, caution was thrown to the winds with the 22 Grand Finale Mission orbits. For five months, we held our collective breath as the spacecraft repeatedly passed – surprisingly unscathed – between the planet and its rings, ending its epic mission on 15 September 2017.
The Cassini mission’s abundant discoveries were down to an army of scientists taking the raw data and turning it into new knowledge. They number in the hundreds, and are based at universities and scientific institutions all over the world, but I’ll mention two of the key figures, without whom the project might not have been anywhere near as successful. First was Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. In other words, the Head Honcho. Linda cut her teeth in planetary science with the Voyager missions, which she joined when the two spacecraft were launched to the outer Solar System in 1977.
I had the honour of meeting Linda in Pasadena when I was visiting the United States for the Great American Total Solar Eclipse in August 2017, less than a month before Cassini’s final dive into Saturn’s atmosphere. After we’d chatted about the science, I asked her whether she’d be sad at the spacecraft’s demise, having spent much of her career preparing for the mission, cheering it on during its seven-year voyage to Saturn, and supervising its science program. ‘No,’ she replied. ‘I’m already focused on what comes next.’ But when I watched the live broadcast from JPL at the mission’s end, along with millions of others around the world, I couldn’t miss the handkerchief she had at the ready. ‘It’s like losing an old friend,’ she told the media afterwards, and I’m not surprised. I felt like that, too, and I was only an enthusiastic bystander.
The other person I’d like to single out is Carolyn Porco, who led Cassini’s Imaging Science Team throughout the mission’s observational phase. Carolyn is much more than an expert scientist, however. She also has the eye of an artist, and many of the half-million or so images obtained by the spacecraft are breathtaking in their beauty. And that’s not just because of the extraordinary subject matter, but because of their composition, detail, colour, lighting and object juxtaposition – all the ingredients of an awesome picture. Of course, the images are also scientifically valuable, greatly enhancing our understanding of the Saturnian system.
Saturn’s atmosphere, for example, beguiles planetary scientists with its complex structure. The planet is a ‘gas giant’, a world with no detectable solid surface, shrouded in dense clouds. Unlike Jupiter, whose cloud belts are visible from Earth even in small telescopes, Saturn has rather subtle markings, because its ‘weather’ occurs at lower atmospheric levels due to the colder temperature at its greater distance from the Sun.
However, a few eagle-eyed amateur astronomers equipped with state-of-the-art electronic detectors on their hobby telescopes were able to pinpoint storms in Saturn’s cloud belts as they developed, alerting Cassini mission scientists to unusual Saturnian meteorology for immediate follow-up. Probably the best known is Trevor Barry of Broken Hill in outback New South Wales, a retired miner I had the good fortune to get to know almost 20 years ago. Inspired by the stars, Trevor embarked on an astronomy degree at Swinburne University after his retirement, and wound up with the university’s Award for Excellence as the top graduate of his year. He has an enviable track record of Saturnian storm-spotting, a talent that quickly convinced Cassini scientists of the value of working with this unassuming and down-to-Earth astronomer. One particularly active mid-latitude storm turned out to be a record-breaker, and Trevor’s scientific rags-to-riches story led him not only to collaborate with Carolyn Porco and her team in Pasadena and visit the high-altitude observatories in Hawaii, but also to star in his own feature segment on Australian national television.
While Saturn’s northern polar hexagon looks almost artificial, this diagram shows how it is formed by a six-fold standing-wave pattern in the planet’s polar jet stream. Atmospheric vortices near the ‘points’ of the hexagon keep it stable.
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As Saturn moved on from its equinox and springtime sunlight began to illuminate the planet’s arctic region, Cassini’s cameras revealed a ferocious hurricane that rages around its north pole. With wind speeds around 500 kilometres per hour, and an ‘eye’ that spans 2000 kilometres, it’s truly a giant among storms. But it sits in the exact centre of something more extraordinary still – a belt of clouds that is perfectly hexagonal in shape.
The six straight sides of this strange geometric pattern are each bigger than Earth. To be honest, in pictures it looks more like something you’d take a spanner to than a natural phenomenon, but it’s now understood to be due to the planet’s polar jet stream. Like Earth’s own jet streams, it meanders from side to side as it circulates, but unlike ours, it is not disturbed by continents and oceans underneath. Thus, it has settled into a stable wave pattern with six ‘peaks’ and ‘troughs’ around its circumference – a circular standing wave forming a perfect hexagon.
Despite the consistent shape, it does display changes in its colouring. Images taken in November 2012, early in the Saturnian northern spring, showed that the interior of the hexagon is bluish in hue, and quite dark. Four years later, the blue had turned to a rich golden colour, similar to the rest of the planet. Scientists think this gold tint comes about because of sunlight-induced chemical reactions in Saturn’s atmosphere, producing more suspended particles (aerosols) and leading to a greater level of haze.
IF STORMY WEATHER IN SATURN’S ATMOSPHERE IS UNEXpected, one doesn’t have to look far away to discover even more bizarre conditions. Many of the planet’s moons are strange worlds, but its largest satellite, Titan, is by far the strangest. At 5150 kilometres in diameter, it is the second biggest moon in the Solar System after Jupiter’s Ganymede – larger than the planet Mercury, and half as big again as our own Moon. It was discovered on 25 March 1655 by the same chap who figured out that Saturn had rings around it – the great Dutch astronomer and mathematician, Christiaan Huygens.
Titan takes 15 days and 22 hours to travel around Saturn, and the same length of time to rotate on its axis. Thus, like our own Moon, it always keeps the same face towards its parent planet – but there the similarity ends. Titan is the only moon in the Solar System to have a thick atmosphere, which stabilises its surface temperature at around –180 °C. And, whereas our Moon’s surface is solid rock overlaid with thin soil, Titan’s surface is rock-hard water-ice over which erosion processes have created a ‘sand’ of ice crystals and solidified hydrocarbons. This material forms long, wind-blown dunes in Titan’s equatorial regions.
There’s considerable evidence that Titan’s icy surface forms a shell that floats above a global ocean of liquid water and ammonia, kept warm by nuclear processes in Titan’s rocky core. We know the ice-shell rotates independently of the core, because the longitudes of geographical features on its surface display a small backwards and forwards motion as Titan orbits Saturn. And, as if that wasn’t weird enough, it’s thought that Titan has a number of freezing volcanoes, spewing out a magma composed of slushy water and ammonia. Only one has been confirmed, however.
Most city-dwellers in sunny climates are familiar with that orange haze that sometimes develops in the atmosphere on windless summer days. My home town of Sydney is famous for it, sitting as it does in a basin between mountains and ocean. The haze is a photochemical smog, caused by the action of the Sun’s ultra-violet rays on hydrocarbons primarily from vehicle exhausts. There are no vehicles on Titan (apart from one now-defunct robotic lander), but its atmosphere has a similar composition – mostly nitrogen, but laced with a brew of methane and other hydrocarbons that cause the opaque orange haze. Thus it’s difficult to map the surface of Titan, even from space.
Despite the hazy atmosphere, we know that Titan has a weather cycle of evaporation and rainfall, similar to that on Earth. However, the moisture in its atmosphere is not water vapour (which would be frozen), but a mix of hydrocarbons that are best thought of as liquid natural gas – ethane, methane and other compounds. Indeed, clouds of this ethane–methane mix usually cover a small percentage of Titan’s surface, and when conditions are right, rain falls from them.
IT WAS DATA FROM THE FLY-BYS OF THE TWO NASA VOYAGER probes in the early 1980s that suggested the possibility of hydrocarbon seas on Titan. By the mid-1990s, American astronomer Carl Sagan and others had suggested there might be ocean-sized bodies of liquid methane on the surface, based on ground-based radar data. Once Cassini arrived in orbit around Saturn in 2004, the hope was that large bodies of liquid would be very quickly detected. The spacecraft carried a small lander called Huygens, and when that touched down on Titan’s surface on 14 January 2005, some expected it to splash down in an ocean. It didn’t, but images sent back during its parachute descent revealed drainage channels leading to what could be a shoreline. Tantalising stuff.
By 2007, however, scientists believed they had definitive evidence of lakes filled with methane, which came from smog-penetrating radar aboard the Cassini orbiter. These lakes were mostly near Titan’s north and south poles, and their existence has now been confirmed beyond doubt by radar and infrared mapping. They pool in basins in the ‘bedrock’ of ice on Titan’s surface, and are the only stable bodies of liquid known anywhere in the Universe, other than those on Earth. The seas and lakes dominate in Titan’s northern arctic, although there are a few in the south. They are large – comparable in area with North America’s Great Lakes in the case of the three biggest, which are designated as maria, or seas. Titan’s largest sea, Kraken Mare, is about three times larger than Lake Michigan-Huron, which, with a surface area of 117 300 square kilometres, is the biggest freshwater lake on our own planet.
Some 30 smaller lakes, ranging from a few kilometres in length up to a couple of hundred, have also been identified. All these polar seas and lakes appear to be fed by methane rainfall (via river-like features), but there are a few lakes in Titan’s equatorial region that are probably fed by springs from a methane and ethane ‘water table’ in places where the ice bedrock is porous.
The radar equipment carried aboard Cassini is also capable of measuring the depth of Titan’s lakes and seas. Average depths vary from two or three metres for the smallest lakes to tens of metres for the seas, with a maximum depth of more than 200 metres (the limit of measurement) for Ligeia Mare, Titan’s second biggest sea. It’s also possible to use radar to detect the average wave height on the lakes and seas, and the measurements that have been made show very small waves – around a few millimetres in height. This suggests either that surface winds are very low, or the liquid in the lakes is oily – or perhaps both.
Although we now know a lot about Titan’s lakes and seas, many tantalising questions remain. One concerns temporary surface features that have been observed in the three large seas – Kraken Mare, Ligeia Mare and Punga Mare. They look like bright patches that seem to come and go. But bearing in mind that these features are detected by radar reflection (where a bright signal indicates a rough surface), some scientists have attributed them to surface ripples on the seas, whipped up by light winds. An alternative hypothesis is that they are methane ‘icebergs’, which form on or near the surface, and then sink from view as the conditions change.
Also hypothesised is the prospect that cyclones occur over the three large seas, with some predicting that Titan’s summer weather could produce the necessary conditions. Perhaps Cassini’s demise came too early in the summer, for none were actually observed. Similarly tumultuous is the ‘Throat of Kraken’, a narrow neck of liquid in Kraken Mare that is expected to generate strong currents, and perhaps even whirlpools, at certain seasons of Titan’s 29-year journey around the Sun.
TITAN IS, INDEED, A STRANGE WORLD, BUT IT MAY HOLD even more dramatic secrets. With suspicions of a rich organic (carbon-containing) chemistry on the surface borne out by observations already made, some scientists believe this frigid place is an analogue for the early Earth, with an atmosphere similar to that of our own planet before life evolved. Others go further, suggesting that there could already be life-forms thriving in the hydrocarbon lakes. They would be quite different from the water-based life we see on our own planet, using liquid methane as their working fluid, breathing hydrogen and feeding on acetylene. Tantalisingly, both these chemicals are depleted at low levels in Titan’s atmosphere.
This is by no means evidence for life on Titan – there are abiotic processes that could equally well produce the same effect. But it is a hint that there may just be life in the Solar System so radically different from life on Earth that it could only have formed independently. And, should such a ‘second Genesis’ be proved correct, it would suggest that life might well be widespread throughout the Universe.
With that intriguing thought in mind, a number of spacecraft have been proposed to further explore Titan, with particular interest in the seas and lakes. They range from a balloon-borne robot floating in Titan’s atmosphere to a robotic submarine to explore the seas. To date, just one has been funded – NASA’s Dragonfly drone rotocopter, announced in June 2019 and scheduled for launch in 2026. It’s likely others will follow.