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AS CASSINI made its approach to Saturn in early 2004, it monitored the solar wind heading for the planet and radio emission from the planet's auroral activity while the Hubble Space Telescope took imagery of the ultraviolet emission. Large gusts in the solar wind on 17 and 25 January changed both the aurora and the radio emission. As viewed from space, an auroral display is a ring of light around a magnetic pole that is emitted by atoms and molecules that are excited by the electrons that flow in the magnetic field. In the case of Earth the emission is mostly from oxygen atoms and nitrogen molecules, but on Saturn it is from atomic and molecular hydrogen.

On 8 March Cassini began to resolve discrete features in the atmosphere, and on 19-20 March it observed the merger of two storms in the southern hemisphere, each of which was about 1,000 km in diameter. Both were drifting westwards, the more northerly one at twice the rate of the southerly one. When they met, they spun around each other in a counterclockwise manner. The resulting storm was elongated in the north-south direction with bright clouds on either end, but within 2 days it had adopted a more circular shape and the bright clouds were in a circumferential halo. It was only the second time that a merger had been observed on Saturn. In addition, there was a distinctive dark circular spot right on the south pole. This matched an infrared ob servation by the Keck Observatory on 4 February showing this location to be warm. In fact, it was the warmest place on the planet. The mystery was not that rn Left: Seen by the Hubble Space Telescope, Saturn's aurora appears as a ring of glowing gases circling the planet's south polar region. Observations in conjunction with those from the Cassini spacecraft suggest that Saturn's auroral storms are driven mainly by the pressure of the solar wind - a stream of charged particles from the Sun - rather than by the Sun's magnetic field. This image acquired on 28 January 2004 showed a strong brightening in the aurora which corresponded with the recent arrival of a large disturbance in the solar wind. Astronomers combined ultraviolet images of Saturn's southern polar region with visible-light images of the planet and its rings to make this picture. Image courtesy NASA, ESA, J. Clarke (Boston University), and Z. Levay (STScI).

the polar region was warm; it had been in full sunlight for several years, heating the polar vortex - a persistent weather pattern akin to a 'jet stream'. But both the distinct boundary of the vortex 30 degrees from the pole and the hot spot at its centre were unexpected. If the increased southern temperatures were the result of the seasonal variation in sunlight, they ought to increase smoothly towards the pole, but this was not the case - the temperature increased abruptly near 70°S, and then again right at the pole. If the abrupt change at 70°S was due to a concentration of particles that trapped solar energy in the upper atmosphere, then this would explain why the 'hot spot' appeared dark in visible light.

Just why the atmospheres of Jupiter and Saturn have an alternating pattern of east-west winds varying in direction with latitude is disputed. In contrast to Earth, whose weather is driven mainly by sunlight, the gas giants are still in a state of gravitational collapse and therefore have an additional energy source in the form of the heat that leaks from their interiors. The challenge was to understand the role of these interior energy sources in sustaining the strong jet-stream winds of the gas giants. According to one theory, the circulation was driven by sunlight heating a shallow upper layer of the atmosphere, but by the other theory the winds extended deep into the interior and were driven by the energy that was leaking out. Neither theory could readily account for the maximum wind speed at the equator.

One test was to measure the long-term sensitivity of the winds to variations in sunlight from seasonal and other influences. Studies had shown Jupiter's winds to be insensitive to seasonal changes, but Saturn was more difficult to monitor because its cloud structure was 'muted' visually. In 1980-1981 the Voyager spacecraft were able to resolve sufficient detail to enable the wind speeds to be measured, and revealed that the equatorial jet stream flowed at an astonishingly rapid 1,700 kilometres per hour. The Hubble Space Telescope could track sufficient detail to make further studies, and observations in 1996-2001 found that, notwithstanding the change in season and the location of the shadow cast on the planet by the ring system, the jets far from the equator had not changed - implying they were deeply rooted. However, at just 990 kilometres per hour the equatorial jet stream was much weaker. One suggestion for this apparent change was that the structures viewed by the Hubble Space Telescope were at higher altitudes, where the wind might be slower.

In mid-May 2004 Cassini measured the temperature fields across the southern hemisphere, providing a 3-dimensional chart of the stratospheric winds that confirmed the equatorial winds do indeed decrease rapidly rn Above: In this image taken on 27 April 2006, Saturn's fascinating meteorology manifests itself in a 'string of pearls' formation, extending over 60,000 km. Seen in new images acquired by Cassini's visual and infrared mapping spectrometer and lit from below by Saturn's internal thermal glow, the bright 'pearls' are actually clearings in Saturn's deep cloud system. More than two dozen occur at 40 degrees north latitude. Each clearing follows another at a regular spacing of some 3.5 degrees in longitude. The regularity indicates that they may be a manifestation of a large planetary wave. Image courtesy NASA/JPL/University of Arizona.

with increasing altitude above the level of the ammonia cloud layer. Evidently the structures seen by the Hubble Space Telescope were indeed at a higher altitude than those seen by the Voyagers, and that the apparent slowing was due to storms projecting their 'tops' to altitudes at which they were more readily seen from Earth. As for what maintained the energetic jets, Cassini provided a clue. Early on, it found the region near 35°S (i.e. the southern edge of the equatorial jet) to be so active that it was dubbed 'storm alley'. One sequence of images showed a number of dark spots 'emerge' from the upper level outflow of a convective storm and yield their energy to the jet stream, suggesting that this was the mechanism by which the energy that leaked from the interior sustained the horizontal winds of the upper atmosphere.

On the Voyager flybys, sporadic radio emissions from Saturn were inferred to be due to electrostatic discharges in storms in the equatorial region, which was in the shadow of the ring system. Cassini first detected such discharges in July 2003, at a distance of 161 million kilometres. They tended to occur in episodes lasting several hours, in some cases involving hundreds of bursts, often recurring in phase with the planet's rotation then ceasing for long periods. It was not until the range had closed sufficiently to see atmospheric detail that it was possible to seek evidence that the radio bursts were correlated with atmospheric storms. In early August it was noted that there had been a

bigger than average 'white storm' in the southern hemisphere at the time of an intense radio episode in mid-July. When the imaging team saw the storm was back in September, the radio team was alerted and they replied that their instrument was monitoring an episode that was even more intense than that in July. This convective storm was so large and bright that the imaging team named it the 'Dragon Storm'. The fact that over an 18-day interval the largest storm was brightest when the radio emission was strongest, with the storm always in the same position during a burst, indicated that they were the same phenomenon. But instead of the burst being detected when the storm rose over the horizon from Cassini's point of view, and peaking when it was on the meridian facing the spacecraft, the emission started before the storm rose and halted as it crossed the terminator into daylight.

Starting with periapsis on 17 February 2005, Cassini's infrared imaging system began a campaign in which it snapped Saturn at 5 microns to use the heat leaking from the deep interior to illuminate the cloud structures in silhouette. Previously, the deep clouds had been sought by viewing in sunlight, but the view had been obscured by the upper level hazes and clouds. At 5 microns it was possible to map both the day and night sides, and there was a mass of structure. Unlike the hazy global bands of the upper atmosphere, many of the deeper clouds were isolated localised features with a variety of sizes and shapes. Observations of these clouds provided a means of measuring the wind speeds at this deeper level, to make a 3-dimensional chart of the atmospheric circulation. A comparison with the 'high altitude' winds indicated there were substantial wind shears in the equatorial zone. In keeping with the finding that above the ammonia clouds the wind speed decreased with increasing altitude, at this 'low altitude' (estimated to be 30 km beneath the ammonia clouds) the winds rn Left: A false colour composite of Saturn's southern atmosphere made from Cassini images taken in near infrared light through filters that sense different amounts of methane gas. Clouds that are deep in the atmosphere are red, while grey indicates high clouds, and brown indicates clouds at intermediate altitudes. The bright feature just above and to the right of centre is a complex convective storm called the 'Dragon Storm'. It lies in a region referred to as 'Storm Alley' because of the high level of storm activity observed there. The Dragon Storm was a powerful source of radio emissions during July and September of2004. Scientists have concluded that the Dragon Storm is a giant thunderstorm whose precipitation generates electricity as it does on Earth. Image courtesy NASA/JPL/Space Science Institute.

were 275 kilometres per hour faster. The results of this method of observation were so dramatic that it was decided to make further observations on subsequent periapsis passes.


When Giovanni Domenico Cassini discovered Iapetus in 1671 he inferred from the fact that it was apparent only on one side of its orbit that for some reason the leading hemisphere was dark. When the Voyagers passed through the system in 1980-1981 they confirmed this, but the dark terrain (which was named Cassini Regio) appeared featureless. Two days after entering orbit of Saturn, the Cassini spacecraft turned its cameras to Iapetus at a range of 3 million kilometres and revealed there to be basins on the dark terrain - evidently the relics of a heavy bombardment that occurred early in the moon's history. Imagery on 17 October from a range of 1 million kilometres showed a chain of 'white dots', seemingly mountain peaks rising to heights of 10 to 20 km, contiguous with a dark linear feature. A flyby on 31 December at a range of 123,400 km established this linear feature to be a ridge that spanned Cassini Regio, and further study showed it to run along the equator and extend onto the trailing hemisphere. In fact, Iapetus is not a precise sphere but an ellipsoid, and if the ridge was a compressional structure created by the manner in which the moon's interior shrank early on, then it would form on the circumference with

the widest diameter, after which tidal effects would first tip the axis perpendicular to the orbital plane and synchronise the axial rotation with its orbital motion. But what of the dark material on the leading hemisphere?

A recent study by the Arecibo radio-telescope in Puerto Rico operating as a radar at a wavelength of 12.6 centimetres was unable to tell the two hemispheres apart. Although the bright side was known to be primarily water ice, it appeared much less reflective at this wavelength than the icy satellites of Jupiter. If there was ammonia mixed in, it would look like clean ice optically but would not reflect microwaves so well. The fact that Cassini Regio was indistinguishable to radar implied that the dark material was just a thin veneer over the ice. Infrared spectroscopy found the bright terrain to be water ice with a small amount of organic (tholin) material; and Cassini Regio was best modelled as a mix of tholin, a hydrogen cyanide polymer, a small amount of water ice and ferric oxide. A distinctive far-ultraviolet absorption feature for water ice was strong for Phoebe (the outer moon suspected by some as being the source of the dark material on Iapetus) and the bright part of Iapetus, but extremely weak for Cassini Regio; but this did not actually rule out Phoebe as the source of the dark material on Iapetus as the volatile constituents could have been liberated during emplacement, thereby concentrating the non-ice material. An inspection of Phoebe by Cassini as it entered the Saturnian system showed this to be a captured body that formed in the outer realms of the Solar System. On becoming trapped in the warmer Saturnian environment, it could well have undergone a period of outgassing. Dark streaks on the bright terrain adjacent to Cassini Regio implied the dark material was delivered ballistically, either by being swept up from space or from a source centred on the leading hemisphere. If of exogenic origin, then the material was deposited after the ridge formed, after the moon had adopted its current spin axis and after its rotation had been synchronised - a timescale which may in turn shed light on when Phoebe was captured.

the orbit of Dione, but is concentrated near the orbit of Enceladus. It was found to be comprised of tiny water-ice grains. Terrestrial studies had established that whereas the material in the 'F' and 'G' rings has a power-law distribution characteristic of debris from impacts, the 'E' ring material is of uniform size.

It had long been suspected that Enceladus was responsible for the 'E' ring, and this was now seen to be the case. But where was the source? Enceladus has such a high albedo that it reflects most of the sunlight, making its surface the coldest in the Saturnian system. The temperature at the subsolar point was expected not to exceed 80K. Even though the south pole was in continual sunlight, the oblique illumination suggested it should be significantly colder, but thermal


The Voyagers had shown Enceladus to have a sharp line of demarcation between an area that was heavily cratered and a seemingly smooth plain which indicated that the small icy moon had been extensively resurfaced. This impression was reinforced by the early views from Cassini. When it flew by at an altitude of 500 km on 9 March 2005, the magnetometer found that Saturn's magnetic field was 'bent' around the moon by electric currents generated by the interaction of the magnetosphere with neutral atoms of gas, indicating the presence of a tenuous envelope. When neutrals were ionised by magnetospheric plasma, they were 'picked up' by the magnetic field and caused oscillations at frequencies that enabled them to be identified as O+, OH+ and H2O+ ions, indicating an envelope of water vapour. With an escape velocity of a mere 212 metres per second, gas would readily leak away if it were not replenished.

The source could be outgassing from the interior through fractures in the crust, geysers or cryovolcanism. It was decided to make the next flyby at 175 km in order to directly sample the envelope. On 14 July Cassini penetrated the electrically conducting envelope, which the magnetometer found to be concentrated at the south pole. Data from other instruments showed that the spacecraft had passed through the fringe of a cloud of water vapour from a localised source in the south polar region. It was evident that water vapour being vented into space was carrying particulates by a process similar to the venting that occurs on an comet. The 'E' ring extends from the orbit of Mimas out to beyond rn Below: From afar, the anti-Saturn hemisphere of the moon Enceladus exhibits a bizarre mixture of softened craters and complex, fractured terrains. This mosaic of 21 Cassini narrow-angle camera images, acquired on 14 July 2005, is a false-colour view that includes images taken at wavelengths from the ultraviolet to the infrared. In false-colour, many long fractures on Enceladus exhibit a pronounced difference in colour (represented here in blue) from the surrounding terrain. Image courtesy NASA/JPL/Space Science Institute.

Enceladus Temperature Map

Predicted Temperatures

Observed Temperatures infrared measurements found it to be the warmest place on the moon. The optical imagery showed the south polar region to be bounded by parallel ridges and valleys and to contain a series of arcuate 'tiger stripes' where the temperature rose to 145K, which was astonishingly hot for such a frigid body. When a re-analysis of early long-range imagery of the moon as a crescent showed a glow at its south polar region, it was decided to make further such observations, and on 27 November 2005 a number of geysers were observed issuing material 500 km into space.

It appears there is liquid water at shallow depth beneath the south pole, and the 'tiger stripes', which run in parallel, 40 km apart, for about 140 km, are fractures through which it is able to reach the surface. It has been suggested that this activity is driven by the heat due to the tidal stress resulting from the eccentric orbit. If the ice contained ammonia, this antifreeze would lower the melting point of water ice by 100°C and reduce its density sufficiently for solid-state convection to occur. As a rising diapir neared the surface it would induce characteristic tectonism.

I Above: These images from Cassini's composite infrared spectrometer show a dramatic 'warm spot' centred on the south pole of Saturn's moon Enceladus - a sign of internal heat leaking out of the icy moon. It had been expected that the south pole would be very cold, as shown in the left-hand panel. The right-hand panel shows that equatorial temperatures are about 80 K (-l93oC), much as expected, but there is a warm spot at the south pole reaching 85 K (-l88oC). That is l5oC warmer than expected. (The poles should be colder than the equator because the Sun shines at such an oblique angle there.) Data suggest that small areas of the pole are at even higher temperatures, well over 110 K (-l63oC). Image courtesy NASA/JPL/GSFC.

I Top right: Images of Saturn's icy moon Enceladus backlit by the Sun show the fountain-like sources of the fine spray of material that extends far above the south polar region. This greatly enhanced and colourised image was taken looking more or less broadside at the so-called 'tiger stripe' fractures observed in earlier Enceladus images. It shows discrete plumes of a variety of apparent sizes above the limb of the moon. It is thought that the jets are geysers erupting from pressurised subsurface reservoirs of liquid water. Image courtesy NASA/JPL/Space Science Institute.

It is clear that although Enceladus has had a long history of activity, this is currently limited to the south polar region. Because a rotating body is more stable if its mass is near its equator, any redistribution of mass within an object will create instability with respect to the axis of rotation. The development of a deeply seated 'hot spot' would therefore induce Enceladus to roll over to position this low-density region on its axis. If diapiric convection is facilitated by the presence of ammonia in the water ice, ammonia ought to be being vented. None was detected, but the fact that nitrogen was present in the cloud over the south pole, and indeed in the 'E' ring, indicated the combination of solar ultraviolet and magnetospheric plasma was rapidly dissociating the ammonia. Other Cassini data showed that the venting varies over a time scale of days. When the spacecraft observed a significant injection of atomic oxygen into the 'E' ring in January 2004, the first hypothesis was that the collision of two yet-to-be-detected moonlets in this vicinity must have released a puff of water ice which had dissociated into its constituent hydrogen and oxygen, but it was now realised that the source must have been an outburst from Enceladus.

The Voyagers noted a periodicity in the rhythm of the radio emission from Saturn, and this was taken to be the rate at which the planet's core was rotating. However, in 1997 the Paris Observatory announced a different radio periodicity. An analysis of Cassini's monitoring between 29 April 2003 and 10 June 2004 showed a periodicity of 10 hours 45 minutes 45 seconds (±36 seconds), which was about 1 per cent longer than the Voyager time. As it was unreasonable

to think the rotation of the planet had slowed, it was suggested that the magnetic field might be similar to that of the Sun, whose rotation varies with latitude, being fastest at the equator, and that the part of the planetary field that controlled the charged particles that issued the radio emission had shifted to a higher latitude. If the magnetic axis of Saturn had been significantly inclined to its rotational axis, it would have been a simple matter to measure the rate at which the magnetic axis was precessing, but at an angle of just 3 degrees such a measurement is difficult. Nevertheless, by the half-way point in Cassini's primary tour the magnetometer had identified a periodicity in the magnetic field itself of 10 hours 47 minutes 6 seconds (±40 seconds) - even slower than the current radio periodicity. In 2007 it was discovered that the gas emitted by the geysers on Enceladus was being ionised in space, with the charged particles forming a disk close around the planet's equator, and that this imposed such 'drag' on the magnetic field as to make the radio periodicity exceed the planet's rotation; in effect, the radio data had been measuring the rotation of this plasma disk, not the planet. The degree of drag would depend on the amount of material in the plasma disk, which in turn would vary with the activity of Enceladus' geysers.

Rings and Moonlets

Imagery taken on 1 June 2004 included two moonlets about 3 to 4 km in size, between the orbits of Mimas and Enceladus. They were spotted two months later by Sébastien Charnoz, a dynamicist at the University of Paris in France. "I had looked for such objects for weeks while at my office, and it was only on holiday using my laptop that my program detected them; this tells me that I should take more holidays!" The IAU named them Methone and Pallene.

For an hour immediately after Saturn Orbit Insertion on 30 June 2004, while Cassini was north of the ring plane, it observed the rings at unprecedented spatial resolution, viewing the shadowed face of the system. Working inwards, the 'F' ring seemed to be more 'dirt' than ice; the 'A' ring was icy near the outside and dirty towards the inside; the 'B' ring was mainly ice; and the 'C' ring was dirty towards the inside. The most opaque parts of the system - in particular the outer part of the 'A' ring and the entire 'B' ring - were cooler and the translucent parts were warmer. This could be explained in terms of sunlight penetrating the sparsely filled rings to warm the material on the 'far side'.

The gravitational interactions between the rings and the moonlets which orbit in and just beyond the outer part of the ring system transfer angular momentum to the moons, causing them to move out and the rings to sag towards the planet. This implies the rings are a fleeting phenomenon having a lifetime of several hundred million years. However, if there is a collisional cascade at work in which large moons struck by asteroids and comets are broken into smaller moons, and so on, the result is a supply of particles to sustain the ring system beyond the time in which it would otherwise fade. Imagery taken while Cassini was north of the ring plane showed the inner edge of Encke's Division to be scalloped. This was later found to be due to the fact that the moonlet Pan, which is responsible for the 325-kilometre-wide gap, travels in an eccentric orbit. The pattern enabled the mass of the moon to be calculated, which in turn led to a density of 0.5 g/cm3. Observations of the perturbations of Atlas gave a similar value. As solid water ice is 0.93 g/cm3, this implied that these moonlets were accretions of icy particles. Sightings of 'spikes' and 'wisps' at the outer edge of the Keeler Gap (which is 250 km inside the outer edge of the 'A' ring) led to the discovery of the 7-kilometre-sized moon Daphnis orbiting in the gap. This was only the second moonlet to be rn Above left: The ice jets of Enceladus send particles hundreds of kilometres above the south pole of this spectacularly active moon. Some of the particles escape to form the diffuse 'E' ring around Saturn. This colour-coded image was enhanced to make the extent of the fainter, larger-scale component of the plume easier to see. The bright strip behind and above Enceladus is the 'E' ring, in which this intriguing body resides. The small round object at far left is a background star. The image was taken in visible light with Cassini's narrow-angle camera on 24 March 2006 at a distance of about 1.9 million km from Enceladus. Image courtesy NASA/JPL/Space Science Institute.

found inside the ring system; the first being Pan. It had a similarly low density.

Monitoring by the spacecraft of stars as they were occulted by the ring system showed that the material in the 'A' ring is concentrated into clumps. This lent support to the theory that the ring material is not simply degrading towards ever finer material, but is also being reaccreted. In fact, imagery taken while Cassini was north of the rings after Saturn Orbit Insertion showed gravitational disturbances in the 'A' ring indicative of the presence of four small embedded moonlets, each of which was calculated to be of the order of 100 metres in size - too small to sweep a clear channel in the rings; their presence was able to be inferred only because their gravitational wakes were superimposed on a particularly smooth section of the ring. If ring material is constantly reaccreting, this will considerably extend the life of the ring system. In total, the ring material is equivalent to an icy moon with a diameter of 200 km - less than half the size of Mimas. The debate about whether the rings are material that did not accrete to create a moon, or the debris of a moon that was shattered, is therefore seen to have been futile, as the system is in an essentially steady state, persisting for billions of years, with different incarnations of moonlets and the rings being present at different times.

I Above: Saturn's shadow stretches completely across the rings in this view, taken on 19January 2007. The view is a mosaic of 36 images - that is 12 separate sets of red, green and blue images - taken over the course of about 2.5 hours. This view looks toward the unlit side of the rings from about 40 degrees above the ring plane. The images in this natural-colour view were obtained with Cassini's wide-angle camera at a distance of about 1.23 million km from Saturn. Image courtesy NASA/JPL/Space Science Institute.

Further evidence of gravitational interactions was noted in an image that Cassini took on 29 October 2004, in the form of a faint 'streamer' of material drawn from the inner edge of the 'F' ring towards the shepherding moonlet Prometheus. In fact, this moon not only generates 'knots', 'kinks' and 'clumps' in the ring, but also 'dark channels'. As Prometheus approaches and recedes from the ring during its 14.7-hour elliptical orbit it perturbs ring particles into elliptical orbits, with the result that one orbital period after the encounter there is a dark channel in the interior portion of the ring, and since these disturbances take time to dissipate they form a repeating pattern along the ring.

The irregularly shaped moonlet Epimetheus was found to have a large number of 'softened' craters on a predominantly water-ice surface. Epimetheus is co-orbital with Janus, and on 21 January 2006 they swapped orbits, with Janus taking the low position. They will exchange again in 2010. There is no danger of a collision during such exchanges, as the moonlets approach no closer than 15,000 km. Their low densities suggest they are loosely consolidated accretions, rather than chips off larger bodies. Cassini found that the irregular moon Hyperion, orbiting much further out, has a density of 0.6 g/cm3, indicating that it, too, is a loose accretion rather than a solid body. Hyperion is just below the size limit at which internal pressure will crush loosely packed ice and close the pore spaces to form solid ice. It is somewhat larger than Epimetheus and Janus, and smaller than Mimas, which is spherical; however, why there should be a reaccreted object at this location in the Saturnian system is not known. Trojans are a class of objects which occupy the gravitationally stable points 60 degrees leading or trailing a larger body. Tethys was already known to have two such moonlets: Telesto leading and Calypso trailing. Cassini found them to be elongated with cratered but smooth surfaces. Dione was also known to have Helene in its leading Trojan point. Cassini discovered 5-kilometre-sized Polydeuces in the trailing point.

Ring 'spokes' comprise dust particles less than about 1 micron in size that collect electrostatic charges in the plasma environment of the rings, then become subject to electric and magnetic forces. In certain conditions, they become negatively charged and are briefly levitated en masse 100 km away from the surface of the rings. When viewed from up-Sun, their shadows are seen projected on the rings, but when observed from down-Sun the forward-scattered sunlight makes them appear bright. The illumination of the ring system changes with Saturn's travel around the Sun, and the presence of the spokes is more likely when the Sun is close to the ring plane. At the time of Cassini's arrival in the Saturnian system the rings were 'open' to the Sun and conditions were unfavourable, but on 5 September 2005 the craft gained its first glimpse of this phenomenon in the shape of several faint narrow radial spokes on the outer part of the shadowed face of the 'B' ring, about to enter the planet's shadow. In the Voyager imagery, the narrow wedge-shaped spokes extended from 10,000 to 20,000 km out across the 'B' ring, but these new spokes were a mere 3,500 km long and 100 km wide. By 2008, however, Cassini's orbit will be steeply inclined to the ring plane, and this will enable it to view the rings almost 'face on', and as spokes become more common it will be well positioned to monitor their motion.

Usually when Cassini passed through Saturn's shadow it was near periapsis, and the occultation lasted only an hour or so, but the occultation on 16 September 2006 occurred while the spacecraft was sampling the 'tail' of the planet's magneto sphere, some 2.2 million kilometres down-Sun, with the result that it spent fully 12

hours in shadow. This offered an opportunity to make a number of intriguing observations of the ring system in silhouette. It was found that there is a tenuous ring associated with Janus and Epimetheus, and another associated with Pallene. In addition, the 'G' ring was revealed to have a very distinct inner edge. Wispy, fingerlike projections were evident extending into the 'E' ring from Enceladus, probably ice particles vented by the geysers at the moon's south pole.


During a flyby of Titan on 26 October 2004 Cassini sampled the outer fringe of this moon's atmosphere. The ratio of the isotopes of nitrogen indicated that 75 per cent of the original nitrogen in the atmosphere may have leaked to space. As water ice is an efficient carrier of primordial noble gases, their absence suggested the nitrogen in the atmosphere derived from the volatisation of ammonia ice by accretional heat as the moon formed, with the ammonia subsequently having been dissociated by solar ultraviolet. The implication was that the part of the solar nebula from which Titan condensed was cool enough for ammonia ice and methane ice, but too warm for a nitrogen clathrate with trapped primordial noble gases.

On the surface, there was a complex interplay of albedo. There were 'streaks' that were suggestive of the movement of (dark) material across the surface, and the fact that these were consistently aligned with the prevailing 'zonal' circulation implied windblown material. It was initially expected that the particulate 'fall out' from the organic haze would be sticky, but modelling suggested these would harden during

I This panoramic view of Saturn combines 165 images from Cassini's wide-angle camera taken over nearly three hours on 16 September 2006, while it drifted in the darkness of Saturn's shadow. Cassini detected two new faint rings: one coincident with the shared orbit of the moons Janus and Epimetheus, and another coincident with Pallene's orbit. The narrowly confined 'G' ring is visible outside the bright main rings. Encircling the entire system is the more extended 'E' ring. The icy plumes of Enceladus, whose eruptions replenish the 'E' ring, betray its position in the 'E' ring's left-side edge. Interior to the 'G' ring and above the brighter main rings is the pale dot of Earth, over a billion kilometres distant.

their long descent and, being non-sticky, would pile up on the surface like fine dust. It had also been thought that the surface must be stagnant, as the energy of insolation was too weak to drive winds in the lower atmosphere, but then it was realised that Saturn's tidal influence on Titan is 400 times greater than that of our Moon on Earth, and 'atmospheric tides' are dominant at the surface. In Titan's dense atmosphere and low gravity, even a wind averaging 1 kilometre per hour could 'bounce' grains along the surface in a process called saltation.

The best views of the surface were provided by the spacecraft's microwave radar, but this was effective only when close to the moon and supplied long narrow swaths of the surface, each of which had an area of approximately 1 per cent of the globe. Statistically, 40 impact craters with diameters larger than 20 km could have been expected in such an area but, to general surprise, none were visible. Because impacts must have occurred, the absence of craters implied that the surface was active. Indeed, not only were there some features with characteristics suggestive of cryovolcanism, the atmosphere contained argon-40, a gas that would be released by such activity. Cassini radar altimetry showed the range of elevation to be confined to 50 metres along one 200-kilometre-long track, indicating a large flat rn Above: As Cassini approached Titan on 21 August 2005, it captured this natural colour view of the moon's orange, global smog. Images taken with the wide-angle camera using red, green and blue spectral filters were combined to create this colour view. The images were acquired at a distance of about 213,000 km from Titan. Right: This image acquired with Cassini's wide-angle camera on 22 August 2005 using a spectral filter centred on infrared wavelengths at 939 nm, reveals the mid-latitudes on Titan's Saturn-facing side. Features within the region seen here - known informally as the 'H' — now have names such as Tsegihi, Aztlan and Quivira. The bright 215-km-wide feature provisionally named Bazaruto Facula is visible right of centre, with a dark, unnamed 80-km-wide crater at its centre. Images courtesy NASA/JPL/Space Science Institute.

region. A radar study by the Arecibo observatory had shown there to be 'specular' reflections from isolated areas 50 to 100 km across, suggesting the presence of either lakes of hydrocarbons or flat areas such as would be left if lakes had dried out. However, if organic particulates had been blown into low-lying areas to form level plains, it was possible that there were areas that looked just like dry lakes but which had formed without the involvement of liquid.

Cassini released the Huygens probe on 24 December 2004 to trace a ballistic arc leading to entry of Titan's atmosphere on 14 January 2005. It had been expected that the probe would emerge from the base of the optically thick haze at an altitude of 70 km, but this did not happen until 45 km, and the view of the surface remained murky until 30 km. Sampling showed a constant ratio of methane to nitrogen in the stratosphere and upper troposphere, but at about 20 km this ratio began to increase, and at 8 km the methane reached saturation. At about 7 km the wind speed decreased to a mild 1 to 2 metres per second and the direction became variable, implying that the probe had descended into a convective region in which the local winds were disconnected from the zonal jet stream. During the final phase of the descent, the downward-looking infrared spectrometer obtained reflectance spectra to determine the composition of the surface in the vicinity of the landing point, and at an altitude of 700 metres it activated a 20-watt lamp in order to 'fill in' the wavelengths of sunlight that were filtered out during passage through the atmosphere. To the imager, the result resembled a car shining its headlamps into fog, suggesting there was a drizzle of liquid methane.

As hoped, the probe came down over a boundary between the light and dark areas. The imagery appeared to show a network of dark drainage channels on a bright area leading down to a shoreline, with several offshore islands on the dark area. The wind carried the probe out over the dark area for touchdown. The ground-level view showed a solid surface littered with 'rocks'. Since the camera was very close to the ground, the sense of perspective was deceptive: the rocks were not the boulders they appeared; they were actually only a few centimetres in size, the nearest being no more than 1 metre away, and they were assuredly not silicate rock but lumps of water ice. The fact that they were rounded and sitting exposed on top of darker finely grained material suggested that they were erosional products that had been washed down the drainage channels onto the plain. The probe impacted the surface at 4.5 metres per second, with a 15-g deceleration over an interval of 40 milliseconds. A penetrometer projecting from the base of the probe had struck a pebble and nudged it aside, then readily penetrated a surface that was neither hard like solid ice, nor readily compressible like a blanket of fluffy aerosols. It was a 'soft' surface, for which plausible candidates were: (1) a solid granular material with little or no cohesion; (2) a 'mud' of ice grains from impact or fluvial erosion wetted by liquid methane; and (3) a 'tar' of finely grained ice and photochemical products. The probe had slipped along the surface for several seconds prior to coming to a halt, tilted at an angle of 10 degrees with its base dug in about 10 centimetres.

The fact that Huygens operated on the surface for over an hour gave a welcome bonus. The measured surface temperature was 93.65 K (±0.25). The lamp for the spectrometer remained illuminated, and would have provided a source of heat.

In addition, a sample inlet on the base was held at +90°C, making Huygens by far the warmest object on the moon. There was a significant increase in the methane fraction several minutes after landing, implying that heat from the probe was boiling volatiles out of the ground. The lens of the downward-looking imager was embedded, but the repeating views of the oblique and side-looking imagers gave hints that the heat from the probe was causing material to splutter across the ground. The implication was that occasional methane rain drained from the elevated terrain onto the low-lying ground, where it rapidly t __ soaked into the porous material, which then served as a reservoir for slow evaporation. The volatiles detected after landing were

rich in organics that had not been noted during the descent, notably ethane and possibly benzene and cyanogen (both results of methane and nitrogen chemistry) and carbon dioxide, all of which were indicative of a complex chemistry in progress in the surface material.

With the Huygens mission achieved, Cassini resumed its remote-sensing of Titan, in particular the clouds that prevailed near the continuously sunlit south pole and the intriguing 'streaky' clouds that were confined to the southern temperate latitudes. On 28 October 2005 Cassini's radar imaged a field of dark 100-metre-tall dunes. The individual dunes ran for hundreds of kilometres, and the field extended at least 1,000 km along the equatorial zone. Calculations showed that the variable tidal wind caused by Saturn's gravitation was combining with the west-to-east zonal circulation to create surface winds that would yield a sinuous pattern of longitudinal dunes that were aligned with the direction of the prevailing wind flow, rn Above: Radar imaging data acquired during Cassini's flyby of 22 July 2006 provide convincing evidence for large bodies of liquid on the surface of Titan. Intensity in this colourised image is proportional to how much radar brightness is returned. The colours are not a representation of what the human eye would see. The lakes, darker than the surrounding terrain, are emphasized here by tinting regions of low backscatter in blue. Radar-brighter regions are shown in tan. The strip of radar imagery is foreshortened to simulate an oblique view of the highest latitude region, seen from a point to its west. The image is centred near 80 degrees north, 35 degrees west and is about 140 km across. Image courtesy NASA/JPL/USGS.

I Left: This composite view shows an image from the descent imager/spectral radiometer (left) taken while the Huygens probe was setting on Titan's surface, alongside a similarly scaled picture taken on the Moon's surface. Objects near the centre of the picture are roughly the size of a man's foot. Objects at the horizon are a fraction of a man's height. The Huygens image was taken on 14 January 2005. Images courtesy ESA/NASA/JPL/University of Arizona.

I This composite image acquired by Cassini's visual and infrared mapping spectrometer during the 29 December 2006 flyby shows a huge cloud system over the north pole of Titan. The image extends from the north pole down to a latitude of 62 degrees north. Such cloud cover was expected, according to atmospheric circulation models of Titan, but it had never been observed before in such detail. The condensates may be the source of liquids that fill the lakes recently discovered by the radar instrument. Image courtesy NASA/JPL/University of Arizona.

I This composite image acquired by Cassini's visual and infrared mapping spectrometer during the 29 December 2006 flyby shows a huge cloud system over the north pole of Titan. The image extends from the north pole down to a latitude of 62 degrees north. Such cloud cover was expected, according to atmospheric circulation models of Titan, but it had never been observed before in such detail. The condensates may be the source of liquids that fill the lakes recently discovered by the radar instrument. Image courtesy NASA/JPL/University of Arizona.

except near elevated terrain that controlled the local wind direction. This process could transfer material from the mid-latitudes to the equatorial zone. Furthermore, the presence of dunes implied the absence of persistent liquids on the surface in the equatorial zone that would serve as sand traps, a conclusion that seemed to conflict with the idea that Titan was a land of lakes. On 22 July 2006, when Cassini's ground track extended to the northern region that had been in darkness since before the spacecraft's arrival in the Saturnian system, the radar discovered dozens of well-defined dark patches up to 1 kilometre in size, some that were tens of kilometres, and one that was approaching 100 km. These were the darkest radar features yet observed, and dark usually signified either a smooth surface or a radar-absorbent material - and a lake of liquid hydrocarbon would be both. And there were channels leading into or out of some of these patches that had characteristics which were consistent with their having been created by flowing liquid. In 2007 coordinated observations by the optical and radar imaging instruments found a similar dark feature that was so large as to justify being dubbed the 'Black Sea' of Titan.

In its study of the circulation of the upper atmosphere, Cassini had earlier found that at mid-to-high northern latitudes the zonal wind inhibited mixing and created an isolated vortex around the pole. Being in continual darkness, the cold stratosphere in this region was sinking, in the process drawing down and concentrating the organics that were produced in the upper haze. On 23 September 2006 the spacecraft discovered a vast tropospheric cloud spanning the dark north polar region that appeared to be composed of ethane, apparently the result of the concentration of organic haze in the descending north polar vortex. Ethane raining into lakes of methane would dissolve. If the temperature dipped low enough, ethane would undoubtedly fall as snow, and might even build up an ice cap. The localisation of methane and ethane at the winter pole implied a seasonal cycle in which the volatile hydrocarbons migrated from one pole to the other over a Titanian year (which lasts

29.5 terrestrial years) and this, in turn, offered an explanation for the current concentration of clouds at the south pole and absence of liquid on the surface at lower latitudes. If the Cassini mission is able to be extended beyond its nominal four years, then it should be possible to monitor the onset of the northern summer, with storms forming in the tropics as the volatiles migrate south, possibly temporarily submerging the now inert Huygens probe on the floor of a lake of rainfall runoff. Between them, the Cassini orbiter and its Huygens probe have certainly lifted the veil on Titan.

Further Reading

Cassini at Saturn: Huygens Results David M. Harland, Springer-Praxis, 2007 Titan Unveiled

Ralph Lorenz and Jacqueline Mitton, Princeton University Press, 2007

Dr David M. Harland gained his BSc in astronomy in 1977, lectured in computer science, worked in industry and managed academic research. In 1995 he 'retired' to write on space themes.

Dr David M. Harland gained his BSc in astronomy in 1977, lectured in computer science, worked in industry and managed academic research. In 1995 he 'retired' to write on space themes.

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