by Duncan Lunan

On 14^th January 2005 the Huygens probe entered Titan’s atmosphere, deploying a large parachute for descent through the upper atmosphere and a smaller stabilising ‘chute for final descent, while sending signals to the Cassini orbiter for relay to Earth.  The full descent took three hours.  Around the base of the clouds it encountered severe turbulence, swinging in arcs of up to 120 degrees, enough to cause temporary drops in the strength of the signals to Cassini.  Due to a technical error, apparently, one of the data channels with information on the winds was not activated.   However, in a remarkable technical feat Huygens’s omnidirectional antenna was tracked from Earth and the windspeeds it experienced were reconstructed from the Doppler shifts. 

In the run-up to the landing several critics of space spending asked me if the conditions couldn’t be reproduced in the laboratory.  To some extent the answer is yes:  Carl Sagan and Jordan Khare were able to duplicate the spectral signatures of Titan’s clouds by energising ‘primal atmospheres’ of nitrogen and methane  (J. Kelly Beatty, ‘Voyager at Saturn, Act II’, Sky & Telescope, November 1981).  Before the Huygens landing, Martin Towner and colleagues at the Open University had created ‘Titan in a bucket’, cooled by liquid nitrogen, to test various atmospheric models.  (‘Simulating Titan’, Frontiers, Particle Physics & Astronomy Research Council, winter 2005.)  But the chambers were so small that gases and liquids had to be simulated in separate chambers, and there’s no way that can adequately simulate physics and chemistry occurring at ultra-low temperatures over the area of a continent.

The atmosphere below the clouds was much stiller, yellow-lit and remarkably clear.  It seems unlikely that Saturn could be seen from Titan’s surface, but surface features have been seen through the clouds in the infra-red.  The greatest surprise from the panoramas Huygens obtained during its parachute descent was that the landscape looked so familiar – at first glance.  Mountains and what seemed to be rivers and beaches looked almost earthlike, and the probe touched down on material which splashed like snow or wet sand.   But it was hydrocarbon slush, possibly with a thin, dry crust.  The mountains appear to be water ice, but the drainage channels running off from the mountains are apparently conduits for liquid methane.  One river is 1,500 km  (930 miles)  long, but on closer examination what seem to be seas and lakes below the mountains are actually solid;  the Cassini radar found no large bodies of liquid on the surface, but they could be methane ice.  The temperatures and pressure are close to the ‘triple point’ of methane where it can exist as solid, liquid or gas, like water on Earth.   In theory, liquid methane could sustain life, though ‘not as we know it’, but there is a problem:  unlike water ice, which floats, methane ice sinks as it forms.  

Huygens was designed to float if it landed in ocean, but Titan’s seas could be frozen from the bottom up.  The dark features in the descent photos are almost smooth and flat, looking very much like frozen seas, but they may be deposits of hydrocarbons washed down to the plains by the methane rivers.  The probe landed on what looks exactly like a beach, except that the surrounding ‘boulders’ are water ice.  The heat from the probe evaporated hydrocarbons from the surface which formed a deposit on the downward-facing camera, and there are indications of a liquid, probably methane, a few centimetres below the surface.  Huygens was designed to survive for at least fifteen minutes on the surface, but in fact data was relayed by Cassini for over an hour, and the omnidirectional signal could be detected at Earth for three hours after that, though no more data was received after Cassini went over the horizon on Titan.  It was an outstanding achievement, in which Britain played a major part:  the Surface Science Package team was led by the Open University, with contributions from the Rutherford Appleton Laboratory and Southampton University.  British Industrial contributors included Logica GMC, IGG Component Technology, IRVIN-GQ and SciSis.

Cassini continued to map Titan by radar on successive flybys;  44 were planned initially, but the final total was 127.   It seems that the whole surface is subjected to a constant methane drizzle, with occasional downpours carving out the river-like channels, and there are lakes of liquid methane or ethane at the north pole.  Nevertheless, tidally generated winds create many areas of hydrocarbon dunes in the equatorial region, the largest 1500 x 200 km in extent.   (Peter Bond, ‘The Sands of Titan’, David Powell, ‘Cassini Sees Lakes on Titan’, Astronomy Now, July & September 2006.)  

A giant impact crater was found, more than 400 miles across, with strange ‘cat scratches’ across its floor which appear to be erosion features, possibly caused by wind-blown ammonia ice.  The atmosphere and surface of Titan are dominated by methane, which is at its triple point in Titan’s dense nitrogen atmosphere and so shifts between solid, liquid and gas, in weather, river and lake systems eerily like Earth’s.  The crustal features are moving relative to one another, apparently floating on a subsurface water ocean which could contain life.   Later flybys of Titan have shown crustal movements to be due to tidal stresses, with the crust rising and falling by more than ten metres.

In temperatures only 100 degrees above absolute zero, water behaves like rock.  The mountains are made of water ice, and evidence has been found suggesting cryovolcanoes, with liquid water acting like lava.  The new model suggests that the entire crust is primarily water ice, 50 km thick, overlying a liquid water ocean up to 250 km deep – making the crust and underlying ocean of Europa almost trivial by comparison.   (‘Titan’s Tides Point to Hidden Ocean’, ESA Space Science News, 28^th June 2012.)

Enceladus

As well as discovering the mysterious spoke-like features above Saturn’s rings, the Voyagers also discovered that large areas of the icy moon Enceladus had been resurfaced in the relatively recent past, indicating that the diffuse E-ring within which Enceladus orbits was being fed by ice crystals emanating from outbreaks of water on the surface.  These were observed by Cassini emanating from blue streaks on the surface near the south pole of Enceladus.  Close passes of Enceladus revealed an atmosphere of water vapour, confirming the water-driven volcanic activity which has resurfaced large areas of the moon.  In a long-distance view Cassini photographed the geysers actually feeding into the E-ring, and it was suggested that they’re powered by intense radioactive heating from a small core at the heart of Enceladus.  More recently it’s been discovered that the core is interacting with the planet’s magnetic field and acting as a brake on it, which means that it must be electrically conducting – so the metallic particles of the spokes may be coming from Enceladus and flowing towards Saturn along the magnetic field lines.  However, geysers of dust have been found on the moons Dione and Tethys, and they too may contribute to the spokes.  

In the first two encounters with Enceladus it was verified that the particles of Saturn’s E-ring come from eruptions of liquid water, and that they come from blue-covered ‘tiger stripes’ near the south pole.  If tidal heating is responsible one would expect the events to be near the equator, just as ‘moonquakes’ are created around the bulge on the lunar equator facing the Earth.  After the third flyby in 2007 it was suggested that the axis of Enceladus has been changed as a result of internal activity, causing the moon to tip over by nearly 90 degrees.   (Kulvinder Singh Chadha, ‘Wobbly Enceladus Tipped Over’, AN, July 2006.)  That may explain why the E-ring appears brighter above and below the ring plane than it does along the orbit of Enceladus itself.   (Peter Bond, ‘New Views of Saturn’s E and G Rings’, AN, Sept. 2006.)  In March 2008 the controllers took a risky option, to send Cassini through one of the plumes only 23 km above the surface.   It posed the risk of breaking up the spacecraft and releasing contamination from its radioactive power source  (William Harwood, ‘The Road Ahead’, AN, July 2006), but it proved worthwhile, because organic compounds were detected, perhaps hinting at life below the surface of Enceladus.

On the next pass, Cassini flew at 30 miles’ altitude over the blue ‘tiger stripes’  (now called sulci), revealing that the geysers are coming from vents in the sides of fractures about 1000 feet deep, with V-shaped inner walls.  The outer flanks of some of the fractures show extensive deposits of fine material deposited by the geysers, while the surrounding terrain is finely fractured and littered with blocks of ice tens of metres in size and larger (the size of small houses).  A big argument is going on about whether the water is liberated by tidal forces, causing friction in crustal plates, or by internal radioactive heating, which might allow the possibility of life in a subsurface ocean.  Mapping the interior by analysis of the tracking data may settle the question, especially following a still closer flyby, only 16 miles up.

Hyperion

Hyperion proved to be the next surprising moon in 2006, looking for all the world like a fossilised sponge.  The wispy features on Rhea which were photographed by the Voyager spacecraft proved to be a network of chasms.  Mimas, one of the icy inner moons, has a crater 130 km in diameter and at least 10 km deep, making it look like the ‘Death Star’ in Star Wars. 

It was provisionally named Arthur by its discoverers, but is now known as Herschel.  It wouldn’t have taken much more to shatter Mimas, and in Comet, by Carl Sagan and Ann Druyan  (Michael Joseph, 1985), there’s a painting by Kim Poor of Mimas broken in half.  Mimas has a diameter of 392 kilometres, and is mostly made of ice, so its broken half wouldn’t stay broken but would slump into a smaller sphere.  Several of Saturn’s moons have Trojan-type companions;  but the photographs of the smaller moons of Saturn became more strange as time went on:  many of them appear stretched, like some asteroids and comet nuclei, some have equatorial ridges like Iapetus, and one seems to be totally smooth.

After reversing the orientation of the orbit with respect to Saturn’s magnetic field, and completing the planned sequence of moon flybys, Cassini used a gravitational slingshot from Titan to take it into an orbit at 75 degrees to Saturn’s equator, to study the planet at higher latitudes and the rings at angles never seen before.  Among many remarkable images obtained, Alpha Centauri was photographed over the limb of the planet in 2008, clearly showing it as a double star.  One of the largest storms ever seen on Saturn dominated its northern hemisphere in 2011-2012.  A view of the planet and rings from the night side in 2013 captured the Earth and Moon, Mars and Venus, grouped around Saturn along with the smaller, inner moons.  Detailed views of the hexagonal storm at the north pole showed its boundary to be a jetstream which was weaving between huge vortices, like the ones photographed since by the Juno orbiter at the poles of Jupiter.  In 2016 interstellar dust was detected, entering the Saturn system at 45,000 miles per hour… the liat of discoveries goes on and on.

Though extended, the mission had to end someday.  One plan was to send it through the Cassini Division to a final plunge into Saturn;  but as the Cassini Division is not empty, that raised the contamination risk again.   Another option, remarkably enough, was to return Cassini to Jupiter and plunge it into Jupiter’s atmosphere – repeating the final plunge of the Galileo probe into Jupiter in September 2003, but this time with pictures.  In the end, the controllers settled for a final sequence of 22 orbits within the rings, grazing the inner edge of the Crêpe Ring, the D-ring whose existence had once been so controversial, before the final plunge into Saturn on September 15^th, 2017.  NASA broadcast the final control room scenes as the signal broke up and was lost – making far less of a production of it than ESA had with the end of the Rosetta mission, in September 2016, but an emotional moment nevertheless.

See also: Saturn and its moons – Part 1

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