Titan and the Huygens Mission Editorial

INTERNATIONAL SPACE SCIENCE INSTITUTE

Published Published by by the the Association Association Pro Pro ISSI ISSI No 15, November 20052005

The human eyes progressively lose munity: he constructed his first Impressum power to accommodate when the telescope in July 1609, and turned phase of biological necessities is an improved instrument to the over. Seneca, himself plagued by heavens in November. With this the loss of vision, described the telescope, he observed the Moon use of a globe of water helping to and found the rings of Saturn. SPATIUM see the letters enlarged and hence Isaac Newton improved his de- Published by the more clearly. The Arab scientist sign by replacing the first lens by Association Pro ISSI Ibn al-Haitam is said to have pro- a mirror, to produce what is now twice a year posed the use of solid transparent called the Newtonian telescope.

material of suitable shape to sup- Due to technical limitations of the INTERNATIONAL SPACE port the vision. European monks manufacturing process, increased SCIENCE of the 13th century translated his magnification had to be achieved INSTITUTE oeuvre in Latin. Not required by increasing the focal length. Association Pro ISSI to fulfil biological, but rather in- While the Galilean telescope had Hallerstrasse 6, CH-3012 Bern tellectual necessities, they were a handy length of 2 m, the Dutch Phone +41 (0)31 631 48 96 strongly interested in his ideas and Christiaan Huygens half a century see in 1267 Roger Bacon, a Francis- later came up with an astronomi- www.issi.unibe.ch/pro-issi.html can friar of Oxford, showed, that cal telescope of 7 m. New meth- for the whole Spatium series small letters can be magnified by ods for grinding and polishing specially shaped pieces of crystals, glass allowed him to make a quan- President from which his colleagues then tum leap in telescope technology Prof. Heinrich Leutwyler, made the first reading stones to be and to discover Titan, the largest University of Bern shifted carefully from one letter to moon of Saturn in 1655. Publisher the next. Dr. Hansjörg Schlaepfer, It is here that our story begins, the legenda schläpfer wort & bild, This promising business opportu- story of a marvellous space mis- CH-8173 Neerach nity was discovered by the glass- sion to land a probe on Titan. We Layout blowers of Murano in the early are indebted to Professor Nicolas Marcel Künzi, marketing · kom- 14th century, who were able to Thomas of the Physikalisches In- munikation, CH-8483 Kollbrunn fabricate the best material for eye- stitut of the Universität Bern for Printing glasses. The new product success- his kind permission to present our Schellenberg Druck AG fully penetrated the market, but at readers herewith a revised version CH-8330 Pfäffikon the same time challenged crafts- of his fascinating lecture on the men all over Europe to grasp their ESA Huygens mission to Titan. share also. Of course, scientists became an important customer segment; they used the lenses not Hansjörg Schlaepfer only for reading letters, but for Zürich, November 2005 additional purposes as well, be it for looking at objects very close or very far. It was Hans Lipper- shey, a Dutch lens maker, who demonstrated the first refracting Front cover telescope in 1608 for military use. An artist’s view of the moon Titan Galileo Galilei introduced the and the Saturnian ring system. instrument in the science com- (Credits: see figure 7)

SPATIUM 15 2 Titan and the Huygens Mission1

Introduction lenged the American student, the Earth circles the Sun at a ra- Stanley Miller in 1953 to execute dius of approximately 150 million a simple experiment: he put some km, the diameter of the Saturn water in a large laboratory bot- orbit measures roughly 1.5 billion tle, added methane and nitrogen km making a journey to Saturn a The fourteenth day of January which were thought to be the lengthy endeavour. ESA therefore 2005 was one of mankind’s major main constituents of the primor- joined forces with the US space milestones, which, like most mile- dial Earth atmosphere before life agency NASA and planned a mis- stones probably, passed unnoticed developed. He simulated the ef- sion to Saturn called Cassini 5, in by the vast majority. On this day, fects of the Sun by ultraviolet ra- honour of the discoverer of the at 11:30 UTC a spacecraft from diation and lightning by means of Saturnian ring system. An inter- planet Earth landed on the surface electrical discharges in the bottle agency agreement was concluded of Titan, after a journey of more and then went quietly to dinner. whereby NASA was to provide than 3,000,000,000 km. There, the During the whole night, his ex- the launch and the mother ship probe remained busy for its short periment was running. When he Cassini, on which the ESA Titan lifetime of a mere seventy minutes came back the next morning, he probe Huygens was attached to before it fell into an eternal sleep. found the gas mixed with a stun- ride to Saturn. In addition, Cassini ning wealth of complex organic was to act as relay station for the Why was Titan chosen from all the molecules including amino acids, Huygens radio signals over its en- numerous heavenly bodies popu- the very building blocks of life. tire mission duration. lating our solar system? Titan, the Was that the key to understanding largest moon of Saturn, was dis- the evolution of life on Earth? The present issue of Spatium is de- covered by Christiaan Huygens 2 voted to this outstanding mission in 1655. In 1908, the Spanish as- Thirty years later, the two US and to the scientific results gained tronomer Comas Sola stipulated spacecraft Voyager 1 and 2 were so far. that Titan must have an atmo- underway to probe the outer solar sphere, which was an outstanding system4 and passed by Saturn and finding as no other moon of the Titan. They not only confirmed solar system is covered by a gase- the presence of methane but also ous shell. It was in 1944, when the of many heavier organic molecules. Dutch Gerard Kuiper 3 detected the 6,190 Å methane absorption These were the results that line in Titan’s atmosphere. This prompted the European scientific again was a revolutionary result: community in the early eighties at least on Earth, methane is as- to propose to ESA a space mission sociated with biological activity. to Titan. However, landing on a In the fifties, much was hypoth- moon in the outer solar system is esized about the evolution of life not a Sunday morning’s walk: the on Earth. These discussions chal- distances are enormous: while

1 Lecture by Prof. Nicolas Thomas for Pro ISSI on 22 March 2005 2 Christiaan Huygens, 1629–1695, Dutch scientist 3 Gerard Kuiper, 1905–1973, Dutch-American astronomer 4 It is interesting to note, that both these spacecraft launched in 1977 are still func- tioning: they have left now the outer limits of the solar system and continue to send back information from distances of 94 AU (14 billion km) for Voyager 1 and 75 AU Figure 1: A self-made telescope ena- (11 billion km) for Voyager 2. bled the Dutch scientist Christiaan 5 Jean-Dominique Cassini (1625–1712), Italian astronomer Huygens to discover Titan in 1655.

SPATIUM 15 3 Titan, the cury, and the only planetary body, What makes Titan so fascinating other than the Earth, to have a is that many processes that occur Mysterious Moon substantial nitrogen-based atmo- on the Earth are thought to have sphere. Many exotic chemical re- analogues on Titan albeit at actions, driven by solar radiation much colder temperatures as it is and the high energy particles of ten times farther away from the Saturn’s magnetosphere, result in Sun. These processes include not Saturn’s giant moon Titan (figure an atmosphere with primitive or- only chemical reactions, but also 2) is the second largest in the solar ganic compounds, which eventu- weather including rain, erosion by system, larger than planet Mer- ally rain down onto the surface. winds and liquids, a greenhouse effect and possibly volcanism. Ti- tan’s key parameters are compared to those of the Earth in the table on page 5.

On Earth, the water cycle drives many important phenomena: it condenses to form clouds in the atmosphere, rains down to the surface to cause fluvial erosions from the bedrock creating surface structures like rivers with complex tributary systems. On Titan, it is too cold for water to exist in liquid form, but methane plays a similar role there. While, however, the water cycle on Earth is closed, this is not the case with the methane cycle on Titan: pho- tons of the solar irradiation and the energy electrons from the Saturnian radiation belt dissoci- ate the methane molecule leav-

ing a CH 3 or CH 2 radical, which quickly reacts to ethane, ethylene and so forth. It is estimated that within a mere 10 million years, the equivalent of the complete atmospheric content of methane is destroyed by these mechanisms, which raises the question regard- Figure 2: A close-up view of Titan recorded as the Cassini spacecraft approached its first close fly-by of Saturn’s smog-shrouded moon on 26 October 2004. Here, red and ing the fresh supply of methane. green colors represent specific infrared wavelengths absorbed by Titan’s atmospheric On Earth, it is life which refreshes methane while bright and dark surface areas are revealed in a more penetrating infra- the methane reservoir, as methane red band. Ultraviolet data showing the extensive upper atmosphere and haze layers is a by-product of the metabolism are seen as blue. Sprawling across the 5,000 km wide moon, the bright continent- sized feature known as Xanadu Regio is near picture center, bordered at the left by of many organisms. But all known contrasting dark terrain. (Credits: Cassini Imaging Team, SSI, JPL, ESA, NASA) forms of life require the presence

SPATIUM 15 4 dry cold atmosphere causes a 300 Earth Titan km thick layer of smog to build up, which is the result of complex chemical interactions between the Atmospheric composition atmospheric hydrocarbons driven 78% N2, 21% O2, <1% H2O 98% N2, 2%CH4 by the sunlight and the high energy particles of Saturn’s radiation Surface pressure 1 bar 1.5 bar belt.

Atmospheric thickness 50 km 300 km (top of stratosphere) The Objectives for the Huygens Mission

Clouds/rain H2O CH4 Based on the knowledge of Titan Radius 6,371 km 2,575 km in the late eighties, the goals for the Huygens mission were set at Axial tilt 23.5° 26.7° understanding the complex processes shaping this distant celes- Distance from Sun 1 AU 9.5 AU tial body, which in many respects mirror those on our home planet Solar irradiance 1,368 W/m 2 15 W/m 2 Earth especially during its pre- biotic phase. The main areas of scientific interest were:

abundance of atmospheric con- of liquid water which can not exist The Surface stituents; isotope ratios for abun- on the frosty Titan: the source dant elements; scenarios of for- of fresh methane remains one of The handful of Voyager images of mation and evolution of Titan the many secrets which Titan Titan revealed little, as they were and its atmosphere continues to keep even after the unable to penetrate Titan’s thick intense probing by Huygens and atmosphere. The Hubble Space vertical and horizontal distribu- Cassini. Telescope acquired a series of in- tions of trace gases; range of com- frared images in the 1990’s show- plex organic molecules; energy ing some patterns on the moon’s sources for atmospheric chemis- The Interior surface, but it was not clear, try; photo-chemistry of the strato- whether these indicated solid con- sphere; formation and composi- The Voyager 1 and 2 spacecraft tinents in oceans of liquid meth- tion of aerosols first determined the average den- ane or whether the surface was sity of Titan by measuring the entirely solid. winds and global temperatures; radius and the mass derived cloud physics, general circulation from the gravitational pull ex- and seasonal effects in Titan’s at- perienced by the spacecraft. The The Atmosphere mosphere; search for lightning result was 1,881 kg · m–3, which discharges corresponds to roughly 30% of The most intriguing feature of the density of the Earth. No mag- Titan is the thick atmosphere physical state, topography and netic field was detected suggesting completely hiding the surface in composition of the surface; inter- that no molten iron core is present. the visible spectrum. The moon’s nal structure of the moon

SPATIUM 15 5 The uncertainties about the physical kg spacecraft with the required parameters governing descent and thrust. The trajectory included Cassini-Huygens landing on Titan were so great four swing-bys, one of the Earth, that the mission planners faced a two of Venus and the last of Jupi- Mission whole host of challenges: ter, see ﬁgure 3. A swing-by is a manoeuvre, whereby the spacecraft passes near a planet. When the The Challenges spacecraft enters the gravitational field of the planet, it is acceler- Orbit: Saturn orbits the Sun at ated; when it leaves the planet, it Space science missions have al- approximately 10 Astronomical is decelerated again. As seen from ways been attractive as they push Units (AU), i.e. at about 1.5 bil- the planet, there is no exchange available technologies to achieve lion km. To reach such a distant of energy, but in the Sun’s inertial performance never reached be- target in a reasonable time frame, system, the spacecraft gains some fore. The Cassini-Huygens mis- advanced celestial mechanics are additional speed at the expense of sion is no exception. The distanc- to be used as present day launch- the planet’s motion. As its mass es involved are enormous and the ers are not able to provide a 5,600 is many times larger than the

Saturn arrival 1 July 2004

Venus swing-by 26 April 1998

Venus swing-by 24 June 1999 n

Deep space

s manoeuvre

nu 3 December 1998

Sun Ve Orbit of Jupiter Orbit of Orbit of Satur Orbit of Earth

Earth swing-by Launch 18 August 1999 15 October 1997

Jupiter swing-by 30 December 2000

Figure 3: The complex trajectory of Cassini-Huygens in a simplified form: Launch was on 15 October 1997. The first swing- by occurred at Venus on 26 April 1998, the second on 26 April 1998, while the Earth swing-by was on 18 August 1999. After this phase the Cassini-Huygens spacecraft left the terrestrial planets and headed towards the giant planet Jupiter, which it passed on 30 December 2000 to arrive at Saturn on 1 July 2004.

SPATIUM 15 6 spacecraft’s mass, the net change sphere. However, the orbit param- though, some information was of speed of the planet can be neg- eters were extremely well-known, lost, due to a command error on lected, while the increase of the so there was never any danger. one of two redundant data chan- spacecraft’s speed is essential for nels linking Huygens with Cassini. the mission. Mission control: Radio signals, al- But finally, taking into account all though propagating with the the challenges met, the mission Mission duration: Notwithstanding speed of light of 300,000 km·s–1, has been an outstanding success, the complex trajectory, the flight require over eighty minutes to not only on technical and scientif- time to Saturn amounted to seven reaching the Saturnian system. ic grounds, but also regarding the years. In addition, Cassini is sched- Any real time mission control ac- successful co-operation between uled to explore the Saturnian sys- tivity is therefore not possible. If the two space agencies ESA and tem for another four years, which the spacecraft were to experience NASA, with their radically differ- brings the mission duration to an immediate problem any neces- ent corporate cultures. 11 years. During this time frame sary corrective commands would the spacecraft requires electrical reach the spacecraft at least 2.5 power to keep the thermal con- hours after the critical situation oc- The Technical Solutions trol system active, the electronics curred. All the mission sequence running, the scientific instruments steps including the entire land- The Huygens Spacecraft working when needed, the tele- ing sequence had to be executed communication system operation- in a fully automatic mode. With The Cassini-Huygens spacecraft al to generate the radio signals today’s technology this would (ﬁgure 4) was one of the largest, carrying data back to Earth and pose no major problem under the heaviest and most complex inter- to receive the commands from known parameters of the Earth. planetary vehicles ever built. The ground control. On an Earth-like On Titan, however, where no main body of the Cassini orbiter orbit, the electrical energy is gen- probe had ever been before, this consisted of a cylindrical stack erated by solar cells converting was an extraordinary challenge to with a lower equipment module, a the sunlight into electrical cur- be met with technologies available propulsion module and an upper rent. At Saturn’s distance from the in the early nineties. equipment module. It was topped Sun, the solar irradiation is a mere by the fixed, four-meter diameter one percent of its value around Ultimately, the successful landing high-gain antenna securing the link the Earth, which makes solar en- of Huygens on Titan has demon- to the Earth. Cassini was equipped ergy generators unfeasible. More strated that the assumptions of the with a suite of instruments, cameras attractive, but also much more scientists were very accurate and and other remote sensing instru- complex, is the energy genera- the performance of the spacecraft ments. The Huygens probe was tion from the heat of the decay of as designed by the engineers was mounted on the side of the Cassini plutonium dioxide radioisotopes. practically nominal: after a flight spacecraft from where it was jetti- Three thermoelectric generators time of seven years landing oc- soned by the spin and eject device. provide Cassini with 300 W at the curred a mere 14 minutes later beginning of the mission in 1998 than expected. The separation The Huygens probe’s structural and 210 W in 2009. speed of Huygens away from system consisted of several sub- Cassini as well as its spin rate were units: the central experiment Of course, the Earth swing-by on exactly in the middle of the al- platform contained the six scien- 18 August 1999 caused major con- lowed tolerance. The Huygens tific experiments. The front shield cerns in the public as the plutoni- probe was operational for 70 min- protected the instruments dur- um could have caused radioactive utes after landing, much more ing entry in the atmosphere and pollution if the spacecraft would than the initially expected two slowed the spacecraft as it entered have entered the Earth’s atmo- or three minutes. Unfortunately the atmosphere at supersonic speed.

SPATIUM 15 7 Space of Zurich, while the back cover was built by APCO Tech- nologies of Vevey (figure 5).

The temperature control system was responsible for maintaining the temperature of the spacecraft within the specified range. The complex trajectory to Saturn exposed the spacecraft to extreme variations in its thermal environment: in the vicinity of Venus, the solar heating is nearly three times greater than near the Earth and the probe’s electronics and instruments required active cooling. On the other hand at Saturn, solar irradiance is hun- dred times less, which required the probe’s heaters to be turned on. The Huygens telecommunication system provided the link between the probe and Cassini after separation until the end of the Huygens mission. As Huygens could not carry a high gain antenna allowing direct contact with ground control due to mass constraints, Cassini acted as a relay station to send the Huygens data back to Earth.

The Huygens payload consisted of Figure 4: The Huygens probe (centre) is mated to the large body of the Cassini spacecraft. (Credits: NASA) six complex instruments address- ing many scientific questions:

The specific shape of the front linked to Cassini via the circular The Huygens Atmospheric shield together with the axial spin spin and eject device. This mecha- Structure Instrument HASI was a assured stability during the entry nism provided Huygens a spin multi-sensor package designed to phase. Aerodynamic friction at rate of 7.5 revolutions per minute measure the physical properties this time heated the protective and an axial velocity of 0.35 me- of Titan’s atmosphere such as tem- tiles of the front shield to 1,200 ºC tres per second. perature, pressure, turbulence and and radio contact to Cassini was atmospheric conductivity. In addi- lost. On top of the experiment It is interesting to be noted here, tion, it searched for lightning. The platform a secondary platform that the Swiss space industry con- sensors consisted of a 3-axis accel- with additional instruments was tributed major subsystems to the erometer, a temperature sensor, a mounted. The after cone con- Huygens probe: the front shield multi-range pressure sensor, a nected the experiment platforms and the spin and eject device were microphone, and an electric field to the back cover, which itself was developed and built by Contraves sensor array.

SPATIUM 15 8 wind direction and magnitudes in Titan’s atmosphere. DWE measured the Doppler shift of the probe relay link signal from the Huygens probe to the Cassini orbiter. The DWE-proper hardware consisted of two ultra-stable oscil- lators, one on the probe and one on the orbiter. A height profile of wind velocity should have been derived from the residual Doppler shift of Huygens radio relay signal as received by Cassini. Unfortu- nately all these data were lost due to a command error on one of the two communication channels between Huygens and Cassini.

The Surface Science Package SSP was a suite of sensors for de- termining the physical properties Figure 5: Installation of the Multi-Layer Insulation (MLI) blankets on Huygens’ front shield and back cover. The intense aerodynamic heating during entry rapidly of the surface at the impact site, vaporised the MLI, and the 20 mm thick sponge-textured brown tiles insulated the and for providing information on carbon fibre internal structure and the descent module from the intense radiation of the composition of the surface ma- the hypersonic shock wave. terial. The instrument included a force transducer for measuring the The Gas Chromatograph and Chromatograph (GCMS). Two impact deceleration and sensors to Mass Spectrometer GCMS meas- samples were collected: one from measure the refraction index, tem- ured the chemical composition of the top of the descent down to the perature and thermal conductivity Titan’s atmosphere from 170 km tropopause (160–40 km) and the of the surface material. It included altitude down to the surface. It second sample in the cloud layer an acoustic sounder for sounding determined the isotope ratios of (23–17 km). At the end of each the atmosphere’s bottom layer the major gaseous constituents. collection period, the filter was and the surface’s physical proper- GCMS also analysed the gas sam- retracted into a pyrolysis furnace, ties before impact. If the probe ples from the Aerosol Collector where the effluent from the cap- would have landed in a liquid, the Pyrolyser (ACP) and investigated tured aerosols was analysed, first sounder was to be used to probe the composition of several candi- at ambient temperature (about the liquid depth. A tilt sensor was date surface materials. 0 ºC), subsequently heated to included to indicate the probe’s 250 ºC and then to 600 ºC in attitude after impact. The Aerosol Collector and Py- order to conduct a step-wise pyrolyser ACP deployed a filter out rolysis. The pyrolysed products The Descent Imager and Spec- in front of the probe to sample were flushed into GCMS for ana- tral Radiometer DISR was the the aerosols during the descent lysis. optical remote sensing instrument and prepared the collected mat- aboard Huygens. It included a set ter (by evaporation, pyrolysis and The Doppler Wind Experiment of upward and downward looking gas product transfer) for analysis DWE was a high-precision track- photometers, visible and IR spec- by the Gas Chromatograph/Mass ing investigation to determine trometers, a solar aureole sensor,

SPATIUM 15 9 2004 Cassini arrived at Saturn and performed a major engine burn to bring it into orbit. One day after, Cassini had a first, albeit distant, encounter with Titan (339,000 km) which provided the first op- portunity to study the moon’s South polar region with its radar.

The spacecraft crossed through the large gap between the F ring and G ring, see figure 7. For this phase, the spacecraft oriented its high gain antenna forward to use it as a shield against the incom- ing dust particles in the ring plane. The main engine burn began shortly after it crossed above the Figure 6: A demanding mission like Huygens-Cassini was of course planned to gather as much information en route to its destination. This image shows Phoebe, one ring plane on 1 July 2004 at 01:12 of Saturn’s many moons taken by Cassini during its close encounter on 11 June 2004. UTC and ended 97 minutes later Phoebe’s irregular shape and retrograde orbit suggest a Kuiper belt origin. (Credits: in order to slow down the space- Cassini Imaging Team, SSI, JPL, ESA, NASA) craft sufficiently to be captured by Saturn’s gravity field. Cassini- Huygens’ closest approach to Sat- a side-looking imager and two near Canberra, Australia. The urn occurred during this burn; down-looking imagers: a medium- launcher boosted the spacecraft its distance from Saturn was then 1 resolution and a high-resolution into a Venus–Venus–Earth–Jupi- a mere 20,000 km, only /6 of its imager. There was also a Sun sen- ter swing-by trajectory toward its diameter. sor that measured the spin rate. final destination of Saturn, see ﬁg- DISR spectral sensitivity covered ure 3. Cassini made its first close Titan the range of 0.3 to 1.7 µm, i.e. encounter on 26 October 2004 1 the visible and the near-infrared After the long interplanetary at a distance of 1,200 km, i.e. /4 wavelengths. cruise, Cassini made its first meas- of the moon’s diameter. A second urement in the Saturn system close encounter followed on 13 with a close fly-by of Phoebe on December 2004. This trajectory, if The Trajectory 11 June 2004, see ﬁgure 6. Phoebe uncorrected, would have led to a is a small moon with a diameter of subsequent fly-by at an altitude of Launch took place on 15 Octo- only 220 kilometres. Its inclined about 4,600 km. A targeting ma- ber 1997 from Cape Canaveral and retrograde orbit around Sat- noeuvre was required, therefore, in Florida by means of a Titan IV- urn suggests that it may have been to achieve the desired intercept B/ Centaur launcher. After 143 s, a Kuiper belt object captured by course for the Huygens probe. the spacecraft detached from the Saturn gravity field. Following Once released there was no way the launcher at a speed of 7,000 the fly-by, a trajectory correction to change its trajectory. This km · h–1. 40 minutes later, the manoeuvre was made on 16 June manoeuvre was executed on 17 spacecraft established the first 2004 to place the Cassini-Huy- December 2004 and placed Cas- radio contact with NASA’s Deep gens spacecraft on a precise inter- sini-Huygens on a direct impact Space Network tracking complex cept course with Saturn. On 1 July trajectory with Titan.

SPATIUM 15 10 Figure 7: False colours help to highlight subtle differences like for example in this wonderful image of the Saturnian ring system. The bluer an area appears, the richer it is in water ice, conversely, the redder an area appears, the richer it is in some sort of dirt. The exact composition of dirt remains unknown, however. This and other images show that inner rings have more dirt than outer rings. This dirt/ice trend could be an important clue to the ring’s origin. The thin red band in the otherwise blue A ring is the Encke Gap. (Credits: UVIS, U. Colorado, ESA, NASA)

On Christmas Day, at 02:00 UTC, tance of 4 million km and reached relative position of Cassini, Huy- the Cassini onboard computer gen- the outer limits of its atmosphere gens and Titan allowed a theoreti- erated the signals that activated on 14 January 2005. At the same cal maximum coverage of 4 hours the spring and eject device which time, Cassini passed Titan at a dis- 30 minutes after landing. had firmly held the Huygens tance of 60,000 km with its high probe for the past seven years and gain antenna directed towards the The Huygens probe reached now provided the probe with ex- Huygens probe for the entire des- Titan with a relative speed of actly the required spin and velocity cent, landing and surface phase. 18,000 km·h–1. Aerodynamic fric- toward its final destination, Titan, Later, Cassini pointed its antenna tion caused the probe to slow see ﬁgure 8. After release, Huygens towards the Earth to relay the to a velocity of 1,400 km·h–1. A coasted towards Titan for a dis- Huygens data back to Earth. The sequence of parachutes was then

SPATIUM 15 11 any useful information, their very appearance, however, was a great sensation as they confirmed that Huygens had executed its entry sequence perfectly.

The flight through the upper atmosphere was a bumpy ride, as the probe rocked far more than expected. In the high-altitude haze, it swung at least 10 to 20 degrees from the vertical; while in the lower layers the probe was much more stable, tilting less than 3 degrees. In this phase, the probe decelerated to approximately 5.4 m·s–1, and drifted sideways at about 1.5 m·s–1. It came as a great surprise that during this descent phase the probe’s spin reversed; a fact which is still unexplained Figure 8: Artist’s concept of the Cassini-Huygens orbiter shows the Huygens probe immediately after separation from Cassini. The probe drifted for about three weeks today. until reaching the upper layers of Titan’s atmosphere. (Credits: NASA/JPL/Caltech) At an altitude of 700 m above the surface the descent lamp of the deployed to further brake the moment, Huygens started to DISR instrument was activated. probe to 300 km · h-1 (ﬁgure 9). The transmit the first radio signals to The purpose of this lamp was to back cover and the front shield Cassini. These reached not only provide a light source with known were jettisoned so that the scien- Cassini, but about 67 minutes later spectral properties to enable sci- tific instruments became exposed also the Green Bank Telescope in entists to determine accurately the to Titan’s atmosphere at a height West Virginia, USA. They were, reflectivity of the surface. At 11: 30 of 160 km above ground. At that as expected, far too faint to carry UTC the Huygens probe touched

SPATIUM 15 12 down on Titan. The first image Mercury, where it is expected to reached the Earth two hours later, allow pinpointing the spacecraft see ﬁgure 11. It confirmed the safe with an error of a few centimetres! landing of Huygens in what seems to be a dry river bed with rounded The surface phase of the mission pebbles. lasted 70 minutes – considerably longer than anticipated as there One of the most stunning results was no failure on the batteries. of the Huygens mission is that Therefore, all five batteries could during its descent through Titan’s be used to power the probe while atmosphere, Earth-based radio-tele- four would have been enough scopes were able to pinpoint its to complete a nominal mission. position by tracking the extremely Furthermore, the landing on the faint carrier radio signal. Connect- surface was quite soft, so that no ed together to implement Very major damage was done to the Long Base Interferometry they probe and its instruments. After were able to track the Huygens the end of battery power the probe’s path through the atmo- probe became silent. sphere with an accuracy of around 100 m, from which the directions and strengths of the winds as a Figure 9: The Huygens probe during the hot phase of entry into Titan’s atmo- function of the probe’s altitude sphere. The probe plunged into the could be inferred. This came as upper layers of the atmosphere at a speed an important and welcome redun- –1 Figure 10 shows the view of Huyges of 18,000 km· h . The velocity was then dancy for the lost communication reduced to about 1,400 km· h–1 in less from a height of 30 km. On the far left, than 2 minutes, thanks to the friction channel carrying the Huygens a boundary seems to exist between some of the heat shield with the atmospheric Doppler Wind Experiment data. sort of smooth dark terrain and a type of gas. (Credits: ESA) choppier terrain in the distance. In the As a consequence, determination image center and on the left, white areas of a spacecraft’s exact position by cover the image that might be a type of means of interferometry is now ground fog. The Huygens probe landed in the dark area of the far right. (Credits: considered for the Agency’s forth- ESA, NASA, Descent Imager/Spectral coming BepiColombo mission to Radiometer Team)

SPATIUM 15 13 Titan: the Results so Far

Titan continues to be an exciting celestial object even after the detailed analysis by Huygens and the continuing visits by Cassini. Space scientists all over the world are currently engaged in the analysis of the data received from Huy- gens and the information provided by Cassini. The main scientific results as of September 2005 are summarized below.

The Interior

It is thought that Titan consists ba- sically of a rocky core of silicates overlaid by a crust mainly of water ice. At the temperatures prevailing on Titan, water ice plays the role of rocks on Earth. As Titan circles Saturn on a highly eccentric orbit,

Figure 11: The ﬁrst scene recorded by the Huygens probe immediately after landing on Titan’s frosty surface. Bathed in an eerie orange light at ground level, rocks strewn about the scene are thought to be composed of water and hydrocarbons frozen solid at an inhos- pitable temperature of –179 °C. The light-toned rock below and left of centre is only about 15 cm across and lies 85 cm away. The shapes of the pebbles indicate erosion processes either by water or by wind or both. After touching down at 4.5 meters per second, the probe is be- lieved to have penetrated around 15 cm into a surface with the consistency of wet sand or clay. (Credits: ESA, NASA, Descent Imager/Spectral Radiometer Team, JPL)

SPATIUM 15 14 strong tidal forces act on its inte- Liquid Surfaces and Volcanoes rior causing local heating on top of the heat generated by the decay Again in contrast to the expecta- of radioactive material in its core, tions, no ocean of liquid methane which may give rise to up-wells of was found with the possible ex- material in the form of cryo-volca- ception of a small lake in the vicin- noes in places which in turn may ity of the South Pole, see figure 12. act as the source of methane – and This, however, necessitates a new other constituents – replenishing theory regarding the replenish- the moon’s atmosphere. It must ment of the methane content in be remembered, however, that the atmosphere. One could be the the term heating here is a relative: out-gassing from cryo-volcanoes. in the very cold environment of A handful of surface features indi- Titan the temperatures reached by cating the existence of volcanoes these mechanisms remain in the have been found, but their behav- order of a few 100 K. iour over time still needs to be Figure 12: The dark patch is possibly one of the long awaited reservoirs observed in order to confirm this of liquid methane on Titan’s surface. interpretation. The energy source The Surface It is situated in the South Polar region feeding this mechanism could be where currently the early summer sea- son started causing strong meteorologi- either the tidal forces due to Ti- The moon’s surface is being cal activity, which may lead to temporary tan’s large eccentricity or decay of mapped globally by Cassini on methane lakes. (Credits: NASA/JPL- radioactive material in its interior. its visits to Titan. The Cassini Caltech/Space Science Institute) radar subsystem can penetrate the smoggy atmosphere undisturbed Dunes, Tectonic Faults and its data are complemented and Fluvial Systems locally by the ground truth infor- during its entry into the thick at- mation provided by the Huygens mosphere, no smaller, but many One salient feature of the moon’s data. larger craters were expected. It surface are thin dark structures came as a great surprise that only extending over hundreds of kilo- The images of the surface taken very few large impact craters metres. These may be caused by the Cassini radar show a com- have been identified so far, im- by tectonic activity. Other fea- plex terrain with large clear and plying that the surface topogra- tures are more like rivers and dark areas. The Xanadu Regio, a phy is of recent time, i.e. no more may be caused by liquids flowing continent-sized light area is the than 130–300 million years old from higher parts of the surface first to have received a name on in places. All large craters have a (50–200 m elevation) down to Titan. The different albedos may very weathered appearance suggest- lowlands. It is thought that the stem from varying materials, tex- ing that the moon’s surface is con- methane rain causes liquid to flow tures, or different slopes of the tinuously and intensively rework- down-hill and to carve rivers into surface. ed by ongoing processes such as the “bedrock” of water ice. These viscous sagging, tectonics, cryo- flows carry other organic material volcanism, and erosion by wind washed out from the atmosphere, Impact Craters and liquids. The icy pebbles imag- which in turn is deposited along ed by Huygens on the surface the rivers, see figure 13. These chan- As any object, which would cause (figure 9) show a rounded appear- nel-like features are the best evi- a crater smaller than roughly 20 ance consistent with strong ero- dence for the presence of liquids km in diameter, would be burned sional effects by winds or liquids. on the surface so far. At the South

SPATIUM 15 15 Pole, where intense meteorological processes take place at present as the southern hemisphere enters summer time, an extensive system of channels has been identified. During its descent Huygens showed many such channels; all appear to have dark material in the valley surrounded by brighter material.

Other surface structures, mainly observed near the Equator, re- mind one of wind blown dunes on Earth. They show sharp west- ern boundary lines together with extended eastern boundaries.

Soil Reﬂectivity

During the descent, Huygens took many images of the surface with unprecedented detail. Due to the opacity of the atmosphere in the wavelengths of the DISR instru- Figure 13: Methane clouds and rain, evaporating lakes, flowing rivers, and water ment, however, surface features ice-volcanoes all likely exist on Saturn’s moon Titan, according to preliminary analy- could be identified only below a ses of images taken by the Huygens lander. A snaking and branching riverbed is iden- height of about 50 km. After land- tified with the dark channel near the top of the above image, while a dark lakebed is identified across the image bottom. Both the riverbed and lakebed were thought to be ing, the surface reflection spec- dry at the time the image was taken but contained a flowing liquid – likely methane trum was measured from 480 – in the recent past. Titan’s surface was found to appear strangely similar to Earth nm to 1,600 nm and calibrated even though it is so cold that water freezes into rock-hard ice. (Credits: ESA, NASA, by means of the known emission Descent Imager/Spectral Radiometer Team [LPL]) spectrum of the DISR lamp, see ﬁgure 14. The peak reflectivity of about 0.18 occurs at 830 nm and and 1,600 nm the reflectivity is of water ice near 1,000 and 1,200 decreases towards both sides. The low (0.06) and flat, consistent nm, and the identity of the sur- red slope in the visible is consist- with water ice. Nevertheless, the face component responsible for ent with organic material, such as decrease in reflectivity from 900 this slope remains unknown. The tholins 6, but the infrared slope is to 1,500 nm does not show the entire set of DISR observations still unexplained. Between 1,500 expected weak absorption bands gives a new view of Titan, and re- inforces the hypothesis that processes on Titan’s surface are more 6 Tholin is a hard, red-brownish substance made of complex organic compounds. similar to those on the surface of Tholins don’t exist naturally on Earth, because the present oxidizing atmosphere blocks their synthesis. However, tholins can be made in the laboratory by subjecting the Earth than anywhere else in mixtures of methane, ammonia, and water vapour to simulated lightning discharges. the solar system.

SPATIUM 15 16 0.20 The near fly-by of Cassini in De- Titan Surface Spectral Reflectivity 0.8 cember 2004 at a distance of a mere 1,000 km from the surface 0.7 allowed its mass spectrometers to

0.15 detect gas from the upper atmo- 0.6 sphere. An astonishingly complex array of short- and long-chain hy- 0.5 drocarbons and nitriles was detect-

0.10 ed, with atomic mass units of up to 0.4 the instrument’s maximum range of 100. The largest chains con- 0.3 tained up to seven carbon atoms.

0.05 0.2 The mass spectrometers showed

0.1 also that Titan’s atmosphere is enriched in the heavy isotope 15 0.00 0.0 of N with respect to the more 600 800 1000 1200 1400 1600 abundant light 14N. The isotope Wavelength [nm] ratio 15N/ 14N is much higher than on Earth. After insertion of Figure 14 shows the surface reﬂectivity as measured after landing (red line). It is compared with a simulation (blue line) of a mixture of large grained low tempera- Cassini in its orbit around Saturn ture water ice, yellow tholins, and an unknown component with a featureless slope its instruments detected the vast between 850 and 1,500 nm. Spectra of two different organic tholins (yellow tholin: torus of gas orbiting Saturn along dashed line and dark tholin: solid black line) are also shown for comparison. Labora- tory activity is ongoing in an attempt to identify or synthesize the missing blue mate- with Titan. As Titan lies within rial. (Credits: M. G. Tomakso and al. Rain, wind and haze; a close-up view from the the magnetosphere of Saturn, it is descent to Titan’s surface, Nature, to be published) constantly bombarded with high energy particles from Saturn’s radiation belt. These particles col- The Atmosphere remain at the altitude of their for- lide with gas in Titan’s atmosphere mation and it is thought that the and cause gas to be ejected, form- The Composition outer haze layers are produced ing the vast cloud of gas encircling by this mechanism. Sedimenta- Saturn. The loss of atmospheric The most striking feature of Ti- tion and mixing begins then in a matter by this process as well as tan’s atmosphere are the various second step. Particles of about the the molecules’ intrinsic thermal layers of haze, see ﬁgure 15. This haze same size stick together to form energy cause the enrichment of is thought to be formed in a two aggregates containing some tens of heavy isotopes because lighter stage-process: The atmospheric monomers up to several hundreds isotopes move faster and there- methane is dissociated by solar pho- leading to very complex organic fore escape the Titan gravity field tons and high-energy electrons molecules and clusters. These more easily than the heavier ones. of Saturn’s radiation belt. The re- cause the homogeneous orange In contrast, the heavy isotope maining radicals then react and haze in the lower atmosphere. In of carbon is not enriched. The create larger organic molecules. contrast to what was expected, 13C/ 12C ratio is slightly lower Then, a first growth step may take Huygens has not found any clear than it is on Earth. This is con- place by accretion leading to small part in the atmosphere. Rather, sistent with the assumption of a particles, roughly spherical mono- throughout the atmosphere there methane replenishment process, mers of 0.05 µm diameter. Due is a more or less dense haze much which tends to keep the isotope to their low mass, these particles like a hazy day in Berne! ratio constant.

SPATIUM 15 17 Clouds and Atmospheric Dynamics

The Titan atmosphere shows signs of clouds, of which the largest con- centrations have been found in the South Polar region, which is currently in early summer. The extra heat from the Sun is thought to create evaporation of methane from the surface. With the heated nitrogen, the methane is brought to higher altitudes in the atmosphere, where it condenses and forms cu- mulus clouds. From fly-by to fly- by these clouds vary significantly. It seems that the South Pole is a region of intense meteorological activity at present. Much fainter clouds were detected at mid latitudes. These lasted for only a few hours. Track- ing these clouds allowed an esti- mate of the wind speed of 34 m · s–1, faster than Titan rotates itself. This super-rotation of the Titan atmosphere had been predicted.

A third category of clouds take the form of east–west streaks hundreds of kilometres long. They seem to originate from fixed posi- tions on the surface and one interpretation is that they are produced by surface processes and in fact may be gas vented from Titan’s Figure 15: This image taken by the Cassini orbiter reveals Titan’s complex layered haze structure. Two very distinct levels are discernible, while later images revealed interior and then swept along by as much as 12 separate layers above the main orange coloured haze. The haze tops the super-rotating winds. roughly at an altitude of 500 km, which is much more than the Earth’s stratosphere extending to about 30 km. (Credits: Cassini Imaging Team, SSI, JPL, ESA, NASA)

Figure 16: The domed feature detailed below is thought to be cryo-volcano, seen in infrared light. The lava welling After landing on the surface, the together with the rivers carved up to form the volcanic mound would Huygens DISR instrument in- out from the bedrock by fluids possibly be a slurry of methane, am- dicated a methane abundance of provide evidence for precipita- monia, and water ice combined with 5% suggesting a relative humid- tion of methane with subsequent other ices and hydrocarbons. The circular feature is roughly thirty kilometers ity of methane of 50%, i.e. far evaporation much like the rain in diameter. (Credits: VIMS Team, U. from saturation. This observation on Earth. Arizona, ESA, NASA)

SPATIUM 15 18 Outlook

Titan has many similarities with the Earth. Its atmosphere is similar to the pre-biotic Earth atmosphere, and the surface processes strongly resemble their equivalents on the actual Earth. These facts make Titan an attractive object for scientific research as it may help to un- derstand, how life developed on Earth 3.5 billion years ago. While it is a fascinating thought that understanding our past may require visiting such a distant object like Titan, its sheer distance from the Earth makes further missions in the near future very improbable especially in a time where the ESA science budget is squeezed. Clear- ly, the next logical step would be a sample return mission to Titan aiming at bringing back some material down to Earth. This, however, will remain a dream for the present generation of scientists to eventually become reality for their descendents...

SPATIUM 15 19 SPATIUM

The Author

Nicolas Gordon Morgan Thomas that time, he was also engaged as made his first lecture at his tender a visiting scientist at the Lunar age of eight. Like his colleagues, and Planetary Laboratory at the he was asked to present a topic of University of Arizona, Tucson, interest; and as at that time Neil and at the Queen’s University of Armstrong just had set foot on Belfast. In March 2003 he was the lunar surface, young Nicolas elected Professor of Experimen- decided to give a presentation on tal Physics at the University of the solar system. This laudable at- Bern. tempt, however, was stopped at Jupiter since his time allowance Nicolas Thomas is the Principle of two minutes was over at the Investigator for the Microscope giant planet. Notwithstanding on the Beagle 2 Lander and the this set-back planetary science Co-Principle Investigator of continued to fascinate the young- ESA’s BepiColombo Laser Alti- ster who later enrolled at the Poly- meter Experiment to Mercury. technic of Wolverhampton, and Over the years, Nicolas Thomas then focused on Experimental has published over 100 publica- Space Physics at the University tions covering the fields of come- of Leicester, where he received a tary, Jupiter and Mars science. Masters Degree. Nicolas Thomas concluded his studies at the Uni- Nicolas Thomas is deeply fasci- versity of York with a thesis on nated by the beauties and mira- the Jovian satellite, Io, which, as cles of our solar system, even, as he confessed recently, continues he underlines, as a non-believer to be his favourite celestial ob- that there is life elsewhere in ject. our solar system. But further out there? That is another story... The further path brought N. We scientists ought to be honest, Thomas to the Max Planck In- he confesses, and have to base our stitute for Aeronomie (MPAe) arguments on facts and not on of Lindau, where he worked on fantasy. The solar system does not the analysis of data from the Hal- need fantastic hypotheses to fas- ley Multicolour Camera aboard cinate people. Just tell them the the Giotto spacecraft. A further truth. That’s beautiful enough. post-doctoral fellowship allowed him to join the Space Science Department of ESA, and then he returned again to MPAe. During