Published by the Association Pro ISSI No. 38, October 2016

INTERNATIONAL SPACE SCIENCE INSTITUTE SPATIUM Published by the Association Pro ISSI No. 38, October 2016

163251_Spatium_38_2016_(001_016).indd 1 26.09.16 15:30 Editorial

14 January 2005 was a slow news memories of the people who made day: nothing happened that justi- the dream a reality. Impressum fied an eye-catching headline. Hence, a calm day for the news One of the Huygens mission’s lead- papers. Yet, a small note amongst ing scientists is Professor John Zar- all the daily trivialities meant re- necki, former Director of the ISSN 2297-5888 (Print) lief from a seven year nightmare Centre for Earth, Planetary, Space ISSN 2297-590X (Online) for numerous space scientists and & Astronomical Research at the engineers all over the world: Huy- Open University, Milton Keynes, gens had safely landed on Titan. England. He was the Principal In- SPATIUM vestigator of the Surface Science Published by the Titan: the sorcerous name for an Package of Huygens, a suite of in- Association Pro ISSI enigmatic moon in the outer solar struments that probed the moon’s system, more than one billion kilo atmosphere during descent and the metres away. Huygens, a small surface properties after landing. It space probe named in honour of is with great pleasure that we pre- Dutch scientist Christiaan Huy- sent our readers today with his re- Association Pro ISSI gens, built by the European space view of what an armada of scien- Hallerstrasse 6, CH-3012 Bern community, had been seven years tists have found out since Huygens Phone +41 (0)31 631 48 96 underway towards its remote des- landed on Titan, eleven years ago. see tination Titan together with its www.issibern.ch/pro-issi.html US-American mate Cassini, com- Even though Titan surprises us for the whole Spatium series missioned to enter a literally un- with its many similarities to our known world. planet, its climate does not really President qualify it as an alternative: the sur- Prof. Adrian Jäggi, The challenges were numerous: face temperature is some 200 °C University of Bern Titan’s distance from Earth is so degrees lower than on Earth! Any great that radio signals need one how, we wish our readers a pleas- Layout and Publisher and a half hour to reach Earth and ing lecture about one of the fasci- Dr. Hansjörg Schlaepfer any corrective measure would need nating worlds in which our solar CH-6614 Brissago the same time again before be system is so incredibly rich. coming effective. Thus, there is no Printing chance of intervening from Earth Hansjörg Schlaepfer Stämpf li AG during the final critical moments Cinuos-chel, October 2016 CH-3001 Bern when the probe approaches its destination with cosmic speed: a fully automatic mode of operation was required. Furthermore, is the surface of Titan liquid or solid? How cold is it there? No one knew. Huygens had to cope with all imaginable scenarios and in fact, it mastered its job excellently, imper- turbably until the end of its short life on the surface, 72 minutes after touchdown. All these and many further challenges remain in the

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163251_Spatium_38_2016_(001_016).indd 2 26.09.16 15:30 The Moon That Thinks It’s A Planet: Titan1

by Prof. John Zarnecki, Professor of Space Science, The Open University, Milton Keynes, England and International Space Science Institute, Bern

probe on Saturn’s largest moon Ti- cornucopia of data. The current is- Prologue tan, a staggering 1,500 million kil- sue of Spatium summarizes some ometres away (Fig. 1). At that time, major findings scientists all over the science community was just the world have gained so far while This issue of Spatium picks up a starting to browse the wealth of the reader is kindly referred to subject that Spatium no. 15 ad- data that Huygens had down- Spatium 15 for technical informa- dressed in November 20052: Titan loaded. In the meantime, the tion on the Cassini-Huygens twin and the Huygens Mission. That stream of science data did not spacecraft and its unique trajectory early report appeared a few months cease, rather, NASA’s Cassini or- to the outer Solar System. after the successful landing of the biter continued circling the Satur- European Space Agency’s Huygens nian system providing a further

Fig. 1: The yellowish moon Titan seen (in blue). Near the rings and appearing The dark space in the A ring is called here by NASA’s Cassini orbiter behind above Titan is Epimetheus, a small the Encke Gap, the domain of the min- Saturn’s A rings (in white) and F ring moon that orbits just outside the F ring. ute moon Pan. (Credit: ESA/NASA).

1 T his text is based on a talk by Prof. John Zarnecki for the Pro ISSI audience on 28 October 2015. It was prepared by Dr. Hansjörg Schlaepfer and reviewed by Prof. Zarnecki. 2 See Spatium no. 15: Titan and the Huygens Mission by Nicolas G. M. Thomas, November 2005.

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163251_Spatium_38_2016_(001_016).indd 3 26.09.16 15:30 In contrast, one thing was clear: tan’s surface, methane is a liquid Introduction the atmosphere is a complex chem- fuelling speculations about lakes ical laboratory producing an as- and seas covering Titan, just like tounding variety of organic mole- on Earth. In short, the methane The Cassini-Huygens mission was cules. Its major constituent is mystery marked Titan as a prime conceived in the early 1990s. That nitrogen, as is the case with the target for a daring space mission was at a time when our knowledge Earth’s atmosphere. In addition, amongst all the objects in the outer

of Titan was scarce due to its enor- one finds methane (CH4), a gas Solar System. mous distance from Earth. Ground- that on our planet is produced by based observations had yielded no biological processes. Would this Now, after the spectacular in-situ more than some blurry pictures, mean that the methane is a sign of measurements made by the Huy- and the images gained by Pioneer present life on Titan? As methane gens lander and the numerous 11 in 1979 and the two Voyager is destroyed in time spans of 50 fly-bys executed by Cassini in the fly-bys in 1980 and 1981 respec- million years by the solar ultra meantime, our portrait of this tively, could not improve the situ- violet radiation under the condi- enigmatic world has sharpened ation drastically. A thick opaque tions prevailing on Titan that was considerably and the intriguing atmosphere obscures the moon an obvious, albeit far-reaching similarities with our planet have (Fig. 2), and it was not known there- conclusion. The presence of meth- become even more astonishing fore, whether Huygens would land ane makes an active source of fresh than before. on a liquid or solid surface. This gas necessary that continuously re- compelled the mission planners to plenishes the amount lost. Further- take a broad variety of scenarios more, under the temperature and into account. pressure regime prevailing on Ti-

Fig. 2: Size comparison of Earth Earth’s Moon, and it is 80 % more mas- mede, and is larger than the smallest with the Moon (left) and Titan (right). sive. It is the second-largest moon in the planet, Mercury, although only 40 % as Titan’s diameter is 50 % larger than Solar System, after Jupiter’s moon Gany- massive. (Credit: NASA)

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163251_Spatium_38_2016_(001_016).indd 4 26.09.16 15:30 Exciting Titan to further reduce the speed down to 300 km/h. At 160 km above Swiss High Technology ground, the heat shield and the on Titan back cover were jettisoned allow- The then Contraves Space On Christmas Day 2004, Cassini’s ing the probe’s instruments to fully Company, Zurich, together on board computer generated a take up their scientific tasks, which with the Vevey based APCO short but nonetheless important they executed with perfect pre- were responsible for three main signal. It activated the Swiss made cision. subsystems of the Cassini-Huy- spring and eject device, which had gens twin spacecraft: once re- attached the Huygens probe safely leased, the spin and eject device to the Cassini main craft for the Titan’s Atmosphere provided the Huygens probe past seven years (see box to the with the required thrust towards right). The mechanism now pro- Titan is unique in many respects. its final destination Titan. The vided the probe the required spin It is the only moon in the Solar Sys- heat shield reduced the probe’s and velocity vector towards its fi- tem that features a fully developed speed during atmospheric entry nal destination, Titan, still 4 mil- atmosphere consisting of more while the back cover shielded lion km away (Fig. 3). Twenty days than just trace gases. It was known the suite of science instruments later, Huygens reached Titan’s to the mission planners that Titan during the seven years of inter- outer atmosphere at a relative speed has a dense, opaque, aerosol-rich planetary cruising. of 18,000 km/h, see Fig. 4 overleaf. atmosphere consisting mostly of The heat shield reduced the veloc- nitrogen mixed with methane and this complex atmosphere at various ity to 1,400 km/h allowing the traces of some ten additional mol- altitudes and determining the tem- probe to eject the first parachutes ecules. Probing the composition of perature and pressure profile were

Fig. 3 Probing the Saturnian system: release of ESA’s Huygens probe (to the destination Titan in the lower back- NASA’s Cassini spacecraft is shown here lower left) heading toward its ultimate ground. (Credit: NASA) in the centre a few moments after the

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163251_Spatium_38_2016_(001_016).indd 5 26.09.16 15:30 Fig. 4: Huygens’ landing sequence: chute, which in turn opened the first ture of –180 °C. For about 70 minutes, The probe dived at a speed of main parachute, as well as the ejection it remained active before falling into an 18,000 km/h into the atmosphere’s of the back cover. At a velocity of eternal silence. The radio signals from outer layers at an altitude of 1,270 km. 95 m/s, the front shield is jettisoned and Titan need some 1.5 hours to reach our The large heat shield served to reduce replaced by a small stabilizer parachute planet. This precludes controlling the the entry velocity down to 1,400 km/h that allows the probe to drift slowly descent sequence from Earth and makes at about 150 km altitude. At this through the atmosphere. Upon surface a fully automatic mode of operation a moment, Huygens’ onboard computer impact, the probe sat calmly on the must. (Credit: after an ESA image) ordered the ejection of the pilot para- moon’s surface at an ambient tempera-

therefore amongst Huygens’ chief perature was in the order of –100 °C ness of the atmosphere, the objectives. with strong variations of some atmospheric pressure at the land- 20 °C due to inversion layers. Be- ing site was about 1.5 times that at low, the temperature increased the surface of the much more mas- Atmospheric Temperature rapidly, reaching a maximum of sive Earth. and Pressure Profiles –87 °C at 250 km. Further down, the temperature decreased steadily The main instrument for probing again throughout the stratosphere, Atmospheric Dynamics the temperature and pressure dur- reaching a minimum of –203 °C at ing descent was the Huygens the boundary between the strato- The term dynamics refers to the Atmo spheric Structure Instrument sphere and the troposphere. Then, movement of the atmospheric par- (HASI). In contrast to expecta- the temperature increased again as ticles at different altitudes. During tions, the atmosphere turned out the probe neared the surface, ris- descent, the Huygens probe was to be highly stratified (Fig. 5). ing to a chilly –180 °C at the land- attached to parachutes and their rel- Above 500 km, the average tem- ing site. Due to the extreme thick- ative movement projected down to

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163251_Spatium_38_2016_(001_016).indd 6 26.09.16 15:30 chemistry instruments aboard. One of the most important was the Gas Chromatograph and Mass Spec- trometer (GCMS) for determining the chemical composition of Ti- tan’s atmosphere during descent. As only very little was known when the mission was planned, the GCMS had to fulfil extreme re- quirements. It had to combine an excellent sensitivity (10 parts in 1 billion molecules) with a very wide range of particles (molecular mass range from 2 to 146 amu, where one atomic mass unit amu is equal 1 12 to ⁄12th of the mass of a C atom). The GCMS performed excellently delivering detailed mass spectra during descent (Fig. 6 on the next page).

Formation and Evolution of Fig. 5: Titan’s upper atmosphere consists of a surprising number of different the Atmosphere3 layers of haze, as shown in this ultraviolet image of Titan’s night side limb gathered by Cassini’s cameras. These layers extend several hundred kilometres above the surface, much higher than the Earth’s atmosphere. (Credit: NASA) Planetary atmospheres are supposed to have formed in either one or a combination of two principal the ground allowed for an estima- winds are supposed to be driven by ways. During Solar System formation of the atmospheric dynamics interactions of the atmosphere with tion, a body’s gravity attracted at various heights. In fact, the probe the solar wind. parts of the surrounding gas and experienced a rough ride: over the dust constituting the protosolar duration of the entire descent, the nebula. The gaseous components probe glided eastward for a distance Chemical Composition of of this matter then built the atmos- of 166 km. The winds were gener- the Atmosphere phere, while the solids contributed ally prograde, that is in the same to the body’s mass. Later, planetes- direction as the moon’s rotation. Of course, the outstanding com- imals – the small evolving bodies The wind speed peaked at roughly plexity of Titan’s atmosphere was circling the early Sun – were cap- 120 m/s at an altitude of about one of the key drivers for launch- tured and their material integrated 120 km. This is much faster than ing a mission to Titan and an ex- into the emerging planetary body. the rotation speed on the moon’s ceptional challenge for the scien- The impact of gas-rich planetesi- equator. This strange effect is called tists and engineers who built the mals is supposed to have contrib- super-rotation, known to exist on instruments. In fact, Huygens had uted further to building up the Venus as well. Such super-rotating an entire suite of atmospheric atmosphere.

3 See also Spatium no. 36: Formation and Evolution of Planetary Atmospheres by Helmut Lammer, November 2015.

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163251_Spatium_38_2016_(001_016).indd 7 26.09.16 15:30 A variation on this theme is accre- nebula at those times. In contrast, Atmospheric Nitrogen tion from a characteristic sub-neb- the rarity of noble gases in the ula in the region surrounding an Earth’s atmosphere is interpreted The most abundant constituent is emerging giant planet. The com- as being a consequence of plane- nitrogen, and its origin remains position of such an atmosphere tesimal influx, and the near ab- one of Titan’s great mysteries. In seems to reflect a special blend of sence of noble gases on Titan pro- the stratosphere, some 300 km solar nebula accretion and degas- vides further support for this above ground, nitrogen reaches sing from planetesimals: Jupiter for hypothesis. concentrations of 98.4 %. As the instance has an endowment of Earth’s atmosphere also contains heavy noble gases and other heavy some 80 % nitrogen, one could in- elements relative to hydrogen that fer that the atmospheres of planets is greater than existed in the solar and moons would contain nitro-

Fig. 6: The chemical composition of wealth of chemical reactions is taking ane). Further notable constituents are Titan’s atmosphere. The two panels place there. Further, the similarity of the noble gas Argon 40Ar, a series of in- present the overall mass spectrum gath- the two diagrams suggests that the sur- creasingly complex organic molecules ered at an altitude of around 120 km and face is soaked with products from the beginning with ethane (C2H6), carbon on the surface. A first glance provides chemistry taking place in the u pper at- dioxide CO2, cyanide (C2N2), benzene an impression of the incredible com- mosphere and raining down onto the (C6H6) up to 140 atomic mass units. plexity of the chemical composition of surface. As expected, the main compo- (Credit: ESA) Titan’s atmosphere testifying that a nents are N2 (nitrogen) and CH4 (meth-

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163251_Spatium_38_2016_(001_016).indd 8 26.09.16 15:30 gen in major amounts. Yet, this is Atmospheric Methane tities of liquid methane are as- not the case; rather, Earth and Ti- sumed to be trapped in ice beneath tan are notable exceptions. This led An even greater mystery is the the surface, possibly reaching the scientists to speculate that the ac- presence of methane in Titan’s at- surface through some form of tual atmosphere of Titan might re- mosphere because the Sun’s ultra- cryo-volcanism. This activity semble that of our planet in its early violet radiation destroys methane, could indeed replace the methane pre-biotic time that is before liv- which would disappear completely lost by photochemistry in the up- ing organisms began enriching it after some 50 million years. There- per atmosphere. with oxygen via photosynthesis. fore, there must be a mechanism at work that continuously replenishes Beyond its very presence, methane In order to allow study of the pos- the losses incurred. stands out by the role it plays on sible origins of nitrogen, the Titan, which strongly resembles GCMS tracked the ratio of a vari- In order to shed light on this se- the role of water on Earth. On our ety of isotopes and trace species cret, the GCMS tracked the levels planet, water evaporates from the during descent. More specifically, of methane in the atmosphere. In- oceans to the atmosphere, from it focussed on heavy noble gases itially, in the highest atmospheric where it rains down to the surface such as argon 36Ar, 38Ar, krypton layers, it found only low concen- again. River systems collect the (Kr), and xenon (Xe). These gases trations of methane. Then, at some water and bring it back to the had been identified earlier in me- 40 km, the concentration started oceans thereby closing the water teorites, in the atmospheres of increasing gradually until at ap- cycle. On Titan, the temperature Earth, Mars, Venus (to some ex- proximately 7 km the concentra- is far too low for liquid water, but tent), and Jupiter. They were pos- tion reached saturation level. Fur- it is in the right range for methane sibly present throughout the solar ther down, methane concentrations to be liquid. In fact, there exists a nebula before the planets and remained relatively constant until complex methane cycle embracing moons formed, and should there- the probe touched down on the the surface lakes, the clouds and the fore have been incorporated into surface. Immediately after landing, methane rains. A further analogy both Saturn and Titan during the the methane concentration in- deserves attention: on Earth, the early stages of planetary system for- creased by about 40 %, while the pressure and temperature values mation. Condensation of gases on nitrogen count rate remained con- are close to the triple point of wa- the young Titan would have re- stant. This suggests the presence of ter, where water can exist either as sulted in the capture of 36Ar, as liquid methane on the surface sput- solid, liquid or gas. On Titan, the well as nitrogen, in proportions tered by the incoming probe or by pressure and temperature values present in the protoplanetary disk the spacecraft heating the surface are very close to the triple point of at those times, for which the com- material. methane, allowing it similarly to position of today’s Sun is a good exist in the solid, liquid or the gas- proxy. However, the depleted ni- While on Earth, methane is nor- eous state. This makes liquid and trogen ratio detected by Huygens mally a product of biochemical solid methane possible at the implies that it was captured as am- processes, measurements of the moon’s surface.

monia (NH3) or in other nitrogen- carbon isotopes contained in Ti- bearing compounds. Subsequent tan’s methane ruled out the possi- photolysis in a hot proto-atmos bility of production by micro- Noble Gases in the Atmopshere phere generated and heated up by organisms. This leads to the the accreting Titan or possibly hypothesis that Titan’s methane Noble gases along with their iso-

impact-driven chemistry of NH3 stems from underground (or sur- topes play a key role when it comes is supposed to have led to the ni- face) reservoirs. It is thought that to hypothesising on the evolution trogen atmosphere we have on Ti- Titan had accreted methane dur- of Titan’s atmosphere. Out of the tan today. ing its formation, and large quan- numerous components found, 40Ar-

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163251_Spatium_38_2016_(001_016).indd 9 26.09.16 15:30 gon stands out, a noble gas that does not exist naturally, but is al- ways a by-product of radioactive decay of potassium in rocky material. The presence of 40Ar is a very strong support in favour of a rocky core at the centre of Titan as we will see below.

Atmospheric Aerosols

Aerosols are tiny solid or liquid particles suspended in a gaseous en- vironment. To address the role of aerosols in Titan’s atmosphere, the Huygens probe carried the Aero- sol Collector and Pyrolyser (ACP) experiment aboard. This instrument collected aerosol particles and heated them in a special oven to vaporise all the volatile components in order to analyse their chemical composition.

The ACP acquired two atmo spheric samples during the descent, one at an altitude of 130 km and the other at 25 km. The main substances found in the aerosols were

ammonia (NH3) and hydrogen cyanide (HCN) confirming that carbon and nitrogen are major constituents of the aerosols. The two samples did not show substan- tial differences, which indicates that the aerosols’ composition was probably the same at both altitudes. Fig. 7: The emergence of aerosols in the atmosphere of Titan. When sun- This finding supports the idea that light (or highly energetic particles from Saturn’s magnetosphere) hits the upper layers of Titan’s atmosphere, the nitrogen and methane molecules are broken up. This the aerosols have a common source results in the formation of massive positive ions and electrons, which trigger a chain in the upper atmosphere, where, of chemical reactions, producing a variety of hydrocarbons. These reactions even- the ultraviolet part of the sunlight tually lead to the production of carbon-based aerosols, large aggregates of atoms splits the atmospheric gases such as and molecules that Huygens found in the lower layers of the atmosphere. Large carbon-based molecules form from aggregations of smaller hydrocarbons high up in methane allowing the emergence the atmosphere. Upon reaching a certain size, they drift down much like snow- of the aerosols (Fig. 7). flakes and eventually give rise to aerosols. (Credit: ESA)

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163251_Spatium_38_2016_(001_016).indd 10 26.09.16 15:31 In parallel to the chemical analy- Dry river beds and lakes probe during the very moment of ses of the aerosols, the Descent Im- landing. Fig. 9 shows the penetra- ager/Spectral Radiometer (DISR) Immediately before touch down, tor signal produced during land- characterised their optical proper- Huygens made the first physical ing. The peak force registered at ties. The results show that they contact with the surface by means 8 ms is thought to be caused by hit- strongly resemble tholins4, mole- of a small penetrometer consisting ting a smaller pebble, as the big cules which can be produced in the of a rod or stick protruding through hard pebbles that can be seen in laboratory by sending electrical the probe’s hull. Its purpose was to Fig. 8 would have broken the in- discharges into mixtures of nitro- measure the forces acting on the strument. The subsequent part of gen and methane. This provides an the signal representing medium indication regarding their produc- strength forces is interpreted as tion mechanisms (Fig. 7). Fig. 8: A close-up view of Huygens’ pushing down to the underlying landing site resembling a dried-up softer, granular material, probably river or lakebed. Rounded cobbles, 10 to 15 cm in diameter and probably made Titan’s equivalent of sand. Not- Titan’s Surface of hydrocarbons and water ice, rest on withstanding the fact that the a darker granular surface. These pebbles penetrometer was quite a simple are rounded which is indicative of an Huygens’ descent lasted about 150 erosion process by liquids, most proba- instrument, its signal provided a minutes. Then, the probe made a bly liquid methane. (Credit: ESA/ great deal of information on the softish landing on Titan’s surface NASA) character of Titan’s surface at the and shortly after sent the first pic- landing site. ture to a nervous audience staring at their TV screens on Earth. It was This information is complemented a long awaited, spectacular mo- by the imagery gained by the Cas- ment: Huygens had survived the sini orbiter during its more than touchdown and started working on 150 fly-bys made since its entry the surface. The first image shows into the Saturnian system. Surpris- a dry river or lakebed with pebbles ingly, the dense haze, that charac- and sand-like material (Fig. 8). The probe recorded an ambient temperature of about -180°C. Despite Fig. 9: The Huygens penetrometer the frosty temperature, the probe’s signal during landing. The horizontal axis is calibrated in units of milli thermal control system protected seconds, while the vertical axis shows the sensitive onboard electronics the force of Titan’s surface acting on the and instruments, and during the penetrator probe. (Credit: ESA) following 72 minutes, a huge amount of unique data was collected and subsequently down- loaded via the Cassini orbiter.

4 T he term “tholin” was coined by the American astronomer, Carl Sagan (1934–1996) to describe substances he obtained in experiments simulating the gas mixtures in Titan’s atmosphere. Tholins are not one specific compound but rather are descriptive of a spectrum of molecules that give a reddish, organic surface covering on certain planetary surfaces.

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163251_Spatium_38_2016_(001_016).indd 11 26.09.16 15:31 terises Titan’s atmosphere, offers Dunes some narrow spectral windows where Cassini’s cameras could peer Besides seas and lakes, there are ad- right down to the surface. In addi- ditional features on Titan that re- tion, Cassini’s radar was able to im- semble areas on Earth: the dunes. age the surface undisturbed by the They cover various parts of Titan’s haze. The results of this survey surface and are probably composed were nothing less than a sensation of sand-sized hydrocarbon and/or (Fig. 10): it turned out that Titan nitrile grains mixed with water ice. and Earth are the only places in the In contrast, however, to dunes on Solar System where stable liquids Fig. 11: A telling image of Titan’s Earth, the particles constituting exist on the surface. While on surface: here, Cassini’s infrared cam- the dunes on Titan are supposed to era looks back after a fly-by. A bright Earth, the lakes are made of water, red spot appears on the surface almost have rained down from the atmos on Titan they probably consist of blinding the instrument. It is the Sun’s phere. Later, they may have been liquid methane. The hypothesis of specular reflection on Cracken Mare, eroded by liquid methane runoff liquid surfaces was corroborated by one of the many methane lakes of Ti- and moved by eolian processes. tan. The picture is a stunning proof of an infrared image gained by chance the existence of liquid surfaces on Ti- (see Fig. 11). tan. (Credit: NASA) Sizes and patterns of the dunes vary as a function of altitude and latitude. The dunes in areas that are more elevated or in higher latitudes Fig. 10: This false colour radar image from the Cassini orbiter shows a surface strip of Titan’s surface. The yellow-hued terrain appears to be peppered with tend to be thinner and more widely blue-tinted lakes and seas of liquid methane. (Credit: NASA) separated, with gaps that have a thinner covering of sand. Dunes in lower altitude and latitude regions are wider, with thicker blankets of sand between them. Comparing these features with dunes on Earth shows that the Kalahari dunes in South Africa and Namibia, located in regions with limited sediments available resemble the first type of dunes while on the other hand, Earth’s Oman dunes in Yemen and Saudi Arabia, where there is abundant sediment available, resemble the second type more.

This altitude effect suggests that the material building the dunes is found mostly in the lowlands of Titan. Saturn’s elliptical orbit around the Sun along with Titan being tidally locked to Saturn make the summers on the moon’s southern hemisphere shorter and warmer than in the northern hemi

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163251_Spatium_38_2016_(001_016).indd 12 26.09.16 15:31 sphere. This leaves the soil in the tan’s surface show that some of its surface, the cavity’s lower bound- south possibly dryer due to in- features are in the wrong place at ary reflecting the ELF signals could creased evaporation during the the wrong time! Obviously, the be an electrically conductive ocean hotter summer time. In contrast, rotation of Titan’s surface is not as of water and ammonia below a in the northern hemisphere, the expected. At a much finer level, non-conducting icy crust. soil hypothetically contains more one can make similar observations moisture, making it more difficult on Earth and Mars, where the at- to build dunes and to move their mosphere and the solid body con- A Rocky Core? particles because they are stickier tinuously transfer momentum be- and heavier. tween each other. With the The indication for a rocky core impressive mass contained in Ti- comes from the Argon isotope 40Ar tan’s thick atmosphere one might detected by the Huygens Gas Titan’s Interior presume a similar process, but it is Chromatograph Mass Spectrome- much bigger than expected. The ter. This noble gas does not exist The data provided by Huygens and only way to explain this observa- naturally, rather, it is the exclusive Cassini allows speculation about tion is to say that the surface is de- decay product of potassium 40K the moon’s interior. The currently coupled from the bulk of the core contained in radioactive rock. This preferred physical models suggest by floating on an ocean of liquid noble gas offers, therefore, a unique a core of solid silicate rock as is the water in between. window to the interior of Titan as case with most bodies of the Solar the only possible source of 40Ar is System. Then, above, follows a Further results point in the same the rock existing deep in Titan’s layer of water ice under high pres- direction: Huygens had instru- interior. sure, and possibly a large subsur- ments aboard that allow the detec- face ocean covered by the moon’s tion of lightning in the moon’s at- icy surface. mosphere. On Earth, thousands of Cryo-volcanism? lightning flashes take place every minute, and each bolt generates a The term cryo-volcano refers to a A Subsurface Ocean? radio crackle. They propagate in geologic feature working at very the form of extremely low fre- low temperatures, where material The argument for a water-based quency (ELF) pulses around the from the underground protrudes ocean below the surface is based on globe in the cavity formed by up to the surface. On Titan, there extremely precise observations of Earth’s surface and the ionosphere, are possibly about half a dozen of Cassini’s trajectories during the a region of electrically charged par- such cryo-volcanoes (see Fig. 12 fly-bys. The orbital parameters of ticles in Earth’s upper atmosphere. overleaf). The presence of 40Ar at these trajectories allow for the es- Although Huygens did not detect the levels seen by Huygens is a timation of the pull of gravity and any lightning or thunderstorms, it strong indication of geological its fluctuations and since gravity is registered an ELF signal at around activities on Titan consistent with an indication of the distribution of 36 Hz. In order to explain this the assumed periodic replenish- matter within the moon, these data unique signal, scientists have pro- ment of atmospheric methane. In allow for the building of models of posed that Titan’s atmosphere fact, cryo-volcanism provides one Titan’s interior. From these obser- might act like a giant electrical cir- possible process for release of meth- vations, one can deduce the exist- cuit where electrical currents are ane and 40Ar from the interior out ence of a global ocean of water. generated in the ionosphere when into the atmosphere. it interacts with Saturn’s magneto- Such an adventurous hypothesis sphere. While Huygens found elec- Besides the circular features inter- needs corroboration by independ- trically conductive ionospheric lay- preted as cryo-volcanoes, there are ent findings. Radar images of Ti- ers at about 60 km above the also a few patterns interpreted as

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163251_Spatium_38_2016_(001_016).indd 13 26.09.16 15:31 impact craters (Fig. 13). On Titan, we see at most ten such features. While most surfaces in the Solar System are peppered with impact craters, Titan’s surface is very sparsely cratered. There are probably two reasons for this. One is the thick atmosphere, which acts as a filter, causing the smaller im- pacting bodies to burn up before reaching the surface. The other factor is the dynamic nature of the surface. Weathering by wind and liquids obliterate the impact craters over long time spans. This is an- other indication, that the surface is active.

Fig. 13: Sinlap is a relatively fresh impact crater with a diameter of about 80 km. (Credit: NASA)

Fig. 12: A surface feature that is in- peratures. Lava welling up to form the with other ices and hydrocarbons. The terpreted as a cryo-volcano, a type of volcanic mound is a semifluid of meth- circular feature is roughly 30 kilome- volcanism operating at very low tem- ane, ammonia, and water ice combined tres in diameter. (Credit: NASA)

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163251_Spatium_38_2016_(001_016).indd 14 26.09.16 15:31 Life on Titan? Outlook veniently at close quarters. From what we know about the winds, it would take something like two The similarities of Earth’s and Ti- The Cassini-Huygens mission is a weeks to execute an entire circuit tan’s atmospheres were one basic seminal success not only from a sci- at the equator to find the most in- motivation for ESA and NASA to entific point of view but also with teresting places where a lander send a mission to Titan. Upcom- regard to the effective transatlan- could then address specific issues. ing speculations about the exist- tic cooperation between the two An alternative could be a probe ence of some unknown form of space agencies ESA and NASA. floating on one of Titan’s many life provided an additional thrust Yet, unsurprisingly, many ques- lakes. In any case, Titan remains to embark on such an outstand- tions remain open. This gives am- an outstandingly compelling des- ingly daring mission. Now, after ple motivation to plan a further tination for future science its completion, the similarities have mission to that miraculous, distant missions. become even more compelling, world which in many respects is so but also the differences between astonishingly similar to ours. Such the two. a new mission could exploit the detailed information delivered by The similarities refer to the organic Cassini-Huygens and the special chemistry taking place in Titan’s conditions prevailing on Titan: its atmosphere that resembles the thick atmosphere and low gravity 1 chemical processes on our early (about ⁄7 of Earth’s) may be an planet that possibly paved the way ideal place for an approach based for the emergence of life. In addi- on aerial vehicles: from a balloon tion, Titan has a greenhouse effect or a Montgolfier, cameras could like Earth contributing to stabilis- observe the moon’s surface con- ing the global climate. Geological similarities are striking as well: liquid bodies, river networks, Fig. 14: Constituents of a hypothetical follow-on mission to explore Titan dunes, mountains and hills, and include an aerial platform, an orbiter and a lander. possibly even some sort of tectonic activity may be at work. The chief difference is temperature: while on Earth, the mean surface temperature is around 17 °C, on Titan it is a mere −180 °C. Therefore, water at the surface exists in the solid state exclusively; yet methane may assume the role of water on Titan as a solvent for living systems. In addition, water-based living systems might evolve in Titan’s warm subsurface ocean. The Cassini- Huygens mission did not provide evidence for any form of life on Ti- tan; in fact, it was not equipped to do so.

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The Author

John Zarnecki grew up in London, In 2000, John Zarnecki, along England. In his early years, space with the other members of the Sur- and space exploration attracted his face Science Package team, moved attention and it is no wonder that to the Open University in Milton the youngster was amongst the pu- Keynes, in central England, where blic when Yuri Gagarin, the first he became Director of the Centre human astronaut visited L ondon in for Earth, Planetary, Space & Astro 1961. John started his studies at nomical Research, some years Queens’ College, Cambridge, later. In 2013, Professor Zarnecki where he graduated with a physics was invited to take up a Director degree before obtaining a docto- ship at the International Space rate at the Mullard Space Science Science Institute in Berne, Laboratory (University College Switzerland. London) in Surrey. Prof. Zarnecki has given long and Prepared with a sound scientific distinguished service to the global education, John Zarnecki started space community, working in nu- his professional career at British merous international teams and Aerospace working in a team de- committees such as for instance for veloping the Hubble Space Teles- ESA’s Senior Review Committee, cope’s Faint Object Camera. Then, charged with selecting the scienti- in 1981, he moved to the Univer- fic themes that would form the ba- sity of Kent in Canterbury to be- sis for ESA’s L2 and L3 missions in come the project manager for the 2028 and 2034, respectively. Dust Impact Detection System on board ESA’s Giotto probe that vi- An outstanding, charismatic per- sited Halley’s Comet. Yet the most sonality, Prof. John Zarnecki has exciting programme was still been granted numerous honours of ahead: the Huygens probe sched- which the Gold Medal of the Ro- uled to land on Saturn’s moon yal Astronomical Society in 2014 Titan, some 1.5 billion kilometres stands out. away. John Zarnecki was appoin- ted Principal Investigator for Huy- gens’ Surface Science Package in 1990. The instrument collected over three and a half hours of data, which, thanks to its efficient en coding, could be stored on board the lander before being relayed by the Cassini orbiter down to Earth.

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