Ordinary Meeting, 2005 March 30

 

Titan Revealed – First Results from Huygens

Huygens, Dr Green opened by explaining, was a European-built landing probe, which had descended into the atmosphere of Saturn's moon Titan on January 14. It had been carried there by NASA's Cassini spacecraft, which remained in orbit, on an ongoing mission to study Saturn's entire system, including the rings, moons and magnetosphere. He added that, weighing five tonnes at launch, standing 6.7 m tall, and having a $3bn price tag, Cassini was something of a dinosaur; it was very unlikely that NASA would send another similarly sized craft to the outer planets again. Indeed, it seemed likely that it had survived NASA's budget cuts of the early 1990s, to which CRAF, for example, had fallen victim, only because of their agreement with ESA to fly Huygens.

A 2.7-m-diameter disc-shaped entry vehicle weighing 318 kg, Huygens had been selected by ESA as its first 'Medium Mission' in 1989; Cassini had been on NASA's drawing boards since the early 1980s. Both had finally been launched from Cape Canaveral on 1997 October 15 aboard a Titan IVB rocket with Centaur upper stage. Their interplanetary trajectory had taken them through two gravitational slingshots from Venus, one from the Earth, and then finally one by Jupiter, before arriving at Saturn on 2004 July 1.

Turning to Huygens' eventual destination, Titan, Dr Green explained that, at 5150 km across, it was larger than both the Moon and Mercury, and that it was the only planetary satellite in the solar system with an atmosphere. It had been discovered telescopically by Christiaan Huygens in 1655, after whom the present probe had been named. However, little had been known about it before this mission: its atmosphere appeared to be composed mostly of nitrogen, with methane and traces of other hydrocarbons, but the surface below was obscured by photochemical haze at 200-km altitude. Voyager had suggested that on the surface, the temperature was ~ -175°C, the pressure ~1.5 times that on Earth, and the gravity one-seventh of terrestrial.

The scientific interest of Titan, he explained, stemmed partly from the belief that it had formed from the same primordial material as the Earth: its atmospheric chemistry might resemble a deep-frozen, preserved copy of that from which life on Earth had developed. It was known that its atmospheric methane could be broken apart by cosmic rays, yielding a reactive ionised form, which would readily polymerise to form hydrocarbons. These in turn were the building blocks for more complex organic molecules. But here lay a puzzle: the estimated lifetime of its atmospheric methane was only one million years – within this time, the bulk of it would become dehydrogenated. The liberated hydrogen atoms would rise up buoyantly through the atmosphere and escape, making the process irreversible. Why then was methane still seen today; was some reservoir continuously replenishing it? As a final curiosity, the surface temperature and pressure lay close to methane's triple point, allowing it to transmute readily between solid, liquid and gaseous forms, potentially driving unusual weather systems and erosion.

Dr Green recalled that Huygens had originally been planned purely as an atmospheric probe; a landing science package had only been added after considerable debate, motivated perhaps by the memory of the 1978 Pioneer Venus Multiprobe mission, one of whose atmospheric probes had continued to function after touching down, provoking embarrassing criticism of the lack of measurements planned for this eventuality.

Upon arrival in the Saturn system, Cassini-Huygens had been carefully aimed to pass northward through the gap between the outer F and G rings. This was perhaps the riskiest moment of the entire mission: whilst this gap was believed to be comparatively void of ring particles, a single collision could have destroyed the craft. To minimise the danger, it had rotated such that its high-gain antenna, its most resilient part, faced the direction of travel during this passage. Then, after a 20-minute orbital insertion burn by Cassini's engine, skimming a mere 20,000 km above Saturn's cloud-tops, it had returned southward through the same gap on the opposite side of the planet, never again to venture so dangerously close to it during its four-year mission.

Originally, it had been planned that Huygens would be released during Cassini's first orbit. The motivation for this was two-fold. Firstly, every orbit carried a small risk of a devastating impact with a ring particle, making it desirable to perform high-priority science as early as possible. Additionally, every manoeuvre by Cassini used more of its valuable fuel reserve while Huygens remained aboard.

However, Dr Green explained that an embarrassing problem, discovered mid-flight, had forced this to change. Huygens was to beam its data back to Cassini, to be later relayed to Earth. The problem was that as its speed changed upon entry into Titan's atmosphere, its radio signal would be redshifted. Cassini's antenna had a wide frequency response to account for this, but engineers had overlooked the effect of the redshift on the data rate, rendering the antenna quite unable to synchronise to Huygens' transmission. This could not be rectified by software patch, and so the geometry of the mission had had to be changed to place Huygens' entry path into Titan's atmosphere perpendicular to its line of sight to Cassini, to minimise the effect.

This was found to be possible if Huygens' release was delayed until Cassini's third orbit; the speaker personally felt that this arrangement had actually been preferable to the original plan. Giving a preview of what was to come, Cassini had imaged Titan from a distance of 338,958 km on July 3, prior to Huygens' release, though this had been too late for any change of landing site. Imaging at infrared wavelengths, sensitive to the presence of water and complex organic material, it had revealed features appearing remarkably like oceans of hydrocarbons.

Turning briefly to Cassini, the speaker displayed a gallery of its finest images, revealing Saturn's weather systems, and the cratered surfaces of its moons, in astounding detail. During its four-year, 74-orbit, mission, it would have close approaches with seven of the 35 known moons. His personal interest lay primarily in the dust in Saturn's system, and so he showed some images of its rings, many taken in the midst of its close encounter with them during orbital insertion. Fine filamentary and wave-like structures had been identified in several of them, caused by gravitational interactions with the moons.

Images of the A ring showed density waves, not dissimilar to the spiral arms of galaxies, while those of the E ring showed, in unprecedented detail, how the moons Prometheus and Pandora on either side shepherded it, carving its sharp edges. Chemical analyses of the F ring had shown similarities with the moon Enceladus within it, supporting ideas that it had formed from the debris of meteoritic impacts. Studies of the grain sizes had revealed a trend of increasingly large particles going outward through the ring system; this might yield interesting insights into its origin, especially when combined with Voyager data to probe its evolution over the intervening two decades.

Returning to Huygens, Dr Green explained that it had been only a three-hour mission. Released from Cassini on 2004 December 25, it had cruised through space with no systems active except a wake-up timer, which had reawakened it on January 14, fifteen minutes prior to its impact with Titan's upper atmosphere, at an estimated altitude of 1,270 km, and speed of 6 km/s. After passing through its peak deceleration, and then opening a pilot chute, its main parachute had opened at an altitude of 160-180 km, slowing its descent to 80 m/s. A spring had released its heat-shield shortly thereafter, revealing the instruments beneath, and allowing the scientific work to begin.

After 15 minutes, its descent having slowed to 40 m/s, a smaller stabiliser parachute had replaced its main parachute, the motive here being simply to get it to the surface within its operational lifetime. After accelerating briefly to 100 m/s, the thickening atmosphere had slowed it to a leisurely 5 m/s before impact with the surface. The mission design lifetime was 153 minutes: 2.5 hours for the probe to descend through the atmosphere, and only three minutes of guaranteed operational time after touchdown – just enough to establish the nature of the surface upon which it had landed. In the event, it had returned data from the surface for 1 hour, 9 minutes and 36 seconds, terminated only when Cassini sank beneath its horizon and could no longer receive its telemetry.

Turning to Huygens' armoury of instruments, the speaker discussed the Huygens Atmospheric Structure Instrument (HASI) first, which had monitored Titan's atmospheric pressure, temperature, and density throughout the descent, as well as the gas' electrical properties. Among its most notable discoveries announced so far was that Titan was thought to be free of lightening.

Chemical analysis of the atmosphere had been undertaken by the Gas Chromatograph Mass Spectrometer (GCMS) experiment, looking especially for the presence of noble gases and organic species. Early results suggested that Titan's noble gases had been significantly depleted compared to those of other solar system bodies. The significance of this lay in the physical similarity of argon to nitrogen: if Titan's primordial argon had escaped, so too should its nitrogen, suggesting its present nitrogen to have formed later; the photodissociation of ammonia was one possible source.

A study of aerosol particles had been conducted by the Aerosol Collector and Pyrolyser (ACP), which had shared the GCMS's mass spectrograph. Its results were yet to be announced.

Perhaps the most eye-catching returns were from the Descent Imager and Spectral Radiometer (DISR), which had operated seven cameras. Some pointed downwards, capturing images of the surface below; others pointed sideways, capturing the horizon; others upwards, imaging the scattering of light around the Sun, from which information could be gleaned about the atmospheric density of aerosol particles. Dr Green remarked upon the challenge that DISR's engineers had faced: Titan's thick atmosphere absorbed 90% of the Sun's light and, together with its remoteness from the Sun, rendered it a very gloomy environment, 1,000 times darker than the Earth. In its descent, Huygens had been moving at high speed and spinning at 7 rpm, making long exposures impossible, while Titan's haziness had presented still further problems.

The speaker showed mosaics of DISR's images where it had been possible to match them together. He urged that these were only preliminary results: the full processing of images taken in such difficult conditions would take many months. Surface features and relief had come into view at an altitude of 20 km, he reported, discrediting previous theories that Titan's surface was a global ocean.

A Doppler Wind Experiment (DWE) had hoped to record information about wind speeds, using the redshift of Huygens' communication signal, as measured by Cassini, to monitor its speed as Titan's winds buffeted it. The same redshift would also be monitored from terrestrial radio observatories, gaining additional measurements of Huygens' speed in a different direction. Dr Green explained that at the time of launch, measuring Huygens' radio signal – at 10 W, comparable in strength to a mobile telephone – from the Earth had not been feasible. It had since become comparatively easy, and so this latter experiment had been planned in flight.

In the event, the Cassini-based DWE experiment had failed. To minimise data loss, Huygens had been designed to transmit two data streams, labelled 'A' and 'B', which were largely duplicates. Channel A had failed due to a software fault on Cassini, and, while most experiments had lost only minimal data where engineers had taken advantage of the two channels to transmit more data, the DWE, measuring the redshift of A, had lost all. Whilst it would have been little consolation for those who had worked on the instrument, the terrestrial experiment had provided a rough indication of the winds which the DWE would have measured.

Dr Green added that Earth-based detection of Huygens' telemetry had been the first indication of the mission's success. Mission scientists had known, upon seeing its signal, that the probe was still intact, and had even known that it had touched down safely when its signal had continued after a sudden redshift change around the predicted landing time, indicating a jolt. But, unable to decode the radio data from such a distance, they had faced an agonising wait for that to be forwarded by Cassini. As Cassini had been unable to turn its main transmitter to the Earth until it had finished listening to Huygens, this had meant a delay of three hours, plus a further 67 minutes while its signal travelled to the Earth. To add to the tension, the data of the doomed channel 'A' had been broadcast ten minutes before that of 'B', telling of the failure of the former before the success of the latter.

Turning finally to his own specialist interest, the Surface Science Package (SSP), with which the Open University had been most actively involved, the speaker explained that, without knowing in advance whether it would land on liquid, mud or solid, its design had been quite a challenge. The result had been a collection of nine separate sensory subsystems; in all three scenarios, some would be useless, while others would make scientifically valuable measurements. The package had been generally tailored towards a liquid landing – eight out of the nine working in that case – because Titan's eccentric orbit around Saturn was more easily explainable if it had a global ocean. If Titan's primordial liquid content had frozen at some point, it would be expected that the resulting solid crust would have tidally deformed as it moved closer and farther from Saturn, dissipating Titan's orbital energy and circularising its orbit within the age of the solar system.

In the event, Huygens had landed on solid ground, where four of the subsystems had been usable. Sonar ranging of the surface had started from an altitude of 100 m, producing a curious double-peaked echo, indicative of surface features on 10-m scales. Unfortunately this behaviour had ceased shortly before touchdown, and so the landing site might have been atypical. Estimates of the altitude dependence of the speed of sound, computed from the echo delay, suggested an increasing methane concentration close to the ground, supporting theories that it vented from Titan's surface. However, early indications were that the shore-like features which DISR had imaged did not neighbour liquid reservoirs at the time of Huygens' visit.

Another sensor had thrust a probe into Titan's soil upon touchdown, measuring the required penetration force. It had detected a sharp spike of resistance upon initial contact with the surface, after which it had slid in rather more easily and with steady force. At first sight this suggested a surface composition not dissimilar to that of crème brûlée, though a bounce from a surface pebble was another possible explanation. Laboratory tests were under way, searching for materials which reproduced this profile using a duplicate of the probe, attached to a rig which accelerated it to an appropriate impact speed.

Other sensors, developed to measure the refractive index and physical density of surface liquid, proved useless. Further experiments, accelerometers and tilt-meters to measure any rocking motion of Huygens due to surface waves, also proved useless on the surface, but returned valuable data during descent; Dr Green noted that they had detected friction with Titan's atmosphere from 1,400 km altitude. They had also detected an unexpected 7-rpm wobble in the craft's motion at high altitude; this might be explained aerodynamically if its centre of mass had been displaced from its geometric centre, causing it to move as it rotated, but the motion had intensified upon the opening of the stabiliser parachute, and that remained unexplainable.

To close, the speaker reminded members that the analysis of Huygens' data legacy had only just begun: many new discoveries would emerge in coming months. Even then, their true significance might not be fully appreciated for years, until new questions were asked of them. Looking ahead, new insights would be provided when Cassini used radar to map the landing site. Its planned orbit would allow it to start this work on its eighth orbit, in 2005 October, though much of it would have to wait until 2007-8.

Following the applause for Dr Green's full account of Huygens' discoveries, the President regretted to announce that there was no time for questions, before inviting Mr Martin Mobberley to present his regular Sky Notes.

Fairfield

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