This year the prestigious Grubb-Parsons Lecture was entitled Touchdown on Titan and detailed the incredible Huygens probe landing on Saturn’s moon Titan, surely one of the most ambitious and arguably the most successful mission in the history of unmanned space exploration.

The lecturer was the Principal Investigator for Huygens, Prof. John Zarnecki of the Open University, who gave an interesting and informative talk that suffered only from running short of time. This is my summary of the talk with my memory supplemented by the various mission websites and of course, Wikipedia.

The Huygens probe hitchhiked its way 1.3billion km across the Solar System (and in fact much more than that due to the number of gravity assists the mission required) riding piggyback on NASA’s Cassini probe which has subsequently being exploring the Saturnian system.

After launching in 1997 Cassini-Huygens entered orbit around Saturn on 1st July 2004. On its third orbit the Huygens probe was released and spent 3 weeks (whilst the Cassini probe was manouvered into position so that it could transmit data back to Earth during the descent) making its way to Titan.

Named after the 17th century Dutch astronomer, Christiaan Huygens, the Huygens probe was designed and built by the European Space Agency (ESA). It was designed as a lander, making a descent through the atmosphere using a series of parachutes before touching down on what was, at the time of design, an unknown surface – it was not known whether the landing site would be an icy mountain range, a plateau, or even a lake or ocean. Our knowledge was compromised by Titan’s thick smoggy atmosphere which had proved inpenetrable to the cameras aboard the Viking 2 probe that passed in 1981.

It was a quite incredible piece of design – launching sensitive instrumentation atop a rocket, spending over 6 years dormant during the cruise across the solar system (the instruments were woken up and tested every 6 months during this period), then landing on a completely alien world, the conditions of which no-one could predict.

The instruments aboard were as follows:

• Huygens Atmospheric Structure Instrument – designed to measure the physical and electrical properties of the atmosphere
• Doppler Wind Experiment – designed to measure atmospheric wind speed
• Descent Imager/Spectral Radiometer – designed to image Titan and investigate suspended particles in the air
• Gas Chromatograph Mass Spectrometer – designed to analyse gases and chemicals in the atmosphere
• Aerosol Collector Pyrolyser – collected and processed samples for the GC/MS
• Surface Science Package – designed to make investigations of the physical properties of Titans surface at the landing site – specifically it would investigate the properties (motion, desnity, temperature etc) of any ocean should one be encountered – this part was designed by John Zarnecki and his team at the Open University

It was intended that the mission wold last for approximately 2.5 hours during which the descent would take place, and then there would be about 3 minutes of data collection after touchdown. In the event the descent happened exactly as planned, although strange anomolies occured in the rotation of the probe which confused those trying to assemble mosaics of images, and the probe was carried significant distances by 400kph winds in the high atmosphere which reversed direction half way down. It landed on what turned out to be an icy, damp ocean or lake bed, and the surface instruments were able to send data back for 2 hours before Cassini was no longer in a position to relay information. Radio Telescopes on Earth were able to detect the probe for even longer than this although sadly no data could be relayed.

The findings from Titan have been astounding, both in their detail and for their scientific value. Images captured during the descent show fascinating terrain with rolling hills, apparent shorelines, and best of all, river systems with tributaries carving channels between the hills. It has since been theorised that these features are caused mainly by erosion by violent hydrocarbon rainfall (though the raindrops are large and move slowly there are a lot of them concentrated at one time, sufficient to cause such gouging).

This is a summary from the ESA website concerning the findings based on the above image.

• Surface ridge around 100m tall
• Channels are evidence of rain
• Dark material are photochemical smog deposits
• Ridges made of frozen hard water ice
• Some evidence for fluid flow in the form of methane
• Evidence for Earth like processes such as precipitation, but with different materials
• Possibility of rainfall relatively recently

The landing site was not an ocean but what is thought to be a drying ocean bed. The most incredible image returned is that from the surface itself. It shows what appear to be rocklike features, but these are in fact ice “cubes” which have been worn smooth by the effect of liquids. Furthermore instruments detected a signficant rise in methane concentrations after landing indicating that the heat of the probe vaporised the methane-ice around it showing that the surface around the lander was damp.

John had to skip through much of the findings, but did highlight some of the results of spectroscopy (showing the hydrocarbons) and gave some idea of the work now being carried out by Cassini to discover the locations of actual liquid oceans, which thanks to detailed radar imaging, have now been confirmed.

During his talk John also emphasised the strength of the team which took Huygens successfully to Titan, and the incredible work that was done to pack so much science into a prope that weighed just over 300kg, and had to be protected through a tough atmospheric descent into a hostile environment.

I find everything about the Cassini-Huygens mission fascinating and awe-inspiring and it was a privilege to get an insight into this mission. John also gave a tantalising glimpse of the next mission to Titan which includes a balloon to allow for a much longer mission, investigating far more of the planet and this time landing in liquid. This is definitely one to keep an eye on.

There is lots more information on the ESA Huygens Mission website