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 des- tination with cosmic speed: a fully automatic mode of operation was required. Furthermore, is the sur- face 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 af- ter touchdown. All these and many further challenges remain in the SPATIUM 37 2 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 This 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. SPATIUM 37 3 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) SPATIUM 37 4 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.