Titan and the Huygens Mission Editorial
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INTERNATIONAL SPACE SCIENCE INSTITUTE Published Published by by the the Association Association Pro Pro ISSI ISSI No 15, November 20052005 Titan and the Huygens Mission Editorial 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. S PATI UM 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 com- plex tributary systems. On Titan, it is too cold for water to exist in liquid form, but methane plays a similar role there. While, how- ever, 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.