Prologue 1 The genesis of Cassini-Huygens w.-h. ip, t. owen, and d. gautier 1 Introduction: Titan viewed from Earth long that he had made with the help of his brother The reasonable man adapts himself to the world; the unrea- Constantjn, Jr. The objective lens was 2.24 inches in sonable one persists in trying to adapt the world to himself. diameter with a focal length of 10.5 feet. This focal Therefore all progress depends on the unreasonable man. length was required to overcome the chromatic aber- – George Bernard Shaw ration of the uncorrected lens. Using a magnification of 50, Huygens noticed a small “star” to the west of The exploration of the solar system has become an Saturn, and used another star in his field of view to important part of the glory and dreams of our civiliza- establish its location relative to the planet. The next tion. The study of the Saturnian system and Titan itself night he saw that the “star” had moved through the can be traced back more than 400 years to when Galileo sky with Saturn and realized that he had discovered the first pointed his telescope to the night sky to seek the planet’s first known satellite (Huygens, 1656; Alexan- mysteries of the universe. The Cassini-Huygens mis- der, 1962). He confirmed his discovery by watching sion represents the first time in human history that a the satellite move around the planet on subsequent spacecraft was sent to spend time in the Saturnian sys- nights, eventually determining its period of revolution.1 tem for close-up observations, bringing humanity to These must have been difficult observations, given the the surface of one of Saturn’s satellites. We believe that long focal length, small diameter, and mediocre quality this extraordinary achievement will be remembered as of the objective lens. The name “Titan” was given to a major milestone in planetary exploration 400 years this satellite in 1847 by Sir John Herschel (son of Sir from now. More than that, it is also the first plane- William, who had discovered Uranus). The Titans were tary science project of truly global scale, with scientists ancient deities who, together with a race of giants, were from three continents joining the effort. How did it defeated in battles with the Olympian gods when the come about? latter made their home in Greece.2 To begin, let us first summarize what we knew about Titan before the birth of the Cassini-Huygens mission. Comprehensive descriptions of Titan studies during the 1 With becoming modesty, Huygens (1689) described his discovery as far period from its discovery in 1655 to the landing of the less important than Galileo’s: Huygens probe in 2001 have been published by Fortes The moons about Jupiter, it is well known, we owe to Galileo, and any one may imagine he was in no small rapture at the discovery. The outermost (1997) and Coustenis et al. (2009). Here we simply but one and brightest of Saturn’s, it chanced to be my lot, with a telescope present a few highlights in chronological order. not less than 12 foot long to have the first sight of in the year 1655. 2 Classical scholars have suggested that the legendary battle between the Titans and giants with the Olympian gods symbolized the conquest of the indigenous people by the Greeks when they moved into the peninsula we 1655: discovery now call Greece. The legend tells us that the goddess Athena personally defeated the giant Enceladus and buried him in Sicily under the volcano we Christian Huygens discovered Titan on March 25, 1655, call Mt. Aetna. His struggles to get out of this grave caused the earthquakes and eruptions of this volcano, an astounding coincidence with the behavior when he was 26 years old. He used a telescope 12 feet of the satellite of Saturn that bears his name. 10 Downloaded from https://www.cambridge.org/core. UCL, Institute of Education, on 09 Aug 2018 at 09:05:08, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/CBO9780511667398.002 1 Introduction: Titan viewed from Earth 11 from a planetary atmosphere. He accepted the obser- vation that Titan had an atmosphere and pointed out that this was possible despite the satellite’s small mass because of its low temperature. 1943: an atmosphere! In 1943 Gerard P. Kuiper brought these hints, specula- tions, and predictions to an end when he discovered the 6190 A˚ and 7260 A˚ bands of CH4 in spectra of Titan he recorded on photographic plates at the McDonald Observatory in Texas (Kuiper, 1944). Kuiper was sys- tematically recording spectra of the outer planets and their satellites so it was easy for him to recognize the methane absorptions in Titan’s spectrum, given their prominence in the spectra of the outer planets. 1943–1993: These major milestones in the history of Figure 1 The hand-drawn picture of Titan by Joan Comas Titan observations were followed by 50 years of often Sola´ in 1907 displaying the limb-darkening effect. conflicting ideas and observations. We skip the details. 1908: an atmosphere? Titan is the only satellite in the solar system with a red-orange color. The cause of this color became In the course of a series of visual investigations of an interesting topic. Kuiper (1944) proposed that it the satellites of Jupiter and Saturn, the Catalonian was the result of a chemical interaction between the astronomer Comas Sola´ (1908) reported limb darkening atmosphere and the surface analogous to the surface on Titan that he interpreted as evidence for an atmo- oxidation of Mars. Polarization observations by Vev- sphere. Figure 1 shows the hand-drawn picture of Titan erka (1973) showed that the orange color of Titan must by Comas Sola.´ This observation remains controver- actually be in a thick cloud or haze layer, not on the sial, given that his drawing of Titan is unlike any image surface. Khare et al. (1984) proposed that the col- obtained by the Voy age r or Cassini-Huygens spacecraft. oration was caused by aerosols composed of a mixture The same must be said of later drawings of Titan made of complex organic molecules that indeed produced an by Lyot, Dollfus, and Camichel using a larger telescope orange color in laboratory experiments. Sagan coined at a better site (Alexander, 1962). Nevertheless, Comas the word tholins for such mixtures, from the Greek Sola´ opened the door to considerations that Titan might word θoλoζ , meaning muddy. These tholins must pre- 3 have an atmosphere. cipitate, forming deposits on the satellite’s surface that could have become several meters deep over 4.6 billion 1916: an atmosphere? years. Then there was the issue of the atmosphere itself. In his book The Dynamical Theory of Gases, Sir James What other gases might be present besides methane? Jeans (1954) investigated the thermal escape of gases Lewis (1971) was the first to propose that Titan might have an atmosphere of N2 produced by photolysis of 3 Comas Sola´ made many more accurate observations, among them the dis- covery of a comet now known as 32P/Comas Sola.´ In 1965, while inspect- ammonia. Atreya et al. (1978) proved that this was ing a photographic image of this comet recorded by S. Gerasimenko to indeed a viable process on Titan. This led to models improve its orbit, K. Churyumov discovered another comet. Their discov- ery is now the target of the ESA Rosetta mission. It is scheduled to orbit of thick nitrogen atmospheres, of which Hunten’s was and deploy a lander to inspect the nucleus of comet 67P/Churyumov- the most successful. Hunten (1978) postulated an atmo- Gerasimenko in 2015. So Comas Sola´ continues to stimulate planetary science. sphere of 20 bars, with a greenhouse effect that could Downloaded from https://www.cambridge.org/core. UCL, Institute of Education, on 09 Aug 2018 at 09:05:08, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/CBO9780511667398.002 12 Prologue 1 The genesis of Cassini-Huygens produce a surface temperature of 200 K. However, the surface was finally sensed directly by Jaffe et al. (1979), who used the Very Large Array (VLA) of radio tele- scopes in New Mexico to obtain a surface temperature of only 87 ± 9K. Titan was clearly a world worthy of further explo- ration and the Voy age r encounters strongly reinforced this conclusion. 2 Titan viewed from space: Pioneer and Voyager Pioneer 11 was the first spacecraft to reach Saturn. A small, spinning spacecraft with a limited payload, it flew past the planet on September 1, 1979. Its most important findings for our story were a confirmation of Titan’s low temperature and verification that a safe passage existed through a gap in the rings. The big news about Titan came from the sophisti- cated, three-axis stabilized Voy age r spacecraft. As the Figure2TitanasseenbyVoy age r 1 . NASA/JPL Photo. Voy age r 1 spacecraft was approaching the Saturn sys- tem in November 1980, both Daniel Gautier and Toby Owen were onsite at JPL as members of the IR spec- to explore in detail the chemistry of its atmosphere trometer (IRIS) and imaging teams, respectively. With by means of in situ measurements and to investigate growing excitement, they watched closely the change the hidden surface. This required the launching of a of Titan’s image from a faint dot to a rounded sphere probe into the atmosphere of the satellite, accompa- surrounded by a yellow-orange smog that hid the sur- nied by radar and an infrared spectrometer that could face (see Figure 2).