The mysteries of Titan
by Andrew J. LePage
|The presence of a substantial atmosphere is the one property of Titan that makes it unique among all the moons in our solar system.
Dutch astronomer Christian Huygens discovered Titan in 1655. It orbits Saturn at a distance of 1,223,000 kilometers (760,000 miles) once every 15.95 days. As telescopes improved in the centuries following its discovery, Titan was found to display tiny disk a bit under one arc second across making it the largest of Saturn’s moons by far. In 1908 Catalan astronomer Jose Comas Sola published his observations of Titan’s pronounced limb darkening. By the Space Age, the estimated diameter of Titan was around 5,000 kilometers (3,100 miles), making it roughly the same size as Mercury and in contention for being our solar system’s largest natural satellite.
Oddly enough, one of the best-determined properties of Titan before the Space Age was its mass. Several studies performed during the first half of the 20th century of how Titan perturbed the motions of Saturn’s moons found a value for the mass of Titan that was within 2% of today’s accepted value of 1.82 times that of Earth’s Moon. This combination of mass and diameter indicated a density that was consistent with a composition of roughly equal parts of rock and ice like Jupiter’s similarly sized moons, Ganymede and Callisto.
By far one of the most interesting observations of Titan made before the Space Age was the spectroscopic detection of methane in 1944 by Dutch-American astronomer Gerard Kuiper. His results implied the presence of an atmosphere with a surface pressure of at least 100 millibars, or about one tenth of Earth’s atmospheric pressure. Such an atmosphere would explain the observed limb darkening of Titan’s disk. Calculations performed by Rupert Wildt of the University of Gottingen during the 1930s had already shown that Titan was massive and cold enough to hold onto an atmosphere composed of gases with a molecular weight of 16 (like methane) and heavier. The presence of a substantial atmosphere is the one property of Titan that makes it unique among all the moons in our solar system.
A view of Titan from Voyager 1 during its historic flyby in November 1980. (credit: NASA)
Driven in large part by the launching of the first lunar and planetary probes in the early years of the Space Age, planetary astronomy experienced a renaissance in the 1960s as NASA launched probes to the Moon and beyond. In 1970 NASA announced its intention to launch a series of probes to the unexplored outer planets using a rare planetary alignment that permitted a spacecraft to fly by successively more distant planets. Called the Grand Tour, this initial plan proved to be far too ambitious and was scaled back in 1972 to a pair of simpler Mariner-class spacecraft to be launched in 1977 to flyby Jupiter then Saturn. Eventually called Voyager, a high priority target for study was Titan.
In order to properly design these spacecraft and their mission, scientists needed to know more about Titan. Detailed examination of the spectrum of Titan in the early 1970s showed that the amount of methane in Titan’s atmosphere was greater than Kuiper had originally estimated. In addition, there was evidence for the presence of another gas—possibly in large amounts—in the atmosphere of Titan that was not detectable from Earth. There was much speculation on what these gases were but one candidate, nitrogen, came into favor through the 1970s. Titan was expected to contain ammonia ice when it formed. If ammonia was released from the interior and into the atmosphere, it would have been destroyed quickly by solar ultraviolet radiation in a process called photolysis to form hydrogen, which would have quickly escaped from Titan, and nitrogen, which was heavy enough for Titan to hold.
|With all the uncertainties in our knowledge of Titan, models of its atmosphere ran the gamut of properties.
Infrared photometry showed the presence of ethane, ethylene, and acetylene in Titan’s atmosphere. These and other more complex hydrocarbons yet to be detected were generated by reactions between fragments of methane molecules generated by photolysis. Infrared and radio measurements also implied temperatures elevated above those expected for a simple blackbody, indicating the presence of a greenhouse effect. Scientists explained results from polarization studies performed in the early 1970s by the presence of clouds or an aerosol haze. A ready source of aerosols on Titan would be complex hydrocarbons generated by methane photolysis. These organic compounds would also explain Titan’s pronounced red color.
Renewed study of the tiny disk of Titan showed that its pronounced limb darkening had led to a systematic underestimation of its apparent size. Observations of Titan as it was occulted by the Moon in 1974 showed that it had an optical diameter of 5,800 kilometers (3,600 miles), which would make Titan the largest moon in the solar system. However, it was widely recognized that this measurement was probably biased by clouds that could be hundreds of kilometers above Titan’s surface. Not only was the true diameter of this moon more uncertain than ever, it was strongly suspected that clouds or haze would obscure the surface from the view of a passing spacecraft.
With all the uncertainties in our knowledge of Titan, models of its atmosphere ran the gamut of properties. At one extreme was a thin atmosphere of primarily methane with a surface pressure of only 20 millibars (about 2% of Earth’s surface pressure) and a temperature of 80 K (–316°F). The temperatures would reach a high of 160 K (–172°F) high in the aerosol-laden Titanian stratosphere in this model. At the other extreme was a model of a thick atmosphere dominated by nitrogen with a methane concentration of less than 7%. In this model the surface pressure was 21 bars (about 21 times that of the Earth) and the temperature reached a comparatively balmy 200 K (–100°F) due to a strong pressure-induced greenhouse effect not unlike the type seen on Venus. Methane clouds would form between 100 and 120 kilometers (62 and 75 miles) above the surface where the atmospheric pressure was still 600 millibars (about 60% of Earth’s surface pressure). Because of Titan’s low surface gravity, the column of gas needed to create elevated surface pressures would be many times higher than was needed on Earth. Although no observations of the time required it, models with even higher surface pressures and temperatures could be contrived assuming higher methane concentrations and the ad hoc presence of other infrared absorbing gases, to the point where Titan would almost be a tiny gas dwarf.
Between the two extremes was an atmospheric model championed by Donald Hunten of the University of Arizona. His model assumed a Titanian atmosphere dominated by nitrogen with less than 7% methane. The atmospheric conditions in this model were close to the triple point of methane, where it can exist simultaneously as a gas, liquid, or solid. As a result, methane on Titan might play the same role as water in Earth’s atmosphere. Clouds of liquid or solid methane would be present that could extend to the surface depending on the atmosphere’s methane concentration. Conditions on the surface entirely depended on the unknown value for radius of Titan’s surface: the smaller the radius, the higher the surface pressure and temperature would be. Using the best pre-Voyager measurement of Titan’s surface temperature of 87 K (–303°F), the surface pressure was expected to be about 2 bars, or twice Earth’s surface pressure.
What the actual surface of Titan would be like was even more uncertain. Given its bulk density, it was expected that the surface would be composed of ice. While ice would be as hard as rock at the temperatures likely to be found on Titan, the geological processes at work on such an alien world were mere speculation in the 1970s. Assuming the least dense model for Titan’s atmosphere, it would be cold enough for methane ice to cover the surface. As models with ever denser atmospheres are considered, the conditions on the surface would allow for liquid methane to exist in bodies whose sizes ranged from isolated puddles to a world-encircling ocean. It all depended on the properties of the atmosphere and the concentration of methane, which were then unknown. Scientists expected that the aerosols in the hazy atmosphere generated by the photolysis of methane would eventually fall out of the atmosphere and accumulate on the surface. After billions of years this material would mantle the icy surface in a layer that could be hundreds of meters thick. Just like its atmosphere, what the landscape of Titan might be like was almost pure speculation.
The surface of Titan as seen by Huygens after its landing on January 14, 2005. (credit: ESA/NASA/JPL/University of Arizona)
The Voyager spacecraft and their mission were designed to provide information not only on Titan but also the other moons in the Jovian and Saturnian systems, as well as the planets themselves. A pair of identical 825-kilogram (1,820-pound) probes were launched using a Titan IIIE-Centaur fitted with a TE 364-4 solid rocket motor to provide the extra kick needed to reach the outer solar system. Voyager 2 was launched first on August 20, 1977, while Voyager 1, flying a faster trajectory that would bring it to Jupiter and beyond first, was launched on September 5.
|During the time of the Voyager encounters, there were no trajectories that allowed a close pass by Titan while preserving a Uranus flyby option.
Each nuclear-powered Voyager probe carried identical sets of instruments to measure the fields, particle, and plasma environments not only near their targets but during the interplanetary cruise as well. Mounted on a pointable scan platform on the end of a boom were various boresighted sensors for infrared radiation, polarimetry, and ultraviolet spectroscopy, as well as wide- and narrow-angle cameras. These instruments could be directed towards a target of interest while Voyager’s dish antenna, 3.7 meters (12.1 feet) in diameter, kept pointing towards the Earth to transmit its findings. Radio transmissions from this antenna could also be used to probe the properties of a body’s atmosphere as a function of altitude as Voyager passed behind it as viewed from the Earth. Radio occultation would also allow a direct determination of the radius of Titan’s solid surface.
The trajectories the Voyager spacecraft took through the Jupiter and Saturn system were tailored to balance a number of sometimes conflicting mission priorities. Because Titan was such an important target in the Saturn system, Voyager 1 flew a trajectory that allowed it to flyby Titan at a range of 4,000 kilometers (2,500 miles) and subsequently pass behind it so that the spacecraft’s radio transmissions could probe the atmosphere, determine the moon’s radius as well as further refine its mass. During its approach to Titan, Voyager’s cameras would be used to image 90% of the illuminated disk at a scale of better than 1.7 kilometers (1.1 miles) per pixel and 50% at a scale of 650 meters per pixel (2,100 feet) or better. These images would allow the nature and extent of Titan’s clouds to be determined as well as observe the Titanian surface through any breaks in those clouds. The close pass would also permit the detection of any magnetic field Titan might possess.
The trajectory for Voyager 2, which would reach Saturn nine months after Voyager 1, included two options. The first would be flown if Voyager 1 failed to meet its objectives at Titan. Voyager 2 would be directed to flyby Titan at a range of 12,000 kilometers (7,500 miles) before passing behind Titan to probe its atmosphere. If Voyager 1 was successful and its sister was still healthy, the second probe was instead free to follow a different trajectory past Saturn that allowed it to make a complementary survey of Saturn and then fly on to Uranus, thus preserving a piece of NASA’s original Grand Tour. During the time of the Voyager encounters, there were no trajectories that allowed a close pass by Titan while preserving a Uranus flyby option.
Voyager 1 started making regular observations of Saturn during its long approach in the summer of 1980. After the Voyagers’ encounters with Jupiter during 1979 and the discovery of the varied characters of Jupiter’s four large Galilean moons, no one knew what to expect at Titan. But the chances of seeing the surface of Titan had dimmed after Pioneer 11 made an initial reconnaissance of Saturn on September 1, 1979. Distant observations of Titan strongly hinted at an opaque, hazy atmosphere. By November Titan started appearing more and more planet-like to the approaching Voyager 1 as more observations were recorded.
|The radio occultation data combined with that from the ultraviolet spectrometer demonstrated that most of the atmosphere was composed of nitrogen, just like the Earth’s atmosphere. Hunten’s model for Titan’s atmosphere was vindicated.
Voyager 1 made its closest approach of 3,915 kilometers to (2,433 miles) from Titan’s surface at 5:41 UT on November 12, 1980, about 18 hours before its closest approach to Saturn. This encounter with Titan would be the closest either Voyager would pass by any body during their long historic flights. As feared, the approach imagery of Titan showed it to be a nearly featureless orb with the surface completely obscured by a deep haze layer composed of organic aerosols. The vidicon-based cameras used by the Voyagers were insensitive to the near-infrared windows eventually exploited by Cassini to glimpse the Titanian surface through this haze. Data from Voyager’s spectrometers showed the presence of methane, ethane, and a veritable zoo of other organic compounds. Nitrogen was also detected in abundance.
An initial examination of Voyager’s radio occultation data hinted that the surface was not detected. The radio beam had apparently been refracted and absorbed too much by Titan’s atmosphere. However, a more detailed examination of the data showed that while the radio signal did indeed fade below detectability during ingress, the surface of Titan was definitely observed during egress. A comparison between the data sets showed that the signal was lost during ingress almost at the surface. The analysis of this data showed Titan to have properties close to today’s accepted values: a diameter of 5,152 kilometers (3,202 miles), a surface temperature of about 94 K (–290°F), and a pressure of 1.47 bars. The radio occultation data combined with that from the ultraviolet spectrometer demonstrated that most of the atmosphere was composed of nitrogen, just like the Earth’s atmosphere. Hunten’s model for Titan’s atmosphere was vindicated. Titan was also firmly established as our solar system’s second largest moon just behind Ganymede. And with the success of this first encounter, Voyager 2 was free to fly to Uranus and beyond.
After Voyager 2 made a distant 664,000-kilometer (412,000-mile) pass by Titan on August 26, 1981, it would be almost 23 years before another spacecraft visited Titan. While the Voyager data answered many basic questions about Titan, they also raised many new questions about the conditions at the still unseen surface. It would take the landing of the ESA Huygens probe on January 14, 2005, and 73 (and counting) targeted flybys by Cassini to begin to show the true nature of Titan’s surface—as well as raise yet more new questions.
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