5.1 Introduction

Titan's atmosphere harbors a suite of hydrocarbons and nitrogen-bearing compounds formed from the dissociation of the two main species, nitrogen (N2)and methane (CH4). It also contains oxygen compounds, likely produced from an influx of water and/or oxygen. The mixing ratios of these photochemical species vary with altitude, latitude, and time as a consequence of various chemical sources and sinks and of the atmospheric transport that redistributes them both vertically and horizontally. It is important to characterize and monitor the distribution of these chemical species because they play an important role in the radiative budget and provide insight into the seasonally varying atmospheric circulation. They can also help us understand the complex chemistry at work in Titan's atmosphere, leading to the formation of thick haze layers, which in turn affect the heat balance and general circulation. This chapter reviews the neutral composition of Titan's atmosphere, from the troposphere up to the thermosphere (~ 1400 km), and its vertical, horizontal, and temporal variations. These topics are interwoven with the origin and evolution, the general circulation, the clouds and weather, and the atmospheric chemistry of Titan that are the subjects of Chapters 1, 4, 6, and 7.

5.1.1 Historical perspective

The first unquestionable evidence for an atmosphere on Titan was the discovery of several absorption bands of methane in near-infrared spectra of the satellite (Kuiper, 1944). But it was not until the 1970s that Titan became an object of intense study.

8.1 Introduction

This chapter summarizes what is known about Titan's extensive photochemical haze, which extends from the surface to about 1000 km altitude. The haze determines the appearance of the moon across a broad spectral range. It dominates the opacity short of 5 μm and it also affects the radiation transfer in the thermal IR. Thus, haze plays a major role in Titan's radiative energy budget. It is also a sink for gas phase photochemistry and a source of surface material, and the haze has much to tell us about atmospheric dynamics.

A great deal has already been written about Titan's haze, summarized most recently by both Tomasko and West (2009) and Lorenz et al. (2009). In this chapter we briefly review the key attributes of the haze determined from in situ measurements made by the Descent Imager/Spectral Radiometer (DISR) instrument and then focus on developments more recent than those reviewed by Tomasko and West (2009). These developments include new observations and analyses, laboratory investigations relevant to the haze's physical characteristics (size, shape, and density), its chemical composition and optical properties, and microphysical model studies that simulate the interaction of haze with the gas phase background and the impact of haze in the radiative energy budget and the atmospheric dynamics.

11.1 Introduction

An ionosphere is the ionized part of the upper atmosphere of a planet or a moon, a transition layer between the space environment and the lower atmosphere. At Titan, the ionosphere was first detected by the Voyager 1 radio occultation experiment (Bird et al., 1997). As Titan is located within Saturn's magnetosphere for most of the time with occasional incursions into its magnetosheath (and even rarer incursions into the solar wind), its ionosphere is a key layer in coupling Titan with Saturn's space environment. The question of whether Titan's ionosphere is produced primarily by solar radiation or electron precipitation from Saturn's magnetosphere has been under debate for several decades (e.g., Nagy and Cravens, 1998). This is not surprising, bearing in mind the complex and dynamic nature of both the magnetospheric forcing and of the magnetic field line configuration at Titan (see Chapter 12). For instance, while Titan does not have any significant intrinsic magnetic field, Saturn's magnetic field lines drape around and permeate its ionosphere. The draping changes significantly with the angle between the solar direction and the co-rotating plasma direction, which varies as Titan orbits around Saturn.

The Cassini spacecraft, which arrived at Saturn in July 2004, has explored Titan's ionosphere in detail through many close fly-bys, the first of which took place in October 2004. The resulting rich datasets from many instruments, combined with comprehensive analyses, have revealed the chemically and dynamically most complex ionosphere in the solar system.

6.1 Introduction

Titan appears alluringly familiar. Its surface is shaped by weather, with lakes, fluvial channels, and dunes (Tomasko et al., 2005; Lorenz et al., 2006; Stofan et al., 2007; Barnes et al., 2007; Lopes et al., 2010). Its atmosphere sports clouds that can grow to over four times the height of terrestrial thunderstorms (Griffith et al., 1998; Brown et al., 2002; Roe et al., 2002; Schaller et al., 2006a). These features result from an uncanny resemblance to Earth; similar to the terrestrial hydro-logical cycle, Titan has a methane cycle, with methane clouds, rain, and seas. On both Earth and Titan, the condensable is supplied by the surface; evaporates into the atmosphere, where it condenses into clouds; redistributes in the atmosphere; and precipitates back to the surface. These processes depend on the partitioning of solar insolation, the atmospheric structure and temperature, the condensable inventory and properties, and the circulation, all of which differ between Earth and Titan (Table 6.1).

On Earth, the equivalent of 2.7 km of water covers the surface and supplies the atmosphere with the equivalent of 2.6 cm of precipitable water. This largely wet surface (70% of the globe) is heated by, on average, 60 percent of the incident sunlight, which passes through the mostly transparent (when cloudless) atmosphere. Sunlight powers weather. Its effects are direct – for example, through the evaporation of surface liquids. In addition, there are indirect impacts – for example, through differential heating across the globe, which ultimately steers the general circulation of the planet, with conditions altered locally by the variable heating associated with surface topography, land-water contrast, and other terrain heterogeneities.

1.1 Introduction

Although Titan is similar in terms of mass and size to Jupiter's moons Ganymede and Callisto, it is different in that it is the only one harboring a massive atmosphere. Moreover, unlike the Jovian system, which is populated with four large moons, Titan is the only large moon around Saturn. The other Saturnian moons are much smaller and have an average density at least 25 percent less than Titan's uncompressed density and much below the density expected for a solar composition (Johnson and Lunine, 2005), although with a large variation from satellite to satellite. Both Jupiter's and Saturn's moon systems are thought to have formed in a disk around the growing giant planet. However, the difference in architecture between the two systems probably reflects different disk characteristics and evolution (e.g., Sasaki et al., 2010), and, in the case of Saturn, possibly the catastrophic loss of one or more Titan-sized moons (Canup, 2010). Moreover, the presence of a massive atmosphere on Titan, as well as the emission of gases from Enceladus' active south polar region (Waite et al., 2009), suggest that the primordial building blocks that comprise the Saturnian system were probably more volatile-rich than those of Jupiter.

The composition of the present-day atmosphere, dominated by nitrogen, with a few percent methane and lesser amounts of other species, probably does not directly reflect the composition of the primordial building blocks and is rather the result of complex evolutionary processes involving internal chemistry and outgassing, impact cratering, photochemistry, escape, crustal storage and recycling, and other processes.

9.1 Introduction and some history

Titan, with its dense atmosphere, low gravity, weak solar insolation, and complex composition, provides a unique example of a planetary upper atmosphere. The large mass of the atmosphere, coupled with low gravity, results in a greatly extended atmosphere where the plane parallel assumption, nearly universal in terrestrial and giant planet atmosphere studies, no longer applies. Moreover, the weak gravity results in large escape rates that may play a significant role in upper atmospheric thermal balance. The weak solar insolation means that in many cases dynamical processes can dominate over solar processes, while at the same time the complex composition causes radiative cooling processes to be more important than in most other planetary upper atmospheres. Most of the time Titan orbits within Saturn's magnetosphere and the interaction with energetic particle populations may significantly alter the upper atmosphere. Measurements by the Cassini spacecraft have allowed us to greatly extend our knowledge of the thermal balance in Titan's upper atmosphere, although the main result so far may be the realization that the simple descriptions employed before Cassini fail to capture the complexity and variability of this enigmatic atmosphere. To understand the progress enabled by Cassini-Huygens measurements, we first review our knowledge of thermal balance in Titan's upper atmosphere based on observations by the Voyager spacecraft and ground-based telescopes.

2.1 Overview

The presence of an atmosphere, initially suggested based on limb darkening by Sola (1904) and later by the presence of methane spectral lines by Kuiper (1944), has long given Titan a special place in the minds of planetary geologists. The first close-up images were obtained by Pioneer 11 in 1979 (Gehrels et al., 1980), confirming a substantial atmosphere. These early observations led to the diversion of the trajectory of the Voyager I spacecraft to a closer encounter with Titan in 1980. Although the visible cameras on Voyager also had difficulty seeing Titan's surface (Richardson et al., 2004), radio occultation experiments suggested a surface pressure of 1.5 bars and temperature near 95 K (Lindal et al., 1983). These results were exciting because, for a methane mixing ratio of a few percent at the surface (Hunten, 1978), they placed methane's partial pressure near its triple point. Thus, like water on Earth, solid, liquid, and gaseous methane could potentially exist in Titan's environment. Ethane, which is the main product of methane photolysis, can also be liquid under these conditions. The presence of condensable volatiles in Titan's thick atmosphere opens the door for active fluvial, lacustrine, and pluvial processes that can shape its landscape with similar morphologies to those we find on Earth.

Prompted by the exciting results of the Voyager mission and the nearly two decades of Earth-based imaging campaigns that followed, NASA/ESA launched the Cassini-Huygens mission to Saturn in 1997. To penetrate Titan's thick atmosphere, Cassini is equipped with a Ku-band radar capable of obtaining images of the surface at a scale of 300 meters.

12.1 Introduction

Titan, Mars, and Venus are three largely unmagnetized planetary bodies with dense atmospheres that are immersed in external and highly dynamic magnetized plasma flows. Mars and Venus interact with the solar wind, whereas Titan usually interacts with the rotating magnetosphere of Saturn, and only occasionally is subject to shocked solar wind during brief excursions into Saturn's magnetosheath (Figure 12.1). Titan's atmosphere is ionized by the energetic plasma flow, together with solar and cosmic ray radiation (see Chapter 11), and the resulting ionosphere provide a conductive environment with which the external plasma flow interacts. The ability of the ionosphere to carry an electrical current plays an important role in the dynamics and energetics of the ionosphere, and through collisions, to the deposition of energy and momentum into the neutral atmosphere. This magnetosphere/ionosphere interaction at Titan involves the formation of an induced magnetosphere around Titan with interaction boundaries that drapes the magnetic field lines into a long tail behind the moon, already detected by the instruments of the Voyager 1 spacecraft (e.g., Ness et al., 1982; Gurnett et al., 1982) during its swift fly-by of Titan's plasma wake. The interaction causes ionospheric convection and facilitates the escape of ionospheric plasma through the tail to the surrounding streaming magnetosphere past Titan. In addition, Titan's vast neutral gas environment becomes partly ionized; the created ions are picked up by the induced convection electric field by the streaming magnetospheric plasma and drift away in a gyrating motion, at the same time mass loading the streaming plasma so it slows down in the neighborhood of the moon.