Athena Coustenis

Titan's atmosphere and hypothesized ocean: A reanalysis of the Voyager 1 radio-occultation and IRIS 7.7-μm data

Voyager1 radio-occultation refractivity profiles of Titan in the 0–200 km altitude range are reanalyzed in order to derive limiting profiles for the equatorial thermal structure, taking into account uncertainties on the mean molecular weight m. The major atmospheric constituents are assumed to be N2 and CH4 (already detected) and Ar which is a plausible additional constituent when m is greater than 28. The stratospheric abundance of CH4 is assumed to be limited by saturation at the tropopause. Spectra calculated from the infrared thermal profiles in the CH4 ν4 band at 7.7 μm are compared with the Voyager IRIS observations. This allows us to constrain both the abundances of major atmospheric components and the stratospheric temperatures. The methane mole fraction is between 0.5 and 3.4% in the stratosphere but may be as high as 21% at the surface, whereas the atmospheric Ar/N2 ratio can be anything between 0 and 0.27. The temperature is between 92.5 and 101°K at the ground level, and between 70.5 and 74.5°K at the tropopause. The maximum uncertainty (±4°K) occurs between 150 and 200 km altitude. A “nominal” temperature and composition profile of Titans atmosphere between the ground and 200 km is given with tabulated values. The implications of composition and surface temperature uncertainties on the abundances of the expected major oceanic constituents are discussed, as well as the consequences for the solubility of noncodenssable atmospheric constituents (H2 and CO) and organic compounds raining out from the atmosphere.

Overview of the coordinated ground-based observations of Titan during the Huygens mission

Coordinated ground-based observations of Titan were performed around or during the Huygens atmospheric probe mission at Titan on 14 January 2005, connecting the momentary in situ observations by the probe with the synoptic coverage provided by continuing ground-based programs. These observations consisted of three different categories: (1) radio telescope tracking of the Huygens signal at 2040 MHz, (2) observations of the atmosphere and surface of Titan, and (3) attempts to observe radiation emitted during the Huygens Probe entry into Titans atmosphere. The Probe radio signal was successfully acquired by a network of terrestrial telescopes, recovering a vertical profile of wind speed in Titans atmosphere from 140 km altitude down to the surface. Ground-based observations brought new information on atmosphere and surface properties of the largest Saturnian moon. No positive detection of phenomena associated with the Probe entry was reported. This paper reviews all these measurements and highlights the achieved results. The ground-based observations, both radio and optical, are of fundamental importance for the interpretation of results from the Huygens mission.

Structural and tidal models of Titan and inferences on cryovolcanism

Titan, Saturns largest satellite, is subject to solid body tides exerted by Saturn on the timescale of its orbital period. The tide-induced internal redistribution of mass results in tidal stress variations, which could play a major role for Titans geologic surface record. We construct models of Titans interior that are consistent with the satellites mean density, polar moment-of-inertia factor, obliquity, and tidal potential Love number k2 as derived from Cassini observations of Titans low-degree gravity field and rotational state. In the presence of a global liquid reservoir, the tidal gravity field is found to be consistent with a subsurface water-ammonia ocean more than 180 km thick and overlain by an outer ice shell of less than 110 km thickness. The model calculations suggest comparatively low ocean ammonia contents of less than 5 wt % and ocean temperatures in excess of 255 K, i.e., higher than previously thought, thereby substantially increasing Titans potential for habitable locations. The calculated diurnal tidal stresses at Titans surface amount to 20 kPa, almost comparable to those expected at Enceladus and Europa. Tidal shear stresses are concentrated in the polar areas, while tensile stresses predominate in the near-equatorial, midlatitude areas of the sub- and anti-Saturnian hemispheres. The characteristic pattern of maximum diurnal tidal stresses is largely compliant with the distribution of active regions such as cryovolcanic candidate areas. The latter could be important for Titans habitability since those may provide possible pathways for liquid water-ammonia outbursts on the surface and the release of methane in the satellites atmosphere.

The Hera Saturn entry probe mission

The Hera Saturn entry probe mission is proposed as an M-class mission led by ESA with a contribution from NASA. It consists of one atmospheric probe to be sent into the atmosphere of Saturn, and a Carrier-Relay spacecraft. In this concept, the Hera probe is composed of ESA and NASA elements, and the Carrier-Relay Spacecraft is delivered by ESA. The probe is powered by batteries, and the Carrier-Relay Spacecraft is powered by solar panels and batteries. We anticipate two major subsystems to be supplied by the United States, either by direct procurement by ESA or by contribution from NASA: the solar electric power system (including solar arrays and the power management and distribution system), and the probe entry system (including the thermal protection shield and aeroshell). Hera is designed to perform in situ measurements of the chemical and isotopic compositions as well as the dynamics of Saturns atmosphere using a single probe, with the goal of improving our understanding of the origin, formation, and evolution of Saturn, the giant planets and their satellite systems, with extrapolation to extrasolar planets. Heras aim is to probe well into the cloud-forming region of the troposphere, below the region accessible to remote sensing, to the locations where certain cosmogenically abundant species are expected to be well mixed. By leading to an improved understanding of the processes by which giant planets formed, including the composition and properties of the local solar nebula at the time and location of giant planet formation, Hera will extend the legacy of the Galileo and Cassini missions by further addressing the creation, formation, and chemical, dynamical, and thermal evolution of the giant planets, the entire solar system including Earth and the other terrestrial planets, and formation of other planetary systems.

Earth-Based Perspective and Pre-Cassini–Huygens Knowledge of Titan

This chapter sets the scene for the current investigation of Titan with Cassini—Huygens, by reviewing the steps that took us there, from the first glimpses through a small telescope to the satellite observations passing by the first hints of an atmosphere about a 100 years ago.

Chapter 38 – Titan

Titan is Saturns largest satellite and the second largest moon in our Solar system. Several space missions (essentially the on-going Cassini-Huygens) and ground-based observations have revealed Titan to be an unique world on its own bearing similarities to our own planet through its atmospheric composition and dynamics, organic chemistry and geomorphological features, among other. The moons astrobiological potential is also significant.

Life Beyond Earth: What is life and where can it exist?

The search for habitable worlds in the Universe entails our understanding of the conditions in which life appeared, survived and developed on Earth. This understanding has been growing consistently since the first geological, atmospheric, oceanographic and biological studies. As stated in The Limits of Organic Life in Planetary Systems , put together by the Committee on the Origins and Evolution of Life of the National Research Council (NRC, 2007): it is now clear that although terrestrial life is conveniently categorized into million of species, studies of the molecular structure of the biosphere show that all organisms that have been examined have a common ancestry. There is no reason to believe, or even to suspect, that life arose on Earth more than once, or that it had biomolecular structures that differed greatly from those shared by the terrestrial life that we know of today. Our planet is not blessed everywhere with conditions favourable to human life, but in spite of the harsh and extreme chemical and temperature ranges that living species have to deal with, we have proof today that life thrives on Earth wherever liquid water and energy sources are available. However, other lifeforms may well exist, as has been suggested by some scientific studies. In what follows in this chapter we try to give an overview of terrestrial life and what it requires, touch upon other possibilities and focus on the environmental conditions necessary for the sustainability of life of the standard definition (Earth-like), before we begin our trip across the Solar System and elsewhere in quest of habitable places.

Life in the Saturnian Neighborhood

The Cassini–Huygens mission has revealed a very active and diverse Saturnian system in which several satellites show promising conditions for habitability and the development and/or maintenance of life. Titan, for example, is the only other body in the solar system besides Earth to possess a dense atmosphere composed essentially of nitrogen (97 %) and in which the combination with methane (2 %) gives rise to a host of organic compounds. The presence of seasonal effects, unique geomorphological features, and a probable internal liquid water ocean make Titan one of the most astrobiologically interesting bodies. Another Cassini discovery of tremendous relevance to astrobiology is the large organic-ladden plumes ejected from Enceladus’ south pole cracks, mainly made of water vapor but suggestive of a complex organic chemistry occurring in the interior of this small satellite, in the presence of liquid water. Future extensive and in situexploration ideas which could help us improve our understanding of these issues in the Kronian system are also discussed here.

Titan Beyond Cassini—Huygens

This chapter reviews the unanswered science questions which remain after the Cassini-Huygens nominal tour as well as the many new questions which has arisen following new discoveries which have been made. Further missions to the Titan system which have been studied are described, in particular that of the most recent study, the Titan Saturn System Mission.