ESA Science & Technology - Publication Archive

The orbital properties of Phoebe, one of Saturn's irregular moons, suggest that it was captured by the ringed planet's gravitational field rather than formed in situ. Phoebe's generally dark surface shows evidence of water ice, but otherwise the surface most closely resembles that of C-type asteroids and small outer Solar System bodies such as Chiron and Pholus that are thought to have originated in the Kuiper belt. A close fly-by of Phoebe by the Cassini-Huygens spacecraft on 11 June 2004 (19 days before the spacecraft entered orbit around Saturn) provided an opportunity to test the hypothesis that this moon did not form in situ during Saturn's formation, but is instead a product of the larger protoplanetary disk or 'solar nebula'. Here we derive the rock-to-ice ratio of Phoebe using its density combined with newly measured oxygen and carbon abundances in the solar photosphere. Phoebe's composition is close to that derived for other solar nebula bodies such as Triton and Pluto, but is very different from that of the regular satellites of Saturn, supporting Phoebe's origin as a captured body from the outer Solar System.

The origin of Phoebe, which is the outermost large satellite of Saturn, is of particular interest because its inclined, retrograde orbit suggests that it was gravitationally captured by Saturn, having accreted outside the region of the solar nebula in which Saturn formed. By contrast, Saturn's regular satellites (with prograde, low-inclination, circular orbits) probably accreted within the sub-nebula in which Saturn itself formed. Here we report imaging spectroscopy of Phoebe resulting from the Cassini-Huygens spacecraft encounter on 11 June 2004. We mapped ferrous-iron-bearing minerals, bound water, trapped CO₂, probable phyllosilicates, organics, nitriles and cyanide compounds. Detection of these compounds on Phoebe makes it one of the most compositionally diverse objects yet observed in our Solar System. It is likely that Phoebe's surface contains primitive materials from the outer Solar System, indicating a surface of cometary origin.

Data from the Cassini-Huygens mission provide convincing evidence that the saturnian moon Phoebe formed elsewhere in the Solar System, and was only later captured by Saturn's gravitational pull.

Titan, the largest moon of Saturn, is the only satellite in the Solar System with a substantial atmosphere. The atmosphere is poorly understood and obscures the surface, leading to intense speculation about Titan's nature. Here we present observations of Titan from the imaging science experiment onboard the Cassini spacecraft that address some of these issues. The images reveal intricate surface albedo features that suggest aeolian, tectonic and fluvial processes; they also show a few circular features that could be impact structures. These observations imply that substantial surface modification has occurred over Titan's history. We have not directly detected liquids on the surface to date. Convective clouds are found to be common near the south pole, and the motion of mid-latitude clouds consistently indicates eastward winds, from which we infer that the troposphere is rotating faster than the surface. A detached haze at an altitude of 500 km is 150-200 km higher than that observed by Voyager, and more tenuous haze layers are also resolved.

Scientists were ecstatic last weekend as Titan, Saturn's largest moon, dramatically revealed itself to have the atmosphere-bearing, hydrocarbon-based landscape they had anticipated. But Huygens, the European Space Agency (ESA) spacecraft that successfully parachuted through Titan's atmosphere on 14 January, has also revealed its share of surprises. The high-risk mission had functioned better than its designers had dared to expect - and they quickly reported that the moon looked even more interesting than they had hoped.

On 14 January 2005, after a marathon seven-year journey through the Solar System aboard the Cassini spacecraft, ESA's Huygens probe successfully descended through the atmosphere of Titan, Saturn's largest moon, and landed safely on its surface. It was mankind's first successful attempt to land a probe on another world in the outer Solar System. Following its release from the Cassini mothership on 25 December, Huygens reached Titan's outer atmosphere after 20 days and a 4 million kilometre cruise. The probe started its descent through Titan's hazy cloud layers from an altitude of about 1270 km at 09:06 UTC. During the following three minutes, Huygens had to decelerate from 18 000 to 1400 km per hour.

Cassini-Huygens, named after the two celebrated scientists, is the joint NASA/ESA/ASI mission to Saturn and its giant moon Titan. It is designed to shed light on many of the unsolved mysteries arising from previous observations and to pursue the detailed exploration of the gas giants after Galileo's successful mission at Jupiter. The exploration of the Saturnian planetary system, the most complex in our Solar System, will help us to make significant progress in our understanding of planetary system formation and evolution, which is also a key step in our search for extra-solar planets.

Contents

4	Solving the puzzles of Saturn and Titan
6	High ambitions for an outstanding planetary mission
8	A long and rich journey
12	What lies beneath?
16	Mysterious Titan
20	Vehicles of discovery

Huygens is being carried as a passenger on NASA's Cassini Orbiter to Saturn, where it will be released to enter the atmosphere of Titan, the planet's largest moon. During the controlled descent phase, its instruments will execute a complex sequence of measurements to study the atmosphere's chemical and physical properties and, if it survives impact, Huygens will collect data on Titan's surface properties. Flight operations will be conducted from the Huygens Probe Operations Centre (HPOC) at ESOC. This article describes the ground-system infrastructure, procedures and constraints involved in operating Huygens over its 6.7-year mission lifetime.

Many engineering challenges had to be overcome in designing the first probe planned to study a moon beyond the Earth's system. An extensive development programme was undertaken, involving several unusual tests, driven by the mission's unique aspects. ESA's Huygens Probe will be delivered to Titan, Saturn's largest satellite, by the Cassini Orbiter in 2004. After a dormant interplanetary journey of 6.7 years - although Huygens will be activated every 6 months for health checks - its aeroshell will decelerate it in less than 3 min from the entry speed of 6 kms^-1 to 400 ms^-1 (Mach 1.5) by about 160 km altitude. From that point, a pre-programmed sequence will trigger parachute deployment and heat-shield ejection. The main scientific mission can then begin, lasting for the whole of the Probe's 2-2.5 h descent.

The Huygens Probe is ESA's element of the joint Cassini/Huygens mission with NASA to the Saturnian system. Huygens will be carried on NASA's Cassini Orbiter to Saturn, where it will be released to enter the atmosphere of Titan, the planet's largest satellite. The Probe's primary scientific phase occurs during the 2-2.5 h parachute descent, when the six onboard instruments execute a complex series of measurements to study the atmosphere's chemical and physical properties. Measurements will also be conducted during the 3 min entry phase, and possibly on Titan's surface if Huygens survives impact. This article provides an overview of the mission and a concise description of the payload.

The photochemistry of hydrocarbons in Titan's atmosphere is modeled by a comprehensive kinetic scheme, containing 732 elementary reactions and 147 species up to C₆₀. Four groups of the hydrocarbons are considered : Polyacetylenes (PA), Polyvinyles (PV), Vinylacetylenes (VA) and Allenes (Polyenes).

We develop a semiempirical grey radiative model to quantify Titan's surface temperature as a function of pressure and composition of a nitrogen-methane-hydrogen atmosphere, solar flux and atmospheric haze. We then use this model, together with non-ideal gas-liquid equilibrium theory to investigate the behavior of the coupled surface-atmosphere system on Titan.

The aim of this work is to deduce physical characteristics of the aerosols that compose the thin detached haze layer of Titan, for which the composition and origin are currently unknown. We have used four images at three different phase angles and three different wavelengths where the detached haze clearly appears. The low optical path in the layer permits assumptions that greatly simplify the treatment and allow us to consider the scattered intensity as the product of the scattering cross section, the phase function at the considered phase angle, and the number of aerosols. Physical arguments are given to support fractal aerosols in the detached haze, and we investigate a wide range of monomer radii, monomer numbers, and imaginary refractive indexes. This study yields information about the spectral variation of the refractive index for the detached haze tholins that are clearly different from those for the main haze. On the other hand, no firm values are given for the monomer radius and number, but these two parameters are strongly linked through a simple relation

We estimate the wind speeds in Titan's thermosphere by considering the various terms of the wind equation, without actually solving it, with a view to anticipating what might be observed by the Cassini spacecraft in 2004. The winds, which are driven by horizontal pressure gradients produced by solar heating, are controlled in the Earth's thermosphere by ion-drag and coriolis force, but in Titan's thermosphere they are mainly controlled by the nonlinear advection and curvature forces.

The very similar D/H ratio in the water of Comets Halley, Hyakutake, and Hale-Bopp (~3 × 10^-4) is 10-20 times larger then the D/H ratio in the solar nebula. Therefore the cometary water had to originate in a giant molecular cloud, where the HDO is enriched by ion-molecule reactions.

We investigated electron dissociative recombination of N⁺₂ ions, electron impact dissociation of N₂ molecules, nonthermal exothermic ionosphere-related photochemical reactions, atmospheric sputtering via solar wind and magnetospheric particles, solar wind pick-up and the loss of ¹⁴N to ¹⁴C via cosmic rays as possible sources of nitrogen isotope fractionation in Titan's atmosphere where this molecule is the principal constituent.