DISR sensor head undertake spectroscopy right across the ultraviolet to near-infrared spectral range to measure the insolation-scattering properties of the aerosols in the orangey haze. It will profile how solar energy is transmitted through the atmosphere. Throughout the descent, the upward-looking spectrometer will peer at the bright patch in the sky as the haze forward-scatters sunlight, and it will measure the absorption spectrum in order to profile the methane mixing ratio. Meanwhile, other sensors will monitor the descent by imaging the surface repeatedly. In fact, DISR incorporates three imagers with low, medium, and high resolution. ''With these three cameras and a spinning spacecraft, we'll time our images in such a way that we'll take an entire hemispheric panorama from above the horizon down to the surface,'' explained P.H. Smith of the University of Arizona's Lunar and Planetary Laboratory, who helped to develop the instrument. Transmitting the panoramas during the final 20 kilometres of the descent will require fully two-thirds of the relay bandwidth. ''The information we'll get out of these images will be absolutely stupendous,'' Smith promised.

Imaging will start soon after the probe sheds its thermal shielding, at an altitude of 170 kilometres, at which time the narrow field of view will provide a resolution of about 1 kilometre. If the probe is drifting in a strong wind, this should be evident in the imagery, and the imagery will serve to map the swath of the ground track. Titan, being ten times farther from the Sun, receives 1 per cent of the insolation received by the Earth, a few per cent of which will filter through the optically thick orangey haze to the surface. At the subsolar point, the illumination should be equivalent to a clear terrestrial day shortly after sunset. Elsewhere on the illuminated hemisphere it will be rather gloomier, but not dark. Unless there is a thick overcast of cloud, there ought to be a broad solar aureole. Although many wavelengths will penetrate to the surface, the methane will absorb strongly, and in the final phase the probe will turn on a lamp to enable it to take a surface spectrum. The field of view of the final look-down image should span about 75 metres, so it ought to be possible to determine precisely where the probe settles and put the measurements of the surface characteristics into a proper context. The principal investigator is M.G. Tomasko of the University of Arizona.

In the same way as the Galileo spacecraft made high-resolution measurements of the Doppler on the radio relay from the probe that it sent into Jupiter's atmosphere, Cassini will monitor its probe. The Doppler Wind Experiment (DWE) will measure the direction and strength of the winds with altitude with a resolution of 1 metre per second, starting when the probe's transmitter switches on and continuing throughout the parachute descent. Zonal (latitudinal) winds are expected to be much faster than the meridional (equator-to-pole) winds. If the data is 'clean', it should be possible to measure the rates at which the probe swings and rotates beneath its parachute. The principal investigator is Michael Bird of the University of Bonn in Germany.

At the end of the descent, the Surface Science Package (SSP) will employ a suite of independent subsystems to determine the physical and chemical properties of the surface. As seven of the sensors require intimate contact with the surface, they are mounted inside, or on, the lower rim of a 100 x 100 millimetre square cross-section cavity (dubbed the 'Top Hat') in the Fore Dome, this having been deemed preferable to either developing a deployment mechanism, or competing with the inlets for the ACP and GCMS instruments and the radar altimeter's antenna for access to the Fore Dome's surface. The sensors not requiring exposure to the surface are mounted internally. In the event that the probe splashes down, many of the sensors are designed to characterise the fluid. Some of the sensors will also be able to make measurements during the descent. The principal investigator is J.C. Zarnecki of the Open University in England. The scientific objectives of the SSP are:

• to determine the physical nature and condition of Titan's surface;

• to measure the abundances of the major constituents to place constraints on theories of atmospheric and oceanic evolution;

• to measure the thermal, optical, acoustic and electrical properties and density of any ocean, providing data to validate physical and chemical models;

• to determine wave properties and ocean-atmosphere interaction; and

• to provide ground-truth for interpreting the main spacecraft's radar mapping and other remote-sensing data.

The Accelerometer (ACC) subsystem comprises two piezoelectric sensors, one of which (ACC-E) is a penetrometer mounted on a spear that will come into contact with the surface. Being internally mounted, the other sensor (ACC-I) will be able to measure descent and surface accelerations. The Tilt (TIL) sensor subsystem works on an electrolytic principle. It uses platinum terminals and a methanol-based liquid enclosed in a sealed glass housing; it also has fluid damping to improve its operation in moderately dynamic environments. All the sensors are mounted internally. One individual sensor element senses the local vertical about a single axis, and pairs of elements determine the tilt angle in any plane. The Thermal Properties (THP) sensor assembly in the Top Hat will measure the temperature and thermal conductivity of the lower atmosphere and any surface fluid. An electrical current passed through platinum wires 5 centimetres in length and 10 and 25 microns in diameter will act to heat the surrounding medium, and a series of resistance measurements will measure the rate of heating of the element at 0.1-second intervals in order to detect the onset of convection. The Acoustic Properties (API) sensors employ piezoelectric ceramic devices similar to those used in marine applications. Two of the three transducers will face each other across the diameter of the Top Hat, and will alternate between transmit and receive modes in order to measure the velocity of sound (API-V). The third transducer is an array of elements pointing vertically down that will operate as an acoustic sounder (API-S) by issuing pulses and then listening for returns from the surface during descent or, following splashdown, from the ocean floor. The Fluid Permittivity (PER) sensor in the Top Hat uses simple electrodes whose capacitance varies with the permittivity of the environment. In the case of a splashdown, a single conductivity measurement (CON) will test for polar molecules in the fluid, and the displacement of a buoyant float will be measured by four strain gauges in a bridge arrangement for the Fluid Density (DEN) measurement. The Refractometer (REF) in the Top Hat is a specially shaped prism fitted with a pair of light-emitting diodes to yield internal or external illumination of its curved surface through light guides. Light passing through its top surface will be fed to a linear diode array, and the refractive index measured from the position of the light/dark transition on the array. Together, these various measurements should yield at least a partial characterisation of either a solid or a liquid surface.

Although the stratospheric zonal winds rage at about 200 knots, the troposphere is expected to be milder. In fact, as a thermal gradient is required to induce the flow of air, and Titan's surface temperature is the same from pole to pole to within a few degrees, conditions at the surface are likely to be stagnant. It will be "cold, dark and quiescent'' forecasts Caitlin Griffith of Northern Arizona University in Flagstaff. A near-infrared study by the UK Infrared Telescope in Hawaii found that the amount of methane in Titan's troposphere waxes and wanes on a timescale of hours.37 The terrestrial weather system is very active because the Earth receives a great deal of insolation energy, and because it rotates sufficiently rapidly for the Coriolis effect to stimulate vorticity. However, Titan's atmosphere is chillier, much more massive and sluggish, and the moon rotates very slowly. A different force is thought to drive its weather system. Perhaps the latent heat that is released when a gas condenses plays a significant role. Clouds on Titan are rare and sparse, but at least one large-scale tropospheric storm system has been noted. In part, this is due to the fact that Titan's upper troposphere is super saturated with methane, and so a cloud particle that forms around a smog particulate at that altitude will rapidly grow into a large raindrop and fall.38 Although sporadic drops will evaporate as they fall, an occasional storm may well dump a large amount of precipitation onto the surface. If Huygens is (un)lucky, it will descend into what passes for the local monsoon.

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