Sunlight plays a key role in driving many important physical processes in planetary physics. Absorption of ultraviolet light drives photochemical reactions, leading to changes in atmospheric composition and to the production of atmospheric aerosols. The size, shape, composition, and distribution of aerosols and cloud particles determine their optical properties and their ability to absorb sunlight and emit thermal infrared radiation, thus playing a key role in the thermal balance of the atmosphere. The net radiative heating or cooling rate provides the forcing for atmospheric dynamics, which in turn can affect the distribution of aerosol and cloud particles and climate. The composition, thermal balance, dynamics, and meteorology of the atmosphere also affect (and are affected by) the nature of the surface. Images of the surface in reflected sunlight together with near infrared reflection spectra can reveal the nature of the surface and its interactions with atmospheric processes. Thus, optical measurements in the wavelength of solar radiation made inside a planetary atmosphere can reveal a great deal about many important physical processes occurring there.

The Descent Imager/Spectral Radiometer (DISR) is the optical instrument that makes measurements at solar wavelengths aboard the Huygens Probe of the Cassini mission. This instrument is being developed in a collaborative effort by scientists from the US, France, and Germany. DISR measures solar radiation using silicon photodiodes, a two-dimensional silicon Charge Coupled Device (CCD) detector and two InGaAs near-infrared linear array detectors. The light is brought to the detectors using fibre optics from many separate sets of foreoptics that collect light from different directions and in different spectral regions. In this way the instrument can make a suite of measurements which are carefully selected to answer key questions concerning the nature of the surface and the composition, meteorology, thermal balance, and clouds and aerosols in the atmosphere of Titan.

DISR Scientific Objectives

Thermal Balance and Dynamics

The first objective of DISR is to measure directly the vertical profile of the solar heating rate. This will be done using measurements of the upward and downward solar flux over the spectral interval from 0.35 to 1.7pm from 160 km to the surface at a vertical resolution of approximately 2 km. The downward flux minus the upward flux gives the net flux, and the difference in the net flux at two altitudes gives the amount of solar energy absorbed by the intervening layer of atmosphere. This basic measurement gives an important quantity for understanding the thermal balance of Titan's atmosphere.

From other Huygens measurements of the temperature profile and the gaseous composition, the science team plans to model the radiative cooling rate at wavelengths in the thermal infrared. An important contribution to this calculation will be the measurements of the size, shape, optical properties, and vertical distribution of aerosol and cloud particles determined by other DISR measurements. The combination of the measured solar heating rate with the computed thermal cooling rate will give the net radiative drive for atmospheric dynamics. Model computations can be used to estimate the wind field from the radiative forcing.

Finally, the science team plans to measure the horizontal wind direction and speed as functions of altitude from images of the surface obtained every few kilometres in altitude which will show directly the drift of the probe over the surface of Titan. The measured wind speed and direction determined by DISR can be compared to the wind field computed from the net radiative forcing determined above.

Distribution and Properties of Aerosol and Cloud Particles

Several properties of the cloud and aerosol particles are important for understanding their interaction with solar and thermal radiation field. The size of the particles compared to the wavelength of the radiation is important for understanding their scattering properties. Measurements of both the forward scattering and polarising nature of the aerosols on Titan have been used to show that spherical panicles can not simultaneously explain these two types of observations. We are therefore interested in knowing particle shape as well as size. The vertical distribution of the particles is obviously important for knowing their influence on the profiles of solar and thermal radiation. Finally, a suite of optical properties are needed as functions of wavelength to permit accurate computations of the interactions of the particles with radiation. These include the optical depth, single scattering albedo, and the shape of the scattering phase function. These properties together with the determinations of size and shape can yield the imaginary refractive index (and possibly constrain the real refractive index also) and thus constrain the composition of the particles.

We plan to measure as many of these properties as possible by combinations of measurements of small angle scattering in the solar aureole in two colours, by measurements of side and back scattering in two colours and two polarisation's, by measurements of the extinction as a function of wavelength from the blue to the near infrared, and by measurements of the diffuse transmission and reflection properties of layers in the atmosphere.

Nature of the Surface

The surface of Titan was hidden from view of the cameras aboard the Pioneer and Voyager spacecraft by the layers of small haze particles suspended in the atmosphere. Nevertheless, intriguing suggestions regarding the nature of the surface have been made, including the possibility that the surface consists of a global ocean of liquid methane ethane. Radar observations and direct observations at longer wavelengths strongly hint that the surface is not a global ocean. The range of fascinating surfaces observed by the Voyager mission on satellites of the outer solar system showed a surprising range of phenomena including craters, glacial flows, frost and ice coverings, and active geysers and volcanoes. These preliminary explorations of the small bodies of the outer solar system suggest that the surface of Titan also may well contain new surprises.

We plan to measure the state (solid or liquid) of the surface near the probe impact site, and to determine the fraction of the surface that is solid and liquid in this region. We plan to measure the topography of the surface, and explore the range of physical phenomena that have formed the surface. We plan to measure the reflection spectra of surface features from the blue to the near infrared in order to constrain the composition of the different types of terrain observed. In addition, we plan to image the surface at resolution scales from hundreds of meters (similar to those accessible from the orbiter) to tens of centimetres over as large an area as possible to study the physical properties occurring on the surface and to understand the interactions of the surface and the atmosphere.

Composition of the Atmosphere

The Huygens Probe contains a mass spectrometer/gas chromatograph to measure directly the composition of the atmosphere. Nevertheless, direct sampling techniques can have problems with constituents that can condense in the atmosphere should a cloud particle enter and slowly evaporate in the sampling system of such an instrument. The DISR will provide an important complementary capability by being able to record the spectrum of the downward streaming sunlight which shows the absorption bands of methane, the most likely condensable constituent. The observations of the visible and near infrared absorption bands of methane will be used to determine the profile of the mixing ratio of methane gas during the descent of the Huygens Probe.

Methane can exist as a solid, liquid, or gas on Titan, and has been suggested to play a role in the meteorology of Titan similar to the role played by water on the Earth. Our measurements of methane mixing profile will be analogous to a relative humidity profile on the Earth.

Finally, the atmosphere of Titan is believed to consist primarily of nitrogen, methane and argon. Our measurements of the mixing ratio of methane together with the determination of total mean molecular weight of the atmosphere by radio occultation measurements made by the Cassini Orbiter will indirectly yield the argon to nitrogen mixing ratio as an important backup to the mass spectrometer measurements planned fort he Huygens Probe.

DISR Instrument Overview

• CCD detector fed by:
  • three imagers
  • upward and downward-looking spectrometers
  • solar aureole camera (2 colours, 2 polarisation states)

• Two linear IR detector arrays
  • upward and downward-looking infrared spectrometers
  • upward and downward-looking violet photometers

• Surface Science Lamp

• Sun Sensor

• Inflight Calibration System

• Hardware and software data compression systems

• Flexible data collection software

• Three frame imagers:
  • Passband from 660 to 1000 nm at 0.06°, 0.12^o and 0.20° per pixel
  • Complete coverage in azimuth from 6° to 96° nadir angle in 36-exposure sets

• Upward and downward-looking IR spectrometer
  • 132 spectral pixels from 850 to 1700 nm
  • Diffuser looking up, image spot at 20° nadir angle looking down

• Upward and downward-looking visible spectrometer:
  • 200 spectral pixels 480 to 960 nm; diffuser with shadow bar looking up
  • 10 40 × 40 resolution elements from 10° to 50° nadir looking down

• Upward and downward-looking violet photometer:
  • Diffusers looking up (with shadow bar) and down; 350 to 480 nm passband

• 4-channel solar aureole camera:
  • 6° wide strip from 25° to 75° zenith angle centred 60° and 180° from azimuth of Sun
  • Passbands at 500 and 935 nm, each in vertical and horizontal linear polarization

• Lamp for measurement of reflection spectrum of surface below 100 m altitude

• Sun sensor to measure solar azimuth and zenith angles and brightness at 939 nm.