The Huygens payload consists of six science instruments, as described below.
Huygens Atmospheric Structure Instrument (HASI). This instrument contains a suite of sensors that will measure the physical and electrical properties of Titan's atmosphere. Accelerometers will measure forces in all three axes as the probe descends through the atmosphere. With the aerodynamic properties of the probe already known, it will be possible to deduce the atmospheric density and to detect wind gusts. In the event of a landing on a liquid surface, the probe motion due to waves will also be measurable. Temperature and pressure sensors will enable the construction of a profile of the thermal structure of the atmosphere. The Permittivity and Electromagnetic Wave Analyzer component will measure the electron and ion (i.e., positively charged particle) conductivities of the atmosphere and search for electromagnetic wave activity. On the surface of Titan, the conductivity and permittivity (i.e., the ratio of electric flux density produced to the strength of the electric field producing the flux) of the surface material will be measured.
Doppler Wind Experiment (DWE). This experiment will use an ultra-stable oscillator to give the probe relay link a very stable carrier frequency. The probe drift caused by winds in Titan's atmosphere will induce a measurable Doppler shift in the carrier signal. The swinging motion of the probe beneath its parachute and other effects such as signal attenuation due to atmospheric properties may also be detected.
Descent Imager/Spectral Radiometer (DISR). This instrument will make a range of imaging and spectral observations using several sensors and fields of view. By measuring the upward and downward flux of radiation, the radiative balance (or imbalance) of the thick Titan atmosphere will be deduced. Solar sensors will measure the light intensity around the Sun due to scattering by aerosols in the atmosphere. This will permit the calculation of the size and number density of the suspended particles. Two imagers (one visible, one infrared) will observe the surface during the latter stages of the descent and, as the probe slowly spins, build up a mosaic of pictures around the landing site. There will also be a side-looking visible imager to get a horizontal view of the horizon and the underside of the cloud deck. For spectral measurements of the surface, the weak sunlight will be augmented by a lamp that will switch on shortly before landing.
Gas ChromatographÐMass Spectrometer (GCMS). This instrument will be a versatile gas chemical analyzer designed to identify and quantify various atmospheric constituents. It will be equipped with gas samplers that will be filled at high altitude for analysis later in the descent when more time is available. The mass spectrometer will construct a spectrum of the molecular masses of the gas, and a more powerful separation of molecular and isotopic species will be accomplished by the gas chromatograph. During descent, the GCMS will also analyze pyrolysis products (i.e., samples altered by heating) passed to it from the Aerosol Collector Pyrolyser. Finally, the GCMS will measure the composition of Titan's surface in the event of a safe landing. This investigation will be made possible by heating the GCMS inlets just prior to impact in order to vaporize the surface material upon contact.
Aerosol Collector and Pyrolyser (ACP). This experiment will draw in aerosol particles from the atmosphere through filters, then heat the trapped samples in ovens (the process of pyrolysis) to vaporize volatiles and decompose the complex organic materials. The products will then be flushed along a pipe to the GCMS instrument for analysis. Two filters will be provided to collect samples at different altitudes.
Surface-Science Package (SSP). The
SSP contains a number of sensors designed to determine the physical
properties of Titan's surface at the point of impact, whether
the surface is solid or liquid. An acoustic sounder, activated
during the last 100 meters of the descent, will continuously determine
the distance to the surface, measuring the rate of descent and
the surface roughness (e.g., due to waves). If the surface is
liquid, the sounder will measure the speed of sound in the "ocean"
and possibly also the subsurface structure (depth). During descent,
measurements of the speed of sound will give information on atmospheric
composition and temperature, and an accelerometer will accurately
record the deceleration profile at impact, indicating the hardness
and structure of the surface. A tilt sensor will measure any pendulum
motion during the descent and will indicate the probe attitude
after landing and show any attitude motion due to waves. If the
surface is, indeed, liquid, other sensors will measure its density,
temperature, refractive index, thermal conductivity, heat capacity,
and electrical permittivity.