The Descent Imager/Spectral Radiometer (DISR) experiment on the Huygens probe to Titan has scientific objectives in four major areas:
1) We want to understand the nature and distribution of photochemical haze in Titan's atmosphere. We plan to measure the size, shape, vertical distribution, and absorption as a function of wavelength of the haze particles to better understand their production and life cycle in Titan's atmosphere.
2) We want to understand the heat balance of Titan's atmosphere. We plan to measure the absorption of sunlight as a function of altitude to understand how much sunlight is absorbed in the atmosphere and how much is absorbed at the ground to support the small greenhouse effect observed in the atmosphere. This absorbed solar energy forms part of the driving force for Titan's winds. DISR will also measure the wind speed directly from the horizontal drift of the probe relative to features seen on the surface in our images.
3) We want to understand the nature of the surface and its interaction with the atmosphere. We plan to obtain some 700 images of the surface at resolutions varying from 150 meters to less than one meter. We will assemble these images into some 20 panoramic mosaics of the surface as seen from altitudes varying from 150 Km to a few hundred meters. We will also measure the reflection spectrum of the surface in several thousand locations in an attempt to catalog the surface reflectivity (and composition) with particular types of terrain seen in the images using the illumination of the sun at the wavelengths where sunlight penetrates to the surface of Titan. DISR also includes a small lamp to provide a continuous illumination of the ground at all wavelengths for measurements of surface reflectivity during the last few hundred meters of the descent for more diagnostic determination of composition.
4) Finally, we want to understand the composition of the atmosphere. In particular, we want to measure the mixing ratio of methane as a function of altitude using the DISR measurements of the atmospheric absorptions as a function of wavelength throughout our descent from 150 Km to the surface. Methane can be present as a liquid, a solid, or a gas on Titan, and can form clouds from which methane rain or methane snow can fall. Measuring the vertical profile of methane is analogous to measuring the relative humidity on the Earth. Taken together, investigations in these four general areas will provide a much more detailed of the chemical and physical processes which have occurred on Titan. Ultimately such studies will lead to a more detailed understanding of the processes that have determined the conditions on all the planets, including the Earth.
This paper comes directly from DISR's Principal Investigator, Dr. Tomasko.