Descent Imager / Spectral Radiometer (DISR)

Instrument of the Huygens probe onboard the Cassini spacecraft

During the descent of the Huygens probe through Titan's atmosphere in January 2005, the Descent Imager / Spectral Radiometer (DISR) tokk images and performed measurements at various spectral ranges and spatial resolutions. These provided information about the composition of the haze layer, the presence of methane clouds and the nature of Titan's surface.

Science objectives

Saturn's moon Titan is the second-largest moon in the Solar System after Jupiter's moon Ganymede, larger even than the planet Mercury - and it is the only moon that has an atmosphere worth mentioning. On the ground, Titan's atmosphere has one-and-a-half times the pressure of Earth's atmosphere, and like it, consists mainly of nitrogen - however, it contains neither oxygen nor carbon dioxide. Instead, it comprises methane and possibly the noble gas Argon.

The temperature on the moon's surface is approximately -180 degrees Celsius, which eliminates the possibility of liquid water. However, methane can occur both in a gaseous and in a liquid state under these conditions. Therefore, methane's role on Titan may be similar to that of water on Earth - with a precipitation cycle involving clouds, rain and evaporation. Lakes or even oceans of methane and ethane are conceivable, but no one knows what Titan's surface actually looks like, as it is hidden beneath an impenetrable, orange haze.

In the wake of the Voyager 1 flyby of Titan, several initiatives were started to promote a mission to Titan. In 1997, the Cassini/Huygens mission was launched, which has reached the Saturnian system in July 2004. The Cassini spacecraft delivered the Huygens probe to Titan in January 2005 and then started to explore the Saturnian system until 2017.

The Huygens payload comprises six instruments, five of were built to carry out measurements during the descent through the atmosphere (starting at about 160 km altitude). One of these instruments is the Descent Imager / Spectral Radiometer (DISR), which took images and spectral measurements in different directions at various spectral ranges and spatial resolutions. The detectors of the DISR are a 512 x 256 CCD for the visible, two linear arrays of 132 photodiodes each for the infrared and two photodiodes for the violet spectral range. Light from the foreoptics of three surface imagers (high resolution, medium resolution and side looking), two visible spectrometers (upward and downward looking) and the Solar Aureole Imager (SA) is conducted by fiber optics to the CCD.

DISR instrument description

The DISR experiment consists of a sensor head box (SH) conducting the optics and the related detectors and of a separate electronics assembly (EA) box connected by a cable ( View of the instrument from above during assembly, optical fibres not connected, without cover) The SH is located on the experiment platform of the Huygens entry probe at a jettisonable cover such that it will be exposed to Titan's atmosphere with an unobstructed view upwards and downwards. It is an optical assembly containing 11 independent scientific data functions.

Measurements are made in the visible wavelength range and in the infrared. Most data at visible wavelengths will be collected by a CCD imaging detector, where each CCD pixel is permanently assigned to a certain data function. Thus, no complex mechanisms are needed to reconfigure the instrument to collect different data. This CCD detectector is a contribution of MPS. In addition, two simple violet radiometers looking up resp. down, each consisting of a PIN diode and a filter, are included. For infrared measurements, a single grating spectrometer is used which is equipped with two linear focal plane detector arrays consisting of InGaAs diodes. They are supported by CCD multiplexers. This component has been developed at DESPA, Observatoire de Paris. A small shutter permits a digital form of synchronous detection and a high signal-to-noise ratio.

Inflight calibration is performed by means of optical fibers connecting all subsystems to a single calibration source (three 1 W tungsten halogen lamps). The main optics and the sensors are mounted on a titanium support frame. Cooling of the device is achieved by conducting heat from the detectors to the ambient Titan atmosphere. Small heaters will stabilize the detector temperature at about 185 K.

The CCD array is divided in nine areas: three are used for the imager (downward looking in the visible and the IR channel, outward looking), four for measurements of the solar aureole at two wavelengths and two polarization angles, and two for the IR measurements (upward and downward looking spectrometers). The CCD was developed under contract of MPS at Loral in Tustin, Ca. It is is a high-resolution 2 phase frame transfer CCD with implied electronic shutter function so that no mechanical shutter is required. The size of the detector (512 x 256 pixels) is well adapted to the data transmission capability since all derived data must be immediately transferred to the Cassini orbiter.

The key functions of the electronics assembly are:

• power supply
• electric interface to the spacecraft
• housekeeping measurements
• detector drivers
• signal processors
• microprocessor and memory
• image compressor

while the sensor head contains only the part of the electric circuitry directy related to the detectors.

The instrument will consume about 15 W excluding the surface science lamp and the hardware data compressor which allows an on-line 6:1 compression of the image data within 750 ms and which will only be switched on for short time intervals.

MPS contribution

The scientists working on DISR at the MPS were:

• Horst Uwe Keller (DISR co-investigator, project lead at MPS)
• Jörg-Rainer Kramm
• Björn Grieger
• Yuri Skorov
• Stefan Schröder

The MPS was responsible for development, qualification and pre-calibration of the CCD detector and its associated operation electronics. The image compression board was developed in co-operation with the Technische Universität Braunschweig, Institut für Datenverarbeitung.

In the scientific analysis of DISR data, the MPS focusses on investigating the aerosol distribution in the atmosphere and the materials on the surface of Titan.

Cooperation Partners

	Lunar and Planetary Laboratory at University of Arizona Contribution: Responsibilty for the entire DISR instrument
	Jet Propulsion Laboratory Contributions: Software for data compression and modeling programs
	United States Geological Survey Contribution: Hardware and software for image display and analysis
	Departement de Recherche Spatiale, Observatoire de Paris Contributions: detectors for IR spectrometers, associated electronics, mechanical obturator, laboratory data and software for analysis and interpretation of spectral reflection measurements of the surface and of atmospheric gases
	Technische Universität Braunschweig, Institut für Datenverarbeitung Contribution: DCT image data processor
	Lockheed Martin Astronautics - Flight Systems Built the propulsion module subsystem for the CASSINI Spacecraft, have also assembled the DISR instrument.

Related links

	Cassini Project at NASA JPL
	Huygens Probe at NASA JPL
	Lunar and Planetary Laboratory (LPL) at University of Arizona
	Saturn at LPL
	Titan at LPL