NSSDCA ID: 2004-006C-04
Mission Name: PhilaeThe COmetary SAmpling and Composition (COSAC) experiment is one of two evolved gas analyzers carried on board the Philae lander. The instrument is designed to analyze the volatile fraction of cometary matter with emphasis on the detection of complex organic molecules. The primary goal of the COSAC experiment is to measure the elemental, isotopic, chemical and mineralogical composition of surface and subsurface cometary material as derived from volatiles to contribute to a better understanding of formation processes of stars and planetary systems. Another primary objective is the study of complex organic molecules with high molecular mass numbers, which may be pre-biotic building blocks of life on Earth, including investigating the chirality of these molecules. COSAC consists of a pyrolytic section, a gas-chromatograph, a mass spectrometer, a manifold and gas supply system, and a unit for control, data handling, and housekeeping. The pyrolytic section consists of two types of ovens mounted on the carrousel of the Sample Drilling and Disribution (SD2) system. Each type is a platinum cylindrical container 3 mm in diameter and 5 mm in height. The medium temperature ovens are designed for heating up to 180 degrees C. They have an optical window for inspection and infrared imaging spectroscopy of the samples by the CIVA microscopic camera. The high temperature ovens do not have windows and can be heated to 600 degrees C. Each oven is self-contained, with its own wound-on platinum heating filament and a chromel-alumel thermo-couple for temperature measurement. The oven can be heated stepwise to temperature levels selected by telecommand. At each of the 64 levels between -100 degrees C and +600 degrees C analyses can be performed as the gas is released. The normal temperature range in which all combinations of instruments can be used is limited to -100 degrees C to +180 degrees C.
Samples from the surface of the comet nucleus are collected by the SD2 system. The lander can rotate, allowing samples to be taken from different parts of the landing site. The drill can return samples from depths of at least 20 cm. The samples are then transferred into the ovens. The height of the sample in the oven is measured and the filled oven is moved to the tapping station which presses a ceramic sphere onto the rim of the oven affording a gas-tight seal. The tapping station is built to hold two neighboring ovens and contains the electrical contacts needed for heating and measurement. Two small stainless steel pipes run through the ceramic sphere to bring gas to the gas chromatograph and mass spectrometer.
The gas chromatograph consists of eight principally identical units. Each of them comprises a gas injector, a capillary column of 10 - 15 m length and 0.15 - 0.25 mm internal diameter, and a thermal conductivity detector. The capillaries are wound to 100 mm internal diameter spools in parallel with resistive wires for heating. Each spool is compacted by heat resistive glue to form a self-supporting structure. The temperature of the columns can be set by telecommand, the default temperature being 30 degrees C as for the whole piping system. The stationary phases coating the interiors of the different columns were chosen for covering a wide range of analytical tasks like separation of inorganic or organic, polar and non-polar compounds, different enantionmers of chiral molecules etc., but they had also to be selected and tested with respect to robustness, e.g. insensitivity to long term storage, aggressive compounds, and water.
The mass spectrometer is a high-resolution multi-pass time-of-flight instrument. It is a linear reflection type with an electron-impact ionization source at one end and a multi-sphere-plate secondary electron multiplier detector at the other end. A time-to-digital converter is used for signal and flight-time integration. The mass/charge spectrum is determined by accelerating ions inside the source into the flight path. In low resolution mode, a single flight path of 370 mm from source to detector is timed. The mass/charge resolution is 350 (FWHM) for ions of mass 70. A higher resolution mode can also be used, which involves multiple reflections between two gridless reflectors at either end of the flight path to extend the distance travelled. An electron impact storage ion source is employed with the electron flux varied by telecommand. The release pulse is 1 ms. The ions are accelerated to 1500 eV at the source, the expected mass range is 12 to 1500 AMU. The ions are post-accelerated in front of the detector to 4000 eV to be registered in he multi-sphere-plate electron multiplier. The time of flight is measured with a resolution of 2 ns using a 33 MHz clock. The manifold and gas system controls the flow of the evolved gas, the carrier gas (helium), and the calibration gas (a mixture of He, Ne, Ar, and Kr). Gas can also be fed from the gas chromatograph detector to the mass spectrometer. The carrier gas is stored in two 330 cubic cm tanks at 30 bar. The calibration gas is contained in a 25 cubic cm tank at 1 bar. The instrument communicates with the control and data management system of Philae through a DPU board including a Harris RTX 2010 processor, a PROM, an EEPROM, a SRAM, and additional controllers. This board is housed inside the Philae Lander Common Electronic Box. The data flow between the processor and the instrument is routed through an additional interface board which also hosts the mass memory of 3Mx16 bit.
Questions and comments about this experiment can be directed to: Dr. David R. Williams
Name | Role | Original Affiliation | |
---|---|---|---|
Dr. Reinhard Roll | General Contact | Max-Planck-Institut fur Aeronomie | roll@linmpi.mpg.de |
Dr. Fred Goesmann | Principal Investigator | Max-Planck-Institut fur Sonnensystemforschung | goesmann@mps.mpg.de |