THE DETERMINATION OF ICE COMPOSITION WITH INSTRUMENTS ON COMETARY LANDERS

The determination of the composition of materials that make up comets is essential in trying to understand the origin of these primitive obj ects. The ices especially could be made in several different astrophys ical settings including the solar nebula, protosatellite nebulae of th e giant planets, and giant molecular clouds that predate the formation of the solar system. Each of these environments makes different ices with different composition. In order to-understand the origin of comet s, one needs to determine the composition of each of the ice phases. F or example, it is of interest to know that comets contain carbon monox ide, CO, but it is much more important to know how much of it is a pur e solid phase, is trapped in clathrate hydrates, or is adsorbed on amo rphous water ice. In addition, knowledge of the isotopic composition o f the constituents will help determine the process that formed the com pounds. Finally, it is important to understand the bulk elemental comp osition of the nucleus. When these data are compared with solar abunda nces, they put strong constraints on the macro-scale processes that fo rmed the comet. A differential scanning calorimeter (DSC) and an evolv ed-gas analyzer (EGA) will make the necessary association between mole cular constituents and their host phases. This combination of instrume nts takes a small (tens of mg) sample of the comet and slowly heats it in a sealed oven. As the temperature is raised, the DSC precisely mea sures the heat required, and delivers the gases to the EGA. Changes in the heat required to raise the temperature at a controlled rate are u sed to identify phase transitions, e.g., crystallization of amorphous ice or melting of hexagonal ice, and the EGA correlates the gases rele ased with the phase transition. The EGA consists of two mass spectrome ters run in tandem. The first mass spectrometer is a magnetic-sector i on-momentum analyzer (MAG), and the second is an electrostatic time-of -flight analyzer (TOF). The TOF acts as a detector for the MAG and ser ves to resolve ambiguities between fragments of similar mass such as C O and N-2. Because most of the compounds of interest for the volatile ices are simple, a gas chromatograph is not needed and thus more integ ration time is available to determine isotopic ratios. A gamma-ray spe ctrometer (GRS) will determine the elemental abundances of the bulk co metary material by determining the flux of gamma rays produced from th e interaction of the cometary material with cosmic-ray produced neutro ns. Because the gamma rays can penetrate a distance of several tens of centimeters, a large volume of material is analyzed. The measured com position is, therefore, much more likely to be representative of the b ulk comet than a very small sample that might have lost some of its vo latiles. Making these measurements on a lander offers substantial adva ntages over trying to address similar objectives from an orbiter. For example, an orbiter instrument can determine the presence and isotopic composition of CO in the cometary coma, but only a lander can determi ne the phase(s) in which the CO is located and separately determine th e isotopic composition of each reservoir of CO. The bulk composition o f the nucleus might be constrained from separate orbiter analyses of d ust and gas in the coma, but the result will be very model dependent, as the ratio of gas to dust in the comet will vary and will not necess arily be equal to the bulk value. (C) 1997 Published by Elsevier Scien ce Ltd.