PLANETARY FOURIER SPECTROMETER - AN INTERFEROMETER FOR ATMOSPHERIC STUDIES ON BOARD MARS 94 MISSION

PFS is a two-channel Fourier spectrometer operating in the infrared wa velengths between 1.25 and 45 mum. The instrument will be used mainly in the study of the Martian atmosphere. The principal goals are the me asurements of the atmospheric temperature and pressure, atmospheric co nstituents, aerosol and clouds, ground pressure for surface topography , optical and thermophysics properties of the Martian soil. PFS will f ly on the Mars 94 spacecraft which should be launched in 1994 and reac h the planet in 1995. It is essentially constituted by two different i nterferometers located in the same box which is divided into two parts . A dichroic placed on the PFS entrance is used to separate the spectr al range into two parts, a division needed by the different optical ma terials which have to be used in each spectral range. The optical layo ut of the experiment is very compact. Each channel uses two cubic mirr ors mounted on an L-structure pivoted on a motor. The motor moves the mechanics and permits the optical-path difference between the arms to be varied. Each interferometer operates in a different spectral range, respectively, between (1.25 divided-by 4.8)mum(8000 divided-by 2083cm -1) and (6 divided-by 45)mum(1666 divided-by 220)cm-1). The spectral r esolution is 2 cm-1. The entrance aperture area is 30 cm2 per channel and the field of view is 2 and 4 degrees. Every measurement lasts abou t 4 s. The time and, therefore, the relative optical-path difference f or the measurement of every point of the interferogram is given by the zero crossings of the interferogram of a reference monochromatic chan nel at 1.2 mum which uses a laser diode as source. The two interferogr ams are double-sided and will have 16384 and 4096 points, respectively , corresponding to spectra of 6250 and 1823 useful points.