Pergamon Adv. Space Res. Vol. 17, No. 12, pp. (12)61-(12)64, 1996

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0 2 7 3 - 1 1 7 7 ( 9 5 ) 0 0 7 5 9 - 8

INFRARED SPECTROMETER PFS FOR THE MARS 94 ORBITER

V. Formisano, l V. I. Moroz, 2 H. Hirsch, 3 P. Orleanski, 4 G. Michel, 5 J. Lopez-Moreno, 6 E. Amata, 1 G. Bellucci, 1 G. Piccioni, 1 G. Chionchio, 1 A. Carusi, 7 A. Coradini, 7 P. Cerroni, 7 M. T. Capria, 7 F. Capaccioni, 7 A. Adriani, 8 M, Vitterbini, 8 F. Angrilli, 9 Go Bianchini, 9 B. Saggin, 9 S. Fonti, 10 E. Bussoletti, 11 D. Mancini, 12 L. Colangeli, 12 A. Grigoriev, 2 B. Moshkin, 2 V. Gnedykh, 2 I. A. Matsygorin, 2 D. Patsaev, 2 Yu. V. Nikolsky, 2 D. V. Titov, 2 L. V. Zasova, 2 I. Khatuntsev, 2 A. Kiselev, 2 G. Arnold, 3 H. Driesher, 3 M. I. Blecka, 4 R. Rodrigo 6 and J Rodriguez-Gomez 6

1 lstituto di Fisica dello Spazio lnterplanetario CNR (IFS1), via Galilei, c.p. 27, 00044 Frascati, Italy 2 Space Research Institute of Russian Academy o f Sciences (IK1), Profsojuznaja 84/32, 117810 Moscow, Russia 3 Deutsche Forschungsanstaltfiir Lufi und Raumfahrt (DLR), Institutfiir Planetenerforschung, Rudower Chausse 5, 12489 Berlin-Adlershof Germany 4 Space Research Center of Polish Academy of Sciences (SRC PAS), Bartycka 18,4, 00-716 Warsaw, Poland 50bservatoire de Paris-Meudon, Departement de Recherche Spatiale (DESPA), Place J. Janssen 5, 92195 Meudon, France 6 lnstituto de Astrofisica de Andalusia CSIC, p.o,b. 3004.18080, Granada, 5pain 7 lstituto di Astrofisica 5"paziale CNR (IAS) Reparto di Planetologia, viale dell' Universita 11, 00185 Rome, ltalv 8 Istituto di Fisica dell' Atmosfera CNR (I[¢A), vta Galilet, c.p. 27, 00044 Frascati, Italy 9 Universita' di Padova, Dipartimento di Ingegneria Meccanica (D1UNP), via Venezia 1, 35131 Padova, Italy IO Universita' degli Studi di Lecce~ Dipartimento di Fiswa, via Arnesano, 73100 Lecce, ltalv 11 Istituto Universitario Navale, lstituto di Fisica Sperimentale, via De Gasperi 5, 80133 Napoli, Italy 12 Osservatorio Astronomico di Capodimonte (OAC), via MoiarieUo 16, 80131 Napoli, Italy

ABSTRACT

PFS (the Planetary Fourier Spectrometer) covers the range 1_25 - 45/am with spectral resolution about 2 cm-t and angular resolution 0.035 - 0.070 tad (10 - 20 km on the Martian surface working at the periapsis). The instrument has two spectral channels: shortwavelength (SW) and longwavelength (LW) with a bounda~' near 5 p.m. The photoconductive detector (PbSe) is used m the SW channel and the pyroelectric in LW channel. Tile main optical units of both channels are rotating intefferometers with cubic mirror comer reflectors. The infrared radiation from Mars is directed to the interferometers by the pointing system that allows to observe selected points on the Martian surface. A "dichroic" plate splits the beam between LW and SW channels. Several hundred spectra will be obtained during one periapsis passage. These spectra will be used for investigation of Martian atmosphere (temperature and pressure vertical profiles, variations of small constituents such as t-I~O and CO, pressure near the surface, aerosol distribution, composition and optical depth) and some of surface properties (thermal, compositional, textural). Scientific thcilifies of six countries (Italy, Russia, Gemltmy, Poland, France trod Spain) ctx~pemte in the work on this experiment.

t12~61

(12)62

SC IEN T/_FIC GOALS

V. Fornusano at al.

PFS (the Planetary Fourier Spectrometer) is one of the first priority, instruments m the scientific payload of the orbiter of the 1%4ARS-94 (recently converted to MARS-96) mission. This is an infrared spectrometer covering the wide range from 1.25 to 45 ~trn vdth tile spectral resolution about 2 cm-~ and spatial resolution as good as several dozens of kilometers if working at the altitude 300 kin. Scientific

goals of this expemnent are: 1) Study of the atmosphere, including the monitoring of three-dimensional temperature fields at the altitude from 0 to 40 km (using retrievals from 15 Inn CO2 band), winds (indirectly, using geostrophic approach), minor constituents (HeO mid CO variations, search of other gases), atmospheric aerosols (dust and ice clouds, hazes, estimation of their optical depth and scattering fimcfion, determination of particles composition, estimation of their sizes), investigation of the radiation balance of the atmosphere and the influence of aerosols on its energetics. 2) Study of the surface, including the wavelength dependence of its brightness temperature and its time and local variations, thermal inertia, determination of some restrictions on the composition of the surface layer and nature of surface condensates, estimation of the surface scattering function and regolith gram sizes, pressure and height local determination (CO2 altimetry), surface-atmosphere exchange processes.

DESCRIPTION OF THE INSTRUMENT

Requirements to the spectrometer from the atmospheric and surface tasks are not the same and can not be opthnized tbr both sets of goals simultaneously: first of all, atmospheric studies need the best feasible spectral resolution but the spatial resolution is much less important; on contrary, for surface studies good spatial resolution is much more important than spectral. PFS is oplimized in this sense for the atmospheric studies, There is another spectrometer in the MARS-94(96) orbiter payload (OMEGA) with characteristics optimized for the surface. The IRIS instrument of the MARINER-9/1/and FS-1/4 of the VENERA-15 orbiter/2/were precursors of PFS. All three instnnnents are Fourier-spectrometers. ?v 'lain differences between PFS and other two are:

1) PFS is double channel spectrometer, one of chammls (LW) is for long wavelength (see a table below) and covers approximately the same range as precursors, the second channel (SW) is for shortwavelength not covered by IRIS and FS-1/4; 2) main optical units of both PFS channels are not Michelson interferometers, they are rotating interferometers with cubic mirror comer reflectors; 3) PFS has scanning/pointing device on the entrance that allows to observe selected regions on the Martian surface. Modes of the pointing device vary from specified points to step by step scanning.

Main parmneters of the PFS channels are: SW LW

Spectral range ~ n 1.3 - 4 6 - 45 cm-I 2500 - 7700 222 - 1667

Spectral resolution cm-~ 2 2 FOV mrad 0.02 0.04 NER W / cm2 sr cm-1 5×10-t0 2×10 -s One measur, cycle duration s 6 6 Detector PbSe photocond. Pyroelectric

Size mm 0.7×0.7, sqtmre O 1.4, round NEP W / ,J1--I~z 1 × 10 -1I 2× lO-tO Temperature K 195 290

hiterlhrometer Double pendulum Reflecting elements Cubic comer reflectors Beamsplitter CaF2 CsJ Max.optic. path differ, mm 5 Reference source Laser diode 1200 nm Optics transmission 0.15 0.15

lnlerferogram Number of points Dinamical rmlge

Data volume Mbit In one cycle (6s) spectr, trans, mode interf, trmls, mode

hi one periapsis sequence spectr, trans, mode interf, trans, mode

Stun for LW and SW ,adlocated by mission for 1 downlink

hffrared Spectrometer PFS

Double sided 16384 2t3

0.103 0.262

4096 21t

0.026 0.066

40 10 104 26

130 80

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Simplified optical scheme is presented on Fig.l. SI and $2 are plane diagonal mirrors of file pointing/scanning system. They are rotated by step motors and chmige the pointing duvx:tion. FM is the KRS phte with the multilayer coating. It is transparent lbr radiation with wavelength more than 6 ~ n and reflects shortwavelength radiation (< 5 pro). W is Si window with the multilayer coating providing cut-off near 1.3 Inn. 3//is the plane mirror directing the radiation into tile SW channel. B1 is a beamsplitter of the SW channel, B2 - o f L W . R I 1 and R12 are cubic mirror comer reflectors of SW channel, R22 and R21 - of LW. C1 and C2 are parabolic mirrors tbcusing radiation on detectors. During the work oi" the interRzrometer two frames holding R I 1 - R I 2 and R214~22 periodically turn around the common axis mad two interferograms are swept through detectors. Such interferometer with rotating comer reflectors is much less sensitive to the mechanical disturbances than a conventional Michelson interli:rometer. More detailed description of the optics of the interferometer was presented in/3, 4/.

. J

,Mars wsll be observed mainly at periapsis passages. SW chamlel will produce data only during the observations of the illuminated part of the phmet, and it appears that 80 Mbit/downlink volmne

(12)64 V. Formisano et al.

nominally allocated for experiment is sutlicient even ff all intefferograms are transmitted to the Earth. However not every periaps;s passage would be followed by downlink, so mode when spectra are transmitted instead of inteffcrograms will be very useful. Special FFT unit (ICM) will make on board computation of spectra from interferogrmns, but file possibility to transmit only interferograms exists anyway. Built-in blackbody source will be used for in-flight calibration of LW channel and SW channel at wavelength greater than 3 ~m. Lamps mid scattering screen illuminated by tim Sun serve tbr in-flight calibration of SW channel at shorter wavelengths. Planning of the observational sequence will depend on the real stability of the instrument properties and in the worst case every observation of Mars will be fc, llowed by observation of the open space and calibration sources. Tile vacuum chamber in IFSI ,,as used for the laboratory calibration of PFS. Special blackbody sources were designed and produced tor this purpose.

SCIENTIFIC COOPEILA, TION

Scientists and en#neers from 6 European countries and 12 institutes participate in the PFS experiment. Flight units were designed and produced in Italy (interferometer and main electronic modules), Russia (1R detectors and pointing/scanning system), Poland (power supply and GSE), Germany (in-flight calibration mli0. Meudon observatory in France is responsible for the laboratory model of the on-board Fourier-transform computer unit, Spanish participants help to Italians ill the electronics design, German colleagues - in the optics design and adjustmeuL Russian and Gemmal - in the design arid production of the groundbased calibration equipment, etc. We use experience and ability of people mid material resources with the high enough flexibility. In spite of the complexity of this wide international cooperation, it works and all formal deliveries for the mission were done without critical delays. In file middle of 1993 some spectra of the terrestrial alanosphere transmission along the path of several meters were obtained by one of the intermediate models of PFS. These and other tests cotffirmed that the mahl design conception is fruitful but showed at the same time that some improvements are possible in mechanics, electronics and software. The two-year delay of the launch gave us a time lbr these improvements. A small photometer for the range 0.4-1.1 pm~ has been added to PFS on latest stage of Zhe work. Its ahn is to measure a weak solar radiation scattered from the interplanetary dust around Mars and, if possible, to detect a hypothetical dust ring along the Phobos orbit. This tiny device was proposed, designed and produced in collaboration with Colorado University (-Prof. L. Esposito). A mlmerical modeling of the radiance from the dust matter in this ring was done by Polish team/5/.

REFERENCES

1. R. A. Hanel, B.I. Conrath, W.A. Hovis, V. Ktmde, P.D. Ix~wtnan, C. Prabhakara and t3. Sctdachman, The infrared spectroscopy experiment for Mariner Mars, 1971, 1carrel 0 12, 48-62 (1970).

2. D. Ocrtel, V.I. Moroz, D. Spankuch, V.M. Linkin, H. Jahn, V.V. Kerzhanovich, H. Becket-Ross, I.A. Matsygorin, K. Stadthaus, A.N. Lipatov, J. Nopirakovski, A. A. Shurupov, W. Doheler, L.V. Zasova, K. Shafer, E.A. Ustinov, J. Guldner and R. Dubois, hffrared spectrometry from Vencra-15 and Venera-16, Adv. Space Res. 0,5, 25-... (19,q7).

3. tl. ttirsch, A. Adrimli, F. Angrilli, F. Capaccioni, S. Fonti. A. Matteuzzi, G. Michel and V. Moroz, PFS Planetary Fourier Spectrometer tbr file Mars 9,1 mission, SPIE vol. 1780 Lens and Optical .9,stem design, 677-685 (1992).

4. H. Hirsch, G. Arnold, V. Formisano, V. I. Moroz and G. Piccioni, Optical definition of tile Planetary Fourier Spectrometer (PFS) - an FTIR spectrometer tbr tire Mars 94 mission, SPIE, vol. 2268, b![rared Spaceborne Remote Sensing (19%1, ill press).

5. M.1. Blecka. A. Jurewicz, Numerical modeling of radiance ill 0.4-1.4 jun range of dusl-gaseous lores around Mars, 30th COSPAR Scientific A,gsembly, 11amburg, Gerntato,,, 11-21 July 1994, Abstracts, p.47 (1994).