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Journalof GeophysicalResearch VOLUME 76 APRIL 1, 1971 NUMBER 10 Mariner6 and 7 UltravioletSpectrometer Experiment: UpperAtmosphere Data, C. A. BARTH,C. W. HORD,J. B. PEARCE,K. K. KELLY, G. P. A•)ERSON,AN•) A. I. STEWART Department o• Astro-Geophysicsand Laboratory Jot Atmospheric and Space Physics University oJ Colorado, Boulder 8030œ Mariner 6 and 7 observationsof the Mars upper atmosphereshow the ultraviolet emission spectrumto consistof the COs+ A-X and B-X bands,the CO a-X and A-X bands,the CO+ B-X bands, the C I 1561- and 1657-A lines, the O I 1304-, 1356-, and 2972-A lines, and the It I 1216-A line. Laboratory measurementsand theoretical calculationsshow that the CO•* bandsystems are producedby th'e--•Combinationof photoionization excitation of COsand fluorescentscattering of CO.?.An knalysisof the vibrationalpopulations of the CO Cameron bands shows that they may be ProdUcedfrom photon or electron dissociative excitation of COs. The vibrational distribution• 'of the CO fourth po•tive bands is the same in the Mars spectrumas in a laboratoryCO, dissociativeexcitation spectrum, except for the bandsthat are self-absorbedby CO in the Mars atmosphere.Since the •C I 1561- and 1657-A lines and the O I 1356- and 2972-A lines may be produced in the laboratory by COs dissociative excitationprocesses, these are the most likely sourcesin the Mars atmosphere.The altitude distribution of all the emissionsis the same, except for the Yi I 1216-A and O I 1304-A lines and part of the COs+ A-X bands. The scale heights indicate a cold, predominantlyCOs atmosphere.The O I 1304-A line emissionextends to much higher altitudes than the':CO• emissionsshowing that atomicoxygen is present.Its profilesuggests that it is excitedby more than one mechanism. Fluorescent scattering of the CO, + A-X and B-X bands indicates the presenceof COs+, but COy+ is not necessarilyan abundantion. Lyman a radiation with a single scale height extendingseveral radii from the planet establishesthe presenceof atomic hydrogen in the Mars exosphere. On July 31 and August 5, 1969, the first make possible the determination of the com- observationsof the ultraviolet spectrumof the positionand structureof the Mars upperatmos- Mars upper atmospherewere madewith ultra- phere. violet spectrometerson board the Mariner 6 The purposeof the presentreport is twofold' and 7 spacecraft.The principal emissionfea- (1) to give the resultsof a detailedspectro- tures were quickly identified and reported in scopic analysis of the data that lead to the the literature [Barth et al., 1969]. A continued identificationof the principal mechanismspro- examination of these-ultraviolet observations has ducing the upper atmosphereemissions, and revealed that they are rich in spectral detail (2) to present the intensities of individual and contain sufficient altitude information to spectral features as a function of altitude so that modelsof the Mars upper atmospheremay Copyright ¸ 1971 by the American GeophysicalUnion. be constructed.A review of the techniquesand 2213 2214 BIRTH ET AL. theory of the ultraviolet spectroscopyof planets by the planet. Sincethe relative velocity of the [Barth, 1969] showsa number of the spectral spacecraft with respect to the planet was 7 emissionsthat were beingsought in the Mariner km/sec, a completespectral scan was obtained 6 and 7 experiment. every 21 km as the effectiveobservation point, i.e., the closestpoint of the tangential line of INSTRUMENT sight, moved deeper into the atmosphere.The The Mariner 6 and 7 ultraviolet spectrometer ultraviolet spectrometerson each Mariner had consistsof a 250-mm focal length Ebert-Fastie the opportunity to view the atmospheretwice, scanningmonochromator, a 250-mmfocal length once looking ahead and once looking sideways, off-axis telescope, and a two-photomultiplier as each spacecraftflew by Mars making a total tube detector system [Pearce et al., 1971]. The of four probes of the upper atmosphere.The spectral range 1900-4300 A was measured geometry of the observationsis shown in Fig- in the first order at 20-A resolution with a ure 1. In the caseof Mariner 6, the solar zenith b«alkali photomultiplier tube, and the spectral angle of the effective observation point was range 1100-2100A was measuredin the second 27ø for the first limb crossingand 0 ø for the order at 10-A resolution with a cesium iodide second.For Mariner 7, the first limb crossing photomultipliertube. A completespectral scan had a solar zenith angle of 44ø and the second including both orders was completed every 3 had a 0 ø angle. In all cases,the observations sec. Extensive light baffling was used to reject were made at an instrument zenith angle of 90ø . light from the bright disc of the planet while SPECTRUM observingthe upper atmosphere. The ultraviolet spectrum of the upper atmos- OBSERVATIONALGEOMETRY phere of Mars consistsof the C02+ A q/u-X 'I/g The spectral emissionsfrom the upper at- and B •u+-X •//g bands, the CO a 3//-X • mosphereof Mars were observedby having Cameronand A q/-X •+ fourth positivebands, the ultraviolet spectrometer look tangentially the CO+ B •;+-X •+ first negative bands, the through the atmosphereas the spacecraftflew C I 1561- and 1657-A lines,the 0 I 1304-, 1356-, and 2972-A lines, and the H I 1216-A Lyman a line. These spectral emissionfeatures are shown INCIDENT in the spectra in Figures 2 and 3. Figure 2 SOLAR RADIATION displays the Mars spectrum between 1900 and 4000 A at 20-A resolution,which was obtained LINE whilethe instrumentwas observing the Mars OF atmosphereat an altitude between 160 and 180 $16HT km abovethe surface.Figure 3 showsthe spectral interval between 1100 and 1800 A at 10-A resolution. This spectrum originated from the 140- to 160-km level of the Mars upper atmos- phere. Both the long and short wavelength spectradisplayed here are the result of summing four individual 3-secspectral scans. EMISSION MECHANISMS The B-X and A-X C0•' bands may be pro- duced in the Mars upper atmosphereby one or more of three mechanisms: (1) fluorescent Fig. 1. Geometry of atmosphericlimb obser- scatteringof solar radiation by carbon dioxide vations. In all four limb crossings,the line of ions; (2) photoionizationexcitation of neutral sight was perpendicularto a radiusvector making carbon dioxide by extreme ultraviolet solar the instrument zenith angle e = 90ø. The solar radiation; and (3) electron impact excitation zenith angle eowas 27ø and 44ø for the first limb crossingsof Mariner 6 and 7, respectively,and 0 ø of neutral carbon dioxideby the photoelectrons for both secondlimb crossings. producedin the photoionizationof the major MARINER 6 AND 7 UV SPECTROMETEREXPERIMENT 2215 600' COa•rI-x'Z o c0• õ•z-•rI -ooo -- e4 •o uo J • 4oo- olq"= • co+ + •' • ii I •,o• im I"o I"o =2v+I' v2v+ I• CO••-•zN 04.' • - I1•o o o o o - • • jja• • • = d d do Z JJ• I'• I•' I"' I'" I'1" I" I" I' • 20 o E I ,,.,,,, .... , 2000 2500 3000 3500 4000 WAVELENGTH,• Fig. 2. Ultraviolet spectrumof the upperatmosphere of Mars, 1900-4000A, 20-A resolution. Spectrumwas obtainedby observingthe atmospheretangentially at an altitude between160 and 180 km. This spectrumwas obtainedfrom the sum of four individual observations. constituentsin the Mars atmosphere. These CO2(Xl Z,+) + hl/• CO2+(B2Z. +) -+-e processesmay be representedby the following X < 686 A set of equations,which also lists the wavelength of the solar radiation or the energyof the elec- CO2(Xl Z,+) + h• • C02+ (A 2n.) + e trons responsiblefor the excitation. x <716A CO•+ (X •IIo) q- Av--• CO•+ (B •Z. +) C02(Xl Z,+) + e • CO•+(B•Z. +) + 2e 2890 A E > 18.1 ev co, +(x •no) + • -• co, +(A 3507 A E• 17.3 ev CO AlII-X IZ+ 5O0 ...;-; o 0 -- oJ i,o I ' I 1''i ' !' I • 4OO •t-. eu ,o• co ,r, ;500 i! i I i ! I zo0 ,00 I I ' I ' I ' I ' I I 1200 1400 1600 1800 WAVELENGTH, ._ Fig.3. Ultravi,olet spectrum oJ.,the'Upper atmosphere of Mars, 1100-1800 A, 10-A resolu- tion.Spectrum was •)btained by Observingthe atmospheretangentially at an iqtitudebetween 140 and 160 km. This spectrum was obtained from the sum of four individual observations. 2216 • BARTH ET AL. 600 trum than photoelectron excitation, becausea large number of photonsare available for the o photoionization excitation and because the threshold for electron impact excitation lies above the energy of most of the photoelectrons. z 400• A dayglowmodel calculation for a pure COrCO,`+ atmosphere illustrates the dominance of the photoionization excitation and because the b.J •2 -,,2 I > A II-X II ' o 1970]. The photoionizationexcitation spectrum •2oo- H o o o o o• -• • producedby 584-A photonsthat was recorded -• H • • • o o oo IJ.I 1" 1" I"' i'" I'1" 1" 1" I' in the laboratory by a Mariner ultraviolet spectrometeris shownin Figure 4. A compari- son between. it and the Mars spectrumin Fig- ure 2 shows that the vibrational distribution in 0 •-• , I " i , [ I ' ' ' ' I the A-X bands is not the same in the two 3000 3500 4000 WAVELENGTH,• spectra. For example, in the Mars spectrum the 0, 0 band is more intense than the 2, 2, Fig. 4. Laboratory photoionization excitation whereasin the laboratory spectrumthe reverse spectrum of CO2 produced with 584-A pho- tons. is true. When the laboratory spectrumwas sub- tracted from the Mars spectrum, the bands at 400 longer wavelengthsthat arise from the lowest vibrational levels were the most intense in the spectrum that remained. The emphasison the z ill lower vibrational levels is indicative of a spec- trum produced by fluorescentscattering, since the higher vibrational levels require shorter ,.,2001 wavelength photons for excitation, and the '• solar spectrum falls off with decreasingwave- length in this part of the spectrum.
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