Journalof
GeophysicalResearch
VOLUME
76
APRIL
NUMBER
1, 1971
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 atmosphereshowthe 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 produced
by th'e--•Combination
of photoionization
excitation
of COsand
fluorescent
scattering
of CO.?.Anknalysis
of the vibrational
populations
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, dissociativeexcitationspectrum,exceptfor 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 emissionextendsto much higher altitudes than the':CO•
emissions
showingthat atomicoxygenis 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
observationsof the ultraviolet spectrumof the
make possiblethe determination of the com-
Mars upper atmosphere
weremadewith ultraviolet spectrometers
on board the Mariner 6
and 7 spacecraft.The principal emissionfea-
phere.
positionand structureof the Mars upperatmosThe purposeof the presentreport is twofold'
(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 principalmechanisms
proexamination 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
spectral features as a function of altitude so
and contain sufficient altitude
Copyright ¸
information
to
1971 by the American GeophysicalUnion.
that modelsof the Mars upperatmosphere
may
be constructed.A reviewof the techniquesand
2213
2214
BIRTH
theory of the ultraviolet spectroscopyof planets
[Barth, 1969] showsa number of the spectral
emissions
that were beingsoughtin the Mariner
6 and 7 experiment.
INSTRUMENT
The Mariner 6 and 7 ultraviolet spectrometer
consistsof a 250-mm focal length Ebert-Fastie
scanningmonochromator,
a 250-mmfocallength
off-axis telescope,and a two-photomultiplier
tube detector system [Pearce et al., 1971]. The
spectral range 1900-4300 A was measured
in the first order at 20-A
resolution with
a
b«alkali photomultiplier tube, and the spectral
range 1100-2100A was measuredin the second
order at 10-A resolution with a cesium iodide
photomultipliertube. A completespectralscan
including both orders was completed every 3
sec. Extensive light baffling was used to reject
light from the bright disc of the planet while
observingthe upper atmosphere.
OBSERVATIONALGEOMETRY
The spectral emissionsfrom the upper atmosphereof Mars were observedby having
the ultraviolet spectrometer look tangentially
through the atmosphereas the spacecraftflew
INCIDENT
SOLAR
RADIATION
LINE
OF
$16HT
ET AL.
by the planet. Sincethe relative velocity of the
spacecraft with respect to the planet was 7
km/sec, a completespectral scan was obtained
every 21 km as the effectiveobservationpoint,
i.e., the closestpoint of the tangential line of
sight, moved deeper into the atmosphere.The
ultraviolet spectrometerson each Mariner had
the opportunity to view the atmospheretwice,
once looking ahead and once looking sideways,
as each spacecraftflew by Mars making a total
of four probes of the upper atmosphere.The
geometry of the observationsis shownin Figure 1. In the caseof Mariner 6, the solar zenith
angle of the effective observation point was
27ø for the first limb crossingand 0 ø for the
second.For Mariner 7, the first limb crossing
had a solar zenith angle of 44ø and the second
had a 0ø angle. In all cases,the observations
were made at an instrumentzenith angleof 90ø.
SPECTRUM
The ultraviolet spectrumof the upper atmosphere of Mars consistsof the C02+ A q/u-X 'I/g
and B •u+-X •//g bands, the CO a 3//-X •
Cameronand A q/-X •+ fourth positivebands,
the CO+ B •;+-X •+ first negative bands,the
C I 1561- and 1657-Alines,the 0 I 1304-, 1356-,
and 2972-A lines, and the H I 1216-A Lyman a
line. These spectralemissionfeatures are shown
in the spectra in Figures 2 and 3. Figure 2
displaysthe Mars spectrumbetween 1900 and
4000 A at 20-A resolution,which was obtained
whilethe instrument
wasobservin
g the Mars
atmosphereat an altitude between 160 and 180
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 atmosphere. Both the long and short wavelength
spectradisplayedhere are the result of summing
four individual 3-secspectralscans.
EMISSION
Fig. 1. Geometry of atmosphericlimb observations. In all four limb crossings,the line of
sightwas perpendicularto a radiusvectormaking
the instrument zenith angle e = 90ø. The solar
zenith angle eowas 27ø and 44ø for the first limb
crossings
of Mariner 6 and 7, respectively,and 0ø
for both secondlimb crossings.
MECHANISMS
The B-X and A-X C0•' bands may be produced in the Mars upper atmosphereby one
or more of three mechanisms: (1) fluorescent
scatteringof solar radiation by carbon dioxide
ions; (2) photoionizationexcitation of neutral
carbon dioxide by extreme ultraviolet solar
radiation; and (3) electron impact excitation
of neutral carbon dioxideby the photoelectrons
producedin the photoionizationof the major
MARINER 6 AND 7 UV SPECTROMETER
EXPERIMENT
600'
COa•rI-x'Z
-ooo
-- e4 •o
o c0• õ•z-•rI
uo
J
iiI•im
I"I"I' I
•• 4oo- olq
• =2v+
co
+ •
•,o "=
o o
v2v+
•
-
•
Z
•
2215
04.
+•-•zN
CO•
I1•o
o o
jja•
o
o
• • = d
•''
-
d
JJ• I'• I•' I"' I'" I'1" I"
20
o
E
•
do
I"I'
I
,,.,,,,.... ,
2000
2500
3000
3500
4000
WAVELENGTH,
•
Fig. 2. Ultraviolet spectrumof the upperatmosphere
of Mars, 1900-4000
A, 20-A resolution.
Spectrumwas obtainedby observingthe atmospheretangentiallyat an altitude between160
and 180km. This spectrumwas obtainedfrom the sum of four individual observations.
constituentsin the Mars atmosphere.These
CO2(Xl Z,+) + hl/• CO2+(B2Z.+) -+-e
processes
may be represented
by the following
set of equations,whichalsoliststhe wavelength
X <
of the solar radiation or the energyof the electrons responsiblefor the excitation.
686 A
CO2(Xl Z,+) + h• • C02+(A 2n.) + e
x <716A
CO•+(X •IIo) q- Av--• CO•+(B •Z. +)
C02(Xl Z,+) + e • CO•+(B•Z. +) + 2e
2890
A
E >
18.1 ev
E•
17.3 ev
co, +(x •no) + • -• co, +(A
3507 A
CO AlII-X
...;-;
5O0
I
'
o 0
I
--
IZ+
oJ
1''i
' !'
co
,r,
!
I
i,o
I •
4OO
;500
•t-.
eu
,o•
i!
i
I
i
zo0
,00
I
I
'
1200
I
1400
'
I
'
I
1600
'
I
I
1800
WAVELENGTH,
._
Fig.3. Ultravi,
oletspectrum
oJ.,the'Upper
atmosphere
of Mars,1100-1800
A, 10-Aresolu-
tion.Spectrum
was•)btained
by Observing
the atmosphere
tangentially
at an iqtitudebetween
140 and 160 km. This spectrumwas obtained from the sum of four individual observations.
2216
• BARTH ET AL.
trum than photoelectron excitation, becausea
large number of photonsare availablefor the
photoionization excitation and because the
threshold for electron impact excitation lies
abovethe energy of most of the photoelectrons.
A dayglowmodelcalculationfor a pure COrCO,`+
atmosphereillustrates the dominanceof the
photoionization excitation and because the
1970]. The photoionizationexcitationspectrum
producedby 584-A photonsthat was recorded
in the laboratory by a Mariner ultraviolet
spectrometeris shownin Figure 4. A comparison between it and the Mars spectrumin Fig-
600
o
z 400•
b.J
•2
>
•2oo-
-•
IJ.I
-,,2
I
A II-X II
' o
H o o o o o• -• •
H •1" •1"
•i'" oI'1" o1"
I"'
oo
1" I'
.
ure 2 shows that the vibrational
0 •-•
,
I "
i
,
3000
[
I
'
'
'
'
3500
I
4000
WAVELENGTH,
•
Fig. 4. Laboratory photoionization excitation
spectrum of CO2 produced with 584-A photons.
400
the A-X
z
,.,2001
'•
0:: 0
I
'
$000
'
'
I
'
5500
'
'
'
I
4000
WAVELENGTH,
•
Fig. 5. Theoretical fluorescent scattering spectrum
for C'O,`+.
Since the relative intensities between the two
electronictransitionsand amongthe extensive
vibrational developmentin the A-X system
contain information about the excitation mecha-
nism, laboratoryand theoreticalstudieswere
conducted for each of the three mechanisms.
The spectraproducedby the impact of 20-ev
electronson neutral carbon dioxide were found
to be similar but not identical to the spectra
in
spectra. For example, in the Mars spectrum
the 0, 0 band is more intense than the 2, 2,
whereasin the laboratory spectrumthe reverse
is true. When the laboratory spectrumwas subtracted from the Mars spectrum,the bands at
longer wavelengthsthat arise from the lowest
vibrational
ill
distribution
bands is not the same in the two
levels were the most intense in the
spectrum that remained. The emphasison the
lower vibrational levels is indicative of a spectrum producedby fluorescentscattering,since
the higher vibrational levels require shorter
wavelength photons for excitation, and the
solar spectrum falls off with decreasingwavelength in this part of the spectrum. A synthetic spectrum that showsthe relative intensities expected from fluorescent scattering is
plotted in Figure 5. This spectrumwas calculated from the transition probabilities measured by Hesser [1968] and the branchingratios
derived by Dalgarno and Degges [1970] from
the work of Poulizac and Dufay [1967]. The
relative intensity of the (0, 0, 0 to 0, 0, 2) band
was determinedfrom the laboratory measurements of Wauchop and Broida [1971].
The two spectra,the photoionizationexcitation spectrum in Figure 4 and the fluorescent
scatteringspectrumin Figure 5, add up quantitatively to match the intensity distributionof
the C02+ B-X
and A-X
bands in the Mars
producedby the impactof 584-A photonson spectrum.At the particular altitude where this
CO2.At equivalentenergies,
the crosssections Mars spectrumwas observed,the B-X (0, O)
for the two processes
were found to be nearly band is produced90% by photoionizationexequal [Wauchopand Broida, 1971; Ajello, citation and 10% by fluorescent scattering.
1971].In analogyto the productionof the N,? Photoionizationexcitationis the strongersource
3914-A band in the earth's atmosphere,photo- of the A-X band system,but individual bands
ionizationexcitationis a more likely sourceof suchas the (0, 0) are producedmore strongly
excitation of the CO,.* bands in the Mars spec- by fluorescentscattering. Thus this analysis
I•IARINER 6 AND 7 UV SPECTROMETER]7•XPERIMENT
showshow the Mars ultraviolet spectrum between2800 and 4000 A may be usedto measure
quantitatively the number of CO•* ions in the
Martian ionosphere,the number of CO• moleculesat the samealtitude,and the rate of photoionization occurring in the Martian upper atmosphere.
The
CO a-X
Cameron
bands are the most
intense emissionin the Mars ultraviolet spectrum. They may be excitedby: (1) the fluorescent scattering of solar radiation by CO molecules in the Mars upper atmosphere; (2) the
photoelectronexcitation of carbon monoxide;
(3) the photodissociationexcitation of CO•
moleculesin the Mars upper atmosphere; (4)
the photoelectronexcitation of carbon dioxide
moleculesto produce excited carbon monoxide
molecules;and (5) the dissociativerecombination of ionized carbon dioxide molecules. These
2217
band is moreintensethan the (0, 0) band,while
in the Mars spectrumthe reverseis true. These
two mechanismsare also inadequateto account
for the intensity of the Cameronbandsquantitatively. The upper state of the Cameronbands,
the a 8II, is metastable; its lifetime has been
measuredto be 7.5 ñ 2 msec [Lawrence, 1971].
Becauseof the small oscillator strength associated with this transition, an amount of carbon
monoxide far in excess of the total number of
moleculesin the Mars upper atmospherewould
be required to produce the intensity of the
Cameron bands observedin the Mars spectrum
from fluorescentscattering.A similar quantitative argument may be made againstproduction
of these bands by the photoelectronexcitation
of carbon monoxide.
The vibrational distributionproducedby the
remainingthree processes
is lesswell known. In
fact, there is not yet experimentalevidencethat
the Cameron bands are produced by photo-
excitation mechanismsare described by the
followingset of equations,where the energyof
the photoelectrons
or the wavelengthof radia- dissociation excitation or dissociative recomtion responsible
for the excitationis alsolisted. bination while they have been produced by
electron impact excitation. To investigate the
CO(X•Z +) -•- h•--+CO(aSII) k _<2063A vibrational populationsfurther, the Cameron
bands were produced in the laboratory by
CO(X •Z+) -•- e --+ CO(asII) -•- e
bombarding carbon dioxide at a pressure of
E •
6.01 ev
C02(X•Zg+) -•- h• --+ CO(a3II) -•- 0 (3p)
k •
1082 A
COe(X'Zg+) -]- e --+ CO(a3II) -•- 0 (3p) -]- e
E>
11.5 ev
C02+(X 'II,) -•- e --+CO(asII) -•- 0 (3p)
Since the vibrational distribution of the CO
bands is not the same for all five
Cameron
mechanisms,the relative populations of the
upper vibrational levels give a clue to the
actualmechanismoperatingin the upper atmosphere of Mars. The vibrational distributions
for fluorescentscattering and photoelectronexcitation
of carbon
monoxide
were
calculated
previously [Barth, 1966, 1969]. Both predicted
spectra differ from the vibrational distribution
of the Cameron bands in the Mars spectrum.
In the fluorescentscattering spectrum, the vibrational population of the upper level falls off
more rapidly than in the Mars spectrum.In
the photoelectronexcitationspectrum,the (1, 0)
10 -8 torr
with
20-ev
electrons.
The
electron-
excited spectrum recordedby a Mariner-type
ultraviolet spectrometeris shown in Figure 6.
The
vibrational
distribution
of the
Cameron
bands in this laboratory sourceand the Mars
ultraviolet spectrum is nearly the same. By
using Franck-Condon factors calculated by
Nicholls [1962], it was determined that the
lowest vibrational level of the upper state was
the most populated.Levelsup through v' -- 6
were observedwith their populationsdropping
off relatively slowly with increasingvibrational
number. When the identified Cameron bands
were substracted from the Mars ultraviolet
spectrum with the aid of the Franck-Condon
factors,somespectral featuresremained.These
are identified as the CO* B '•-X • first negative bands. They are also produced in the
laboratory by the impact of electronson carbon
dioxide.A laboratory spectrum that emphasizes
thesebandsis shownin Figure 7 as recordedby
a Mariner-type spectrometer.In the figure the
amplitude of the first negativebandshas been
adjustedto match the intensity presentin the
Mars spectrumin Figure 2. First negativebands
2218
BART•
600]
CO
a$•-xl•
]oootxl to
ET A•,.
TABLE 1. Vibrational Populations of the Upper
States of the Cameron Bands aqI-X•;, the First
Negative Bands B•'I;-X•E, and the Fourth Positive Bands AqI-X•I; + in the Mars Spectrum.
Excited
State
CO+
z
--
BzE+-X
200-
0
I
....
I
2000
'
2500
WAVELENGTH,
Fig. 6. Laboratory electron excitation spectrum
of CO• producedwith 20-ev electrons.
>. 400z
Relative Population
a3II, v• = 0
1.00
1
2
3
4
5
6
7
0.67
0.36
0.29
0.18
0.10
0.10
0.10
B•,v • - 0
1.00
1
2
0.90
0.21
AqI, v• - 0
1.00
1
2
3
4
5
6
0.77
0.46
0.32
0.17
0.15
0.14
.
z
200-
CO+ B;'•;+_X•E +
oO.--tO
•
-
!
•
0
I
I"
0
I'
the dominant mechanismin the Mars upper
atmosphere.If the electronimpact dissociative
excitationcrosssectionis much larger than the
photodissociativeexcitation cross section,then
photoelectronexcitation may be the dominant
source of the Cameron bands.
o
2000
2500
WAVELENGTH,
•,
Fig. 7. Laboratory electron excitation spectrum
of COs producedwith 50-ev electrons.
The dissociative recombination of CO•.* is
energeticallycapableof producingthe Cameron
bands with an extensive number of vibrational
levelspopulated,but the laboratoryexperiment
to measure radiation
from this reaction has not
been performed.However, with the CO•* denare also presentin the laboratory spectrumin sitiesfor the Mars ionospherethat are derived
Figure 6. The relativepopulationsof the vibra- from the CO•.*A-X bands, and the recombinational levels of the CO a 8II state and the
tion rate measuredin the laboratory [Welle•
CO* B •Z statein the Mars spectrumare given and Biondi• 1967], it does not appear that
in Table 1.
sufficientrecombinations
can take placeto proOn the basis of the laboratory experience duce the observedintensity even if all recomwith the CO•.* band systems where similar binationsproducethe Cameronbands.
spectra are producedwith similar crosssections
The CO A-X fourth positivebandsmay be
by electronsand photonsof equal energy,it producedin the Mars upper atmosphere
by the
may be expectedthat photodissociation
excita- same five processesthat are potential sources
tion of carbon dioxide by extreme ultraviolet of the Cameronbands: (1) fluorescentscatterradiation will producethe Cameronbandswith ing by CO, .(2) photoelectronexcitationof CO,
a vibrationaldistributionsimilar to that pro- (3) photodissociation
excitation of CO•, (4)
duced by electrons.If the cross sectionsfor photoelectrondissociative
excitationof CO•, and
photon and electron impact are similar, then (5) dissociative recombination of CO•.*. The
an argument analogousto the one used with following equationsdescribethese mechanisms
the CO•.*bandswouldsuggestthat photodisso- and the wavelengthsof the solar radiationreciative excitation of carbon dioxide would be
sponsiblefor their excitation.
MARINER 6 AND 7 UV SPECTROMETER
EXPERIMENT
CO(X'Z +) -•- h•--•CO(AIII)
k_• 1545A which was obtained in the laboratory with a
Mariner-type spectrometer,showsbands arising from vibrationallevelsfrom v' -- 0 up to
CO(X l Z+) + e • CO(A lII) -[- e
E >
8.02
v' -- 6. The lowest level has the greatest popu-
ev
lation,but all levelsup through6 are populated
as well. When the laboratory spectrumis compared with the Mars spectrumin Figure 3, an
analysisshowsthat the vibrationalpopulations
in the two sourcesare nearly the same, althoughthe spectrathemselveshave differences.
What are missing in the Mars spectrum, as
compared with the laboratory spectrum, are
fourth positivebandswhoselower vibrational
C02(X'Z +) -[- h•-• CO(AlII) q- O(3p)
k <
co(x
+) + e
CO(A
2219
920 A
+ o(P) + e
E>
13.5
ev
C02+(X2IIg)+ e--• CO(AlII)+ O(3P)
In contrast to the Cameron bands, the first
mechanism,the fluorescentscatteringof solar
radiation by carbon monoxidemolecules,cannot be eliminated on a quantitative basis. In
fact, becauseof the large oscillatorstrengthof
the A-X transition,this band systemis ideally
level is the lowestvibrational level of the ground
electronicstate, the v"-- 0 level. The reason
why they are missingmust be that sufficient
CO moleculesare present in the Mars upper
atmosphereso that self-absorptionis taking
place in transitions connectedto the ground
suited to be a measure of the amount of carbon
vibrational
level.
The measuredvibrational populationsof the
[Barth, 1969]. As will be seenfrom the-analysis upper state of the fourth positivebands are
of the vibrational distribution, however, the alsogivenin Table 1. The populationdistribubulk of the excitation of the fourth positive tion in the Mars spectrumis differentfrom the
band in the Mars spectrumcannot come from distributionthat would be producedby mechfluorescentscatteringby CO molecules.
A simi- anismsI and 2 (fluorescentscatteringand pholar argumentcanbe madeagainstphotoelectron toelectron excitation of CO) and these data
thus may be used to eliminatethese mechaexcitationof CO being a strongcontributor.
How the vibrational distribution of the fourth
nismsas principalcontributors.
Dissociativerepositivebands may be used to identify the combinationof CO•*, (mechanism5) is also
sourceof excitationwas exploredby using the eliminatedby thesemeasuredvibrationalpoputechniquesdevelopedfor the previous band lations, since this mechanismis energetically
monoxide in the Mars
upper atmosphere
systems;namely,the productionof this band onlyableto excitethe v' -- 0 and I levels,and,
the observed Mars spectra show substantial
systemin the laboratoryby bombarding
up throughthe v' -- 3 level. This
with 20-ev electrons.The spectrumin Figure 8, populations
500
-
CO A•II- X •y-+
400
oooooo
-
¸
I I I'1'
500
-
200
-
wt,-.
I
m(,o
I''
--
eJ
I
' I'
co
ro
! i
.3CI
-- ......
,oo_
0
cu
I
-'
•-
•zo0
'
I
'
I
•4oo
'
I
'
I
•6oo
'
!
'
I
'
•800
WAVELENGTH,
•
Fig. 8. Labora•o•
electron excitation spectrum of GO• producedwith 20mv electrons.
2220
BARTH ET AL.
leavesonly mechanisms
3 and 4 (photodissoei-excitationof molecularhydrogen[Barth ½t ed.,
ative excitationand photoelectrondissoeiative 1965]. Photoelectron excitation of either the
excitation of C02) as sourcesof the Mars atom or moleculeis not significantbecauseof
fourth positivebands.No doubt both meeh- the small fractional abundance of either form
anism_s
are operating,but, followingthe argu- of hydrogenin the Mars ionosphere.The most
mentsgivenpreviouslyfor the Cameronbands plausible first choice of excitation for the
(that in dissoeiativeexcitation processes,
the 1216-A line is resonantscatteringof solar Lyexcitationcrosssectionsare approximatelythe man a radiation.
samefor photonsor electronsof equalenergy),
ALTITUDE DETERMINATION
the major sourceof fourth positiveband exciWhile
the
distance from the earth to the
tation shouldbe photodissoeiative
excitation
causethe numberof photonsavailableis greater
Mariner spacecraft flying by Mars could be
measured to a fraction of a kilometer [Anderthan the numberof photoeleetr0ns,.
When a syntheticspectrumof the fourth son et ed., 1970], it was not possibleto deter-
positive
bands
wassubtr
acted';fr0m
theMars mine the height above the surfaceof Mars
of
spectrum,emissionlines from atomic carbon the tangential line of sight of the ultraviolet
remained.From the positionsof a number of spectrometerto better than tens of kilometers
the ultraviolet lines of C I marked in the figure,
it may be seenthat there is a dear cut identi-
from the spacecraftorientation. However, the
distance between successiveultraviolet spec-
fication of the 1561- and 1657-A lines. Carbon
trometer measurements was determined to better
linesalsoappearin the laboratory
spectrumthan a kilometer, sincethosemeasurementswere
producedby the impact of 20-ev electronson
CO• as can be seenin Figure 8. The mostlikely
dependent on the motion of the spacecraft.
Thus scale heights of particular emissionfea-
source of excitation of the carbon lines in the
tures
could be determined
with
sufficient
ac-
Mars spectrumis photodissoeiative
excitation curacy to be useful in formulating atmospheric
or photoelectron
dissoeiative
excitationof ear- models.
bon dioxide. If there were free carbon atoms
in the Martian atmosphere,the 1561- and
1657-A lines could be produced by resonant
scattering[Barth, 1969]. In any case,the measured intensityof the carbonlines may be used
to determinean upper limit to the numberof
carbonatomspresentin the upper atmosphere.
The atomic oxygen lines at 1304, 1356, and
2972 A may be produced in the Mars upper
atmosphereby (1) resonantscatteringof solar
It is possible,however,to determinea nominal altitude scaleby assuminga simpleatmospheric model and by usingthe ultraviolet data
themselves
to
determine
the
altitude
scale
[Stewart, 1971]. The radio occultationexperiment that measuredthe electrondensityin the
Mars ionospheredeterminedthat the maximum
electron
concentration
occurred at an altitude
of 135 km at a location where the solar zenith
angle was 57ø [Fjeldbo et ed.,1970]. Sincethis
radiation by atomic oxygen, (2) photoelectron height measurementdirectly dependedon the
excitationof atomic oxygen, (3) photodissocia- tracking of the spacecraft,it was possibleto
tive excitationof carbondioxide,and (4) photo- achievean accuracyto within I km. By assumelectron dissociative excitation of carbon dioxing that the upper atmosphereis nearly pure
ide. Only the •P-•S transitionthat producesthe COo.and that the maximum electron density
1302-, 1304-, and 1306-A triplet is an allowed occurs at the same altitude as the maximum
transitionand thus is the only seriouscandidate rate of photoionization,it is possibleto calculate
for the resonant scattering mechanism. The that the total column density of COo.above
•P-sStransitionthat producesthe 1356-, 1358-A 135 km is 4.2 X 10•6 molecules cm-ø'.As will be
doublet and the •P-•S transition that produces seen in the next section, the Cameron bands,
the 2972-A lines are forbidden transitions but
besidesbeing the most intense emission,were
may be producedby the photoelectronand observedover the most extensivealtitude range
photodissociation
mechanisms.
The atomic hy- of any of the ultraviolet emissions.By using
drogen 1216-A Lyman a line may be excited COo.absorption coefficientsand the model atby (1) resonantscatteringof solar radiation mosphere,the location of a maximum in the
by atomic hydrogen,or (2) photodissoeiativeCameron
band emission was calculated to be at
MARINER 6 AND 7 UV SPECTROMETEREXrERIMEZ•T
240
135 km, assumingthat photodissociation
was
2221
-
ß co2*õ 2y.-2
o coz*
.
the excitation mechanism. Since the observations
220
were made edge-onthrough a sphericalatmosphere, the maximum in the intensity distribu-
-
ß o
o
o
200
tion occurs 10 km lower at 125 km. The altitude variations of the Cameron bands for each
o
o
Oo
180
of the four limb crossings
were then fitted to a
calculation appropriate to an overhead sun
even in the caseswhere the solar zenith angle
was 27ø and 44ø. This procedure then established the altitude scale for all the upper atmosphereemissions.
o
ß
o
ß
160
o
140
!
120
! •,,,•,•
!
! ,,•,,,y
io i
0o
t
, ,,,,,,!
io z
io3
SLANT INTENSITYt kR
ALTITUDE VARIATION
Observations
made on each of the two limb
crossingson Mariners 6 and 7 yielded the
variation with altitude of the spectralemissions
from the 100-km level of the upper atmosphere
to as far up as the particular constituentcould
be detected.For the airglowthat originatesWith
the carbon dioxide molecule,the emissionsextend upward to 220 km, near the top of the
thermosphere.Atomic oxygen and atomic hydrogen emissionsare present in the exosphere
as.well, extendingoutward to an altitude of 700
km for atomic oxygen and to a planetocentric
distanceof 25,000km for atomichydrogen.
Fig. 9. Slant intensity of the C02+ B-X and A-X
band sytems as a function of altitude.
excitation of CO•, the altitude profile of the
emissionis proportionalat high altitudesto the
density distribution of neutral carbon dioxide.
The maximum intensity that is measurednear
125 km is over 600 kR, or 6 X 10• photons
cm-• sec-•, of airglow between 1903 and 2843 A
consistingprincipally of the Cameron bands,
although there is a small contributionfrom the
CO* first negative bands. When this emission
rate is adjustedto give the zenith rate'for this
emission,the value obtained is 20 kR or 2 X
10TMphotonscm-• sec-LThis is equal to a sizeable fraction of the total number of photons
cm-• sec-• in the solarflux at wavelengthsshorter
than 1082 A, the photodissociativethreshold
The intensity of the CO• B-X and A-X
bandsas viewedtangentiallythroughthe atmosphere is shownin Figure 9. The scaleheight of
the A-X bandsis greater than that of the B-X
bands, since,as the spectroscopic
analysishas for the Cameron bands.
shown, fluorescentscatteringby CO•+ makes a
The altitudevariationof the CO A-X fourth'
major contribution to the A-X emissions, positivebandswas determinedby fitting a synwhereasthe photoionizationexcitation of CO•
is the major sourceof the B-X bands.The scale
240 height of the ion is usually greater than that of
co• •-• •
the neutral speciesfor either photochemicalor
220
o (2,0)
øø
ß(0,1) ß ß
diffusive equilibrium. The two different scale
2O0
heights for the two mechanismsmay be seen
more clearly in Figure 10, where the intensity
oo
ß
180
o
ß
of two bands of the A-X transition that originate from different upper vibrational levels are
øoo
ß
160
plotted. The (2, 0) band originating principally
from photoionizationexcitation has a smaller
140
scaleheight than the (0, 1) band, which originatespredominantlyfrom fluorescentscattering.
o
o
o
o
The most intense emission in the ultraviolet
120
10-I
, , ,,,,,,1
I0 o
p , , ,,,,,I •N ' ''''"!
l0 t
SLANT INTENSITY,
kR
10o
' ' ' '''"!
10I
portion of the spectrum,the CO a-X Cameron
bands, is plotted as a function of altitude in
Fig. 10. Slant'intendty of the CO•* A-X (2, 0)
Figure 11. Since this emissionoriginatesfrom and (0, 1) bands as a function of altitude. Note
photodissociation
or photoele.
ctron dissociative discontinuity in the intensity scale.
2222
220
BARTI-I ET AL.
-
of the point at the highestaltitude measured
whosedeviationmay or may not be real. The
most likely altitude dependenceof this line is
the sameas that of the other emissions
arising
from dissociativeexcitationprocesses
on C02.
The slant intensity of the 2972-A atomic
oxygenline is shownin Figure 11 as a function
of altitude. Sinceits altitude dependence
clearly
2OO
ß
o
o
ß
o
180
ß
o
ß
o
T
160
o
140
o OI 2972
follows that
ß COe3•-X
120 -
Ioo
i0-!
I00
i0 t
SLANT
102
INTENSITY,
103'
kR
Fig. 11. Slant intensity of the CO a-X Cameron bands and the O I 2972-A
of altitude.
line as a function
of the CO a-X
Cameron bands
plotted in the samefigure,the excitationmechanism for the line most probably is the same
as for the Cameronbands;namely,the dissociation of CO• by photonsor electrons.Above the
altitude where the maximumintensity occurs,
about 125 km, the 2972-A intensityfollowsthe
carbon dioxide altitude distribution.In Figure 12, the 1356-Aatomicoxygenhas the same
altitude dependenceas the carbon monoxide
bands, thus suggestingthat this line also is
thetic spectrumincludingthe (v', 0) bandsto
the Mars spectranormalizingon the v', v" -v• O)
produced from CO• in a dissociativeexcitation
bands. The emission rate of the atomic
process.
carbon
lines was determined by subtracting the synthetic spectralfrom the Mars spectra.The slant
intensities as a function of altitude of the fourth
positive bands and the C I 1657-A line are
plotted as a function of altitude in Figure 12.
These emissions reach a maximum
near
145
km, becausebelow that altitude the airglow at
this wavelengthis absorbedby CO2. At higher
altitudes,the fourth positivebands also follow
the same scale height as the Cameron bands,
which is the neutral carbon dioxidescaleheight.
The altitude profile of the atomic carbon line
The altitude distributionof the atomic oxygen 1304-A line is quite different from that of
the other atomicoxygenemissions,
thus indicating that its excitationprocessis different from
those producing the 1356- and 2972-A lines.
In Figures 13 and 14, the slant intensity of the
1304-A emissionis plotted as a function of altitude for the two limb crossingsfor each of
the two spacecraft.This atomicoxygenline was
detectableall the way up to 700 km in contrast
to the altitude rangeof all of the other spectral
emissionsdescribedso far, which terminate at
alsofqllows
thisscaleheightwiththe exception approximately 240 km. Such an altitude distribution
220
ß co A Irl -x i•:
o C! 1657[
x O[ 1356[
200
o
ß
180
x
o
ß
160
o
ß
140
o
120
ß
x
I00
........
10-2
I
.....
I0-1
I:1,1 , ? , ,,,,,I
I00
IOt
SLANT INTENSITY, kR
Fig. 12. Slant intensity of the CO A-X fourth
positive bands, the C I 1657-A line, and the O I
1356-A line as a function of altitude.
indicates that
the
1304-A
emission
originatesfrom a processthat excitesatomic
oxygenitself. This is true certainly above 240
km and may be true over the entire observed
altitude range. Examination of the data indicates that there is not a single well-defined
scale height, but instead there is an altitude
variation that suggeststhat there are two excitation processes,one with a maximum at very
high altitudesin the range 500 to 700 km and
the other with a maximumin the range 100 to
300 km. In analogywith the excitationprocesses
producingthe 1304-Aline in the earth'sairglow,
the high-altitude excitation processon Mars
must be the resonantscatteringof solar radiation by atomic oxygen and the low-altitude
source,the photoelectronexcitationof atomic
oxygen.Photon or electronimpact dissociative
•IARIi•ER • Ai•D 7 •
700 •
ß
600 -
MARINER 6
o
oß
•PECTROMETERF•XPERIMEi•T
•o
o[ 1304
•
•o
Iist LIMB
I•o o
•=
0 2ndLIMB
•
•
500 -
,.........
t t •[
MARINER
}t
HZ
z
ß
E
'
t•
2223
o
e•
o
m 400 -
ß
•
o
ß
•
•
o
o0
•00 -
!
• •
PLANETOCENTRIC
DISTANCE
(101km)
•0
%
•oo
2oo-
•
•.
•m•
ß
o
ß
o
]5. •]a•
[•[e•s•[• of •he
• H•e •s • fu•c•o•
of
d•s[•ce
for •H•e•
6. A v•]ue
of 3•
•
fo• •he
_
0•
i0•
SLANT
i0•
•NT[NS•TY,
•
Y•. ]3. •]• •e•s•[•of•he0 [ ]30•-A
H•e •eso• sc•e•gofso]••d•io• by.a•-Om•C
asa function
of altitude
forthetwolimbcross-hydrogen
in theMarsexosphere.
ings
ofMariner
6.
D[scuss[oN
While the data that have been presentedcon-
excitationof carbondioxidemay alsocontribute
to the low-altitudesource.
L•an a radiationwasobserved
all the way
out to morethan 20,000km abovethe surface
of Mars.Theslantintensity
asa function
of the
rain su•cient informationto deter•ne • the
structureand composition
of the upper •tmosphereof Mars, quantitativeinterpretation
of
the spectrale•ssionsand the construction
of
modelatmospheres
is left for future publica-
distance
fromthe centerof theplanetis sho•
tio•. However,a qualitativeexamnation0f
in Figure 15 for Mariner 6 and Figure 16 for
these ultraviolet observationsallows several
Mariner7. A valueof 300R for the L•an a
sky background
hasbeensubtracted
fromthe
observedintensityin preparingthesefigures
[Barth, 1970].The existence
of a singlescale
importantconclusions
to bedrawn concerning
the Mars upper atmosphere.
First, with the
exception
of the H I 1216-A,O I 1304-A5nes
and a portionof the CO•*A-X bands,all the
heightin thisaltitudedistribution
suggests
the emissions
maybe andprobably
are produced
mostplausibleexcitationmechanism
to be the
by actionof solarphotonsand photoelectrons
on carbon dioxide. Second,with these same exceptions,all the scaleheightsare the sameand
700-.
'
MAE•N[E
7
*
o[ •o• •
•oo 2 .•
ß •s•UMS
o%
E
500-
o 2•dLIMB
o
aresmall.
These
twoconclusions
mean
thatthe
upper atmosphere is essentially undissociated
andcold.
Third,
since
atomic
oxygen
emksions
ß
0 ß
•• 400-
•o
•
•
• ••
[
•
ß 0o
MARINER
7
HZ
-
• 0
z
•
,
200
:oo
i01
Y[•. H. •]•
•
• fu•c[[o•
........
:
i0•
,.,
,•,,,,:
SLANT
INTENSITY,
R
•[e•s•[• ,of•heO [ ]30•-AH•e
of •t•[ude
fo• [•e •o
o.•
10•
Hmb cross-
•
PLANETOCE•RIC
DISTA•E
(10
• kin)
•[•. ]6. •]aD•[D[eDS[[•
O• [•e • [ ]2]•-•
•m•
[•ce
• H•e • • fu•ct•o•of •]•etoce•tHcd•s-
fo• ••er
.
T. A v•]ue of 300 •
fo• t•e
2224
BARTH ET AL.
TABLE 2. Slant Intensitiesof the CO•+ B-X Band, the CO•+ A-X Band System,the A-X (2,0) Band,
and the A-X (0,1) Band as a Functionof Altitude for the Four Limb Crossings.
CO2+
Altitude,
km
B-X, kR
Crossing
Mariner 6
First Limb
217
194
Mariner 6
Second Limb
171
148
211
189
167
--
Mariner
7
First Limb
C02 +
Altitude,
km
A-X, kR
15
27
85
>33
211
189
166
143
184
162
139
-195
176
156
137
205
183
162
139
8
23
34
15.4
__
200
2.4
181
161
6.9
15.5
__
Mariner 7
Second Limb
210
188
166
3.0
4.6
14.3
8.
15.
30.
104.
19.
51.
> 142.
-25.
28.
57.
> 146.
14.
24.
49.
> 165.
are observedabove250 km, somedissociation
of
the COO.has occurred.Fourth, becausepart of
the COO.*A-X
A-X
Altitude,
km
214
191
168
145
186
164
141
217
197
178
158
139
207
185
164
2,0, kR
0.6
1.1
1.9
8.1
0.9
3.3
8.4
0.7
0.8
0.9
1.9
8.0
0.6
1.1
2.5
A-X
Altitude,
km
0,1, kR
210
187
164
141
182
160
137
213
193
174
154
135
203
181
160
137
0.5
0.7
1.3
4.3
1.2
2.8
4.8
0.9
1.0
1.5
2.8
4.5
0.6 •
1.2
2.6
5.5
as water that is currentlyoperatingin the Mars
atmosphere.
emission is attributed to fluores-
cent scattering,ionized carbon dioxideis a constituentof Mars' ionosphere,
but not necessarily
APPENDIX
The slant intensities of the various emission
the major constituent.Last, the presenceof features
as a function
of altitudearegivenin
atomic hydrogenin the exosphereof a planet tabular form in this appendix.Table 2 lists the
with such a comparatively low gravitational intensitiesfor the COo.*bands, the B-X band,
field means that there must be a source of
the A-X band system,the A-X (2, 0) band,
dissociation
of a hydrogen-bearing
moleculesuch and the A-X (0, 1) band; Table 3 lists the CO
TABLE 3.
Slant Intensities of the CO a-X Band Systemand the O 1 2972-A Line as a Function of Altitude
for the Four Limb Crossings.
CO
Mariner
6
First Limb Crossing
Altitude, km
a-X, kR
Altitude, km
2972-A, kR
200
16.
55.
251.
511.
420.
38.
111.
380.
193
170
147
124
--188
166
143
1.1
2.9
10.2
21.0
-1.2
4.0
10.3
673.
48.
116.
388.
567.
120
180
160
141
122
>20.0
2.5
4.3
11.6
21.5
177
154
131
108
Mariner
6
Second Limb Crossing
195
173
150
127
Mariner
7
First Limb Crossing
187
167
148
129
109
Mariner
7
Second Limb Crossing
0 I
194
173
151
128
106
465.
--
--
60.
123.
187
165
2.5
4.6
409,
143
11.0
712.
121
20.6
526.
--
--
MARINER 6 AND 7 UV SPECTROMETER
]•XPERIMENT
TABLE
4.
Slant
Intensities
of the
CO A-X
Band System,the O I 1356-ALine and the C I
1657-A Line as a Function of Altitude.*
CO
Altitude, km
A-X, kR
200
175
150
125
100
O. 6
2.7
8.6
6.9
3.0
OI
CI
1356 A, kR 1657 A,kR
2225
those emissionfeatures below 1900 A, with the
exceptionof the 1304-A line, the slant intensities for the four limb crossingsare averaged
and the altitude given is the averagealtitude
for the four observations.Becauseof the large
number of data points, the Lyman a 1216-A
data are not givenin tabularform but are available,alongwith the rest of thesedata, from the
National SpaceScienceData Center.
Possibleerrors in the measuredintensity of
the Mars upper atmospherespectral emissions
* The intensities for the four limb crossingshave
been averagedand are given as a function of the may result from four causes:(1) the absolute
calibrationof the spectralresponseof the inaverage altitude.
strument, (2) the subtraction of the backof the
a-X band systemand the 0 I 2972-A line; groundproducedby the off-axisresponse
instrument, (3) the elimination of impulsive
Table 4 the CO A-X band system,the 0 I
1356-Aline, and the C I 1657-Aline; and Table noise generatedby the photomultipliertubes,
5 the 0 I 1304-A line. In all instances,the and (4) the subtractionof the electronicoff-set
slant intensity,'which is given in kilorayleighs, producedby imbalancein the amplifiercircuits.
is the apparent columnemissionrate that is The method of calibration and the estimate of
observedby viewingthe sphericalatmosphere the calibrationerrors are given in the instrufrom the outside, perpendicularto a radius ment paper [Pearceet al., 1971]. Becauseof a
vector.A kilorayleighis 10• photonssecfrom a lack of photometricstandardsfor wavelengths
square centimeter column. It is equal to 4•r lessthan 3000 A, systematicerrorsin the calitimes the apparent surfacebrightnessof the bration may be discoveredin the future as
standardsare developedand adopted.For this
atmosphere
in 10• photonssec
-• cm-'ster-L
For emissionsat wavelengthslonger than samereason,the measuredvaluesof solarflux,
2800A, a substantialbackground
hasbeensub- which are usedin conjunctionwith the planetracted from the spectral data. The origin of tary airglow data to determineupper atmosmay alsobe revised.However,
this backgroundis the off-axisresponse
of the pheredensities,
instrument to the light from the bright disc the relative changein intensitywith altitude,
of the planet. The spectralcharacterof the which is used to determine scale heights, will
not be affected by changesin the absolute
disc is known from Mariner measurements made
subsequentto the atmosphericlimb observa- standards.
The three remainingcauses,the subtraction
tions and it consists of the Fraunhofer specphotomultiplier
noise,and
trum of reflectedsunlight.The intensitieslisted of off-axisresponse,
electronicoff-set,are susceptible
to subjective
assume that the emission is uniform over the
O. 1
0.3
0.4
O. 4
0.3
O. 3
0.5
1.7
1.5
0.8
0.23ø X 2.3 ø field of view of the instrument and
that the slit is aligned tangentially with the
sphericalatmosphere.Correctionsfor theseassumptionscan be made by usingthe analytic
formalismgivenin a discussion
of the geometry
of planetaryspacecraft
observations
[Hord et
al., 1970]. For the two limb crossings
eachfor
judgement
in thereduction
process.
Thesubtraction process
hasthe largesteffecton the data
at high altitudeswherethe intensityis small
and at low altitudes where the backgroundis
large.The errorsinvolvedin the subtraction
process
doaffectthevariation
of intensity
with
altitude. These errors may best be evaluated
Mariners6 and 7, the slant rangeto the point
where the line of sight was perpendicularto
by a repetition
of thereduction
process,
which
is possible
by usingthe dataavailable
through
the radius vector was 7600, 6000, 8900, and
the NationalSpaceScienceData Center,
6200km, respectively,
and in all casesthe slit
wasalignedtangentiallywithin 3ø.
For the spectralrange above 1900 A, the
the 1216-ALymana line,therewererelatively
For all of the emissions
with the exceptionof
few measurements
made over the critical alti-
andan evaluation
of the statistical
slant intensityof the emission
featuresis given tuderegion
was not possible.
for each individual limb crossing,whereasfor variationof the measurements
2226
BARTH ET AL.
TABLE
5.
Slant Intensities of the 1304-A Line as a Function of Altitude for the Four Limb Crossings.*
Mariner6
Mariner'7
First
Second
First
Second
Altitude,
km
0 I,
1304 A, g
Altitude,
km
0I,
1304 A, g
Altitude,
km
0 I
1304 A, g
Altitude,
km
0 I,
1304 A, g
704
681
11.
23.
37.
54.
700
677
11.
3.
684
664
682
660
3.
8.
655
632
610
587
564
542
519
496
473
450
428
405
383
360
337
315
292
270
247
224
202
23.
56.
55.
87.
80.
86.
69.
103.
89.
130.
205.
254.
344.
410.
473.
540.
561.
558.
679.
676.
693.
644
624
604
585
565
545
525
505
486
466
446
426
406
387
367
347
327
307
287
267
248
3.
15.
28.
36.
175.
162.
127.
168.
232.
247.
278.
344.
370.
638
616
594
572
550
528
505
484
462
440
418
395
373
351
329
307
285
263
241
219
197
3.
24.
21.
27.
20.
43.
52.
76.
99.
130.
152.
211.
259.
292.
331.
438.
403.
414.
410.
421.
382.
659
636
614
591
568
545
523
500
477
454
432
409
387
364
341
319
296
274
251
228
205
142.
200.
212.
328.
395.
460.
588.
582.
579.
183
570.
179
781.
2?9
399.
175
439.
160
137
114
574
562.
650.
156
133
110
897.
830.
800.
209
190
170
410.
453.
469.
153
131
109
455.
558.
507.
151
131
111
427.
460.
427.
59.
63.
68.
58.
61.
62.
63.
88.
86.
116.
32.
39.
29.
48.
48.
81.
104.
132.
132.
142.
.
* The intensitieshavebeenderivedfrom a runningfive-pointaveragewherethe altitude is that of the
central point.
However,for the Lymana measurements,
where Propulsion Laboratory, the radio occultation exthe scale height is very large comparedto the
altitude interval betweenmeasurements,the root
mean square variation, which was dctcrmined
periment; and H. M. Schurmeier, A. G. Herriman,
and C. E. Kohlhase, the Mariner project staff.
This work was supported by the National Aeronautics and Space Administration.
for groupsof 50 measurements,
is given in
Figures 15 and 16.
:.
Acknowledgments.
The'ability to achievefour
limb crossingswith a well-aligned slit from two
spacecraft moving at 7 km/sec relative to Mars
was the result of the highly cooperative, responsible interacti. on of the several scientists conduct-
ing experimentson the spacecraft,and the Mariner
project staff. The principalswere R. B. Leighton
and B.C. Murray of the California Institute of
Technology, the television experiment; G. C.
Pimentel and K. C. Herr of the University of
California at Berkeley, the infrared spectrometer
.experiment;
G. Neugebauer
and G. Miinch of the
California Institute of Technology,•
the infrared
radiometer experiment; A. J. Kliore of the Jet
ß
,
ß
The Editor wishes to thank A. Dalgarno and
another referee for their assistance.
in evaluating
this paper.
REFERENCES
Ajello, J. M., Emission cross sectionsof CO, by
•electron impact in the interval of 1260 A to
4500 A, Part 2, submitted to J. Chem. Phys.,
1971.
Anderson, J. D., L. Efron, and S. K. Wong,
Martian
mass and earth-moon
mass ratio from
coherent S-band tracking of Mariners 6 and 7,
Science,167, 277, 1970.
Barth, C. A., The ultraviolet spectroscopyof
planets, in The Middle Ultraviolet: Its Science
MARINER 6 AND 7 UV
SPECTROMETER EXPERIMENT
and Technology, edited by A. E. S. Green,
John Wiley, New York, 1966.
Barth, C. A., Planetary ultraviolet spectroscopy,
Appl. Opt., 8, 1295, 1969.
Barth, C. A., Mariner 6 measurements of the
Lyman alpha sky background, Astrophys. J.
Lett., 161, 181, 1970.
Barth, C. A., L. Wallace, and J. B. Pearce,
Mariner 5 measurement of Lyman alpha radiation near Venus, J. Geophys. Res., 73, 2541,
1968.
Barth, C. A., W. G. Fastie, C. W. I-Iord, J. B.
Pearce, K. K. Kelly, A. I. Stewart, G. E.
Thomas, G. P. Anderson, and O. F. Raper,
Mariner 6: Ultraviolet spectrum of Mars
upper atmosphere,Science,165, 1004, 1969.
Dalgarno, A., and T. C. Degges, CO•• dayglow on
Mars and Venus, paper presented at IAU Symposium 40, Marfa, Texas, 1970.
Dalgarno, A., T. C. Degges, and A. I. Stewart,
Mariner 6: Origin of Mars ionized carbon
dioxide ultraviolet spectrum, Science, 167, 1490,
1970.
Fjeldbo, G., A. Kliore, and B. Seidel, The Mariner 1969 occultation measurementsof the upper
atmosphere .of Mars, Radio Sci., 5, 381, 1970.
Hesser, J. E., Absolute transition probabilities in
ultraviolet molecular spectra, J. Chem. Phys.,
48, 2518, 1968.
2227
Hord, C. W., C. A. Barth, and J. B. Pearce,
Ultraviolet spectroscopyexperiment for Mariner Mars 1971, Icarus, 12, 63, 1970.
Lawrence, G. M., Quenching and radiation rates
of CO (a•r), submitted to Chem. Phys. Lett.,
1971.
Nicholls, R. W., Laboratory astrophy•cs, J. Quant.
$pectrosc. Radiat. Transfer, 2, 433, 1962.
Pearce, J. B., K. A. Gause, E. F. Mackey, K. K.
Kelly, W. G. Fastie, and C. A. Barth, The
Mariner 6 and 7 ultraviolet spectrometers,
Appl. Opt., 10, April, 1971.
Poulizac, M. C., and M. Dufay, An evaluation of
CO•+ •II• --) •IIg vibrational transition probabilities, Astrophys. Lett., 1, 17, 1967.
Stewart, A. I., Interpretation of some results of
the Mariner 6 and 7 ultraviolet spectrometer
results,submitted to J. Geophys.Res., 1971.
Wauchop, T. S., and H. P. Broida, Cross sections
for the production of fiu•)rescenceof CO•+ in
the photoionization of CO•. by 58.4-NM radiation, J. Geophys. Res., 76, 21, 1971.
Weller, C. S., and M. A. Biondi, Measurements of
dissociative
recombination
of
CO• + ions with
electrons, Phys. Rev. Lett., 19, 52, 1967.
(Received December 1, 1970;
accepted December 29, 1970.)