VOL. 82, NO. 28
JOURNAL OF GEOPHYSICAL
RESEARCH
SEPTEMBER 30, 1977
Photochemistryand Evolution of Mars' Atmosphere:
A Viking Perspective
MICHAEL B. MCELROY, TEN YING KONG, AND YUK LING YUNG
Centerfor Earth and Planetary Physics,Harvard University
Cambridge,Massachusetts02138
Viking measurementsof the Martian upper atmosphereindicate thermospherictemperaturesbelow
200øK, temperaturesmuchcolderthan thoseimplied by remotesensingexperimentson Mariner 6, 7, and
9 and Mars 3. The variability in thermospherictemperaturemay reflectan important dynamicalcoupling
of upper and lower regionsof the Martian atmosphere.Absorption of extremeultraviolet solar radiation
can account for observedfeaturesof the ionosphereand providesan important sourceof fast N and O
atoms which may escapethe planet'sgravitational field. Isotopic measurementsof oxygenand nitrogen
imposeusefulconstraintson modelsfor planetaryevolution.It appearsthat the abundanceof N= in Mars'
past atmospheremay have exceededthe abundanceof CO= in the presentatmosphereand that the planet
also has copious sourcesof H=O. The planet acquired its nitrogen atmosphere early in its history. The
degassingrate for nitrogenin the presentepochmust be lessthan the time-averageddegassingrate by at
least a factor of 20.
1.
-
INTRODUCTION
section 3. In accord with earlier models [Parkinson and
Hunten, 1972;McElroy and Donahue,1972;Liu and Donahue,
1976;Kongand McElroy, 1977a,b] we assumethat photolysis
of CO•. is balancedmainly by gas phasereactionof CO with
OH. Models are constrained to agree with recent measurements(B. A. Thrush, private communicationto R. T. Watson,
1977) of the rate constantfor reactionof OH with HO•. and are
The entry scienceexperimentson Viking provide a wealth of
new information on the structureand composition of Mars'
upper atmosphere.They may be used, in combination with
remote sensingdata from earlier spacecraft,to developa reasonablyconsistentmodel for Martian aeronomy.
It is clear that escapeprocesses
have played a major role in
the evolution of Mars' atmosphere.Recombinationof Os+ in adjustedto providea hydrogenescapeflux of 1.2 X 108atoms
the planetary exosphereprovidesa significantsourcefor fast cm-•' s-• in agreementwith fluxes measuredby Mariner 9
[Barthet al., 1972].Resultsare developedto illustratepossible
atoms which can escapethe planet's gravitational field. Comvariations
in upper atmosphericcompositionover a Martian
positional data inferred from the retarding potential analyzer
year.
experiment [Nier et al., 1976b;W. B. Hanson, private commuMeasurementsof the isotopiccompositionof oxygenand
nication, 1977]indicatethat O•.+ is the major constituentof the
nitrogen
may be used to place important constraintson the
Martian ionosphere and suggestan average escape rate of
evolution
of Mars' atmosphere.The observedenrichmentof
about 6 X 10? oxygen atoms cm-•' s-•. The chemistry of the
as comparedto that
bulk atmosphereis regulatedby oxygenescapeon a time scale •SNrelativeto •4N for Mars' atmosphere
of the order of 105 years in such a manner as to ensure an of the earth impliesthat Mars had•a much lar.ger nitrogen
escaperate for H atoms of the magnitude of 1.2 X 108atoms atmospherein the past. In a similar manner the lack of a
cm-•' s-•. Hydrogen moleculesare formed in the lower atmo- detectableenrichmentof the isotopicratio •80/•60 in Mars'
sphere by reaction of H with HO•.. Escaping H atoms are atmospheremay be taken to imply significantexchangebetween the atmosphereand an extensivesurfaceor subsurface
released
by ionospheric
reactions
involvingH•.andCO•.+.
Models for the Martian ionosphereare developedin section
2 and are shown to agree satisfactorilywith in situ measurementsby Viking. The upperatmospheremeasuredby Viking is
unusually cold. The scaleheight of CO•. is about 8 km, which
may be comparedwith scaleheightsin the range 15-22 km as
inferred from the ultraviolet spectrometerexperiment on
Mariner 9 [Stewartet al., 1972].It is clearthat the temperature
of Mars' upper atmosphereis quite variable, ranging from as
low as 120øK to perhaps as high as 400øK. This variability
may be seenalso in the topsideplasma scaleheightsas measured by Mariner 4 [Kliore et al., 1965], Mariner 6 and 7
[Kliore et al., 1969;Fjeldboet al., 1970],and Mariner 9 [Kliore,
1974]. The general characteristicsof the ionospheremay be
reproduced by a relatively simple photochemical model if
proper account is taken of the variability of the extreme ultraviolet solar flux. It is unlikely, however, that sucha simple
model can account for the observedvariation in atmospheric
temperature.
The photochemistryof Mars' atmosphereis discussedin
Copyright ¸
reservoircontaininga volatileform of oxygen,mostprobably
H•.O. Measurements of noble gases in Mars' atmosphere
[Owenand Biemann, 1976; Owen et al., 1976] poseadditional
constraintson planetary evolution, and the implicationsof
thesedata are explored in section4.
2.
Carbon dioxide is the major constituent of Mars' atmosphere over the ionospherically important height range
120-180 km [Nier et al., 1976b;Nier and McElroy, 1976, 1977].
Electronsare producedmainly by photo-ionization of CO•.:
hv + CO•.-• CO•.+ + e
(1)
The primary photo-ion, CO•.+, may be removed either by
dissociative recombination,
CO•.+ +e-
CO+
O
(2)
or by reactionwith O [Stewart,1972;McElroyandMcConnell,
1971; Fehsenfeldet al., 1970],
CO•.+ + O -• O•.+ + CO
1977 by the American GeophysicalUnion.
Paper number 7S0558.
IONOSPHERIC CHEMISTRY AND STRUCTURE
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tendedmissionI (May 7 to June25, 1972).A limitednumber
of computations
werecarriedout underconditions
appropriate for Mariner 9. For the extended mission I in 1972 a
Vikinglike thermal structure was used for the middle and
e
lowerportionsof Mars' atmosphere.
For thestandardmission
a considerably
warmeratmosphere
wasassumed
(for altitudes
below 100km) in order to accountfor the largeamountof
dustpresent
in theatmosphere
at thattime[Klioreetal., 1972].
If onedefinesan effectivetemperatureTerrgivenby
i
120NO•//
110/ /
100
•
105
•/ • • I
•1 I •
[
• I
104
for the atmosphere
belowthe ionospheric
peakZm, thenthe
effectivetemperature(Terr) for the standardmissionis about
20øK warmer than the value assumed for the first extended
mission.
An excellentfit to the Mariner data may be obtainedif the
EUV flux is assumedto vary accordingto the relation
lOs
F•uv
Ion NumberDensity(cm-•)
O.0222F•uvø(F•o.7-25)/R •'
(4)
[1976],
Fig. 1. Numberdensities
for the Martian ionosphere.
Resultswere (F•uvø is the solarEUV flux taken from Hinteregger
units)denotesthe planet'sradialdisobtainedby usingdensities
for neutralspecies
measured
by Viking 1 and R (in astronomical
temperatures
are
[NierandMcElroy,1976,1977].Corresponding
ionospheric
reactions tancefromthesun)andif upperatmospheric
are given in Table 1.
fixedby usingplasmascaleheightsreportedby Klioreet al.
[1973].Calculated
valuesfor peakelectrondensityare com-
Reaction (3) ensuresthat 02 + should be the dominant com-
paredwith observations
in Figure2. Estimatedvaluesfor the
ponentof Mars' ionosphereat all heightsbelowabout300 kin.
A modelionospherederivedby usingdensitiesfor neutral
vations in Figure 3. The variation of EUV flux with solar
heightof theionospheric
maximumarecompared
withobser-
species
measured
by Viking 1 [NierandMcElroy,1976,1977]
is shownin Figure1. A summaryof importantreactionsand
relevantrate constantsis givenin Table 1. Valuesfor the flux
of sunlightat extremeultravioletwavelengths
weretakenfrom
activityimpliedby (4) is somewhat
largerthan the rangeof
intensitiesobservedfor dayglowemissionin the Cameron
bandsof CO [Stewartet al., 1972]and is also largerthan
variationsobservedby Hinteregger[1970] for the chromo-
by theAtmosphere
Exploreraeronomysatellites.
The Viking measurements
weretakenduringa periodof
The discrepancy
may not be serious,
however.It mayreflect
the roleof high-excitation
solarlines(for example,linesfrom
Fe XV andFeXVI) anddifferences
in spectral
re•gions
impor-
a paperby Hinteregger[1976]and reflectrecentmeasurements sphericEUV emissionlinesat 1025,977, 630, 584, and 304
exceptionally
low solaractivity.The solarflux at 10.7cm had a
valueof 69.4X 10-22W m-2 Hz-• at 1 AU onJuly20, 1976,
which may be comparedwith a flux of 75.7 X 10-2•' W m-2
Hz-• at 1AU onSeptember
3, 1976,duringtheentryof Viking
tant for photo-ionization and excitation of Cameron bandsin
the Martian atmosphere.We may note that the trend with
zenith anglefor both peak electrondensityand peakiono-
missionI is satisfactorily
2, and fluxesin the range(110-145) X 10-22W m-2 Hz-• for sphericheightduringthe extended
bythemodel.Interpretation
of thevariationof peak
the Mariner 9 standardmission(November14 to December described
23, 1971)or (110-169)X 10-22W m-2 Hz-• duringthe ex-
2.0
TABLE 1. Important IonosphericReactionsin the Martian
Ionosphere
'E 1.6
Reaction
Rate Coefficient
•:) 1A
CO2+hv-•CO2 + +e
N2+hv-•N2
+ +e
0 + hv -• 0 + + e
CO+hv-•CO
+ +e
CO2+ +O-•CO+O2
+
CO2+ +O-•CO2+O
+
N2+ + CO2-• N2 + CO2+
O + +CO2-•CO+O2
+
CO + + CO• -• CO + CO2+
CO2+ + NO-• CO2 + NO +
02 + +NO-•02+NO
+
CO2 + +e•CO+O
02 + + e -• O + O
NO + +e-•N+O
J• = 2.4 X 10-7
J2=8.7X
10-8
J3= 1.2X 10-7
J4=4.4X
10-7
k• = 1.6 X 10-•ø
k2 = 1.0 X 10-•ø
k3 = 9 X 10-•ø
k5 = IX 10-•
k6 = I X 10-•
k7 = 1.2 X 10-•ø
k8 = 6.3 X 10-•ø
k•=3.8X
10-7
k•o= 2.2 X 10-7(300/Te)
k• = 4.3 X 10-7(300/Te)ø'•7
Te denoteselectrontemperature,
takento be equalto the valuefor
neutraltemperature
in the presentstudy.Photodissociation
rates(J)
at the top of the atmosphere
are givenin reciprocalseconds.
Twobody reactionrates(k) are in cubiccentimeters
per second.This
table is takenfrom McElroy et al. [1976].
•- 1.2
7
•
1.0
0.8
0.6
0.4
0.2
I
50
I
60
I
70
I
80
I
90
SOLAR ZENITH ANGLE (DEGREE)
Fig. 2. Peak electrondensityin the Martian ionosphere
versus
solarzenithangle.Shadedregionsrepresentdata obtainedby the
Mariner 9 radio occultationexperimentduringits standardmission
(47o-57ø) and extendedmissionI (>72 ø) [Klioreet al., 1973].Solid
curvesare densitiesfrom theoreticalcomputationsas describedin the
text. SolarEUV fluxwasassumed
to varywithF•o.7,
andR according
to (4).
MCELROY ET AL.: PHOTOCHEMISTRYAND EVOLUTION OF MARS ATMOSPHERE
altitude with zenith angle during the standard mission is complicated becauseof temporal variations in the structureof the
lower atmospherewhich occurredduring this period. Mariner
9 arrived at Mars when the planet was envelopedby a global
dust storm [Kliore et al., 1972]. The solid curve for the standard mission in Figure 3 assumesthat the effectivetemperature was 20øK warmer than values applicable for the extended mission.The trend of peak altitude with zenith angle
differssignificantlyfrom the observedtrend. We believe,however, that the discrepancymay be attributed to a gradual
clearing of dust accompaniedby steadycooling of the lower
atmosphere.If we assumethat the effective temperature declined by 8 øK over the first 6 weeksof the Mariner mission,we
obtain the trend indicated by the dashed line in Figure 3, a
result evidently consistentwith observation [Conrath, 1975].
The comparisonsshown in Figures2 and 3 indicate that solar
EUV radiation may provide the dominant sourceof ionization
for Mars.
It is more difficult to accountfor the rangeof valuesinferred
for thermospherictemperature.The intensityof the CO Cameron band emission is nicely correlated with observedvariations in the 10.7-cm solar flux. In contrast,the scaleheight of
the Cameron bands exhibits no such correlation [Stewart et
al., 1972]. There can be little doubt, however, that the thermospherictemperaturevariesconsiderablywith time. Figure4
summarizesthe available information, including data from
Mariner 4, 6, 7, and 9 [Kliore et al., 1965, 1969;Fjeldboet al.,
1970; D. E. Anderson and C. W. Hord, 1971; Kliore, 1974;
]5O
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o
_•1•0
•--120
110
i
I
50
i
60
I
70
SOLAR
ZENITH
ANGLE
I
80
90
(DEGREE)
Fig. 3. Altitude of the ionospheric maximum (above a 3-mbar
pressurelevel) versussolar zenith angle. Notation and conditions are
the same as those in Figure 2. The solid curve for the standard mission
assumesthat the effectivetemperatureTerrwas 20øK warmer than the
value applicable for the extendedmission.The dashedcurve assumes
that the effectivetemperaturedeclinedby 8øK over the first 6 weeksof
the Mariner
9 mission.
Molecular hydrogen, produced by
H + HO2-'
H2 + O2
(6)
has a time constant of approximately 103 years. We shall
assumethat the chemistry of the bulk atmospheremay be
adequatelydescribedby using the average model given in
Figure 5.
'Reactionsimportant for a carbon-hydrogen-oxygen
atmosphereare summarizedin Table 2. Odd hydrogencompounds
are suppliedby photochemicaldecompositionof H,O, either
by photolysis,
Stewart et al., 1972], Mars 3 [lzakot), 1973], and Viking 1 and 2
[Nier and McElroy, 1976; McElroy et al., 1976]. The lowest
temperatureswere observedfor Viking 1 and 2 and Mariner 4.
Noting the absenceof a correlation of airglow scale height
with solar activity, at leastover short time periodson Mariner
9, one is temptedto postulatethat the temperatureof Mars'
upper atmospheremay be affectedto a considerableextentby
processes
originating in the lower atmosphere[Stewart et al.,
hv + H20-•OH
+ H
1972]. The Viking data indicate that gravity wavesexcited in
the lower atmospheremay propagate to high altitudes on or by reaction with O(•D),
Mars [McElroy et al., 1976]. Thesewavesmay deliver signifiO(1D) + H20-, OH + OH
cant amounts of energy to the upper atmosphereand may be
responsible,at least in part, for the high-altitude mixing deR(A.U)
tected by Viking [McElroy et al., 1976; Nier and McElroy,
1.558
1.6661.65•'
1.554 1.451
1.,,t811.,,t88
1977]. Excitation of thesewavesshould be favored when Mars
is closestto the sun and may be enhancedfurther by the
additional aerosol burden known to be presentin the air at
that time [Gieraschand Goody,1972]. Further data are clearly
requiredin order to resolvethis issue.It must be pointedout,
however,that the data in Figure 4 could also be usedto argue
#4
a grosscorrelation of thermospherictemperaturewith solar
/
vI
activity. It would be difficult,though,to accountfor the range
'-[--vf'
of temperaturesexhibitedin Figure 4 if EUV solar radiation
shouldbe the only important thermosphericheat source[see
100
Stewart et al., 1972].
500[ I I
I
I
(7)
(8)
1.558
I I #9
(DAYGLOW)I
/#9
(SBAND)
#_6,1
.--"'-.•(s
BAND)
I
3.
,,,lill'l''''''''l::''''''l''''''l'
PHOTOCHEMISTRY
18½
21½
36½
(DEGREE)
The chemistry of the bulk Martian atmosphereshould be
relatively insensitiveto short-periodchangesin the temperature of the upper atmosphere.Carbon monoxide, formed by
photodissociationof CO2, has a time constant of about 3
years, and a somewhat longer time constant applies for 02,
formed mainly by
O + OH--,O2
+ H
(5)
Fig. 4. Temperatureof Mars' upperatmosphereobtainedduring
the missionsof Mariner 4, 6, 7, and 9, Mars 3 and 5, and Viking 1 and
2. Data (eachuncertaintybeingindicatedby an error bar) are plotted
againstthe planetocentricsolar longitudeof Mars, LB,and the radial
distancefrom the sun, R, during the period of each mission.Shaded
regions representtemperaturesderived from topside plasma scale
heights obtained by the Mariner 9 radio occultation experiment
[Kliore et al., 1973]. Dashedcurveindicatesa possibleseasonalvariation for temperaturein the Martian upper atmosphere.
4382
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TEMPERATURE
I00
140
180
220
160 Hal
(øK)
260
[CO
l
,300
,340
abundanceof troposphericH atoms. The abundanceof tropospheric H is set by balance between
,380
/t
7
and is therefore
60
I
T
_
40
20
PO I0• I0• I0? 10
e 10
• i0•ø 10
• i0•a i0• i0•4 10
• 0•
NUMBER DENSITY (cm -•)
Fig. 5. Time-averagedmodel for the Martian atmosphere.
Water may be reformed by
OH + HO2 • H20 + O2
(9)
There is, however, a small net sink for H20 at low altitudes
associatedwith formation of H2 by reaction (6). Hydrogen is
transportedupward and escapesmainly in atomic form, atoms
being releasedby reaction of CO2+ with H2:
CO2+ + H2 • CO2H + + H
(10)
followed by
CO2H+ + e•CO2
+ H
(11)
Hydrogen escapeis limited by the supply of H2 from below.
Production of H2 is determined by (6) and is limited by the
CO + OH -• CO2 + H
(12)
H + O2 + CO2 • HO2 + CO2
(13)
and
sensitive
to the net oxidation
Densities for H, O H, HO2, H202, O, and O3 as calculated for
Mars' atmosphereare given in Figure 6. Computational details are describedby Kong and McElroy [1977a]. Rates for
TABLE 2. Chemical Reactionsin the Neutral Martian Atmosphere
Reaction
No.
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
Reaction
hv + CO2-• CO + O
CO + O(sP) + CO2 -• CO2 + CO2
CO + OH -• CO2 + H
H + O2 + CO2-• HO2 + CO2
O + HO2-• OH + O2
O + O + CO2-• O2 + CO2
HO2 + HO2-• H202 + O2
hv + H202-• OH + OH
O + OH-• O2 + H
hv + H20--• OH + H
O('D) + H,O • OH + OH
O('D) + H2--, OH + H
(13a)
hv + Os• O2('As)+ O('D)
(I 3b)
(14)
(15)
(16)
(17)
(18)
(19)
(20)
(21)
he + Os-• O2 + O(sP)
OH + HO2 -• H20 + O2
H + HO2-• H2 + O2
he + O2-• O + O
O + O2 + CO2-• Os + CO2
O + Os-• O2 + O2
H + Os--• OH + O2
O('D) + CO2-• o(ae) + CO2
O2('A,)--• O2(S2;,
-) + hv
(22)
o2(•a,) + co2-•
state of the
atmosphere.The oxidation state of the atmosphereis determined by the relative magnitudeof H and O escaperates.The
escaperatesare regulatedat the presentepoch[McElroy, 1972;
McElroy and Donahue, 1972; McElroy and Kong, 1976] to
ensurea relatively steadyoxidation state, H atomsbeingtransported upward as H2 and O atoms being supplied to the
exospheremainly as componentsof upward flowing CO2. The
hydrogen escaperate is set ultimately by the rate at which
oxygen escapes.The escape rate for O is set by the rate at
which CO2 is photo-ionizedin the exosphereand shouldmaintain a relatively steadyvalue over large intervalsof geologic
time. At the presentepoch,water evaporatesfrom the surface,
is processedphotochemicallyby the atmosphere,and escapes
to spaceat a steadyrate of 6 X 107moleculescm-2 s-1. If this
rate had applied over the past 4.5 X 109years, Mars would
have lost to space an amount of H20 sufficientto coat the
surfaceof the planet with ice to an averagedepth of about 2.5
m. It shouldbe emphasized,however, that the net quantity of
H20 processedby the planet could significantlyexceedthis
figureif heterogeneous
reactionsat the planetarysurfacemight
be shownto representa sink for O of a magnitudecomparable
to or larger than that due to escape[e.g., Huguenin, 1973a,b,
1974, 1976].
o2(szg-) + co2
Rate Expression
2 X 10 -s7
9 X 1O- ,sexp (- 500/T)
2 X 10-S'(T/273) -'.s
7 X 10-"
3 x '10-SS(T/300)-z9
5.5 X 10 -'2
5 X 10-"
3X 10 -'ø
1.9 X 10 -'ø
8.24 X 10-" exp (- 150/T)
9 X 10-'2 exp (-333/T)
1.4 X 10-SS(T/300)-z5
1.32 X 10-•' exp (-2140/T)
2.6 X 10 -•
1.8 X 10 -'ø
2.6 X 10 -4
4X10
-'8}
<8 X 10 -2ø
< 1.5 X 10 -2ø
Units for rate constantsare s-• for unimolecularreactions,cms s-• for bimolecularreactions,and cm•
s-' for termolecularreactions.Photodissociationrates (s-•) are calculatedfrom solar flux and relevant
cross-section
data. Rate for reaction(14) is taken from the recentmeasurementby Thrush[1977], and
the referencesfor the rest of the reactionsare given by Kong and McElroy [1977a].
MCELROY ET AL.: PHOTOCHEMI•TnV
AND EVOLUTIONOF MARS ATMOSPHERE
7O
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6O
\ hv + COe
\
50
CO+ OH
,., 40
\
\
• •o
\
2O
PRODUCTION
....
103
104
105
10s
10?
10e
NUMBER DENSITY (cm-3)
10e
LOSS
10]0
I0•;
105
10s
PRODUCTION
ANDLOSSRATES
FORCO
z (cm-3sec
-• )
Fig. 6. Densitydistributionsof H, OH, HO•., H•.O•.,O, and O3in
the Mars atmosphere.Resultswere obtainedby usingthe averaged Fig. 8. Productionand lossratesfor CO in the Martian atmosphere
(conditionsare the sameas those in Figure 6).
modelatmosphere
givenin Figure5. Surfacewasassumed
to be inert,
and meanwater vaporabundancein the atmospherewastaken to be
10 precipitable•m.
N(2D) + CO•.-• NO + CO
reactions contributing to the budget of odd hydrogen are
illustrated as functions of altitude in Figure 7. Sourcesand
sinks for CO•. and O•. are summarizedin Figures 8 and 9, and
the variationof upper atmosphericH and H•. with exospheric
temperatureis illustratedby Figure 10.
It may be of interestto considerthe responseof the atmo-
sphereto a largetransientsourcefor H•.O, whichmightarise,
for example,duringperiodsof activevolcanism.An injection
of H•.O at high altitudescouldlead to a significantsourcefor
H•., formeddirectlyby photodissociation
at wavelengthsnear
Lyman a. The sourcestrengthcouldbe as largeas 5 X 10•ø
moleculescm-•' s-•. Hydrogen formed in this manner could
escapereadily to space,leavingoxygenin the atmospherein
concentrationssufficientperhaps even to perturb the total
atmosphericpressure.Mars could acquire a relatively long
lived transient atmospherewith O•. as a major constituent.
This atmospherewould relaxby escape.The associatedrelaxation time could be relativelylong, however,sincethe O escape
rate cannot exceed the value noted earlier, 6 X 10? atoms cm -2
S-1
The chemistryof a nitrogen-hydrogen-oxygen-carbon
system was investigatedrecently by Yung et el. [1977]. Nitric
oxide is formed in the upper atmosphereby reactionof N(2D)
with CO•,
I
•...>•co•+
H•
I
- -
I
, '
5
(14)
and is removed mainly by
NOS) + NO-•
N•. + O
(15)
Odd nitrogenatomsare releasedby ionosphericreactionsand
by electron impact dissociationof N•.. A relatively simple
model gives resultsin satisfactoryaccord with measurements
of upper atmosphericNO reported by Nier et el. [1976e].
Resultsare shownin Figure 11, which includesseveraltheoretical curves which differ mainly in assumptionsmade with
regardto the yield of N(•'D) in electronimpact dissociationof
N•. [McElroy et el., 1976]. Densities for major forms of odd
nitrogenin the lower atmosphereare shown[after Yunget el.,
1977] in Figure 12. Densities computed for the lower atmosphereare sensitiveto assumptionsmade with regard to the
role of surfacechemistry.We assumedthat HNO•. and HNO3
could be incorporatedin surfacemineralsat rates(molecules
cm-•' s-1) givenby -ynt;,where3' denotesan activitycoefficient,
n indicatesthe density(cm-3) of HNO•. + HNOa, and t; is an
appropriatethermal velocity(cm s-•). The resultsin Figure 12
were derived with 3` set equal to 10-•'.
4.
MODELS
FOR PLANETARY EVOLUTION
Measurementsof isotopiccompositionmay be usedto imposeimportant constraintson the rangeof permissiblemodels
60•
[I• [[[,],[]%]
[[[[N•
I•l
]] [Illl[,I[I
'--......H•z-I-HOz
• O+O+CO,./
Xhz/+O,.
•
+OH
0+03• '.X•,
y
• 30
t/.+.o• [o.+.o•
q20-
•
[[
lo
0-z
....
I
10-'
....
I
10
ø
, ,,,I
101
,•,
10z
10a
10•
10
•
PRODUCTIONAND LOSS RATES FOR ODD H (c•3sec -})
Fig. ?. Productionand lossratesfor odd H in the lower Martian
atmosphere(conditionsare the sameas thosein Figure 6).
-
h % ...i
10ø
10]
10z
103
'(',,, 10•
105
lOs
PRODUCTION
ANDLOSSRATESFOR0z (cm-3sec
-•)
Fig. 9. Production
and lossratesfor O•.in the Martianatmosphere
(conditionsare the sameas thosein Figure 6).
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I
50
<[
H exobase
rn
o
-
•4o
x
• lo' -
-
z
lo
z
O2
1 I0
103
10
4
EXOSPHERIC
TEMPERATURE
10`5
10
e
10
?
10s
10
e
NUMBER DENSITIES(crn-3)
170 210 250 290 330 370 410
(øK)
Fig. 12. Number densitiesof N, NO, NO2, NOs, N205, HNO2,
Fig. 10. Densit), variationsof H and H2 at critical level versus HNO•, and N20 in the lower atmosphereof Mars. The surfacereacexospherictemperature.Ionosphericstructurewas assumedto be tivity coefficient-yfor HNO2 and HNO• equals1 X 10-2. Dissociation
of N2 is assumedto proceedthroughe + N2 -• e + N(2D) + NOS).
fixed in the present model.
The model atmosphereusedis describedby Yunget al. [1977].
for planetaryevolution.Mars has lost significantamountsof
oxygenand nitrogenover geologictime. Oxygen escapeproceedsby production of fast atoms in the exosphere,atoms
being formed primarily by the sequence,reaction (3), or
and nitrogen isotopes present in Mars' exosphere. Carbon
atoms may also escape. Fast atoms in this case could be
formed by
e +CO-•e+C
(16)
O + + CO•.-• O•.+ + CO
e+
followed by
CO=-•e
+C+
+O
(20)
O=
(21)
O•.+ +e-•O+
-•e+C+O+O
(17)
O
e + CO•.+ --} C + 02
Nitrogen atoms with sufficientvelocity for escapemay be
formed by
(22)
or
e+
N•. -•e+
(18)
N + N
e + CO +-•c
+ O
(23)
and
Diffusiveseparationin the Martian thermospherewill result
in a preferentialsupplyof light isotopesto the exosphere.The
e + N,.+ --, N + N
(19)
deficiencyof the heavier isotopesat the critical level may be
and we may note that thesereactionsare sufficientlyenergetic characterizedby a parameterR definedby the relation [McEIthat they shouldproceedwith equal efficiencyfor all oxygen roy and Yung, 1976]
R = fc/fo
o VIKING
DAT/•
(24)
wheref,, denotesthe abundanceof the heavy relativeto the
light isotopesat the criticallevelandf0 denotesthe analogous
quantity for the bulk atmosphere.The deficiencyparametersR
may be readily derived as a function of the eddy diffusion
coefficientK, taken to model effectsof mixing near the turbo-
1
pause.Values for R as functionsof K are shown in Figure 13.
Curve A gives the values of R for •80 relative to •60; curve B
givessimilar information for •SN/•4N and •aC/•'C. Analysisof
the Viking data [McElroy et al., 1976;Nierand McElroy, 1977]
suggestsa value for K of about 108cm•' s-•
NO
Consider a reservoir which contains an initial concentration
of gasb(0) atomscm-•', of knownisotopiccomposition
f0(0).
The concentrationof gas in the reservoirat time t, b(t), will
vary with time according to the equation
i
105
106
10?
i
1
lOs
db/dt = -(O, + O=)
(25)
NO NumberDensify(cm-:•)
Fig. 11. Comparisonof computedand measurednumber densities
of NO in the upper atmosphereof Mars. Curve a is obtainedby using
crosssectionsfor electron impact dissociationof N2 as measuredby
Winters with a quantum yield for N(2D) set equal to 50%. Curvesb
and c allow for uncertaintiesin Winters' crosssectionsat low energy
and in the quantum yield for N(2D), as describedin more detail by
McElroy et al. [1976].
where 4• denotesthe rate at which gas escapesto space(cm-•'
s-l), an isotopicallydependentquantityaswasnotedabove;4•.
definesthe loss rate (cm-•' s-•) for all isotopicallyinsensitive
removal processes.The time evolution of the bulk isotopic
compositionis given then by
b dfo/at = ( 1 - R)½•fo
(26)
MCELROY ET AL.' PHOTOCHEMISTRYAND EVOLUTION OF MARS ATMOSPHERE
Consider the application of this simple model to study the
time evolution of '60 and '80. The escaperate for O has a
magnitudeof 6 X 107atoms cm-: s-', as discussedabove.The
additional flux ½: may be used to model loss of O due to
oxidation of surface rocks. We shall assume that '80/'60
I
4385
I
I
is
enriched in the present Martian atmosphereby lessthan 5%
[Nier and McElroy, 1977] with respect to initial conditions.
The manner in which the enrichmentof '•O shouldvary with
respectto the initial reservoirsizeis illustratedin Figure 14. It
is clear that Mars' atmosphere must be in contact with a
reservoircontaininga sourceof oxygenat least as large as4.5
X 10:5atomscm-:. It is probablethat this reservoirreflectsthe
presenceof a relatively large concentrationof atmospherically
exchangeablesubsurfaceH:O [McElroy and Yung, 1976].
Viking's measurementof isotopicallyenriched'SN may be
usedin a similar manner to place a lower bound on the initial
concentration of volatile nitrogen. An enrichment of 1.62
[Nier and McElroy, 1977] requiresan initial N: concentration
of no less than 7.8 X 10:: moleculescm-:, equivalent to a
partial pressureof 1.3 mbar. The time evolution of Martian
o
>
1.2 I08
o
_i 5-
3 x 10'r
4-
nitrogen
willbesensitive
to surface
reactions
involving
HNO:
and HNOa as describedearlier. The enrichmentas predicted
for 'SN will depend on assumptionsmade with regard to the
magnitudeof the initial sourceof volatile N and its isotopic
composition, surface reactivity, and escape efficiency. Two
possiblemodelsare illustratedin Figure 15. The surfacereactivity 7 (for HNO: and HNOa) was taken to be 3 X 10-: (case
A) and 1 X 10-: (case B). More detailed discussionof the
possiblerange of valuesfor 7 is given by Yunget al. [1977].
The present calculationsimply an initial N: abundance of
about 1.7 X 10:4moleculescm-:, equivalentto a partial pressure of 30 mbar. Considerableuncertaintyis attachedto this
value, arising in part from our assumption that the eddy
diffusioncoefficientK is constantin time and in part from the
lack of precisionin our estimatefor the escapeefficiencyof N.
Our analysisnonethelessprovidesa reasonableestimate(factor of 3) for the initial nitrogenabundance.One might argue
that K is set mainly by dynamical processescontrolled by
insolation and topography. Uncertainties in the escaperate
(and consequentlyin 7) may be removed by suitable laboratory experimentation.
The observedenrichmentof 'SN may be usedto placelimits
3
1.03
I
I
I
1.04
1.05
1.06
1.07
ENRICHMENT FACTOR fo(t)/f,o(O)
Fig. 14. Total oxygen reservoir at t = 0 as a function of the
enrichmentfactorfo(t)/fo(O)at t = 4.5 b.y. for 4•. = 1.2 X 10e,3 X 107,
and 0 atomscm-•' s-•, respectively.
on the rate at which the atmospheremay gain nitrogenbecause
of slow, steadydegassingfrom the interior. Here, as before,we
denote the initial atmosphericabundanceof nitrogen by a
quantity b(0) atomscm-2. The time independentsteadysource
loo
A
A : 'r/=.16
7': .03
1 -
B : 'r/: .08
y: .01
o
1
2
3
4
TIME (b.y.)
.6}
06 I I tIIIII]
10? I I IIIII1[
]08 I I I IIII1[
109 I I I IIIII]
10iO I I IIIIII
EDDY DIFFUSIONCOEFFICIENT K(cm2s-I)
Fig. 15. Abundance of N•. (in millibars) as a function of time (in
units of 109 years). The enrichment factor at t = 4.5 b.y. is 1.62 for
both curves, in accord with measurements.For case A the escape
efficiencyrt for the reaction e + N•. -• e + N + N to produce an
escapingatom is 0.16; the surfacereactivity coefficient3• (for HNO•.
and HNO3) is 0.03. The correspondingvaluesfor r/and 3• in caseB are
Fig. 13. Values of R, the deficiencyparameterfor the heavier
isotopein the exosphere,
asa functionof the eddydiffu.
sioncoefficient 0.08 and 0.01, respectively.The eddy diffusioncoefficientK usedin
K. CurveA is appropriatefor '80/'eO. CurveB appliesto 'SN/"N and thesecalculationsequals 1 X 108cm•' s-•. This figure is adaptedfrom
'3C/"C.
McElroy et al. [1976].
386
ENTRY SCIENCE
ioo
I
i
i
estimated at 106atoms cm-•' s-1. The escaperate in the past
dependson the instantaneousabundanceof N•. and wascalcu-
i
lated
_
above.
It
follows
that
Mars
must have
rekian and Clark, 1975; Owen et al., 1976; Rasool and Le
i.o
o.o,_
I
I
Sergeant, 1977]. Information for earth is summarizedin Table
3. An acceptable model for Mars requires either selective
degassingof nitrogen relative to noble gases or a volatile
composition for Mars significantly different from that in-
\\
2.8
x105atoms
cm-2s
-I
0'10
as described
acquired its nitrogen atmosphere early in the evolutionary
history of the planet. The degassingrate for nitrogen at the
present epoch must be less than the average degassingrate
over the planet'shistory by a factor of at least 20. One might
supposethat a similar conversionshould hold for other volatiles with the exceptionof radiogenicgasessuchas 4øAr.
It may be of interest now to examine the possibility of a
common origin for the volatile budgetsof Mars and earth. A
numberof papershaveappearedin the recentliteratureattributing planetary volatilesto different classesof chondrites[Tu-
I
I
2
X
dicated for earth in Table 3.
I
3
4
TIME (b.y.)
Fig. 16. Abundance of N• as a function of time. Enrichment at t =
4.5 b.y. is 1.62; escapeefficiencyr• = 0.16 for both curves.Case A
assumes'7 = 0 and S = 3.6 X 105atoms cm-• s-•. CaseB assumes'7 =
0.01 and S = 2.8 X 105 atoms cm -• s-•
of nitrogenis definedby a quantityS atomscm-•' s-i, and the
abundanceof atmosphericnitrogenat time t, b(t), satisfies
the
relation
Supposethat the initial volatile compositionsof earth and
Mars were similar. A differencein the formation temperatures
of the two planetscould lead to differential rates for releaseof
H•.O, CO•., N•., a6Ar, Kr, and Xe. Laboratory experiments
[Zahringer, 1962; Heymann, 1971] suggestthat noble gasesin
meteorites are released at relatively elevated temperatures,
above about 900øK for a6Ar,Kr, and Xe. Supposethat nitrogen were present in more volatile forms, either as interstitial
atoms or moleculesor as componentsof the organic complexesidentified in carbonaceouschondrites[Moore, 1971].In
this case, Mars could have acquired an early atmosphererich
in H•.O, CO•., and N•.. Thermal constraints would have limited
the gas phase concentrations of H•.O and CO•.. Molecular
nitrogen might have accumulatedas the major constituentof
where the various symbolshave the significancediscussed the atmosphere,while H•.O and CO•.would have beenstoredin
earlier.The time evolutionof the isotopiccompositionis given condensed form in near-surface regions of the planet. An
now by
illustrative model for this situation, case A, is included in
Table 3. We assumehere that most of the noble gasesremain
b dfo/dt = (1 - R)Olfo - (fo - fo*)S
(28)
trapped by the bulk planet. 4øAris releasedat a rate (g/g s-1)
wherefo* definesthe enrichmentassociatedwith the sourceS, less than that appropriate for earth by a factor of 20. The
assumedto be equalto fo(0). Resultsfor severalcombinations smallersourcefor 4øArmay reflecteither a smallerconcentraof the parameters½, and S are shownin Figure 16. It is clear tion of crustal 4øKon Mars or a slower degassingrate or both.
that S cannotexceedabout4 X 105atomscm-•' s-1, whichmay Note that with this model the atmosphereacquiresits argon
be comparedwith the escaperate in the presentatmosphere, and nitrogen at quite distinct phasesof planetary evolution.
db/dt = -(ckl + ck•.)+ S
TABLE
H•O
Earth*
Martian atmosphere,
present'['
Martian atmosphereand
crusts
(27)
3.
Models for the Evolution
C
N
of Martian
Ne
Volatiles
a6Ar
iøAr
3.5 X 10 -ll
1.9 X 10 13
1.1 X 10 -8
5.7 X 10 -iø
2.6 X 10 -l•
2.2 X 10 -14
3.6 X 10 -13
3.5 X 10 -15
small
2.7 X 10 -14
small
3.5 X 10-15
2.8 X l0 -4
1.7 X 10 -5
--•2.2 X 10-•
1.1 X 10-s
7.7 X 10 -7
6.2 X 10 -•ø
3.0 X 10 -5
2.9 X 10 -5
2.9 X 10 -6
2.9 X l0 -6
1.3 • 10 -7
2.4 X 10 -s
small
6.0 X l0 -14
small
1.9 X 10 -13
small
5.7 X 10 -lø
3.0 X 10 -5
3.0 X 10 -5
2.9 X 10 -5
2.9 X 10-6
2.9 X 10 -6
2.9 X 10 -6
small
1.3 X 10 -*
2.4 X 10 -s
6.0 X 10 -•l
small
6.0X 10 -14
5.9 X 10 -•
small
1.9 X 10
small
small
5.7X 10 -lø
3.0 X 10 -5
2.9 X 10 -5
2.9 X 10 -6
2.9 X 10 -6
1.3 X 10-7
2.4 X 10-8
6.0 X 10-14
6.0 X l0 -li
1.9 X 10-•3
1.9 X 10-13
small
5.7 X l0 -1ø
1.1 X 10 -ll
<1.8 X 10 -13
Kr
Xe
Case A
Initial
Present
Case B
Initial
Intermediate
Present
2.9 X 10small
2.7 X 10
1.7 X 10 -la
small
3.5 X 10 -15
CaseC
Initial
Present
Units are gramsof volatileper gram of total planetarymaterial.
*From McElroy [1976]and Turekianand Clark [1975].
]'From Nier andMcElroy [1977]and Owenet al. [1976].
•:Discussed
in this paper.
2.7 X l0 -14
2.7 X 10-14
3.5 X 10
3.5 X 10-15
MCELROY ET AL.: PHOTOCHEMISTRYAND EVOLUTION OF MARS ATMOSPHERE
4387
The nitrogen concentration included in the table reflectsour
best estimate based on studies of escape as constrained to
satisfythe Viking •SN observation.The H•.O and CO•.concentrations are definedby scalingthe terrestrialabundanceslisted
in the first row of the table. An H•.O concentrationof magnitude 4.7 X 10-5 g/g would require a planet-wideice layer of
thickness200 m. Alternatively, it could be accommodatedby a
crustalmaterial containing3% H•.O if this crusthad an average
thicknessof 3 km. If one were to adopt the lower bound as
discussedearlier for the initial nitrogen concentration, the
abundancesof H•.O, CO•.,and N•. could be reducedby a factor
assumptionof time differential degassingfor N•. and noble
gasesimplicit in models A and B. Differencesbetween Mars
and earth might be attributed to relatively more extensive
degassingin the latter case, degassingextendingto greater
depthswherethe material may be deficientin low-temperature
condensates.It is interestingto note in this context that the
nitrogen to noble gas ratios inferred here for the primitive
of 15.
Mars are similar to values found for a wide class of meteorites
Case B considers a rather different
ceed 5 X 10-6 g/g. The concentrationsfor CO•. and H•.O in
case C were obtained by scalingfrom N•. by using terrestrial
ratios for H•.O: CO•.: N•..
Model C seemsthe most plausible. It avoids the ad hoc
model in which we as-
[Gibson, 1969; Eugster et al., 1969; Mazor et al., 1970; Heymann, 1971; Van Schmus, 1974]. It suggeststhat scaling of
have been less volatile than 36Ar, Kr, and Xe. This situation noble gas abundancesfrom earth to other planets may be
couldhave arisenif nitrogenhad beenpresentmainly in stable hazardous. Measurements of noble gas abundancesin the
compounds such as sinoite (Si•.N•.O) or osbornite (TIN), as atmosphere of Venus, scheduledfor the upcoming Pioneer
appearsto be the casefor enstatitechondritesand achondrites probe, shouldprovide additional insights.Major uncertainties
[C. A. Andersonet al., 1964;K. Keil and C. A. Anderson,1965; remain in the interim.
Bannister, 1941]. Then devolatilization of the early planet
5.
SUMMARY
might have favored releaseof H•.O, CO•., and noble gases,
Viking results,in combination with earlier data from Marinitrogen being releasedsubsequentlyduring the period when
the planet underwent major differentiation. The concentra- ner 4, 6, 7, and 9, have been used to develop a relatively
tions of H•.O and CO•. in the primitive atmospherewould be comprehensivemodel of Martian aeronomy. Escapeof H, O,
limited for caseB, as for caseA, by thermal constraints,and and N played an important role in the evolution of Mars'
the planet would have developedan initial atmosphererich in atmosphere.It is not possible,however, to identify a unique
36Ar,Kr, and Xe. Noble gasesin this systemwould be exposed combination of parametersto define the initial inventory of
directlyto the solar wind. Under presentsolarwind conditions Martian volatiles, though the planet appears to have undera magneticfield of •20 'y would be required to shield the gone a period of rapid early degassing,at least for N•.. Meaupper atmosphereof Mars from suchan interaction.It is clear surementsof noble gasesin the presentMartian environment
that Mars doesnot possessa magneticfield of this magnitude are especiallypuzzling.The possibilitythat the compositionof
today, nor is it likely that it ever did. The number of particles Mars' atmospheremay 'have been influencedby interactions
that can be sweptaway by the solarwind is limited by the rate with the solar wind over the early stagesof planetaryevolution
of photo-ionization and by mass loading of the solar wind shouldnot be ignored.The absenceof a significantMartian
itself. It may be shown [Michel, 1971] that the latter factor magnetic
fieldmayhavecontribute
d to differences
in theevodeterminesthe maximum escaperate. Michel [1971] gives a lutionary pathsof Mars and earth, a possibilitywhich could be
largely model independentformula for the masslossrate,
illuminated further by mass spectrometricmeasurementson
sume that nitrogen in the early Martian systemmay in fact
the scheduled Pioneer mission to Venus.
dM/dt = 0.86(T/lOa)(40/m) g/s
(29)
where dM/dt denotes the total mass loss rate expressedin
gramsper second,T denotesthe temperatureof the exosphere
in degreesKelvin, and m denotesthe molecular weight of the
escapinggas. Applying Michel's simpleformula, we can show
that the time requiredfor about 1 X 10•'øatomscm-•' ofa6Ar to
escapeis lessthan 0.5 b.y., and similarly short times would be
associatedwith the removal of Kr and Xe. Light noble gases
would be stripped first, followed by the heavier components.
There would be an associatedloss of CO•. and H•.O, whose
extent would depend fairly critically on the duration of this
hypotheticalearly evolutionaryphase.The time interval associated with this phasewould be relatively brief if the early sun
were more activethan is assumedhere. Propertiesof caseB are
summarized
in Table 3.
Case C in Table 3 summarizes
a model in which we relax the
requirement that volatiles on Mars and earth should have
similar compositions.We assumehere that all volatile compounds may be releasedwith equal efficiency.The nitrogen
model requiresthat the planet undergoa period of rapid initial
degassing.The early atmosphereshould be rich in N•. and
noble gasesand would be protected from the scavengingeffects of the solar wind. Thus we must assume that Mars was
assembledfrom material rich in N•. relative to noblegases,and
the various parameters in the table reflect this view. As was
noted earlier, the H•.O concentration for all models must ex-
Acknowledgments.This work was supportedby the National
Aeronauticsand SpaceAdministration under grant NAS-1-10492 and
by the National ScienceFoundation under grant NSF-ATM75-22723,
both to Harvard University. We are indebted to E. Anders, T. Owen,
and S. Wofsy for illuminating discussio-s.
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