JOURNAL OF GEOPHYSICAL
RESEARCH. VOL. 103, NO. El0, PAGES 22,689-22,694, SEPTEMBER 25, 1998
Atmospheric loss since the onset of the Martian geologic
record' Combined role of impact erosion and sputtering
David A. Brain and Bruce M. Jakosky
Laboratory for Atmosphericand SpacePhysics,Universityof Colorado,Boulder
Abstract. Chemicaland geomorphicevidencesuggests
that Mars' atmospherehas
undergone
significantlossor modificationsincethe onsetof the Martian geologic
record.Analysesof proposedlossprocesses
havebeenunableto individuallyaccount
for lossof enoughatmosphereto havesupportedthe presence
of liquid surfacewater
at the formationtime of the oldestobservedsurfaceunits. Here, we give a simple
calculationof the combinedeffectsof major atmospheric
lossprocesses.
Considering
only impact erosionand using resultsfrom an analytical model by Melosh and
Vickery[1989],we obtain an expression
for surfacepressureas a functionof local
crater density. Based on tabulated crater densities,the processof atmospheric
crateringcan accountfor a lossof 50-90% of the Martian atmospheresincethe
onsetof the geologicrecord. Stable isotopefractionationmeasurements
suggest
that lossof •090% of atmosphericspeciesto spacehas occurredvia solar wind
pick-up-ionsputtering [Jakoskyand Jones,1997]. Combined,•095-99%of Mars'
atmospherecouldhavebeen lost to space.Adsorptionof CO2 in the regolithand
sequestration
in the polesasiceor clathratecouldaccountfor mostof the remaining
loss.Thesecalculations
suggest
that the lossprocesses
of impacterosion,sputtering,
and sequestrationare together capableof explainingthe inferred transition from
an early atmosphereon Mars capable of supporting stable liquid surfacewater to
today's climate.
1. Introduction
Three main processesare thought to have removed
significant
amounts of Martian atmosphereover time:
Geologicfeatureson the Martian surfaceindicate that
ejection
of
atmospheric gas by impacts, loss to space
the climate has changedthrough time. Martian valdue to pick-up-ion sputtering, and sequestration into
ley networks, for example, are located predominantly
the regolith or polar caps. We examine the potential
in old crater-terrain regions,formed during the heavy
bombardment [Baker et al., 1992]. Thesevalley net- for a combination of these processesto have removed
works are seen on surfacesdating from the Noachian sufficientatmosphereto explain the transition in the climate. First, we considerthe effectsof ejection of gas to
epochin Martian geologichistory [Tanaka,1986]. A
widely accepted interpretation of the existenceof val- spaceby large impacts on the atmosphericevolution of
that there
ley networksis that liquid water wasoncestable(or, at Mars. Meloshand Vickery[1989]concluded
could have been nearly I bar of primordial atmosphere
least,morestable)on the surface [Bakeret al., 1992].
Another argument for climate changecomesfrom the 4.5 Gyr ago on Mars. The geologicevidence for climate changecomesonly from existing geologicfeatures
on Mars, however, none of which are likely to predate
about 4 Gyr ago. Here, we examinethe role that impact
erosion
played sincethe time of the onsetof the Martian
[CraddockandMaxwell,1993;Carr, 1996;Fanaleet al.,
geologicrecord about 4 Gyr ago. We do this by using
1992]. The highererosionratesand the evidencefor liquid water are thought to require a warmer climate and the observed cratering record to constrain directly the
a thickeratmosphere
than is presenttoday [e.g.,Fanale role impact erosion played in depleting the atmosphere
et al., 1992]. Severalatmosphericlossprocesses
have to its present amount. Next, we combine the loss from
been proposedto explain the necessaryclimate change, impact and sputtering processesover Martian history.
but none has been able to independently account for Third, we consider loss in the form of sequestrationof
CO2 into subsurfaceor polar cap reservoirs,and factor
sufficient loss or alteration.
the estimated sequesteredinventory into our estimates
of total lossand original atmosphericpressure.Finally,
Copyright 1998 by the American GeophysicalUnion.
we discussimplications of these calculationsfor understanding the possible evolution of the Martian atmoPaper number 98JE02074.
0148-0227/98/98JE-02074509.00
sphere and climate.
paucity of small impact craters on older surfaces,as well
as evidencefor erosionof large craters at rates substantially greater than can occur under the current climate
22,689
22,690
BRAIN AND JAKOSKY: CLIMATE CHANGE ON MARS
2. Impact Loss Model
where M and P denote atmospheric mass and pressure,
respectively, and M0 is the present atmospheric mass
and P0 is the present atmosphericsurfacepressure;t is
Previous work on the effect of impact erosion has
determined that impacts are capable of ejecting some age as in equation(1); b is the slopeof the cumulative
fraction of a planer's atmosphereto space,dependent impactor distribution which best predicts the current
upon propertiesof both the impactor and the planer's lunar crater distribution and has a value of 0.47; t, is
atmosphere
[Meloshand Vickery,1989;Walker,1987]. the time required to reducethe atmosphericpressureto
Relatively small impactors can remove a mass of at- zero, calculatedto be 63.1 Gyr by assuminga constant
mosphereapproximatelyequivalentto the massinter- cratering rate that coincideswith the best fit lunar cra•ceptedby theimpactor
asit travels
through
theat- tering rate; B is 2300 and was estimated by fitting to
mosphere[Walker,1987]. Meloshand Vickery[1989] lunar crater densities.The left-handside(s)of (2) give
showedthat sufficientlylarge impactorsare capableof
removinga more substantialatmosphericmassas the
expandingvapor cloud producedby impact dragsambient atmosphereaway from the planet at speedsexceedingthe escapevelocity.The amountof atmosphere
ejectedduring somelarge impactscould be equal to
the massof atmosphereabovethe plane tangentto the
pointof impact [Meloshand Vickery,1989].For the
current Martian atmosphere,silicate impactors larger
than •3 km and travelingfaster than •14 km/s will
removea "tangent-planemass"of atmosphereby this
process[Meloshand Vickery,1989].
Meloshand Vickery[1989]examinedthe role of im-
algebraic "enhancement"factors for massand pressure
as a function of time, indicating the mass of the Martian atmosphereat a given time, relative to the current
Martian atmosphere.
Meloshand Vickery[1989]usedequation(2) to note
that Mars' atmosphere4.5 Gyr ago could have been
-• 60-100 times as thick as the current atmosphere,in
the vicinity of i bar and that this value might havebeen
large enoughto have allowedstable liquid water on the
Martian surface. They also recognizedthat the importance of impact erosiondecreasedrapidly with time, so
that it was not a significantprocessby the end of the
late heavy bombardment3.5 Gyr ago.
pact erosionby large projectileson the evolutionof the
If we are to understand the transition that occurred
Martian atmosphere.Using a modifiedpowerlaw rela- in the Martian climate, however, we are interested in
tion for the time-dependentimpactor flux, they derived the atmospheric loss that occurred subsequentto the
an analytic expressionfor the densityof Martian craters onset of the geologicrecord, or the earliest time from
>4 km as a function of time. The number of craters
which we have geologicevidence of a thicker climate.
>4 km, denotedN(>4 km), is usedasa statisticallyre- Thus we want to look at impact erosiononly from the
liable measureof the number of impactorslarge enough time of the oldest surface features, rather than from 4.5
to erode the entire atmosphereabovethe tangent plane; Gyr ago. To do this usingthe analyticexpressions
(1)
the function doesnot imply that all cratersgreaterthan and (2) in their presentform wouldrequireknowledge
4 km resultedfrom impacts capableof causingbulk ero- of the agesof the oldest existing Martian terrain, which
sion, nor that all 4 km craters will be retained through are not well constrained. However, the existing model
time. The lunar crateringrecordand absoluteageswere providestwo expressions,each containing time as a free
usedto constrainthis function(Figure la):
parameter. We are able to remove time from the two
equations, leaving one expressionfor the surface pres-
N(> 4km)- 2.68110-a[t+4.57x10-?(eXt-1)](1)
sure enhancement
factor
as a function
of local crater
Here, t is the age of the crateredsurface,in Gyr, and density. The resultant expressionis implicit in P, but
)• -- 4.53Gyr-1. Numerical
valuesin the expressioneasily solvednumerically as shown in Figure lc:
comefrom applicationof the revisedSchmidt-Holsapple
scalinglaw [Schmidtand Housen,1987]and assump-
P0= (1- •,(At,
[1-e_•])
P(t)
1 Be
-z'6•
l/b(3)
tions about the Martian cratering rate relative to that
of the Moon. The expressionshowsthat crater density
on a planetary surfaceevolveslinearly in time, with a
superimposedexponential dependencethat decaysas
where C1 = 4.57x 10-7 , Co.= 2.68 x 10-5 and all
the planet agesand availableimpactorsare usedup.
•--W[•C1
½A[0,+½%]]_/•[C1
-•-•2
] (4)
Fromequation(1), Meloshand Vickery[1989]deter- other variablesare as statedfor equations(1) and (2).
mined the flux of impactorslarge enoughfor the vapor
cloudto removeatmosphere,and integratedit to obtain
an expressionfor the atmosphericmassas a functionof
time. Their expressionfor atmosphericmassrelative
to today's atmosphericmass is equivalentto the atmosphericsurfacepressurerelative to today's surface
pressure(Figure lb):
M0 = P(t)
P0= (l+t•At,
[1-eAt
] (2)
Matra(t)
t Be
-z'6A
W denotesthe Lambda function and is given by
=
Equations(3)-(5) are particularlyrelevantsincewe
now need only know the local crater density of a surface
region to determine the atmosphericsurfacepressureat
the time that region was forming; the precisesurface
age is no longer required. In essence,we can directly
determine the effectson the atmosphereof the impact
craters
observed
on the oldest surfaces.
BRAIN AND JAKOSKY: CLIMATE CHANGE ON MARS
22,691
Crater Density vs Time
10-2
10-3
10-4
10-5
10-6
-4
-3
-2
-1
0
Time From Present (Gyr)
Atmospheric Pressure vs Time
lOO
"'
'"
N NNH
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-4
-3
-2
,
,
,
,
-1
Time From Present (Gyr)
Crater Density vs Surface Pressure
lOO
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.
0000
' ' i b, 004
' '' i 6.006
' ' i 6,008
' ' i 6. 0I10i i 6 , 012
MeasuredCumulative
CraterDensityN(>4km)(kin-2)
Figure 1. (a) Plot of cumulativecrater densityN(>4 km) overthe historyof the Martian
surface,as derivedby Meloshand Vickery[1989].The expression
(givenin equation(1)) is well
matchedto lunar crater densities.(b) Plot of the Martian atmosphericpressureenhancement
factor,P(t)/Po, overthe past 4.53 Gyp,as derivedby Meloshand Vickery[1989]and givenin
equation(2). The geologicepochsare noted,assuming
a-2 powerlaw size-frequency
distribution.
The boundaries between each age are approximate and are estimated from the work of Tanaka
[1986]and appliedto the modelto yield P(t)/Po. (c) Plot of surfacepressureenhancement
factor as a functionof surfacecrater densityN(>4 km). The implicit expression
is derivedby
eliminatingtime as a parameterfrom equations(1) and (2).
22,692
BRAIN AND JAKOSKY: CLIMATE CHANGE ON MARS
3.
Results
3.1
Ancient
Table1. Estimates
ofN (>4km)(inunits
ofkm
'2)
Surface
Crater
Reference
Densities
Craddock
andMaxwell[1993
]
Scott
andTanaka
[1981]
Hartmann
etal. [1981
]
The first step in applying the simplified impact erosion model
from
above to Mars
is to obtain
Lower Bound
a reliable
estimate of the crater density on the oldest surfaces.
Three sources are used to determine a range in crater
6.84x 10'4
8.92x 104
7 x 10'4
UpperBound
3 x 10'3
1.43x104
3 x 10'3
densities. From each sourcewe obtain N(>4 kin), or
lowing for reasonableuncertainty in this correction,we
the cumulative number of craters with diameter greater
estimate
N (> 4 km) - 7 x l0-4 - 3 x 10-3 km-2. It
than 4 km at the time of formation of the oldest geologic
should be noted that the locations of the surface units
units on Mars. In some casesthis requires a conversion
between
incremental
and cumulative
crater
densities.
used to obtain crater densities from this reference were
unspecified.
We also correct for preferentialerosionof small craters
A more recent estimate of crater densityon the oldoverthe historyof the Martian surfaceby usinglargedi- est Martian surfaces comes from Craddock and Maxwell
ameter crater density estimates and assuminga power
law distribution in crater density; the assumedpower [1993].Crater densitiesare givenfor crateredand dissectedplateau sequences,distinguishingbetween fresh
law distribution is either a canonical-2 power law disand modifiedcraters and examiningcorrelationbetween
tribution or is a distribution in agreementwith figures
degradation
and elevation of the cratered terrain. The
included in the sources.
total area investigated is extensive and yields crater
Hartmannet al. [1981]obtainedcratersize-frequency
distributions
for a number of surface units on Mars.
Figure2 showsan incrementalcratersize-frequency
dis-
densities
N(>4 kin) = 6.84x 10-4- 3 x 10-3 km-2.
This range is obtained using two methods and taking
the highest and lowestresulting crater densities.First,
tribution for the most heavily crateredregionsstudied.
N(>4 kin) is scaledfromvaluesfor N(>16 kin) givenin
Convertingto a cumulativedistribution,correctingfor
Craddockand Maxwell's Table 1, assuminga-2 power
the effectsof removalof small cratersby erosionand allaw distribution; scalingfrom N(>16 kin) shouldnot
require a correction for small crater erosion. Second,
N(>4 kin) is estimated using Craddockand Maxwell's
Figure 1 by correctingfor erosionof craterssmallerthan
20-30 km in diameter.
Additional
o
data on ancient Martian
crater densities
comefrom Scottand Tanaka[1981]. Basedon counts
of Early Noachian craters taken in the Tharsis region
-1
of Mars, the oldest units there exhibit crater densities
N(>4 kin)= 1.26x 10-3-1.43 x 10-3 km-2. Thisrange
%,:,
',•
%
is obtained by scalingN(>4 kin) from valuesfor "old
terra regions"givenin Scottand Tanaka's[1981]Table
I for craters of varying size. A -2 powerlaw distribution
was assumed,and the highest and lowestresultingdensities were kept. It should be noted that the Tharsis
region of Mars in general is relatively young, and the
•
expanses of ancient cratered terrain are not extensive
-5
in area.
The crater
determined
6
densities
on the oldest Martian
terrain
from the three sources are shown in Table 1.
For the remainder of this work we take the low and high
values from Table 1 as the range in crater density on
the oldestMartian terrain. Thus we usea craterdensity
0.25
I
I
0.5
I
estimate
of N(>4 km) - 6.84x 10-q - 3 x 10-3 km-2.
Diameter (km)
Figure 2.
Incremental size-frequencycrater density
3.2 Atmospheric
Loss From Large Impacts
The inferred crater densities are used in the simpli-
distributionof Hartmann et al. [1981]for the region fiedmodelexpression
(equations(3) and(4), andFigure
labeled "heavilycrateredplains". To obtain N(>4 kin) lc) to yield P/Po, the enhancementfactor in surface
for use in the impact erosionmodel, we correct for preferential removal of small craters by assumingthat the pressure between the current atmosphere and the atpower law distribution associated with larger craters mosphere at the formation time of the oldest surfaces.
(N(>32 km)) appliesfor cratersof every size. We es- From Figure lc, the range in crater density corresponds
timate from the correcteddistributionthat N(>4 kin) to an enhancement factor in surfacepressureof 2.1-8.7.
=7 x 10-4 - 3 x 10-3 km -2
This enhancementfactor suggeststhat the total amount
BRAIN AND JAKOSKY: CLIMATE CHANGE ON MARS
22,693
of atmosphereon Mars that was presentat the onset of
the geologicrecordhas beensubsequentlyreducedby a
field [Hutchinset al., 1997].In addition,the absolute
factor of 2.1-8.7 from large impact erosion.
The range in enhancementfactor implies that Mars
has lost about 50-90% of its early atmosphereby large
impacts. This loss is significantly different from the
98-99.5% loss over 4.5 Gyr suggestedby Melosh and
mates of the amount
amount of gas lost to space is uncertain. Current estilost differ from earlier estimates
in the way the differentisotopicconstraintsare applied
[Hutchinsand Jakosky,1996]. We assumehere that
the isotopicanalysisprovidesa strong indication only
of the fraction of volatiles lost to space and that the
Vickery[1989].The craterdensitywouldneedto be > absoluteamount of loss,which is extremely uncertain,
0.01km-2 (almostanorderofmagnitude
greater
than is sufficientto accountfor muchof the lossrequiredby
the highestcrater densityvalueslisted above)in order the calculation below; the uncertaintiesin the absolute
for the surface pressure to have been in the vicinity
sputtering loss rate allow these assumptions. We take
of 1 bar at the time of onset of the geologicrecord. 90% as the nominalvaluefor bulk atmospheric
lossby
Clearly, a much smaller fraction of Martian atmosphere sputtering.
hasbeenremovedby largeimpactssincethe onsetof the
Both sputtering and impact erosionact on the atmogeologicrecordthan overthe past4.5 Gyr. This is not a spherein a relative way; the amount removedby either
surprisingresult but suggeststhat impact erosionalone processis always dependentupon the total atmospheric
is not capable of accounting for the required amount abundance. Bulk atmosphericremoval by impact does
of atmospheric loss since the formation of the oldest not preferentially remove light isotopes,while sputterMartian
surface units.
ing does not affect the impact erosion process. ThereIn addition, we can use the crater densities from fore we assumethat each processacts independentlyof
different regions to derive the history of atmospheric the other and that the lossesmultiply rather than add
pressuredue to impacts through the Martian geologic (seebelow). Starting from an initial surfacepressure,
epochs(Figure lb). Tanaka[1986]givescratercounts sputtering alonewould reducethe pressureby 90%. Imfor surface units formed during the different Martian
pact erosionwould remove50-90%of the remainingatepochs. From Figure lb, impact erosionhad a signif- mosphere,leaving1-5% of the originalatmosphere,baricant influence during the Noachian, but its influence ring other processes.Thus 95-99% of the atmosphere
rapidly declinedduring the Hesperianand Amazonian would have been lost overall.
The uncertainty in the timing of the sputteringloss
epochs. Becausethe distinction between the geologic
affects
the resultsto someextent; here we assumethat
epochsis tied closelyto the number of impact craters
sputtering
lossacts only from the start of the geologic
on the surface, this conclusionis independent of the
record.
A
later
"start of the clock" for sputteringloss
absolute timescale in the cratering history.
could occur due to the presenceof an intrinsic Mar3.3 Atmospheric Loss From Impact Erosion
tian magnetic field or impact delivery of volatiles, and
and Pick-up Ion Sputtering
would result in a smallercombinedloss.A strongmagWe have shown that Mars could have lost •050-90% of
netic field would cause the solar wind to stand off from
its atmosphere by impact erosionsincethe earliest existing surfaceformed; this amount of lossdoesnot indicate
the planet and would thereby inhibit sputtering loss
the presenceof an early atmospherethick enoughto ac-
by measurementsof remanent magnetismmade in the
[Hutchinset al., 1997].Sucha fieldis, in fact,suggested
count for the abundant evidence for fluvial features seen
Martian meteoriteALH84001 [Kirschrinket al., 1997]
on the surface[Fanaleet al., 1992].However,whenthe and by direct measurementsof the remanentmagnetic
results are coupled with the inferred lossfrom pick-up- field overthe ancientsurfaces
on Mars [Acu•a et al.,
ion sputtering, we come closerto reasonablyaccounting 1998]. However,useof the isotoperatiosas an indica-
for the climatic transition suggestedby geomorphicev-
tor of the efficacyof sputtering losstells us the fraction
of gas lost in total, independentof the time of onset of
In the sputtering process, photochemical processes sputtering loss.
ionize atoms in the upper atmosphere, which are then
The assumption that the impact ejection and the
accelerateddue to the magnetic field in the solar wind. sputtering lossprocessesof atmosphericremoval are enThese pick-up ions can collide at very high velocities tirely independentis not strictly correct. The minimum
with other upper atmospheric species and eject them size of an impactor required to eject gas from the atto space [Luhmannand Kozyra,1991]. This sputter- mospheredependson the mass of atmospherepresent
ing process preferentially removes lighter atmospheric [Meloshand Vickery,1989]. Thus atmospheric
craterisotopes,which have a larger atmosphericscale height ing will be a more efficientprocessif sputteringlossoc[Jakosky
and Jones,1997].By measuring
isotoperatios curs prior to it or simultaneouslywith it. In that sense,
of atmosphericspeciesan estimate can be made of the our estimates are likely to be a lower bound on the toamount of atmospherethat has been lost by this pro- tal amount of atmospherethat has been lost. If the
cess. Observed isotope ratios of Ar, C, H, and N sug- early estimatesregardingthe time history of the maridence.
gestthe likely lossof 85-95%of eachgas [Jakoskyand tian magneticfield [Acu•a et al., 1998]hold up, then
Jones,1997].The timingof the sputteringlossisuncer- the bulk of the sputtering lossfollowedthe bulk of the
tain due to the possibleinfluence of an early magnetic
cratering loss;in this case, our assumedmultiplicative
22,694
BRAIN AND JAKOSKY: CLIMATE CHANGE ON MARS
behavior is valid, and our estimate of atmosphericloss
Maxwell and Michael Carr for acting as Editor for this
will be correct.
manuscript. Their comments and those of Janet Luhmann
and Bob Craddock were appreciated.
4. Discussion
and
References
Conclusions
The calculations presentedhere suggestthat a combination of impact erosion and pick-up ion sputtering
can remove 95-99% of the atmospherethat was present
at the time of the onset of the geologicrecord, about
4 Gyr ago. If no other processeshave acted, then the
present 6 mbar pressure would indicate that the Martian surface pressure was in the range 120-600 mbar
at the formation
time
of the oldest surface units.
This
value is probably not large enoughto support stable liq-
Acufia, M.H., et al., Magnetic field and plasmaobservations
at Mars: Initial resultsof the Mars Global SurveyorMission, Science, 279, 1676-1680, 1998.
Baker, V.R., M.H. Cart, V.C. Gulick, C.R. Williams, and
M.S. Marley, Channels and valley networks,in Mars,
edited by H.H. Kieffer, et al., pp. 493-522, Univ. of Ariz.
Press, Tucson, 1992.
Carr, M.H., Water on Mars, Oxford Univ. Press,New York,
1996.
Christensen, P.R., et al., Results from the Mars Global Surveyor thermal emissionspectrometer,Science,279, 1692-
1697, 1998.
uid surfacewater at any time in Mars' history [Fanale
et al., 1992].Alternatively,if we assumethat the atmo- Craddock, R.A., and T.A. Maxwell, Geomorphicevolution
sphere was as thick as 3 bar at one time, for example,
then 95-99% lossby sputteringand impact would leave
30-150 mbar still in the atmosphere.
It is quite plausible that the regolith or polar caps
could contain most of this remaining gas. For example,
30-40 mbar of CO2 could be adsorbed in the regolith
of the Martian highlands through ancient fluvial processes,J. Geophys.Res., 98, 3453-3468, 1993.
Fanale, F.P., S.E. Postawko,J.B. Pollack, M.H. Carr, and
R.O. Pepin, Mars: Epochal climate changeand volatile
history, in Mars, edited by H.H. Kieffer, et al., pp. 11351179, Univ. of Ariz. Press, Tucson, 1992.
Hartmann, W.K., et al., Chronologyof planetaryvolcanism
by comparativestudiesof planetary cratering,in Basaltic
[Zent and Quinn,1995],and as muchas 100-200mbar
Volcanismon the Terrestrial Planets, Basaltic Volcanism
could be sequesteredas ice or clathrate in the poles
Study Proj., Pergamon, Tarrytown, N.Y., 1981.
(though a few tens of mbar may be a more plausible Hutchins, K.S., and B.M. Jakosky, Evolution of Martian
atmosphericargon: Implications for sourcesof volatiles,
upper limit) [Mellon, 1996]. Although an unknown
J. Geophys.Res., 101, 14,933-14,949, 1996.
amount of CO2 may be present as carbonate minerals
Hutchins,K.S., B.M. Jakosky,and J.G. Luhmann,Impact of
in the crust, a substantial carbonate reservoir is not
a paleomagneticfield on sputtering lossof Martian atmoneeded.
sphericargonand neon, J. Geophys.Res., 102, 9183-9189,
1997.
These calculations imply that impact erosionwas an
Jakosky,
B.M., and J.H. Jones, The history of Martian
important processearly in Mars' geologichistory but
volatiles,
Rev. Geophys.,35, 1-16, 1997.
that it cannot account for enough atmosphericlossto
Kirschvink, J.L., A.T.' Maine, and H. Vali, Paleomagnetic
have allowed the formation of the observedvalley netEvidence of a low-temperature origin of carbonatein the
works on Mars' oldest surfaces.Similarly, sputtering is
Martian meteorite ALH84001, Science, 275, 1629-1633,
a processthat probably has acted throughout most of
Martian history and has profoundly affectedthe isotopic
composition of Mars' atmosphere, but it is not efficient
enough acting alone to have produced the inferred climate changethrough atmosphericloss. Only when we
combine the three lossprocessesof large impacts, pickup ion sputtering, and sequestration into the surface
are we able to identify a plausible means of removing
several bars of early atmosphereon Mars.
The present analysis is not unique, and there are
considerableuncertainties in quantifying all of the processesby which gas can be removed from the atmosphere. However, it does suggestthat plausible mechanisms exist that can provide for a transition from a
thick, warm, early climate to the later cold, dry climate. Interestingly, we can do this without requiring
that large surface or subsurface deposits of carbonate
minerals contain the CO2 from the early atmosphere;
the absence of detectable carbonate spectral features
[e.g.,Christensen
et al., 1998]thereforedoesnot present
a significant problem.
Acknowledgments.
This research was supported in
part by the NASA Planetary Atmospheresprogramthrough
grant NAGW-3995 and the Exobiology program through
grant NAG5-4530. B.M.J. thanks Associate Editors Ted
1997.
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Melosh, H.J., and A.M.Vickery, Impact erosionof the pri-
mordial atmosphereof Mars, Nature, 338, 487-489, 1989.
Schmidt, R.M., and K.R. Housen, Int. J. Impact Eng., 5,
543-560, 1987.
Scott, D.H., and K.L. Tanaka, Mars: Paleostratigraphic
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David A. Brain and Bruce M. Jakosky, Laboratory
for Atmospheric and Space Physics, University of Colorado, Campus Box 392, Boulder, CO 80303-0392(e-maih
[email protected])
(Received April 30, 1997; revisedMay 30, 1998;
acceptedJune 16, 1998.)