Sep 30, 1977 - nate among various choices of ALFO can be attributed to the inability of Mie .... equals approximately 0.65 [Huffman and Stapp, 1973]. Thus as.
VOL. 82, NO. 28
JOURNAL OF GEOPHYSICAL RESEARCH
SEPTEMBER30, 1977
Propertiesof Aerosolsin the Martian Atmosphere,asInferred From Viking Lander Imaging Data JAMESB. POLLACK, • DAVID COLBURN, • RALPHKAHN,9'JUNEHUNTER,aWARRENVAN CAMP,aC. E. CARLSTON, '•AND M.
R. WOLF s
Observationsof the Martian sky, Phobos,and the sun were taken with the Viking lander imaging camerasto obtain informationon the propertiesof the atmosphericaerosols.Atmosphericopticaldepths were derived from the observationsof the brightnessof the celestial objects. Information on the absorptioncoefficient,meansize,and shapeof the aerosolswasderivedfrom studiesof the skybrightness. For this purposewe used a multiple-scatteringcomputer code that employed a recently developed techniquefor treating scatteringby nonsphericalparticles.By monitoringthe brightnessof the twilight sky we obtained information on the vertical distribution of the particles.Three types of aerosolsare inferred to have beenpresentover the landersduring the summerand fall seasonin their hemisphere.A groundfog madeof water iceparticleswaspresentthroughoutthisperiod.It formedlate at nightduring the summerseasonand dissipatedduring the morning.We infer that duringthe summerthe frost point temperaturewas 195øKand the water vapor volumemixingratio equaledabout 1 X 10-4 nearthe ground at VL-2. Assumingthat condensationoccursonly on suspendedsoil particles,we estimate that the averageparticle radius of the fog was about 2 #m and that the fog'sdepthequaledapproximately0.4 km. A higher-levelice cloud was prominent only during the fall season,when it was a sporadicsourceof atmosphericopacityat VL-2. The formation of upperlevelwater ice cloudsduringthe summermay have been inhibited by dust heating of the atmosphere.Suspendedsoil particleswere presentthroughoutthe period of observation.During the summerthey constitutedthe only major sourceof opacity in the afternoon and most of the night. The cross-section weightedmean radiusof theseaerosolsis about 0.4 #m. They havea nonsphericalbut equidimensional shapeand roughsurfaces.Thesesoil particleshavea scaleheightof about 10 km, whichis comparableto the gasscaleheight,and they extendto an altitudeof at least30 km. The principal opaquemineral in theseparticlesis magnetite,which constitutes10%+ 5% by volume of this material. We proposethat soil particles,as well as any associatedwater ice, are eliminatedfrom the atmosphere,in part, by their actingas condensationsitesfor the growth of CO•. ice particlesin the winter polar regions.The resultantCO•.-H•.O-dustparticleis muchlargerand thereforehas a much higher fallout velocity than an uncoateddust or water ice particle.
sphere, usually totally envelopethis hemisphere,and sometimes spreadto the northern hemisphereas well. Analysis of the infrared interferometer spectrometerand ultraviolet spectrometer observations conducted from the Mariner 9 orbiter have provided a determination of some of the properties of the suspendeddust particlesduring the decayingphaseof the 1971-1972 global dust storm. At its peak the optical depthwasabout 1.5, and the opticaldepthmore or less monotonically declined to a value of about 0.2 over a 3month time period [Pang and Hord, 1973; Toon et al., 1977]. The differentialdistributionof particle sizeshad a slopeof-4 for particle radii between1 and 10 urn, and surprisingly,the Aerosols made of soil material and water ice are known to distribution remainedapproximatelyconstantover the durabe presentin the Martian atmosphere,while the occurrenceof tion of the decayphase[Toonet al., 1977].This resulthasbeen carbon dioxideice is suspected.We now briefly discusswhat is interpretedas indicatingthat eddy mixing rather than Stokescurrently known about each of these three aerosol species, Cunninghamfallout controlsthe elimination of dust particles beginningwith the small suspendedsoil particles. Such par- from the atmosphere.The particlesare made of either a mixticlesare injectedinto the atmosphereduring the time of great ture of acidicsilicatematerialsor elsea mixture of clay minerdust storms,which begin when Mars is closeto its perihelion als and'perhapsbasalts[Toon et al., 1977]. Chemical analyses orbital position [Leovy et al., 1973a].The correspondingsea- of the soil at the Viking lander sites indicate that it is made son on Mars is late spring in the southernhemisphereor, either of iron rich clays or a mixture of basalts and carboequivalently,L8 equalsabout 250ø, whereL8 is the aerocentric naceouschondritic material [Baird et al., 1976]. In addition, longitudeof the sun relative to springequinox in the northern the surfacematerial contains0.1-1% chemicallybound water hemisphere.Global dust stormsbegin in the southernhemi- [Biemannet al., 1976].The smaller-sizedsuspendeddust particlesmay be madeof similarmaterial.This conclusionmay be consistentwith the above cited resultsof Toon et al. [1977]. • SpaceScienceDivision,NASA AmesResearch Center,Moffett In addition to global dust storms, localized dust stormsare Field, California 94035. known to occur occasionallythroughout the Martian year •'Centerfor Earth and PlanetaryPhysics,Harvard University,Cam[Leovy et al., 1972;Saganet al., 1972]. Theselocalizedevents bridge, Massachusetts 02138. can place modestquantitiesof dustin the atmosphereat times a Informatics, Inc., Palo Alto, California 94305. 4 Martin Marietta Corp., Denver, Colorado 80201. well removed from the time of the global storms.On the basis 5Jet PropulsionLaboratory,Pasadena,California 91103. of the exceptionalclarity of surfacefeaturesseenon Mariner 9 pictures obtained during its extended mission, Leovy et al. Copyright¸ 1977by the AmericanGeophysicalUnion.
Observationsof the Martian sky and celestialobjectshave been made with the Viking lander imaging camerasto obtain estimatesof the composition,size, vertical distribution, and optical depth of the aerosolsabovethe two lander sites.Preliminary estimates of these quantities have been given by Mutch et al. [1976a, b, c]. In this paper we examine a much larger set of data than was used in the earlier papers and subjectthe measurementsto a more thorough analysis.In the remainder of this introductory sectionwe briefly review prior information about the aerosolsin the Martian atmosphereand then indicate the manner in which the lander imaging experiment can add to this knowledge.
Papernumber7S0559.
4479
4480
LANDER IMAGING
[1973b]estimatedthat the optical depth was lessthan 0.04 for Ls values of 30-100, a time interval that slightly overlapsthe beginningof the Viking mission. Water ice clouds are known to form both in the polar regions during their fall and winter seasonsand near topographically elevated places during summer [Martin and McKinney, 1974; Leovy et al., 1973b]. The polar cloudsare referredto asthe 'polar hood.' It exhibitssignificantvariability in its location on time periodsof the order of days and even hours [Martin and McKinney, 1974].On the averagethe northern polar hood hasbeenobservedto extendto latitudesas low as about 40ø. Thus during fall and winter seasonsin the northern hemispherewe might expectoccasionallyto observe the polar hood above the Lander 2 site (48øN) but not above the Lander 1 site (22øN). During the summer,water ice cloudshave beenobservedto form and intensifyduringthe day aboveelevatedregions,such as the Tharsisshieldvolcanos[Leovyet al., 1973b].Analysisof suchcloudsviewed by the infrared interferometerspectrometer experiment showedthat the cross-sectionweightedmean radius of the ice particleswas about 2 #m and the integrated vertical column densityof water icewas about 0.5 precipitable microns (pr #m) [Curran et al., 1973]. The Viking orbiter cameras have also seen what appears to be low lying fogs within a few depressedareasin the equatorial region [Carr et al., 1976]. Detached high-altitude haze layers have been observed on both Mariner 9 and Viking orbiter photographs [Leovy et al. 1972;Carr et al., 1976].This structureis obvious only at certain seasonsand latitudes.Near the beginningof the Viking mission the detached haze layer was prominent at southern latitudes, where its height and optical depth had valuesof about 40 km and 0.01, respectively[Carr et al., 1976]. Conceivably, this haze layer could be formed from the condensationof CO2 gas rather than H20 vapor. Finally, general circulationcalculationsindicatethat the air temperaturein the inner portion of the winter polar cap regions(latitudesof > 60ø) falls to the frost point of CO•, and therefore CO• ice particlesmay form there [Pollack et al., 1976]. With the aboveresultsin mind, let us considerthe opportunitiesfor studyingatmosphericaerosolswith the Viking lander cameras.By virtue of their geometryand photometricquality, observationswith the lander cameraslead to a good determination of a number of important aerosolproperties.The camerashave a linear responseto incidentradiation;their absolute calibration is well determinedas a result of extensivegroundbased tests as well as ones made on Mars [Patterson, 1977]; and accuratephotometriccomparisonscan be made acrossa given picture, sincethe samediode viewsthe whole scene.One of the diodesis a sun diode, which allowsimagingof the sun. Such images permit an unambiguous determination of the optical depth of the atmosphereduring the day, while images of Phobos obtained with other diodes provide comparable data during the night. By way of contrast, the derivation of optical depth from orbiter or ground-basedobservationsof Mars is usually very model dependentsimply becauseof the more unfavorablegeometricalarrangementof observer,atmosphere,and light source.Also by virtue of their geometrythe lander camerascan view the Martian sky closeto the sun. Such observationsprovide inforr•ation on the size of the aerosols. Also lander images.of the sky are not contaminatedby light originating from the surface, whereassuch situationscannot generallybe realizedfrom other platforms. The atmosphericobservationsdiscussedin this paper span the time period from mid-July 1976 to the end of February
1977. During this time period the seasonat the Viking 1 (22.5øN, 48.0øW) and the Viking 2 (47.9øN, 225.9øW) sites changedfrom earlysummerto midfall(Ls = 100-230).Below we first present values of the optical depth obtained from imagesof the sun and Phobos,then give estimatesof particle sizeand shapefoundfrom skybrightness pictures,nextdiscuss compositionalinformation obtainedfrom sky color measurements, and finally investigatethe vertical distribution of the aerosolswith the aid of photographsin which the sky brightness was monitored as a function of time before sunrise or after sunset. In each case we will discuss the manner in which
the desiredphysicalvariableis extractedfrom the observations and thenpresentthe valuesobtainedalongwith somepreliminary interpretations.Following thesesectionswe discussour resultsin the more generalcontextof their possibleimplicationsfor Martian meteorology.In the lastsectionof thispaper our main conclusions are summarized. OPTICAL DEPTH Procedure
Observations of the sun and Phobos were obtained to deter-
mine the optical depth of the atmosphereduring the day and night. Optical depth valuesr werederivedin a straightforward fashion from Beer's law, accordingto which the observed intensityof theseobjectsI is related to r by I = I0 exp [-r/M(e)]
(1)
whereI0 is the value that would have been obtained had r = 0, M is an air mass factor, and e is the elevation angle of the celestialobject, as measuredfrom the horizon for a hypothetically fiat surfacealigned perpendicularto the local gravity vector. Except for very small valuesof e, M(e) = sin e. The value for r can be found from (1) eitherby usingthe observed intensitiesfrom a pair of picturestaken closetogetherin time
but with the objectat significantlydifferentelevationanglesor alternativelyby knowingthe valueof I0 and usingthe observed intensityof a singlepicture.Becauseof picturebudgetlimitationsa numberof singlesunpictureswere obtained.We therefore averagedthe I0 values derived from pairs of pictures obtainedwith a given cameraand diode on the sameday at different elevation anglesand used these averagevalues to determiner for eachpicture.Comparisonof r valuesfoundby the single-picture methodfor sunpicturestaken closetogether in time indicated that on the averagethesevalueshave an uncertainty of about ñ5%.
The valuesof the sun'selevationangleneededfor (1) were obtained from an ephemerisprepared by S. Synnott and T. Duxbury of the Jet PropulsionLaboratory. We adoptedthis approachinsteadof usingthe observedpositionsof the sunin the picturesbecausethe latter procedurewould have involved makingcorrectionsfor the tilt of the landerwith respectto the localsurfaceand for the tilt of the localsurfacewith respectto the local geoid.The chief uncertaintyin the ephemerisvalues stemsfrom an uncertainty in the location of the lander. This error translatesinto an error of a few tenthsof a degreein the value of e, an error consistent with the difference between
observedand predictedpositionsof the sunand Phobosin the pictures. Images of the sun and Phobos were obtained with diodes whoseinstantaneous field of view was0.12ø. In obtainingthese picturesthe cameraswere almost alwayssteppedby 0.04ø in azimuth
and elevation
so as to increase the number of discrete
readingson the objectand to increasethe probabilitythat one
POLLACK ET AL.: AEROSOLSIN THE MARTIAN ATMOSPHERE
VIKING I00
I10
Ls
I:•0
130
LANDER
I
140 150 16•0 17•018•0190 200 210 220 250
4481
Phobospicturesweretaken with all six color diodes.For each diode,pictureswere obtainedat severalelevationanglesso as to provideinformationon the wavelengthdependence of the optical depth. Results
0 AM
Figures 1 and 2 showthe variation of optical depth r with time at the Lander 1 and 2 sites, respectively.The bottom horizontal axis givestime in units of sol number, the number .8 of solar days on Mars from the time of landing. Sol 0 correspondsto the day of landingat eachsite.SinceViking 2 landed .4 a.• at its site45 daysafter Viking 1 landedat its site,the zeropoint for the bottom horizontal axis in Figure 2 has been displaced i i i i i i i 0 20i 40i 6; 80i I00 120 140 160 180 200 220 240 by 45 sols.Consequently,commonhorizontalpositionson the SOL NUMBER two figures correspondto the same seasonaldata. The seaFig. 1. Optical depthas a functionof time at the Viking Lander 1 site.Separatecurvesare shownfor resultsfrom (A.M.) morningsun sonalcoordinateLs is given alongthe upper horizontal axisof picturesand (P.M.) afternoonsun pictures.The bottom horizontal thesefigures.Two separatecurvesare givenin eachfigure.One axis shows time in terms of sols, the number of Martian solar days curve, labeled A.M., was derived from sun pictures taken from the time of landing(sol 0). The top horizontalaxisshowstime in during the morning, while the other curve, labeledP.M., was terms of the aerocentriclongitude of the sun Ls, which providesa found from afternoon sun pictures.Typically, A.M. sun pic1.6
r-i PM
1.2
measureof the seasonaldate on Mars. Ls had a value of zero at spring equinoxin the northern hemisphere.
tures were obtained 1-2 hours after sunrise, and P.M. sun
pictureswere otained 1-2 hoursbeforesunset.When several A.M. or P.M. sun pictureswere taken on the same day, we sample was taken when the object was almost on the optical averagedtogetherthe optical depth valuesfound from those axis of the camera. As is seenfrom the surface of Mars, the sun pictureshaving adequatesignal strengthto obtain the value subtendsan angle of about 0.3ø, while Phobos typically mea- graphedin thesefigures.Individual data points are shownby suresabout 0.1 ø across.In the caseof the sun pictureswe used circlesand squaresfor the A.M. and P.M. optical depth valthe nine brightest intensity samplesto derive the position of ues, respectively. the optical axis in the image plane. In so doing, we took One striking result shownby Figures 1 and 2 is the high accountof the camera smearingfunction and the known solar value of the optical depth at the two landing sitesthroughout limb darkening [Allen, 1955]. Using this information and the the periodcoveredby the observations. The lowestvalueof the derivedposition of the optical axis,we then found the intensity optical depth was 0.18. During the early portionsof the misreading that we would have gotten for a sample exactly cen- sion,r had an averagevalue of about0.5 and 0.3 at the Lander tered on the optical axis.Also a small correctionwas made to 1 and 2 sites,respectively.At both sites,r increasedsignififind this central intensity for a specifiedfixed size for the sun. cantly at later times.Thesefindingscanbe contrastedwith the The intensitiesso found had the effectsof discretesampling much lower values of optical depth derived in an indirect and varying solar size removed and therefore formed a com- fashionfrom Mariner 9 orbital imagingdata obtainedduring a mon data set from which the optical depth could be found. In seasonalperiod that overlapsthe beginningof the lander misthe caseof Phobosa similar procedurewas used.However, we sion, aswas discussedin the introductionto this paper. Almost employedthe photometric function of Phobos'ssurfacegiven all of the optical depth is due to extinctionby atmospheric by Nolandand Veverka [1977] and convertedthe derivedsur- aerosols.Using formulae given by Hansen and Travis [1974] face brightness to 0 ø phase angle by using their phase and a mean surfacepressureof 7.5 mbar at the two sites[Hess coefficient. Because the phase angle varies appreciably for et al., 1976a,b, c], we find that the molecularRayleighscatterpairs of Phobos pictures,the optical depthsfound from these ing optical depth is only about 1.5 X 10-2 at the effective pictureshave a larger uncertaintythan thosederivedfrom sun wavelengthof the sun diode. pictures. The optical depth curvesof Figures 1 and 2 exhibit three The following procedure was adopted for modeling the smearing function of the camera. We assumedthat a given VIKING LANDER 2 point in objectspacemappedinto a circleof uniform intensity 120 1.30 140 150 160 170 180 190 200 210 220 230 in image space.The instantaneouscontribution of that point to the observedsamplevalue is therefore proportional to the product of the photometric function evaluatedat that point and the area Of overlap betweenthe blur circle and the ino AM stantaneousfield of view. This product was integratedover the [] PM objectof interestand over the finite lengthof time of sampling, during which the image moves slightly and continuously in elevation [Huck et al., 1973]. The size of the blur circle was found empirically by obtaininga least squaresfit to the entire image of the sun. The sun pictureswere taken with a diode coveredwith the 2 40 60 80 I00 i 120 140 160 180 200 SOL NUMBER standard red filter, whose effectivewavelengthis about 0.67 #m [Huck et al., 1977]. Most of the Phobos pictures were Fig. 2. Optical depth as a functionof time at the Viking Lander 2 obtained with the blue diode so as to maximize the signal. Its site. The A.M. and P.M. curves and the horizontal axes have the same effective wavelength is 0.50 um. However, on one night, meaningas they do in Figure 1.
4482
LANDERIMAGING
(o)
SOl
(b)
24-25
SOL 28-29
o.õ o.õ
SUN PHOBOS
o SUN A PHOBOS
0.4
0.3
SUNSET
0.2. 02.-
SUNSET
0.1
181002d:00 22:00 I I I 04:00 24:36,0:00 021:00
18O0
Oi:00 0;:00
20:,00
22:00
24:36,0:.00
02 O0
04:00
06:00
08 O0
TIME (hours)
TIME (hours)
Fig. 3. (a) Variation of opticaldepthfrom the late afternoonof sol24 to the early morningof sol25 at the VL-2 site,as inferred from photographsof Phobos(triangles)and the sun (circles).The time coordinategiveshoursin terrestrialunits. (b) Similar data obtainedon sols28 and 29 at the VL-2 site.
different types of temporal changes:diurnal, random, and seasonal.Below we discusseach of thesetypesof variations. The diurnal variation is characterizedby the A.M. r values lying systematicallyabove the P.M. values.
the optical depth increaseoccurring between this time and sunrise.The slightlydifferenttemporal behaviorof the curves on sols'25 and 29 is most likely due to weather phenomena, i.e., fluctuationsin the temperatureand humidity characterAdditional characterization of this diurnal difference has isticsof the air massesover the landing site. beenachievedby taking photographsof Phobosor the sunat The declining phase is illustrated in Figures 4a and 4b, various times on the same day so as to determine when the which refer to a time period about 150 solslater than that for optical depth changesoccur. Figures 3a and 3b illustrate the Figure 3. The seasonis now the start of fall (LB= 190). Figures temporal behaviorof the optical depth over the portion of a 4a and 4b wereconstructedfrom opticaldepth resultsobtained day in which it is increasing.Thesediagramswere constructed from sun pictures at the Lander 2 site on sols 160 and 184, from photographsof Phobos obtained late at night at the respectively.We seethat most of the decreasein opticaldepth Lander 2 site on sols 25 and 29, respectively.In deriving takes place in the late morning and early afternoon. Whether optical depthsfrom individualPhobospictureson thesenights the optical depth increasesslightly or remains level in the we usedan Io value obtainedfrom Phobospicturestaken at an middle of the afternooncannot be establishedfrom Figure 4b earlier time of the night on sol 48, when no optical depth becauseof the low signalto noiseratio characterizingthe last changesshould have occurred (see (1)). Besidesthe optical two sun pictures. depth values given by the Phobos observations(triangles), We interpret the diurnal variationsin optical depth shown Figures 3a and 3b also show optical depth values in the late in Figures 1-4 as beingdue to a ground fog that forms during afternoon and early morning (circles), which were obtained the coldestportion of the day (the late night), survivesduring from the solar optical depth curvesgiven in Figure 2. The the early morning, and then dissipatesby the middle of the vertical arrows along the horizontal axis show the times of afternoon. Such a fog has been predicted on theoretical sunrise and sunset. These observations refer to midsummer in grounds [Flasar and Goody, 1976;Hess, 1976]. the northern hemisphere(LB = 110). In addition to consistenttrendsin the optical depth curves Accordingto Figures3a and 3b the increasein optical depth of Figures 1 and 2, there were also irregular variations. For commences around 0200 hours local lander time, almost all of example,betweensol 48 and sol 52 at the secondlandingsite, (a)
SOL 160
.9-
(b)
8130 9:00
IdO0
11100
li:00
,0.-00
SOL 184
II':00
TIME(hours)
'
13i:00
14':00
TIME(hours)
Fig. 4. (a) Variation of opticaldepthoverpart of the sunlightportionof sol 160at theVL-2 site.(b) Similarinformation on sol 184, also at the VL-2 site.
POLLACK ET AL.: AEROSOLS IN THE MARTIAN ATMOSPHERE
theP.M. opticaldepthincreased byabout0.2.Twosolslaterit had decreasedby about 0.1. On eachof these3 sols,several sunpicturesweretaken, and the opticaldepthsderivedfrom the individual pictures agree quite well with one another. Thereforewe believethesechangesto be real. In a crudesense we interpret thesechangesto be due to weather, i.e., the passageof differentair massespast the landingsites. Figures5a and 5b illustratea very dramatic-increase in the opticaldepthof the atmosphere that took placeat the second -landersite on sol 161 (LB = 205) within an interval of just 2 hours.Figure 5a showsa pictureof the surfaceobtainedat a local lander time of 1100 hours, while Figure 5b shows a
picturetaken at 1310hourson the sameday (0000 hoursis local midnight).The samegain and offsetsand the sametype of diode were usedfor both pictures.Thesephotographshave been reproducedso as to illustrateapproximatelytheir true
4483
relativebrightnesses. We seethat the surfacebrightnesswas much lower in the later picture.This effectcannotbe attributedto the photometricfunctionof the surface:otherlander imagesshowthe surfaceto backscatterincidentillumination
preferentially. Sincetheareacovered by thelaterpictureis much closer to the antisolar point than that in the earlier
picture,the photometricfunctionwould producean effect oppositeto the one observed.This inferenceof a true darkeningof thesceneis alsosupported by theobviouspresence of shadowsin the earlierpictureand a lack of notableshadowsin the later one. Both the changesin averagesceneillumination and the visibility of shadowscan be seenin a quantitative fashionfrom a comparisonof the histograms of scenebrightnessgivenin thegraphsnextto the upperright-handportions of the picture.
We interpretthe changesseenin Figure5 as beingdue to a
Fig. 5a. Photographof the surfaceof Mars at the VL-2 siteon sol 161at 1100hourslocallanderthne.
4484
LANDER IMAGING
contrast, no sudden increasein wind speedwas noted by the
large increasein the optical depth of the atmosphereover the 2-hour period separatingthe two picturesand further attribute this increaseto the passageof the polar hood over the second lander. Such an increasein optical depth would both decrease
meteorologyexperimentin this time period,so the occurrence of a local dust storm as the cause for the optical depth increase seemsunlikely (J. Tillman, private communication,
theamountof lightreaching thesurface andleadto theresult 1977). Additional evidencefor the occasionalpresenceof the polar hood above the second lander at this seasonis given by a sequenceof daily optical depth measurements initiated on sol 187. There were marked, erratic fluctuations in the optical depth from one day to the next, changesas large as +1
that a larger fraction of this light is skylight instead of the attentuated direct solar.beam.The latter characteristicimplies a more omnidirectional scene illuminance, which would reduce the contrast
of shadows and lead to less variance
in scene
brightness.The latitude and seasonof these observationsare consistentwith the suggestion that the opticaldepthincreaseis
occurringon a time scaleof I day to severaldays.By wayof
due to the passageoverheadof the polar hood, as is the time scaleof the change[Martin and McKinney, 1974]. By way of
•!
'e'2
222
.;4
.
:
:•
g*, I•:7')':.•"•,:":E :e-.4.
54 '• 2:48 2E•e'
'}
.:
contrast,no suchlarge fluctuationswere noted in a lessfrequent monitoringof the optical depth at the first landingsite.
44.5-
.
..
Fig. 5b. A photographsimilar to that in Figure 5a obtained at 1300hours on the same day. The two photographs (camera events21C198 and 21C200) have been developedto display approximatelytheir true relative brightnesses. The histogramin the upperright-handcornerof eachpictureshowsthe distributionof brightnessin eachpicture.For the exact location of the pictures, refer to Mutch [1977, Figure 1].
POLLACKET AL.: AEROSOLSIN THE MARTIAN ATMOSPHERE
The aboveobservationsare consistentwith the known properties of the polar hood, which were discussedat the beginning of this paper. The third type of temporal variability is a seasonalone. Toward the beginningof the mission,there was.sometendency for the optical depth values given in Figures 1 and 2 to decrease, while at later times, there was a marked increase in
optical depth.Thesetrendsoccurredat similar timesat the two landingsitesand so may characterizethe seasonalbehaviorof many equatorial and mid-latitude regions of the northern hemisphere. Someinsightinto the causefor the increasein optical depth from summer
to fall can be obtained
from
a correlation
of
lander and orbiter information. As is illustratedin Figure 1, a very dramatic increasein optical depth occurred at the first landing site betweenLs = 205 and 215. Photog/aphsobtained from the Viking orbiter show that a very violent dust storm beganin the Thaumasiaregion of the southernhemisphereat Ls = 205. By L• = 215 the entire southernhemispherewas coveredwith a thick dust cloud (G. Briggs,private communication, 1977). Consequently,the dramatic increasein optical depth observedat VL-1 in this time interval is most likely due to a spreadingof the dust cloudinto the northernhemisphere. In part, this dust storm may also be responsiblefor the increaseseenover VL-2 at a similar time (see Figure 2). However, the presenceof erratic fluctuationsin optical depth at VL-2 noted at a slightlylater time raisesthe possibilitythat the polar hood may have also contributed to the increasein optical depth at the VL-2 site shown in Figure 2. More generally, dust storm activity in the southernhemisphere may be the dominant factor behind the seasonalincreasein the optical depth observedduring the fall at both landing sites.Orbiter observationsshowthat local dust storms occurredin the southernhemisphereover this entire season (G. Briggs, private communication,1977). These, however, did not becomeglobal in scale,as did the Thaumasia storm described earlier. Their
cumulative
effect could be a slow rise
in the global dust content, consistentwith the lander results. Finally, we have obtained estimatesof the wavelengthdependenceof the optical depth by observingPhoboswith the various
color and infrared
diodes on sol 48 at the Lander
2
site. Theseresultsare shownby the trianglesin Figure 6, where
WAVELENGTH (•m)
4485
optical depth is givenas a functionof the effectivewavelength of the blue, green,red, and IR 1 (near infrared) diodes(the IR2 and IR3 diodeshave an effectivewavelengthsimilar to that of the IR1 diode). Also shown in this figure is the optical depth value derivedfrom sunpicturesobtainedin the late afternoon of the sameday that the Phobospictureswere taken (circle). Its effectivewavelengthis the sameasthat of the red diode.We see that the optical depth is approximatelyindependentof wavelength,as is illustratedby the fit of the horizontal line to the data points. PARTICLE PROPERTIES
Procedure
We have usedphotographsof the sky brightnessto obtain informationon severalparticleproperties.From the observed angular variation of sky brightnesscloseto the sun we obtained estimatesof the mean particle size. Similar data at largerdistancesfrom the sunprovidesomeinformationon the shapeof the particles.To carryout thisanalysis,we haveused a computerprogram that hasthe followingcharacteristics: the multiple-scatteringproblem was accuratelysolved,allowance for the possiblenonsphericityof the particleswas made in the derivation of their single-scatteringcharacteristics,and the radiativeinteractionbetweenthe atmosphereand the ground was taken into account. Below, each of these items is dis-
cussed.We then describehow the desiredparticleproperties were obtained from the observations.
.Except possiblyfor the specialgeometry of the twilight situation, which is discussedin a later section,it is clear from
the sizablevaluesof the opticaldepththat a single-scattering descriptionof the sky brightnessis inadequateand that allow-
anceshouldbe madefor multiplescattering. Given the singlescatteringpropertiesof the particles,as found in a manner.to be describedshortly,we solvedthe multiple-scattering problem for a planeparallelatmosphere in an accuratefashionby usinga computerprogrambasedon the doublingprinciple [Hansen,1969].Asidefrom the .single-scattering propertiesthe only other input informationneededto carry out this calculation was a specificationof the optical depth, which was obtained from the resultsof the previoussection.In performingthismultiple-scattering computationwe assumed that the single-scattering propertiesof the particleswere invariant with altitude. Suchan assumptionmay have limited validity duringthe morning,whenice particlesmay be locatedpreferentially closerto the ground than suspended soil particles. Note, however,that the skylightat the bottom of the atmospheredoesnot dependon the altitudeprofileof the aerosols' number density. To determinethe single-scattering characteristics of the particles,we useda recentlydevelopedalgorithmthat allowsfor the possiblenonsphericalshapeof the particles[J. B. Pollack and J. N. Cuzzi, manuscriptin preparation,1977;Cuzzi and Pollack,1977].This procedurewasbaseduponan analysisof laboratory measurements of the scatteringcharacteristics of irregularlyshaped,randomly orientedparticles.We briefly review the main featuresof this semiempiricalmodel. When
the sizeparametera is lessthan someupperboundALFO, irregularlyshaped,randomlyorientedparticleswerefoundto haveapproximately the samephasefunctionfor scattering as do spheresof equal volume and refractiveindex. Parametera
Fig. 6. Optical depthas a functionof wavelength,as derivedfrom observationsof Phobos at VL-2 on sol 48 at 2300 hours (triangles). Also shownis the opticaldepthfound from sunimagestaken 4• hours earlier (circle). The horizontal line exhibitsthe fit of a wavelength independentoptical depth to these results.
istheratioof a particle'scircumference, 2•rr,to thewavelength X. Thus Mie scatteringtheorycan be usedfor particlesin this size regime. Typically, A LFO rangesfrom values of 3 to 8, largervaluescharacterizing smootherparticles.
4486
LANI)ERIMAGING
The scatteringcharacteristics of larger-sized particles,i.e., a > ALFa, is divided into three components:a diffractedcomponent, an externally reflectedcomponent,and an internally refracted and transmittedcomponent.For randomly oriented particlesthe first two of thesewere equivalentto their counterparts for spheres.Physicalopticstheory was usedto calculate the diffracted component,and Fresnel'sreflectionlaws were employed to find the externally reflected part. Laboratory measurementssuggestedthat the third component,R(O), has the following simple behavior: In (R) • 0, where 0 is the scatteringangle.The slope of this linear behavior was deter-
This procedure automatically yielded the fraction of the observedskylight due to light first reflectedfrom the ground as well as the fraction of the observedground light due to diffuse skylight. We adapted the Hapke-Irvine photometric function to describethe scatteringpropertiesof the surface,in part because this law gave a good fit to the Mariner 9 observationsof Phobos[Nolandand Veverka,1977].This functionisgivenby
I = [(2B0cos/)/(cos i + cose)]f(a)
(2)
where I is the brightness;B0 is the geometricalbedo;and i, e,
minedby a parameter FTB whichwasequaltoj'09øR dO/•9o•8øRand a are the angle of incidence,angle of view, and phase dO.Typically, FTB is about 2. Finally, the scatteringportion of the interactioncrosssection,but not the absorptionpart, was enhancedby a factor SAR to allow for the larger surfaceto volume ratio of irregular particles as compared to spheres. Typically, SAR was about 1.3.
angle,respectively. Furthermore,f(a = 0) = 1, andlog(f) can be approximatedashavinga lineardependence on a. With f in
magnitude units the linear coefficient/5 has a value of about 0.02 magnitudes/degfor Phobos. We used this value as our initial value but found an improved estimateof 0.01 from the In our calculations we used nominal values of 6, 2, and 1.3 surfacepictures. for ALFa, FTB, and SAR. To studythe behavior of spherical By comparing calculated brightnesseswith the observed particles,we simply setALFa = co.For the real part of the brightnessesof the sky and ground we proceededin an iteraindex of refractionwe usedeither 1.5, a value representativeof tive fashionto determinethe value of a numberof the poorly soil material, or 1.31, the value of water ice at visible waveknown modelparameters.Theseparametersincludethe imagilengths. Usually, the first of thesewas used. The imaginary nary index of refraction nt, the cross-sectionweightedmean index was found by matching the absolute value of the sky particleradiusRM, the irregularparticle scatteringparameters brightnessgiven by the calculationswith the observedvalue. ALFa and FTB, the surface geometric albedo B0, and the Figure 7 shows a representativeexample of the fit of the surfacephase angle coefficient/5.To do this, we beganwith semiempiricaltheory (solid line) to laboratory measurements nominal choicesof theseparameters,adjustednt to match the (solid circles with error bars) for an ensembleof randomly absolutevalue of the sky brightnessat an angular distanceof oriented cubes having a values between 1.9 and 17.8. Also about 50ø from the sun, found RM by matchingthe variation shown for comparisonis the phasefunction found from Mie of sky brightnesscloseto the sun,determinedALFa and FTB theory for an ensembleof equal volume spheres.The vertical fromdataon theangularvariationof skybrightness at largediscoordinateis the phasefunctionp(O). tancesfrom the sun,estimated/5from the variation of ground Using the above semiempiricaltheory for scatteringby ir- brightness with angulardistancefrom the antisolarpoint,and regularly shapedparticles and the doubling method, we ob- finally derivedB0from theabsolutevalueof thegroundbrighttained the scatteringand transmissionbehavior of the atmo- ness.This cyclewas then repeateduntil convergence wasobsphereby itself. We then usedthe adding method to combine tained.In all our calculations we usedthe particlesizedistributheseresultswith the scatteringcharacteristicsof the surfaceto tion of Hansen and Hovenier [1974], since one of the find the total sky brightnessas viewedfrom the surfaceand the parametersof this distributionequalsRM. The otherparametop of the atmosphereas well as the total amount of reflected ter, b, which providesa measureof the width of the distribuground light [LacisandHansen,1974;Pollackand Toon, 1974]. tion, was set equal to 0.15, a value appropriatefor a wide PHASE
FUNCTION
FOR SCATTERING
distribution.Sensitivitystudiesperformedwith single-scattering calculationsindicatedthat whilethe skybrightness closeto the sundependedsensitivelyon RM, it was very insensitive to the choiceof b; i.e., in practice,only RM can be derivedfrom
BY CUBES
the observations.
log p(8)
SEMIEMPIRICAL
Here, asin subsequent sections,the raw digitalnumbersthat make up a givenpicturewere convertedto actual brightness values by using ground-basedcalibration files [Huck et al., 1975] togetherwith a correctionfor the wavelengthselective degradationof the diodescausedby the radioactivepower sourceon the landers[Patterson,1977].In all cases,brightness wasexpressed asthe ratio of the observedbrightnessto that of a perfectlyreflectingLambertsurface,normallyilluminatedby
THEORY
o
•'" "--• -• •""--MIE
sunlight at M ars's distancefrom the sun.
• ZERULLANDGIESEDATA II CUBES, 19• 18ø and z• > 10 km.
The basicinput parameters usedin the twilightanalysis
program include theopticalconstants, particlesizedistribu-
i
i
I
i
I
I
I
I
I
iii I2 I
I
O I 2 3 4 5 6 7 8 9 IO I 13 14 t• (deg) 19:0619:1019:1519:2019:2519:,•019:,519:4019:4619:511955 20:0120:0620:1120:16(hr) TIME
Fig. 13b. Shadow edgealtitudeasa functionof locallandertime
tion function,atmospheric opticaldepth,verticaldistribution andsolardepression angleb for anevening twilightonsol41at VL-I. of the particlenumberdensityn(z), and geometrical parame. Sunsetoccursat 1906 hours. Two curvesare showncorrespondingto ters, suchas e. The real part of the index of refractionwas two choicesof camera elevation angle e.
4492
LANDERIMAGING (a) .400
(b)
-
.100
0 DATA o DATA
I•
•
InH= 5.0km
• •
0 H=10.O km
0 H=IO. Okm /x H=15.0 km q- H=20.0 km
.3OO
r-I H = 5.0 km
A H=15.0 km + H=20.O km
.075
LiJ Ld Z i.-
•
Z
• .050
.2oo
.025
-
.100 - OBSERVED •
I
5
0
5
•0
•5
15
I
I
25
35
ZI (km)
Z• (km)
Fig. 14. (a) Skybrightness at twilightasa functionof shadowedgeheightz• for a fixedangleof scattering 0 of 20ø.The observations (diamonds) werederivedfroma twilightrescanpicturetakenduringthefirst25minof twilighton theevening of sol41 at VL-1 (cameraevent12B114).The observed valuesreferto resultsobtainedwith thereddiode.The othercurves in thisfigureshowthe resultsof theoretical calculations for modelshavingan exponential distributionof aerosolnumber density,withvaryingchoices of scaleheightH. Eachtheoretical curvehasbeennormalized to agreewiththeobserved value at z• = 6 km. (b) SameasFigure14aexceptthat theobservations commence shortlyaftertheendof thepictureusedin that figureandspanthenext25 minof twilight(cameraevent12B115).For theexactlocationof thetwotwilightpictures, refer to Mutch[1977,Figure1]. Eachtheoreticalcurvehasbeennormalizedto agreewith the observed valueat z• - 21 km. models having an exponential distribution function for the aerosolnumber density,as given by (3). The theoreticalcurves differ in the choiceof the aerosolscaleheightH. Each of them have been normalized to agreewith the observedbrightnessat a particular altitude: 6 km for Figure 14aand 21 km for Figure
ground fog, the polar hood, and suspendedsoil particles. Below
we summarize
the evidence
for each of these com-
ponentsand then discusssomeof their possibleimplications. Evidence for the presenceof a ground fog is given by the following data: Throughout the period of observations(Ls = 14b. 100-230) the optical depthis systematicallylargerin the mornConsidering first Figure 14b, we see that the shapeof the ing than in the afternoon (see Figures 1 and 2). During the observedcurve is matched almost exactly by the theoretical earlierportion of this period the diurnal optical depthincrease curve having H - 10 km. Noticeably poorer fits are given by commencesin the late night, at a time when the air temperthe theoretical curves having both larger and smaller scale ature closeto the ground is near its minimum value.Almost all heights. The model with H = 10 km also approximately the optical depth increaseoccursprior to sunrise.At a somematchesthe data shown in Figure 14a, which cover a lower what later seasonaldate, dissipationoccursprincipally in the altitude region than those in Figure 14b. Deviations of this late morning (seeFigures3 and 4). Such a temporal behavior theoretical curve from the observed values at the smallest z• is crudelyconsistentwith that expectedfor a fog on theoretical region of Figure 14a may be due in part to the increasing grounds [Flaser and Goody, 1976; Hess, 1976]. Furthermore, importance of multiple scatteringat thesealtitudes, as is dis- the diurnal variation in the red to blue brightnessof the sky cussedabove. While the data of Figure 14a provide a good showscolor changesconsistentwith this hypothesis(see Figure 12). discriminantfor the smallervaluesof H, they do not do so for Acceptingthe above identification,let us try to characterize the larger values of H. We concludethat the aerosolshave a the ground fog further. Accordingto Figure 3 the fog beginsto scale height of about 10 km between the surface and an altitude of 30 km. form at approximately0200 hours local lander time at VL-2, when L.• = 130. Measurementsmade from the Viking meteThe resultsof the above comparisonprovide indirect information on the compositionof the aerosolspresentnear sunset orology experimentindicatethat the air temperatureat 1.8 m for the summerseason.Aerosolsare presentfrom the ground above the surface equaled about 195øK at this time [Hess et to quite high altitudes.They exhibit a smoothvertical distribu- al., 1976b,c]. Since the fog is expectedto begin forming close tion with a scale height of about 10 km, which is comparable to the ground [Flaser and Goody, 1976;Hess, 1976], this value to the gas scaleheight; i.e., to first approximation the aerosol should be quite closeto the frost point temperature.Furthermore, it seemsreasonable to assume that condensation occurs mixing ratio is constant. Such a behavior is consistentwith when the relative humidity is not much above 100%, because that expectedfor suspendedsoil particles but differs from a layered structurethat might be expectedfor water ice clouds. of the presenceof small dust particlesthat can serveas nucleWe conclude that soil particles constitute almost all of the ation centers.Using the vapor pressurecurve of water vapor particlespresentin the afternoon during the summerseasonat above ice, the surface pressureat the VL-2 site [Hess et al., the VL-1 site. 1976c],and the perfect gas law, we find that the partial presDISCUSSION
Our analysisof the Viking landerimagingdata suggests that three types of aerosolswere presentover the landing sites:a
sureof watervaporP.•o, the densityof watervaporp.,o, and the watervapormixingratioa.•o equaledapproximately 7X 10-4 mbar, 8 X 10-•ø g/cm3, and 10-4, respectively,near the groundjust prior to condensation.
POLLACK ET AL.: AEROSOLSIN THE MARTIAN ATMOSPHERE
We next obtain
a crude estimate
of the amount
of water
vapor in a vertical column above VL-2. According to the temperatureprofile determinedduring the descentof the second lander the frost point is not reachedabovethe boundary layer until an altitude of about 30 km for a constantwater vapor mixing ratio of 10-4 [Seiffand Kirk, 1977]. If we therefore assumethe water vapor to be uniformly mixed at lower
altitudesand employthe aboveestimateof pH,o,we find that there are approximately 8 pr um in a vertical column. This value is in reasonable accord with numbers obtained from the
orbiter water vapor experiment[Farmer et al., 1977]. It should be notedthat our value pertainsto the amount of water vapor presentlate at night, at a time when somewater vapor present during the day may havebeen absorbedonto the top layersof the soil [Flaser and Goody, 1976]. Also the orbiter value may be somewhat ............. :----' owing to scatteringby the atmosp,,•r,• a•!as•u .... '-• :• aerosols,whoseoptical depth is nontrivial. We can estimatethe depth of the ice fog by assumingthat condensationtakes place only on the suspendedsoil particles. To do this, we first estimatethe number of soil particlesn close to the ground. This number is related to the dust particles' optical depth ra, mean size •a, and scaleheight Ha by (4)
where a value of twice the geometriccrosssectionhas been usedto allow for the diffractioncomponent.Accordingto our analysis,ra = 0.3, •a = 0.4 t•m, and Ha = l0 km. Hence n -• 30 particles/cma. If condensationoccursonly on dust particles,
the radiusof the resultingice-dustparticlesrt is relatedto and the fractionalamount of condensation f by
fP.,o = (4•r/3)(r? - •a3)np,½e
(5)
wherep•e is the densityof ice (0.92 g/cma). The temperature declinesby about 4øK from 0200 to 0430 hours, when sunrise occurs and the minimum temperature is reached. Since the saturationpressureat 291øK is about half its value at 295øK,
4493
the frostpoint of watervapor for an assumeduniformmixing ratio of 10-4 [Pollack et al., 1976]. Such a mixing ratio approximatelyequalsthat inferredfrom our observations of the ground fog and also leadsto column abundances of water vaporcomparableto the averageMartian value[Farmeret al., 1977]. In our discussionabove of the ice fog conditionsduring the
summerwe pointedout that in realitythe frost point temperature abovethe boundarylayer was not reacheduntil a surprisinglyhigh altitude of 30 km for a uniform water vapor mixing ratio of 10-4. This conclusionwas basedon temperatureprofilesobserved duringthe descentof the Viking landers. It differs from theoretical profiles for dust free atmospheres in beingmoresubadiabatic in thetroposphere thanthe theoretical ones. This difference could be attributed
to the
•,• •'•.... d soil },......... whichabsorbsomeof the •"•"' of the ••usp• incident sunlight and so stabilize the temperature structure [Gieraschand Goody,1972].Thus the presenceof substantial
quantitiesof suspended soilparticlesin thewarmerregionsof Mars maymarkedlyinhibittheformationof watericeclouds. Finally, we consider the suspendedsoil particles, which dominate the diurnally constantaerosolcomponentduring the summer seasonand make a significantcontribution to this componentin the fall. Evidencefor the presenceof this material is given by the absorptioncharacteristics of the aerosols and their vertical distribution, as inferred from an analysisof imagesobtained during the summerseasonin the afternoon. We found that the aerosols'imaginary indicesof refraction were quite substantialfor all six diodes(see Table 1). Such valuesare consistentwith the imaginary indicesthat might be expectedfrom soil particles,but ice particlesare much more transparentat visible wavelengths.At the times under discussion the aerosols were found to have a smooth vertical
structure, whose scale height was comparable to the atmosphericscaleheight.Aerosolswere detectedup to altitudesof at least 30 km. Such a behavior
is consistent with that of
suspendedsoil particlesbut is incompatiblewith that of water thisvalueof f andvaluesgivenearlierforp.•o, •a,n, andt>•c•, ice particles,which shouldhave a more layeredstructure. We now discusspossible sourcesand sinks for the suswe find that rt is about 1.5 um. Finally, we can use the pended soil particles.As we mentionedin the introduction, observedenhancementin optical depthbetweenmorningand Mariner 9 observationsof the decayphaseof the 1971-1972 afternoon,/Xr, to estimatethe depth of the ice fog h. Thesetwo global dust storm showedthat the optical depth declinedfrom variablesare related by 1.5 to 0.2 during the first 3 months of observations.The Ar = 2•'r?nh (6) seasonaldate at the end of this period was about 200 days Setting Ar = 0.15 from Figure 3 and employingour estimates earlier than the seasonaldate at the start of the Viking mission. Furthermore,the optical depth in the afternoon was typically of r• and n, we find that h -• 0.4 km. We next considerthe polar hood, which was occasionally 0.2-0.4 during the summerseasonat the Viking sites.Therepresentat VL-2 during the fall season.Evidencefor its pres- fore we considerit highly unlikely that the soil particlespresenceat VL-2 is given by large erratic fluctuationsin optical ent over the landing siteswere remnantsfrom the last global depth that occurred on a time scale of a day as well as a duststorm.It•is more likely that they weregeneratedmore dramatic darkening of the surfacethat happenedin just a 2- recentlyfrom a number of local dust storms.Evidencefor the hour time interval on sol 161 (see Figure 5). No such large existenceof suchstormsis givenboth by directobservationsof random variations were observed at VL-1. These observations them on Mariner 9 and Viking orbiter images [Leovy et al., are consistentwith earth-based studiesof the seasonal,posi- 1972, 1973b;G. Briggs,private communication,1977] and by tional, and temporalcharacteristics of the polar hood [Martin Mariner 9 photographsof numerouslocalizedsurfacealbedo changes[Saganet al., 1972]. and McKinney, 1974]. It hasbeen thought thatthesoilparticles areeliminated by The polar hood is thoughtto be an ice condensation cloud, the ice phasebeingeitherCO•.,H•.O, or a mixtureof thesetwo vertical eddy mixing, which caused them eventually to be materials[Leovy et al., 1972].Thus the lander data gave evi- depositedas a more or lessuniform blanket acrossthe surface denceof the presenceof upper level ice clouds(in contrastto of Mars. While we do not disputethat someeliminationoccurs the groundfog) only duringthe fall seasonaboveVL-2. That in this manner, we wish to suggesta secondmechanismthat water ice cloudsare not more ubiquitousis at first surprising; may alsobe important. We proposethat somesoil particlesare theoretical calculations indicate that for almost all seasons and removed from the atmosphereas a result of their servingas latitudesthe temperatureof the uppertroposphereliesbelow nucleation centers for the formation of carbon dioxide ice
the frost point temperature,f approximatelyequals•. Using
4494
LANDER IMAGING
particlesin the polar regionof the winter hemisphere.As we showbelow, suchparticlesare muchlargerthan the soil particles themselves and are therefore
more able to sediment
gravitationally out of the atmosphere.Prior to the condensationof the carbondioxide,water ice probablyformson the soil particles.But the increasein particle size from the water condensationis probablynot greatenoughfor the combined particleto fall rapidly out of the atmosphere.Thus the formation of carbon dioxide ice particleson existingatmosphericaerosolsmay serveto eliminate both water and dust from the atmosphere,both materialsgenerallybeingpresentin such condensation
centers.
To explore the above hypothesis,we first estimatethe increasein the sizeof the dust particlesdue to ice formation and then relate thesesizesto fallout velocities.Calculationsgiven earlier in this sectionshow that water vapor condensationon the soil particlestypically resultsin growth to a particle radius of about 2 #m. While thesecalculationsreferredspecificallyto the ice particlesin the ground fog, the sizeso found is a good approximationto a numberof other situationsinvolvingwater ice formation: The final radius given by (5) dependson the cube root of the amount of available water vapor; (5) can also be used to estimatethe size of carbon dioxide particles that form on water ice-dustparticles.We simplyreplacef and
part, of the meridional transportationof dust particlesto the winter polar region,whereCO•.condensationeffectivelyleads to their removal from the atmosphere.This mechanismis consistentwith the puzzlinginferencethat the sizedistribution of the dustparticlesdid not changeduring the decayphaseof the global dust storm of 1971-1972, as was mentionedin the introduction.If CO•. condensationis the chief agentby which dustparticlesare removedfrom the atmosphere,then the time scalefor the decayof a global duststorm is determinedby the time scalefor atmosphericmeridional motions. Such a time scale would be independentof the size of individual dust particles.Finally, our mechanismmay have relevancefor the formation of the polar laminae and debris mantle, as is dis-
cussedin detail by J. B. Pollack(manuscriptin preparation, 1977). CONCLUSIONS
In this sectionwe summarizeour major results.Three types of particleswere presentabove the landing sitesduring the periodfrom L8 - 100 to 230: a water icegroundfog, a higherlevel ice cloud (the polar hood), and suspendedsoil particles. Below we describethe properties of each of theseclassesof aerosolsas derived from the lander images.We also present someof the implicationsof theseresults,whichwere discussed o.:o by theirCO•.counterparts anduseearlierestimates of the in the precedingsection. A groundfog composedof water iceparticleswaspresentat other quantitiesappearingin this equation.Accordingto the general circulation calculation of Pollack et al. [1976], atmo- both landingsitesthroughoutthe entireperiod of the observaspherictemperaturesreachthe frost point of CO•.in the inner tions. Observationstaken near the beginningof the missionat portion (>•60ø latitude) of the seasonalcap duringwinter. The VL-2 indicatethat the fog beginsto form at about 0200 hours rate of condensationof CO•. in this zone is about } g/cm•'/d. If and that most of its growth takes place prior to sunrise. we assumethat all the condensationoccursin the atmosphere Observations taken towards the end of this period at VL-2 and that the scaleheight of the condensedCO•. is comparable imply that mostof the dissipationof the fog takesplacein the to an atmosphericscale height, then the rate of CO•. con- late morning.During the summerseasonwe estimatethat the densationis approximately 3 X 10-? g/cm3/d. Finally, if we frost point temperatureis about 195øK and that the water assumethat freshair is broughtinto thisregionevery10 days vapor mixing ratio closeto the ground at night equalsabout [Pollacket al., 1976],thenfp½o•• 3 X 10-• g/cm3.Usingthis 1 X 10-4 prior to fog formation.Assumingthat condensation value in (5), we find that the radius of the resultingCO•. ice occursonly on the suspendedsoil particles,we find that the particlesequalsabout 25 #m. Thus the CO•. ice particlesare averageradiusof the fog particlesis about 2 #m and the depth much larger than either the original soil particlesor the com- of the fog is about 0.4 km. A higher-levelice cloud is not prominentduringthe summer bined water ice-soil particles. We next estimatethe fallout velocitiesof the various par- season,but it is occasionallypresent during the fall season ticles of interest. According to the calculationsof Arvidson above the VL-2 site. We speculatethat the absorptionof [1972] a CO•. ice particle having the radius estimated above sunlightby the suspendedsoil particleswarms the middle and would have a terminal velocity of severaltens of centimeters upper parts of the troposphereand so inhibitswater ice cloud per second.Consequently,it would fall quickly to the ground. formation in the summer.The opacity of the ice cloud above By way of contrast,the smaller-sizeddustparticlesand water VL-2 showsmarked temporal variations associatedperhaps ice-dust particles have terminal velocitiesthat are about 2 with weather disturbances. Suspendedsoil particleswere presentthroughoutthe misorders of magnitude smaller. The above calculations show that CO•. condensation can sion. They constituted the dominant source of the noneffectivelyremove dust and water ice particlesfrom the atmo- diurnally varying component of the atmosphericopacity. weightedaverageradiusof sphere,and as a result, dust and water ice may be preferen- Theseparticleshavea cross-section tially depositedin the winter polar regions.This inferenceis 0.4 #m, are nonsphericalbut equidimensionalin shape,and consistentwith resultsfrom Viking orbiter photographstaken have rough surfaces.They are distributedin a smoothmanner early in the mission,which showthe atmospherein the south- from the surface to altitudes in excess of 30 km. This distribuern hemisphereto be much more transparentthan the atmo- tion can be characterizedby an exponential dependenceon spherein the other hemisphere[Carr et al., 1976].At this time, altitude, with a scale height of 10 kin. This value is quite carbon dioxide was condensingin the southernpolar regions. comparable to the gas scale height. The principal opaque Our hypothesisalso providesan explanationfor why the sea- mineral in the soil particlesis magnetite,whoseabundanceby sonalCO•.icecap and the permanentH•.O icecap havealbedos volume is 10% + 5%. In addition to beingeliminatedfrom the significantlylessthan unity [Briggs,1974;Kieffer et al., 1976]: atmosphereby eddy mixing processesin all regionsof the Both ice depositscontain dust particlesintimately mixed with planet, the soil particlesmay alsobe eliminated,in association the ice becauseice condensationin the atmosphereoccurs with water ice, through their actingas condensationnucleifor on suspendeddust particles.We also wish to point out that CO•. ice in the winter polar atmosphericregion.This mechathe clearing of global dust storms might be the result, in nism impliesa preferentialdepositionof dustand water ice in
POLLACK ET AL.: AEROSOLS IN THE MARTIAN ATMOSPHERE
the polar regionsand may provide an explanationfor the low albedo of the CO•. and H•.O surfaceice depositsas well as for the relative clarity of the winter hemisphere.In addition, it may play a role in the formation of the polar laminae and subpolardebris mantles.
4495
J. E. Tillman, Early meteorologicalresultsfrom the Viking 2 lander, Science, 194, 1352, 1976c.
Huck, F. O., S. F. Katzberg, D. J. Jobson,and C. L. Fales, Jr., An analysisof the facsimilecameraresponseto radiant point sources, NASA Tech. Note, D 7389, 1973.
Huck, F. O., E. E. Burcher,E. J. Taylor, and S. D. Wall, Radiometric performanceof the Viking Mars landercameras,NASA Tech.Rep.,
TMX-72692, 1975. Acknowledgments. We are very grateful to Richard Goody for suggestingto us that the lander camerascould be usedto look for a Huck, F. O., R. E. Arvidson, D. J. Jobson, S. K. Park, W. R. Patterson,and S. D. Wall, Spectrophotometricand color estimates ground fog and for encouragingthe participationof one of us(R.K.), of the Viking lander sites,J. Geophys.Res., 82, this issue,1977. to many other membersof the Lander Imaging Team and of the Jet PropulsionLaboratory for all their work in obtainingand processing Huffman, D. K., and J. L. Stapp, Optical measurementson solidsof possibleinterstellarimportance,in InterstellarDust and Related the imagesusedin this paper, and to Kenneth Bilskyfor his help in the Topics,edited by M. Greenbergand H. C. van de Hulst, p. 297, data reduction and analyses.The work of one of us (R.K.) was D. Reidel, Hingham, Mass., 1973. supportedin part by NASA grant NGL-22-007-228. Huguenin, R. L., The formation of geothite and hydrated clay minerals on Mars, J. Geophys.Res., 79, 3895, 1974. REFERENCES Irvine, W. M., and J. B. Pollack, Infrared optical propertiesof water Allen, C. W., AstrophysicalQuantities,p. 169,Athlone, London, 1955. and ice spheres,Icarus, 8, 324, 1968. Arvidson, R. E., Aeolian processeson Mars: Erosivevelocities,settl- Kieffer, H. H., S.C. Chase, Jr., T. Z. Martin, E. D. Miner, and F. D. ing velocities,and yellow clouds, Geol. Soc. Amer. Bull., 83, 1503, Palluconi, Martian north pole summertemperatures:Dirty water 1972.
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(ReceivedApril 12, 1977; revisedJune 16, 1977; acceptedJune 16, 1977.)
