The Surface of Mars 4. South Polar

VOL. 76, NO. 2 JOURNAL OF GEOPHYSICAL RESEARCH JANUARY 10, 1971

The Surface of Mars 4. SouthPolar Cap

ROBEaTP. SHARP,BRucE C. MunnA¾, ROBEaTB. LAUrEnCE A. SODE•BLOM •D J•MES A. Cu•s

Division of GeologicalSciences and Division of Physics,Mathematics and Astronomy California Institute of Technology,Pasadena 91109

The south polar cap of Mars occupiesa region of cratered terrain. Immediately outside the shrinking cap craters appear no more modified than those in areas farther north that are not annually frost covered. Craters showing through the frost mantle are locally as abundant, as elsewhereon Mars. Only in a central region close to the pole are craters sparse. Both far- and near-encounter views reveal a highly irregular pole-cap edge. Photos of the same sector taken six days apart are near duplicates,suggesting that the irregularity is primarily ground controlled. No evidence of the classicalpolar collar is seen. Within the marginal zone, frost is preserved largely in crater bottoms and on slopes inclined away from the sun. Preferential retention in low spots supports the earlier suggestionthat the Mountains of Mitchel may actually be depressions.An argument based on insolation as the prime factor in frost wastage and the narrow width of the marginal zone suggeststhat slopesof topographicfeatures therein are mostly gentle, on the order of a few degrees.The frost cover of the pole-capinterior may range widely in thickness,obscuring parts of some craters and seeminglyenhancing topographic visibility elsewhere, possiblythrough variations in thicknessand reflectivity. Unusually bright areas on the cap surface, and differences in luminance between bright rims and the more somber floors of craters and other depressions,may be due in large part to differences in related frost textures and to the local history of evaporation and sublimation. Irregularly angular depressionswithin the pole- cap frost termed 'etch pits' may be the product of differential ablation or the undermining by wind of a slabby surficial crust. Encircling the south pole is a region of subdued relief with a paucity of craters, which displays enigmatic quasi-linear markings believed to be ground features. Although no satisfactory explanation of these markings has been formulated, it seemslikely that this region has been occupiedrepeatedly by perennial massesof CO2 ice, formed and maintained during those phasesof the martian precessionalcycle that resulted in short cool summersin the southern hemisphere. Such ice massesmay play a role in producing the unusual features of the central polar region. Physical relationships suggest a local maximum frost thickness as great as tens of meters. The possibility should be kept in mind that remnants of perennial CO2 ice of still greater thickness may exist locally, for example, in the 'etch pit' area.

The south polar cap of Mars was first seenby relationships of the polar cap as revealed in man more than 300 years ago, and its annual these photos are described.Implications of these waxing and waning phases have been observed observationswith respectto the physical nature, for over 200 years by earth-based optical astron- distribution, thickness, and behavior of the omers [Slipher, 1962]. In 1969 Mariners 6 and 7, pole-cap frost are examined. The origin of through numerous far-encounter photographs features seemingly unique to the cap is explored. (Figures 1 and 2), and Mariner 7, through ten The near-encounterphotomosaic (Figure 3) is near-encounter frames (Figure 3), gave the one of the more informative and striking products closestand most satisfyingviews of this Martian of the Mariner 1969 TV experiment. This feature. near-encounterpolar cap photography involved The characteristic physical features and an in-flight reprogramming of the on-board computer directing a slue of the instrument • Contribution 1893, Division of Geological platform to afford the sciencesystems a view of Sciences, California Institute of Technology, Pasadena 91109. the polar cap. Locations of features or areas on polar-region Copyright • 1971 by the American Geophysical lfnion. photos are more effectively expressedin terms 357 358 of right or left than east or west. Therefore over areas of various shapesand sizes up to readers referring to individual photos in this 300 km across.Individual features appear at section should make sure that the orientation fixed positions and with correspondingsize is consistent with that of the corresponding and shape on successivefar-encounter frames photoin the mosaics(Figures 3 and 4). (Figure 1) taken about 60 rain apart. These featurespresumably represent real differencesin FAR-ENcOUNTERVIEWS reflectivityon the cap's surface or a differencein Far-encounter frames show parts of the the atmosphereover it. pole-capedge and interior not coveredby near- encounterphotos, aswell as the regional configu- NEAR-ENcouNTERV•;ws rations of the pole-capmargin, and they also Near-encounterviews of the polar cap were record possibletemporal variations in frost takenfrom spacecraft positions north of 45%, so cover, associatedatmospheric phenomena, or that obliquityis high (400-48ø from vertical), photometricfunctions. and slant distanceswere large (4925-6340 kin). Far-encounter pictures show considerable irregularityinthe pole-cap edge on a 50-200km Featureso[the Marginal Zone scale.The relief of underlyingcratered terrain is Treatment of the pole-capmargin is facilitated clearly one cause,and other topographicrelief by division into three parts: the extramarginal is inferredto pla•ya part. Cratersexceeding 100 area, the outer marginalzone, and the inner km in diameter are visible just within the pole- marginal zone. These areas are transitory, cap margin on frames taken from distancesin moving inward with the shrinking cap. Along excessof 500,000km (6F36, Figure 1). the 330ø longitudeline (Figure 4), the extra- All far-encounterviews of the polar cap show marginalarea extendssouth to 57%, the outer fuzzinessand lack of definition toward the marginalzone lies between57 ø and 58.5ø , and morningterminator on the west; comparethe the inner marginalzone is between58.5 ø and eastand westedges of the planetarydisc (Figures 60.5ø . 1 and 2). A localizedmorning haze might account Extramarginal area. This area comprises for this fuzziness and for the limb darkening the bare ground immediately beyond the outer over the polar cap appearingprincipally on its limit of recognizablefrost patchesat the time of morning side. However, Leow et al. [1971] viewing (earth date, August 5, 1969) but lying suggestthat the limb darkening is due to a within the region covered each year by the photometricfunction of the planetary surface extendedpolar cap. Most of the extramarginal and not to a generalatmospheric haze. area displays craters (7Nll, 7N13, Figure 3) Comparison of successiveviews (Figure 1) seeminglycomparable in abundanceand charac- suggeststhat changesin the size, shape, and ter to other well-cratered regions on Mars. density of irregular, wispy projectionsfrom the However,the westernthird of 7Nll seemsdevoid pole-cap edge may occur from hour to hour. of craters.Whether this area is truly featureless, However,photos taken 1 rain apart .(Figure 2) as suggestedby B frame 7N10, or is simply show differences about as great, and pairs of obscuredby local clouds or haze is not known. views of correspondingsectors of the pole-cap The possibilityof a localizedhaze is supported margin taken six days apart show no greater by a light patch at this location on a true- changes (Figure 2). This suggeststhat the intensity version of 7Nll (Figure 4), although seeming variations may be the product of Leow et al. [1971] suggestthat this lightness differencesin lighting, viewing angles, and could be the product of a surfacephotometric instrumental response.Indeed, six-day photo function sensitive to the solar incident angle. l•airs show a consistency in pole-cap edge The remainder of the extramarginal zone configuration that hardly seems compatible visible on 7Nll and 7N13 (Figure 3) exhibits with anything as ephemeral as clouds or mist, craters seemingly no different from craters unless they are strongly controlled by ground farther north along the Mariner 7 path outside features. the area of annual pole-cap coverage (7N9, The interior of the cap displays an irregular 7N7). No unusual topographic forms are seen mottling involving modest differencesin albedo in the extramarginal zone. SURFACE OF •IARS--SOUTH POLAR CAP 359

Fig. 1. Mariner 6 far-encounterviews of south polar cap, enlarged to a common scale. Each successiveframe representsan elapsedtime ranging between 56 and 65 minutes and a plantetary rotation between 14ø and 16ø. Crater at A in 6F37 about 110 km across.Small dark spot,s are geometricreference marks on vidicon tube. A, B, and C are simply marginal irregularities selected as referencepoints. Approximate longitudes: A, 209øE; B, 240øE; C, 133øE. 360 SHARP ET AL.

Outer marginal zone. This zone is character- The outer marginal zone displays some ized by preservation of disconnected frost scattered, irregular, dense, white patches (see patches. In maximum discriminable photo right center, 7Nll), suggestive of optically versions (Figure 3) it lies within a spurious thick clouds. However, these patches do not dark band produced by an automatic gain display shadowsor have the outlinesor structures control (AGC) in the spacecraft photographic normally expected of uplifted or convective system. As the scan trace in the TV system clouds [Leovy et al., 1971]. The patches may passedoff the cap, slow responseof the AGC represent unusual accumulations of frost associa- to the great contrastproduced a black band in ted with irregular topographic features. the photos.This is an artifact of the systemand Inner marginal zone. In this zone the frost not the classical polar collar. Photo versions cover, although irregular and ragged at its outer from which the AGC effect is removed (Figure 4) edge, is essentially continuousexcept for islands show no suggestionof a polar collar at the time of frost-free or nearly frost-free ground. These of Mariner 7 flyby but are not absolute proof islands register nearly black becauseof a three- that such a collar is lacking. fold contrast enhancement in the TV system. Within the outer marginal zone, frost is Craters seem more abundant than in most of preservedin crater bottoms, in other presumed the polar cap area because of accentuation by low spots,and on slopesinclined away from the the irregularly distributed frost. Many appear sun. Craterswith a completefrost coveron their as dark-line ellipses,owing to obliquity of view, floor appear as solid elliptical white spots,others or as solid dark crescents concave north. Both with only fractional floor coverings appear as forms stand out against the prevailing white irregular white areas, and some with frost background. Some craters display a black dot only on south-facinginner walls appear as white within a central white area that could be a bare crescents concave southward. Craters on B spot atop a central peak. frame 7N12 within this zone are grotesquely distorted by the combination of low lighting (37 ø) and the enhancement of localized frost. Polar-Cap Interior Ground relief is also emphasized, making the In terms of area, this is the largest pole-cap surface appear rougher than elsewhereon Mars. subdivision. It has an essentially continuous

7F 72.

7F75

6F41 6F 40

Fig. 2. Comparative far-encounter views of soulh polar cap. 7F72 and 7F75 each taken 6 days after 6F34 and 6F36, but 6F41 and 6F40 taken only one minute apart. Central longitudes: 7F72, 244øE; 7F75, 210øE; 6F41, 146øE. Mariner 7 camera system yielded sharper contrast. SURFACEOF MARS--SOUTH POLAR CAP 361

Fig. 3. Mosaicof maximumdiscriminable near-encounter photos of southpolar cap. Location of B framesindicated by black insetrectangles; sun elevation above horizon at lower right corner of B-frames. Latitude-longitude grid given on Figure 4. Location of mosaic shown on far-encounter inset. 362 SHARP ET AL.

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Fig. 4. True-intensity mosaic, without automatic gain control, south polar cap. Compare Figure 3. SVRF^CEOF M^Rs--SovTH PoL^• C^P 363

frost cover, an abundance of craters, and a best shownin the centerof 7N15. Like craters, variety of other topographicfeatures. etch pits have lighter rims and darker floors, Most craters of the polar-cap interior display but unlike craters they are irregularly angular bright rims and somberfloors. The floorsare not in outline. Vertical relief is not yet known, but frost free, for they do not match the blackness more quantitative evaluation of slopes and of bare ground at the pole-cap margin. The heights may be possible with advanced data intercrater surface is lighter than crater floors processing.The 'elephant's footprint' (lower but not as bright as crater rims. right of 7N14), judgingfrom its low relief, may Outlines of larger craters appear irregular be a frost-filled crater with shallow etch pits. and ragged compared to unfrosted craters. South of the principal area of etch features is Extending outward from a few medium-sized a seriesof short, parallel linear featuresaligned craters (see upper center 7N19) is a radial in • WNW direction, herein termed beaded pattern of short ridgesand furrows.Some craters lineations from the tiny nodes they connect show small white central dots. (Figure 5). Less regular lineations of several Craters larger than 15 km appear to be fiat trends are seen in other parts of the pole-cap floored, but many smaller craters are bowl interior. The pole-capinterior displaysa number shaped. A number, even with fiat floors and of sm•ll subduedfeatures of positive relief (see diameters of 30-50 km, have distinct rims. A 7N14 for examples). p•rt of some craters is obscured,although the A frame 7N19 shows several noteworthy remainder is clearly visible, a phenomenonnot features. In the upper center part at about common to craters elsewhere on M•rs. In the 72øS, 55øE (also in upper right corner of 7N17 upper right corner of 7N19, craters are as and Figure 5) is a crater,roughly 15 km across, numerous•s in the most heavily cr•tered Martian nearly surroundedby an irregularly outlined areas. feature of positive relief that looks like a pile The pole-cap interior displays features not of volcanic flows extruded from a central vent. observed elsewhere. Most unusual are irregular However, the crater is larger than most ter- depressionshere termed 'etch pits' for descriptive restrial centralvents, and the surroundingrelief purposes.Elongated forms of similar character might be the productof etching.Roughly 250 km are termed 'etch furrows.' 'Etch' features are to the west is • belt featuring • number of short

-*--644 km o 0 Zone o

Fig. 5. Location sketch of regionsand representativefeatures appearingon 7N17. 364 SH^RP ET irregular ridges bearing roughly northeast. In within the central polar region, and the instep the upper left corner of the frame, an area of of the 'footprint' is the trace of the boundary irregular hummocky terrain looks much like a described above. Other craters scalloping this lunar ejecta sheet, although it cannot be identi- boundary are also strongly expressed. fied with any particular crater. Similar terrain The most unusual central polar feature is an is seen on a smaller scale around some of the assemblageof quasi-linear markings, shown best larger craters of the pole-cap interior. It seems in the south central part of 7N17. These are close likely that a frost cover of irregular thickness to and nearly centered on the pole (Figure 5). and differences in texture here accentuates, to Individual markings have lengths up to 300 km the point of visibility, minor topographicfeatures and widths of 10 km. They spread out eastward probably present elsewhereon Mars, but hard to and convergewestward with considerablecurva- see. ture. The outermost has a sharp buckle at about Several parts of the polar cap interior show 85øS, 30øE. Some en echelon segmentationand irregular areas of unusual luminance, 20 to a more confusedpattern appear at the west end 120 km across. These might be optically thick of the assemblage.Faint traces of theselinearions clouds, but they lack shadowsand proper shape swinging around the opposite side of the pole and structures. Leovy et al. [1971] are not con- can be seen on some photo versions, and the vinced that they are clouds, so other explana- pattern extends far eastward into 7N19 for a tions are sought. One might normally infer total east-west length of 900 kin. that the bright spots are higher areas favorably inclined to catch the sun. However, some lack INTERPRETATIONS,IMPLICATIONS, AND SPECULA- a darker side away from the sun, and they, at TIONS least, are not high standing. Some white masses are streaky, and a highly irregular band of this Differential Preservationof Frost, Marginal Zone. type extends through the upper right part of Lingering of frost in crater bottoms in the 7N17 and the central left part of 7N19. This is outer marginal zone may be the result of protec- not far from locations commonly cited for the tion from solar radiation and from winds and Mountains of Mitchel (Figure 5), although of a possibly greater initial thickness owing to geometrically the band does not match their larger vapor supply and wind drifting. Whatever shape. the causes,preservation of frost in low areas is consistent with the analysis of Sagan and Central Polar Region Pollacl• [1966, 1968] and the suggestion of Near the south pole is a distinctive sector O'Leary and Rea [1967] that the Mountains (Figure 5). It has a paucity of craters, an assem- of Mitchel may really be the 'depressionsof blage of unusual quasi-parallel,linear features, Mitchel.' Frames 7N17 and 7N19 show several and a locally well-defined border lying between large craters, some seemingly irregular terrain, 78 ø to 80øS. This province appears to encircle and a highly irregular bright band in the area the pole, but Mariner 7 near-encounter frames cited as harboring the Mountains of Mitchel at show clearly only the 165ø sector across the about 73øS, between longitudes 67ø and 93øE central meridian between 320 ø and 125øE. [Green, 1879; Slipher, 1962]. Locations are not The central polar region appears largely on accurate enough to support a direct correlation 7N17 and 7N19. Its boundary in the central and to any specificfeatures, and the cap is still too eastern part of 7N19 is an abrupt, irregular, far advanced to allow the Mitchel features to crater-scallopedarc of perceptible vertical relief. appear as separated forms. Insofar as frost Westward, the contact, less well defined, is preservation is a function of vapor-pressure drawn primarily on a contrast in crater abund- relationships [Sagan and Pollacid, 1966], it is ance (Figure 5). consistent with the conclusion, now strongly Only a few craters are seen in the central supported by infrared radiometer [Neugebauer polar region, and most of them are indistinct. A et al., 1969] and infrared spectrometer[Herr and striking exception is the southern, or forefoot, Pimentel, 1969] observations, that the Martian crater of the pair comprising the 'giant's foot- polar caps are composed primarily of COs. step' (7N19, 7N20). The forefoot crater lies In the inner marginal zone, crater rims and SURFACE OF MARS--SOUTH: POLAR CAP 365

other high-standing features range from thinly cover is not consistent with the preservation of frosted to completely frost free. Frost on these solid frost blankets on crater floors in the mar- features may have been initially thin for the ginal zones.A likely alternative is that the frost converse of reasons suggested for thick frost in on depressionfloors has a lower reflectivity than crater bottoms. Exposure to solar radiation and frost on crater rims or on intercrater surfaces. possibly some wind erosion have evidently Frost in depressionsin the waning phase of the further reduced or removed it. Evaporation of pole-cap cycle may experiencea history of sub- frost here appears to be strongly affected by limation and evaporation that gives it a different surface inclination, for surfacesinclined toward texture. Adiabatic compressionand expansion of the sun become frost free, whereas fiat surfaces atmospheric gas moving over features of topo- nearby remain frost covered. This is shown by graphic relief may play a role in this textural the geometrical form of bare-ground patches differentiation. that are parts of craters. Parts of high-standing The brightnessof high-standing areas may be features are also likely to become frost free due to greater insolation on sun-facing slopes, because they have sun-facing slopes. Thus the possibly abetted by high-reflecting textures. small dark spots near the centers of some frost- White dots seen in the centers of a number of floored craters in the marginal zone may be medium-size interior pole-cap craters may thus central peaks,not just aberrant bare spotson the representcentral peaks. Alternatively, they may crater floors. represent frost accumulationsof high reflectiv- These relationships suggest that solar radia- ity on the crater floor, especiallyin the instance tion is a prime factor of frost wastage in the of two or three craters with unusually large marginal zone. Coupled with the narrownessof bright central markings (left center, 7N19). the marginal zone, this assumption permits the Some isolated, irregularly shaped patches and inference that topographic slopes therein are bands of higher than average luminanee on the surprisingly gentle. The argument is as follows. interior pole-capsurface (see 7N17 for examples) Assume an initial frost cover of uniform thick- appear to be high standing, but others, lacking a ness,wastage of frost only by solar radiation, and shaded side, may simply represent patches of the atmosphere playing no role. Under these frost, which for somelocal meteorologicalreason, conditions a slope such as the inner wall of a have a highly reflective texture. crater, inclined south at 5ø , would experience nearly the same amount of insolation as a Origin o• 'Etch' Features correspondingslope inclined 5ø northward but Etch features have negative relief, highly located10 ø farther south.Under the assumptions angular outlines, and irregular shapes. Since made, they should become frost free at about nothing geometrically comparable is seen else- the sametime, and the marginal zoneof the polar where on the Martian surface, etch features are cap shouldbe about 10ø wide. In actual fact it is assumed,as a workinghypothesis, to existwholly no more than 5 ø wide. Thus, unlessfrost thick- within the frost cover. If the elephant's foot- ness and wastage in the marginal zone are print of 7N14 is a frost-filled crater, it supports strongly influenced by factors other than solar this hypothesis.Differential evaporationor some radiation, slopes of individual topographic mechanical erosion processmight produce etch features therein are likely to be something less features. Wind is a possibleerosive agent, as it than 5 ø. can scour in unusual fashion. Some etch pits are large (90 km across) but no larger than colJan Frost Characteristicso• Polar Cap I•terior hollows on earth. Differential ablation might In the pole-cap interior, high features, such as form the pits, owing to localizedthin accumula- crater rims and isolated knobs, are unusually tions of dark-surface debris such as dust. This bright compared with the surrounding surface, might account for the lower albedo of etch pit and both are brighter than the bottoms of most floors. Dollfus [1961] has commentedon residual craters and 'etch pits.' The floors of these accumulation of dust on the polar frost through depressionsare lighter than bare ground at the evaporation. Whatever the mechanism, the pole-cap margin, and the likelihood that they sharpnessand angularity of etch features suggest have only a thin or spatially intermittent frost undermining of a surficial slabby crust. 366 S• • ^•.

Wind may have helped shapesmaller features south pole area have been occupied repeatedly of the pole-cap interior. The beaded lineations by massesof ice, each lasting for a significant (Figure 5, 7N15, 7N17) look wind molded, and part of the 50,000-year precessionalcycle. There fine detail on larger crater rims resemblesscallop- is no way of knowing how long conditions on ing and fluting by wind scour.Much of the pole- Mars have favored polar-cap formation, but it cap surface looks swept becauseof faint line- is a reasonable inference that the cumulative ations and elongationof minor features,possibly time of perennial ice-mass occupation of the produced through scour and drifting by wind. south pole area may total millions, possiblybill- ions, of years. The presenceof suchbodies during Quasi-Linear Markings of Central Polar Region a cumulative temporal span of this magnitude The long quasi-linearmarkings of the central might account for the unusual smoothnessof the polar region are truely enigmatic. They are central polar region and the paucity and faintness thought to be groundfeatures, not clouds,owing of craters, as well as having somethingto do with to sharp boundaries,shape, shading, and lack of development of the quasi-linear features. Con- shadows.If the markingsare grooves,the shading ceivably, these features might be morainal in is proper, but if they are ridges,the sunny side character, or they might be the edges of platy is not bright. This couldbe the result of excessive layers of remnant ice. evaporation or wind erosionon the sunny side, producingvery thin or intermittently distributed Thicknessand Permanenceof Frost or Ice frost, or of frost with an unuaually low reflective The thicknessof frost composingthe Martian texture. The features might even be scarps,with polar caps has long been and remains a point of a bright face and a band at the foot appearing concern. Previous estimates range from 1 mm to dark for someunknown reason.A prolongedstare 1 or 2 meters [de Vaucouleurs,1954; Michaux, at 7N17 with this conceptin mind indeed makes 1967; Glasstone,1968]. the features look like scarps. Some craters of good size (60-kin diameter) Whether troughs, ridges,or scarps,the matter appear to be partly obscuredby frost, and small of their genesisinvites speculation.The features craters appear less numerous within the polar- may be composedwholly of frost, snow, ice, or cap area than on some other parts of Mars. rock material. The possibility of dunes comes Nonetheless, many craters show through the readily to mind, but these Martian features are frost covering (Figure 3), some even as small as less linear and less regular than most large ter- 1.4 and possibly 0.8 km in diameter, (see lower restrial longitudinal dunes. The individual fea- left quadrant of 7N16). Thus frost thickness is tures with a width up to 10 km and a spacingup locally great enoughto modify crater appearance to 50 km are on a much larger scale than either but in many places is not thick enough to blot terrestrial longitudinal .ortransverse dune ridges. the craters out. These relationships suggest The wind pattern required to produce dunes of significant variations in thickness, perhaps re- this arrangementwould have been roughly radial lated to wind drifting. or concentric to the south pole, probably both The 'elephant's footprint' of 7N14 looks like if Baghold [1941] is correct in his analysis of a crater nearly filled with frost in which etch pits longitudinal dunes. have subsequentlybeen developed.If these and One intriguing line of thought is this. The phase other etch features on the polar cap exist wholly of the precessionalcycle of Mars is currently within the frost layer, they suggesta thickness unfavorable to preservationof a south polar cap, of at least tens of meters. Thus, solelyfrom phys- owing to long hot summers.Even so, a small rem- ical relationships seen on the TV photos, we nant of the cap a few degreesin diameter prob- conclude that localized maximum frost thick- ably survives,centered at about 84%, 330øE, nessesare more likely to be on the order of tens in most if not all years (Slipher [1962]; B. A. of meters than millimeters or centimeters. Smith, personalcommunication). In phasesof the It is also possiblethat the polar regions may precessionalcycle favoring short cool summers, currently harbor local accumulationsof old C02 it seems inevitable that a much larger mass of ice of even greater thickness.This material might perennial ice lingered in the south pole region. be dirty enough to escape detection when the This could mean that some part or parts of the annual frost disappears,and it might have some- SURFACE OF MARS--SOUTH POLAR CAP 367

thing to do with etch pits and the quasi-linear not yet possible.Several features seen in pole-cap marks of the central polar region. pictures are interpretable in terms of wind- formed drifts, but evidencefor massiveperennial QuantitativeEstimate of Solid CO• depositsrelated to the precessionalcycle is less The condensation,evaporation, transport, and definite. The more prominent etch pits and the storage of CO• throughout the 687-day annual quasi-linear streaks near the south pole deserve periodand the 50,000-yeareffective precessional consideration as possible indicators of such de- cycleof Mars may follow one of severalpatterns, posits. The topographic relief suggestedby the dependingon total CO• in the system.In order larger etch pits seemstoo great to be accounted of increasingCO• mass(values given in g/cm-" for by annual wind accumulation, but quan- refer to the mass per cm" column, averaged over titative estimates of the relief are not available. the planet), the possibilitiesare: Evidence of permanenceof etch features or of the 1. Partial pressureof CO• lessthan saturated quasi-linear streaks near the pole, if such were vapor pressureat about 80øK: No CO• frost forthcoming from Mariner 1971 data, would be condensation occurs anywhere at any time most important to their interpretation. [Leightonand Murray, 1966, Figure 2]. COMMENTS PERTINENT TO THE MARINER 2. Total mass greater than abovebut less than 1971 MARS MISSIONS •10g/cm-": Annual CO• frost condensation occursat both poles,but no perennialfrost exists The 1971 flights to Mars are scheduled to at either pole except possibly in local wind- arrive at about the beginning of southern summer formed drifts [Leightonand Murray, 1966, p. 139]. and the initiation of northern winter. This means 3. Totalmass between •10 g/cm-"and •100 that the edge of the south ploar cap will have g/crn-":Aperennial CO• frostcap normally exists recededfar beyond the position observedduring at onepole, alternating between S and N through the 1969 flyby, and the north polar cap will be in the precessionalcycle. Local perennial deposits an initial phaseof expansion.As now programmed might occurat both polesowing to wind-formed both Missions A and B of the 1971 Mariner will drifts, dust blanketing,or both. provide TV and other instrumental coverageof 4. Total mass greater than •,100 g/cm-2: the polar cap areas. Perennial C02 frost caps would lie at both poles. A stated aim of mission B is to examine with Significant massesof C02 might be effectively care and detail the extramarginal and marginal hidden under dust blankets. zones of the south polar cap and to monitor changesand developmentsin these as the season The condition of Mars at present is either that advances.It will be particularly interesting to see of possibility3 or possibility4. The masslimits if the features described in the outer and inner given are approximate,based on a simplemodel marginal zones and in the polar-cap interior that neglects atmospheric heat transport and actually develop one after the other in the suc- cloud effects.The limit at about 100 g/cm-•' cession inferred. Confirmation or modification of derives from the difference in average insolation inferences drawn as to relationships between at the two polesowing to the orbital eccentricity. frost preservation, topography, and exposureto This effect limits interhemisphereC02 transport the sun would be important. Further evidence to about 30 g/cm-•' per martian annual cycle that frost is preferentially preserved in the under present conditions. lowest spotswould be most valuable. Dust layers less than a few millimeters thick In addition to watching for signsof the polar may enhanceevaporation of CO• by increased collar in the extramarginal area, it would be heat adsorption, but layers thicker than a few desirable to inspect the seemingly craterless centimeters should diminish evaporation by westernpart of 7Nll to seeif it wasobscured by insulation. Dust depositson the cap could attain cloudsor haze in 1969. The polar margin is also significantthicknesses in the courseof the 50,000- one of the best places to watch for optically year precessionalcycle, an accumulationof only thick clouds. 1/• per year yielding 1 cm in 104 years. Within the pole-cap interior, etch features The actual situation on Mars remains uncer- shouldbe carefullywatched, as changesin their tain, and a choice between conditions3 and 4 is appearancecould give an indication of their 368 SHARP ET AL.

nature and origin. Lineations should be moni- REFERENCES tored, and temporal changesin surface features Bagnold, R. A., The Physics of Blown Sand and on the cap possiblyrelated to wind action should Desert Dunes, 265 pp., Methuen, London, 1941. be watched for. Changes in reflectivity inde- de Vaucouleurs, Gerard, The Planet Mars, (Engl. pendent of lighting conditions,if any are seen, Transl.) 91 pp., Faber and Faber, London, 1954. Dollfus, Audouin, Visual and photographic studies would also be of interest. of planets at the Pic du Midi, in The Solar Particular attention should be given to de- System, vol. $, Planets and Satellites,edited by termining whether the quasi-linearfeatures of the G. P. Kuiper and B. M. Middlehurst, chap. 15, central polar region are ephemeralor permanent. pp. 534-571, University of Chicago Press, Any data obtained on changes of appearance Chicago, 1961. Glasstone, Samuel, The Book of Mars, 315 pp., with the season,time of day, or other temporal U.S. Govt. Print. Office, 1968. conditionscould throw light on their nature and Green, N. E., Observations of Mars at Madeira, origin. Finally, it will be extremely interestingto in Auglist and September 1877, Mere. Roy. comparephotographs of the same plot of ground Astrom. Soc., 44, 123-140, 1879. Herr, K. C., and G. C. Pimentel, Infrared absorption in the frost-mantled and the frost-free states. near three microns recorded over the polar cap The south polar cap and environs provide of Mars, Science,166 (3904), 496-499, 1969. unusually interesting areas for Mariner 1971 Leighton, R. B., and B.C. Murray, Behavior of study. Meteorological conditions permitting, carbon dioxide and other volatiles on Mars observations of the north polar cap should be Science,153 (3732), 136-144, 1966. Leovy, C. B., B. A. Smith, A. T. Young, and R. B. just as interesting. It will be in a waxing phase, Leighton, Mariner Mars 1969: Atmospheric hence differences are to be anticipated; but results, J. Geophys. Res., 76, this issue, 1971. •imilarities with south polar cap features would Michaux, C. M., Handbook of the Physical Pro- be significant, if found. perties of the Planet Mars, 167 pp., NASA, SP. $030, 1967. Acknowledgments. We are deeply indebted to Neugebauer, G., G. Mfinch, S. C. Chase, H. all personswhose combined efforts made the Mariner Hatzenbeler, E. Miner, and D. Schofield,Mariner 1969 flights to Mars a success.With respect to 1969: Preliminary results of the infrared radiom- the series of four articles on Martian surface features eter experiment: Science,166 (3901), 98-99, 1969. published in this issue, we specifically acknowledge O'Leary, B. T., and D. G. Rea, Mars: Influence the valuable aid of the following: G. E. Danielson, of topography on formation of temporary bright S. A. Collins, J. J. van der Woude, T. C. Rind- patches, Science,155 (3760), 317-319, 1967. fleiseh, J. A. Dunne, R. C. Dewar, and Patricia Sagan, Carl, and J. B. Pollack, Elevation differences Conklin, all of Caltech and California Institute on Mars, Smithson. Inst. Astrophys. Observ. of Technology JPL. Our colleaguesof the Mariner Spec. Rep. 22J, 45 pp., 1966. TV team, M. E. Davies, A. H. Herriman, N.H. Sagan, Carl, and J. B. Pollack, Elevation differences Horowitz, C. B. Leovy, B. A. Smith, and A. T. on Mars, J. Geophys.Res., 73, 1373-1387, 1968. Young have provided counsel and information. Slipher, E. C., The PhotographicStory of Mars, The participation of three authors (Murray, 168 pp., Sky Publishing, Cambridge, Mass., Leighton, and Sharp) has been underwritten by 1962. the California Institute of Technology. Cutts has been partly supportedby NASA-105-69836 and (Received August 10, 1970; Soderblom by NGL-05-002-003. revised September 7, 1970.)