Observational consequences of Martian wind regimes

zc~.~us 15, 253-278 (1971)

Observational Consequences of Martian Wind Regimes CARL SAGAN, JOSEPH VEVEI%KA, A~D P E T E R GIERASCH 1 Laboratory for Planetary Studies, Cornell University, Ithaca, New York 14850 R e c e i v e d M a y 17,1971 T h e c o n n e c t i o n w i t h p a s t a n d f u t u r e o b s e r v a t i o n s of Mars of t h e h i g h v e l o c i t y relief w i n d s d e d u c e d b y G i e r a s c h a n d S a g a n (1971) is e x a m i n e d w i t h t h e assist a n c e of a large t o p o g r a p h i c m a p of Mars. A u n i m o d a l h y p o s o m e t r i e c u r v e is d e r i v e d . Seasons a n d locales o f w i n d velocities a t t h e h a l f surface p r e s s u r e level > 8 0 m sec -I, sufficient t o lift d u s t a t t h e surface, are identified. T h e following s u g g e s t i o n s are m a d e ; t h e y c a n b e c h e c k e d b y o b s e r v a t i o n s w i t h t h e M a r i n e r Mars 1971 o r b i t e r s : (1) W h i t e clouds are c o r r e l a t e d w i t h e l e v a t i o n s a n d are p e r h a p s d u e to a d i a b a t i c cooling of rising parcels of air. (2) W h i t e s t r e a k s n e a r N i x O l y m p i c a follow t o p o g r a p h i c c o n t o u r lines a n d m a y b e aeolian deposits of fines; similarly, l a t e a f t e r n o o n b r i g h t e n i n g of E l y s i u m , A m a z o n i s - C a n d o r , a n d H e l l a s are p o s s i b l y c o n n e c t e d w i t h l a t e a f t e r n o o n e n h a n c e m e n t of relief winds, p r o d u c i n g a l o w - a l t i t u d e d u s t pall. (3) T h e a b s e n c e of surface f e a t u r e s in H e l l a s is due, e i t h e r t h r o u g h erosion or o b s c u r a t i o n , t o s u c h a p a l l ; p l a t e a u a n d b a s i n w i n d s m a y p r o d u c e s i m i l a r palls a t a p p r o p r i a t e seasons in A m a z o n i s - C a n d o r a n d in E l y s i u m . (4) A t l e a s t some yellow clouds are f u n n e l e d b y t o p o g r a p h y t h r o u g h lowlands. (5) D u s t devils s h o u l d o c c u r p r e f e r e n t i a l l y n e a r p e r i h e l i o n a n d m a y b e r e s p o n s i b l e for t h e large d u s t clouds o b s e r v e d n e a r perihelic e p p o s i t i o n s . (6) T h e p r e d o m i n a n c e of d a r k a r e a s in t h e s o u t h e r n h e m i s p h e r e is d u e to t h e p r e f e r e n t i a l aeolian d e m l d a t i o n of slopes t h e r e . D a r k a r e a s in general t e n d to be regions of steep slopes w h i c h h a v e b e e n b l o w n clean of s m a l l b r i g h t fines. Since s m a l l particles will t e n d to be m o r e oxidized t h a n large particles, M a r t i a n a l b e d o differences are f i m d a m e n t a l l y d u e to p a r t i c l e size f r a c t i o n a t i o n a n d are o n l y i n d i r e c t l y c a u s e d b y o x i d a t i o n s t a t e differences. (7) S e a s o n a l a n d secular c h a n g e s - - a s i n d i c a t e d , e.g., in a case s t u d y of S y r t i s Major- are, at. least in some cases, closely connected with windblown dust. INTRODUCTION

The connection between topographical relief and winds on Mars has been explored in a preliminary way in a companion paper (Gierasch and Sagan, 1971; henceforth, Paper I). The essential point is that on Mars, unlike on Earth, the vertical relief is comparable to an atmospheric scale height. The atmospheric isotherms follow the relief, and horizontal winds encountering slopes enter a region of steep thermal gradients. The relief winds which are predicted are estimated in Paper I from a simple scaling analysis to be (at 1 P e r m a n e n t a d d r e s s : I n s t i t u t e for Geophysical Fluid Dynamics, Florida State University, T a l l a h a s s e e , F l o r i d a .

typical tropospheric altitudes--say, at the half surface pressure level) in the range of 40-90m sec -1 and to add to (or subtract from) the global circulation winds on Mars due to unequal heating of pole and equator. Particularly at those times and places where global and slope winds add, the resulting net velocities will almost certainly be adequate to lift fine dust from the surface. A mean wind above the surface boundary layer of some 80m see -1 has been estimated as necessary to initiate grain movement by suspension, saltation, and creep on Mars (Sagan and Pollack, 1967, 1969; henceforth, Paper H.) In Paper II it was necessary to argue from visual observations of Mars that the winds 253

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CARI: SA(IAN, J O S E P H V E V E R K A , AND P E T E R GIEgAS(~tt

were v e r y likely a d e q u a t e for grain motion. The exist, ence of relief winds on Mars makes fre(luent dust storms now v e r y likely. I n an opaque g r a n u l a t e d material, small 1)artMes t e n d to 1)e brighter than large particles because <)f the transition from single to multiple scattering as the optical d e p t h through a. single 1)article 1)ecomes sma.ll. Thus it is possible t h a t albedo differences on Mars b e t ~ e e n twight and dark areas, and diurnal, seasonal and secular changes in albedo, may t)e connected with the deposition and removal of fines (Paper I I and references given there). Acc,ordingly it seems possible t h a t the:re is a strong, if complex, correlation among a range of diverse Martian phenomena : the p l a n e t a r y topogral)hy, the local a n d global winds, the yellow clouds, and the diurnal, seasonal and secular changes. Because Martian white ehmds arc almost certainly condensation ch) uds. their appearance and motions will also be con: n e t t e d with relief and winds. It is the purpose of the present paper to explore in a t)reliminary way these possible inter= relations. 13eeause some of the observational material is quite limited, most of the correlations and interactions which we propose must be t e n t a t i v e . B u t the forthcoming missions of the 1971 Mariner Mars orbiters. even if only partially successful. should increase the available observational mat;erial I)y many orders ot' magnitude. The S-band occultation, ultraviolet and infrared sl)ectrometer, and television exI)eriments will all provide topographic data. In addition a wide range of groun(ltm.se(l r a d a r and ('.()e speetroaltimetr.v
discussion is then necessarily limited I)3 the available data, and a major refinement both of observations and of conch|si
~UMMAI{Y ()F ~ ; [ N I ) I{E(;IMES

In Table I is summarized the slope thermal winds and the seasonal (~<)riolis winds calculated in P a p e r I. as well as the global circulation winds as calculated b y l:eovy and Mintz (1969). The calculations are parametcrized b y the Rossby number. Ro / ' ( 2 1 9 L s i n 0) -1 , the ratio of inertial to Coriolis accelerations. Here /~ and L are. respectively, the characteristic wind speed and horizontal length

scale, .(2 is the planetary angular rotational velocity, and 0 is the planetocentric latitude. For completeness we have included some small scale p h e n o m e n a which a p p e a r to us t<> be iml)ortant : (a) T h e r m a l l y driven small scale t u r b u lence should be strongest over regions wit h the highest ground t e m p e r a t u r e , and where large scale winds are lightest. (b) Gierasch ( I .(t71 ) suggests t h a t turbulence m a v lead to the formation of afternoon C()., clouds at altitudes of about 25kin. We might e x p e c t these clouds t~, form most strongly over regions where turbulence is strongest. (e) l)ust devils may form. If t h e y d(). then we might exl)ect from terrestrial exl)erience (Williams, l.q4S : Sinclair, 1966: el. also Neubauer. 1966) t h a t t h e y will I)c most fre(luent where ground t e m p e r a t u r e s arc highest and winds lightest. We shall use these considerations t{, guide us t,o estimates of where on the planet, with respect to seas
C O N S E Q U E N C E S OF M A R T I A N W I N D S

255

TABLE I SUMMARY OF ANTICIPATED MARTIAN W I N D NEGIMES a

Dust devils b Winter slopes /~o-: 1

Thermal turbulence c

No. L a r g e seale W e a k w i n d plus w e a k insolation should supress d u s t devils

Obstacle winda

N o n l i n e a r slope thermal wind e

Coriolis thermal wind:

Yes. M a x i n m m N o a r o u n d poleward ends of ridges. Could increase m e a n wind by factor o f 2 or 3

Yes. C o u l d be 90m s i

Yes. A b o u t 40 5 ( ) m s-1. W e s t wind, b o t h N. & S. hemispheres. A d d s to 1~o < 1 slope winds : ~Y~~ 140m s -1

Directior~ : N. h e m i s p h e r e Hill B a s i n ~ Mj 4 R e v e r s e in S. hemisphere

Winter flats

No. L a r g e scale W q a k w i n d plus w e a k insolation should suppress (lust, devils

~O.

No.

Winter slopes No > 1

No. L a r g e scale VCeak w i n d plus w e a k insolat, ion should suppress d u s t devils

No

Yes. Could be No ~40m s-L Maximum early m o r n i n g , downhill direction

Smnmer slopes No <~ 1

Yes ? Slope winds might suppress d u s t devils

Yes. I n t e n s i t y insolation. P o s s i b l e cold tropopause layer

Yes. M a x i m m n No a r o u n d poleward ends of ridges. Could increase m e a n wind by factor o f 2 or 3

Yes. Could be 9 0 m s -1. Direction : N. h e m i s p h e r e Hill B a s i n R e v e r s e in S. hemisphere

Summer slopes Re > 1

Yes? Slope winds might suppress d u s t devils

5Yes. I n t e n s i t y oc insolation. Possible cold tropopause layer

XO

Yes. Could be No ~ 4 0 m s -1. M a x i m u m emqy morning, downhill direction

Summer flats

Yes. I n t e n s i t y cc insolation

Yes. I n t e n s i t y o~ insolation. Possible cold tropopause layer

No

No

E q u a t o r i a l Yes? Slope (Re winds might always suppress >1) slopes d u s t devils

Yes. I n t e n s i t y cc insolation. Possible cold tropanse layer

No

Weak. 1 0 m s east

Yes. Could be No ~ 4 0 m s -1. M a x i m u m early morning, downhill direction

1,

256

CARL SAGAN, J O S E P H V E V E R K A , AND P E T E R G I E R A S C H TABLE 1 - - c o n t i n u e d

Dust devilsb

Thermal turbulence c

Equatorial Yes. flats intensity .~. insolati(m

Yes. Intensity

Obstacle wind a No

Nonlinear slope thermM wind e No

Coriolis thermal winds No

,:'- insolatiom Possible cohl tropopause layer

Increasing horizontal scale --~. a A t the highest horizontal scah, s, elevation differences comparable to a scale height have been assmned; where the elevation differences "/re less the winds will be less. b The contents of this cohmm are particularly speculative, and are included only because dust devils are potentially vet'5: efficient mechanisms for raising particles. High surface temperatures increase the incidence and strengths of dust devils, and strong mean winds inhibit formation. (In addition to the references already given, see R y a n mid Carroll, 1970.) c Thermal turbulence is turbulent mixing caused by hot air rising from under cold air. Gierasch and Goody (1968) assert that. such mixing may extend 15-20kin high in the Martian atmosphere. It will be disrupted to some extent by strong mean winds. I t now seems possible that strong turbulent mixing can cause a cold upper troposphere through adiabatic overshoot (Gierasch, 1971). a Obstacle winds are atmospheric flow patterns which arise when a mean wind comes upon a large scale obstacle, i.e., one large enough for Coriolis forces to play a significant role. I t is observed on E a r t h t h a t winds tend to go around rather than over such obstacles, and that they do so asymmetrically (cf. Fig. l ). Any size obstacle produces a disturbance in the mean wind field, but the local intensification on polar edges of large obstacles is of special interest. e Nonlinear slope thermM winds have been estimated, as discussed in Paper I, by neglecting Coriolis forces in the molnentum equations (because Ro :-1), and assuming a balance between acceleration and pressure gradients. These arc drainage winds, cool or warm air moving respectively doum or up slopes. f Coriolis thermal winds, calcldatcd from the thermal wind equation, as discussed in Paper I. The nmnbers shown for fiats are taken from Lcovy and Mintz (1969). F i n a l l y we shall i n t r o d u c e a c a t e g o r y of winds to cover the complicated phenomen~ which may occur when a mean wind i m p i n g e s o n t o p o g r a p h i c r e l i e f - - w e call these phenomena obstacle winds oi' corner effects; t h e y are to be c o n t r a s t e d

w i t h t h e t o p o g r a p h i c w i n d s d i s c u s s e d ill P a p e r I, w h i c h w e r e l o c a l l y g e n e r a t e d b y the t h e r m a l effects of t o p o g r a p h y . W i n d s t h r o u g h m o u n t a i n gaps, or a r o u n d m o u n t a i n r i d g e s o n E a r t h , fall i n t o t h e o b s t a c l e w i n d category. T h e y are discussed, for

J ~ ~tense

wind

Pole

J Equator FIG. 1. Schematic diagram of the production of obstacle winds on tile poleward sides of large-scale obstructions to tile mean wind flow.

CONSEQUENCES OF MARTIA~ WINDS

example, by Godske et al. (1957), and have previously been proposed for Mars by Sagan and Pollack (1968). One particular effect is t h a t for obstacles large enough so t h a t Coriolis forces are important, the flow is diverted asymmetrically, producing a local intensification at the poleward edges of ridges which can exceed the mean wind by significant amounts (cf. Fig. 1). This is again a phenomenon which we cannot hope to treat in detail because of its great complexity, but which we should bear in mind; it is an additional probable source of Martian wind velocities larger than have previously beer~ discussed. Blumsack ( 1971 ) shows t h a t for an idealized Martian obstacle wind the velocity intensification at the polar edges of ridges may be a factor of 2 or 3 over the mean winds. In addition, mountain wave and katabatic winds may be important locally. Since all the types of topography listed in Table I, including deep basins and high obstacles, are known to exist on Mars (see, for example, Pettengill et al., 1969; Lincoln Laboratory, 1970), a wide variety of wind regimes are to be expected on the planet. An entirely adequate prediction of these wind regimes cannot be expected until a complete topographic mapping of the planet is in hand. GRAIN TRANSPORT AND DUST STORMS

A wide range of observations--the photometric and polarimetric properties of the Martian surface material, the thermal inertias of the epilith derived from infrared radiometry, the low radar reflectivity of Martian bright areas, and observations of the motion of yellow clouds including their Stokes-Cunningham lifetimes--points strongly to a highly pulverized Martian surface material with mean particle radii between a few tens and a few hundreds of microns (Pollack and Sagan, 1969). In Paper I I is calculated the surface threshold velocity required for dust mobility on Mars as a function of particle radius and Martian surface pressure. For 5mb pressures, it is found t h a t the minimum frictional velocity which will initiate dust movement is some 3.8m see-1 ;

257

the particles which first move at such velocities are about 250/~ in radius. For a 15mb surface pressure, the corresponding figures are 1.9m see -1 and about 180/x. As the wind velocities increase over these values, both larger and smaller particles are moved. In addition, the low angle ballistic motion of a 200 g particle, called saltation, will by momentum exchange inject smaller suspendible particles into the atmosphere after collision with the surface. To connect these threshold velocities within the surface boundary layer to the winds at, say, the half surface pressure level above the surface boundary layer, requires a theory. In Paper I I the Prandtl mixing length theory was employed, which gives a logarithmic variation of wind velocity with altitude through the stress boundary layer, assuming no turbulent mixing within the boundary layer. But on Mars the temperature gradients are larger and the radiation time constants are shorter than on Earth, implying much more turbulent mixing than in the terrestrial atmosphere (Gierasch and Goody, 1968). The two cases, 11o turbulent mixing and strong turbulent mixing, are compared schematically in Fig. 2. While an exact calculation is not possible at the present time, we see t h a t the greater turbulent mixing on Mars tends to lead to larger velocities near the surface for a given velocity near the top of the surface boundary layer. Accordingly even smaller free air velocities t h a n calculated in Paper I I may be adequate to initiate dust motion on Mars. In Paper I I an average free flow velocity of 80m sec -1 over a 10rob surface is shown able to initiate dust motion. This is a mean velocity; some velocity dispersion due to gusts, which can produce instantaneously higher threshold velocities, can be expected. In the following discussion we consider a velocity of 80m sec -1 above the surface boundary layer as adequate to just initiate grain movement (of 200/~ radius grains) at the surface. We consider this to be a conservative number. Because of saltation spray the movement of 200tt particles will primarily lead to the suspension of smaller

258

CARLSAGAN~JOSEPHVEVERKA,AND :PETERGIERASCH z

Smellturbulent.,./ eddyd i f f u s /

/Large turbulent eddydiffusivity

J

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F t , . 2. Schenultie coniparison of velocity gradients within the surface boundary layer in the pl'OSollco and in t,ho Itl)senee o[' tui'bulelil iIiJXJllg. [11 t,he f/)l'll]Ol" ('i/so higher velocities lit'Ill" the gl'Olllld Illid lllOrO |'rt?(ltlOllt, gl'~lill nlotiOll lifo

.xpeeted.

particles into tile atmosphere, width can ttlen be carried over sizable distances by the wind. W e consider the surface t h r e s h o l d v e l o c i t y to be p r o p o r t i o n a l to the free flow velocity a b o v e the stress boundary layer. F o r a l o m b surface pressure a. fl':ec ttow wind of 1 2 0 n l s e c ~ will initiate d u s t m o b i l i t y in partic, les b e t w e e n 60 and S0o in radius. Again s a l t a t i o n leads t.o SUSl)Onsions of snlaller l)artieles. T h e free flow winds m a y lie r e x a m ple. d u s t devils. As is well k n o w n t'ronl terrestrial exllerience, the ]tot/notion of a cloud iS O:t~OII ll/llCh loss t h a n the vclocities within the cloud. This is esl>ecially evident, t7)r dust devils. Accordingly we s l a y n o t in general deduce r e l e w m t winds filr dust raising f r o m the net motion of yell
h a v e c o n s t r u c t e d a 70cm by 170cnl topographical m a p <)f Mars w i t h a vertical scale of 3 illmlknl. The map is constructed Of stlccessive p o l v s t v r e n o shcets o/lt w i t h

a liol; wire. Topographica, l detail ean titan casily be ]/ainted on. Successive layers are joined by all opoxv adhesive. The iitap Call lie u p d a t e d b y excising segillCliLS w i t h a whittling knife. The polystyrene, Illat, orial pernlits the intrusion of pins which can be tised to indicate the posit, ions of observed clouds and to tag tinte-variallle phenoillella. The liigtl) used for the st,tidies described in t,ho p r c s e l l t paper is showll ill Figs. :1 and 4 : tile elevation d a t a al'e {a,kel/ tl'(tii/ I>etf0ongill el a[. (1969). lAncohi Lal)ol'ator 3" (197o), a l l d H e r r ('t tt[. (1970) tho first, two hcing g r o u n d b a s e d r a d a r obsorw~tions, and t,he last being the toi)ograph3" I'OStlltS troll1 the M a r i l l e r (J and

7 infl'ared spectroll/e{er e,xperilllel/t iloar 2.06F wavolengt, h. (~roundbased carbon dioxide s l / e c t r o a l t i m e t r y was considered to be insutliciently reliable for our purposes. R ayleigh scattering elevation data

C O N S E Q U E N C E S OF MARTIAN W I N D S

from the Mariner 6 and 7 ultraviolet speetromet?rs (B~rth a n d Hord, 1971) will be included in future such studies. I n the absence of a c o n t e m p o r a r y ocean oll Mars, there is a problem in defining the zero altitude level. A range of m u t u a l l y

259

exclusive conventions have been used b y various authors. We propose the simplifying c o n v e n t i o n of considering 0 ° longitude, 0 ° latitude t9 be at 0 k m elevation (of. Fig. 5). The h y p s o m e t r i c curve (a g r a p h

FIG. 3. View of the Martian Topographic Map, looking West, with Elysium in the foreground, North to the right. Each pin marker corresponds to a white cloud reported in the Lowell cloud motion survey.

260

CARL SAGAIq~ J O S E P K V E V E R K A , AND P E T E R GIERASCH

displaying the f r e q u e n c y distribution of elevations) of the equatorial region covered b y E a r t h b a s e d r a d a r is shown in Fig. 5. Unlike the hypsometric curve of the E a r t h , this curve is definitely unimodal, suggesting the absence of continental blocks and ocean basins on Mars. This is consistent

with the a p p a r e n t absence of m o u n t a i n chains and suggests t h a t the t o p o g r a p h y of Mars m a y result largely from i m p a c t cratering and erosion. Marcus (1970) has recently a t t e m p t e d to predict the distrib u t i o n of elevations on a cratered p l a n e t a r y surface. Although his model is necessarily

Fro. 4. Another view of the Martian Topographic Map, looking East, with the Candor-Ainazoms plateau in the foreground. North is to the left..

'iI

261

CONSEQUENCES OF MARTIAN WINDS

6

Ah (km) 4

-2

I I0

I 20

I 30

ZSA (%) :FIG. 5. H y p s o m e t r i c c u r v e o f t h e region o f Mars c o v e r e d b y E a r t h b a s e d r a d a r (i.e., a s w a t h e x t e n d ing f r o m t h e e q u a t o r to + 3 0 ° N ) . T h e zero e l e v a t i o n is d e f i n e d h e r e as t h e e l e v a t i o n a t 0 ° l o n g i t u d e 0 ° l a t i t u d e . U n l i k e t h e case for t h e E a r t h , t h e h y p s o m e t r i c c u r v e o f Mars a p p e a r s t o be u n i m o d a l ,

simplified, and does not include erosion (except by cratering), his model predicts t h a t the hypsometric curve of such a surface should be unimodal and asymmetric, with a long tail in the direction of high elevations. The hypsometrie curve of Mars does indeed display such characteristics. But it cannot be claimed t h a t the topography of Mars is entirely controlled by impact cratering. I t is difficult to see how impacts could generate a high polygonal plateau such as Elysium ; in addition, there is clear evidence in the Mariner 6 and 7 photography of noncrater topography: the chaotic terrain and the curved ridges near the south pole, for example. LOCATION AND MOTIONS OF W H I T E CLOUDS

ON MARS The discussion in this section is based on data from the Lowell cloud motion survey (Baum and Martin, 1971; Martin

and Baum, 1969), a study based on twentyeight groups of plates from the Lowell collection which show well-defined t r a m sient white spots (assumed to be clouds) which can be followed from day to day. A small number of these spots may nevertheless be yellow clouds. The study covers fifteen oppositions (1907 to 1958) and consists of 95 cloud histories. The cloud positions observed on individual days are shown in Figs. 3 and 4, with closeups of the Candor-Amazonis and Elysium areas shown in Figs. 6 and 7. A histogram of the number of clouds per unit area at a given elevation versus elevation is shown in Fig. 8; it is evident t h a t the white clouds show a strong preference for high elevations. In fact, most of them occur over the Candor-Amazonis and Elysium regions, known to be among the highest areas on Mars. This suggests t h a t other white cloud concentrations on the Lowell map, especially Arcadia-Tempe, and the region around

F'l(:. 6. A ch},~eup o[" t h ~ ( ! a ~ , d o r - A m a z o n i s regi()u of t h e M a r s t~p~)gr~phical mtip. sh~)xvitl~ Ill(, p~,siti(~li ,ff white, c l o u d s ace~r(iblg t~ th,, ].,wtqt el,)ud m~ti,)~ Stlt'Vt,V ( M a r t i n :~t)d B ~ m n . 19~i(): l~b~um ~ l , | M~)rti~L 1971). Nix (}lympicu is lh,, lnv~z~, ('v,~t~w-slml)~,ct ob.j~,et

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F~¢;. 7. A eloseup of the E l y s i u m region of t h e Mars t o p o g r a p h i c a l m a p , s h m v i n g t h e position of w h i t e clouds ~ee~n'ding to t h e Lowell cloud m o t i o n s u r v e y (M,~rtin a n d B a u m , 1969 ; B a u m a n d M a r t i n 1971).

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:~64

CARL SAGAN~ JOSEPla[ VEVERKAj AND P E T E R GIERASCIf

Phoenicis Lacus, are also elevated areas. Thus the Candor-Amazonis plateau may e x t e nd south to Phoenicis Laeus, and perhaps north towards Arcadia and Tempe. Unambiguous cloud motions occur frequently over the Arcadia-Tempe region. Blue photographs indicate easterly motions of about 5km/hr. A good example is Sheet No. 17 of Martin and Baum (1969) which covers the period of J u n e 1 to J une 13, 1937, late summer in the northern hemisphere of Mars. Thus we conclude t h a t commonly between latitudes +40 ° and +60°N the predominant motion in late summer is easterly. These observations are in excellent accord with the winds expected from the general circulation for summer fiats, where (see Table I) easterly winds of 10m sec -1 velocity are expected. The observed cloud motions are about 2 m sec-1; we have already remarked t ha t cloud motions tend to be slower t han the driving wind velocities.

Baum and Martin (1969) have noted t hat white clouds are much more common in the northern than in the southern hemisphere. In fact most white clouds occur over Arcadia, Tempe, Amazonis, Elysium, and Candor. The only other significant areas of white cloud formation, Tharsis. Tithonius Lacus, and Phoenicis Lacus, are all within 15 ° of the equator. It appears likely t h a t the entire Amazonis-Candor-Tharsis complex is a white cloud area largely because of its high elevation. Two possibilities occur for explaining these high altitude clouds. If mean wind velocities are low, small-scale turbulence m ay produce a convective overshoot and condensation clouds (Gierasch, 1971). Alternatively these high altitude white clouds m ay be due directly to orographic winds, carrying parcels of air laden with condensibles up slopes ; condensation then occurs at altitude because of adiabatic cooling. Since these clouds are observed

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10

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Ah ( k m ) F r o . 8. H i s t o g r a m o f t h e n u m b e r o f w h i t e c l o u d s r e p o r t e d p e r u n i t a r e a i n t h e Lowell s t u d 5, of 15 o p p o s i t i o n s f r o m 1907 a n d 1958, as a f u n c t i o n of e l e v a t i o n . T h e t o p o g r a p h y of t h i s s t r i p b e t w e e n 0 ° a n d + 2 5 ° N l a t i t u d e is t a k e n fl'om t h e L i n c o l n L a b o r a t o r y (1970) r a d a r s u r v e y .

CONSEQUENCES OF MARTIAN WINDS

at the end of the summer, t h e y a p p e a r to be due to rising and cooling slope winds : Gierasch shows t h a t the convective overshoot mechanism would be strongest when the g r o u n d surface is warmest, i.e., in midsummer. Since the predicted motions (Table I) are restricted to the region of the slopes, there is no contradiction between t h e o r y and the absence of rapid motion from resolution-limited groundbased observations ; b u t fairly rapid motion of white clouds in the Candor-Amazonis and Elysium regions should be looked for at high resolution in the Mariner Mars 1971 mission. We also note the a p p e a r a n c e in the Lowell d a t a of a few cases of white clouds in Tithonius L a c u s - - w h i c h m a y be conn e c t e d with the adiabatic cooling of parcels of air rising from the inside to the

265

outside of this object while traversing it, assuming it has a rough crater morphology. W H I T E STREAKS AND D I U R N A L

VARIATIONS One of the most u n e x p e c t e d findings from Mariner 6 and 7 was the set of white streaks on the Amazonis-Candor plateau, particularly in the vicinity of Nix Olympica (see Fig. 9). The streaks t e n d to follow elevation contours (cf. Fig. 6). The possibility t h e n arises t h a t t h e y are aeolian deposits of fine particles, either of minerals or frost, which are channeled along the prevailing wind direction. The possible diurnal v a r i a t i o n in these streaks is at the present time unknown, and is an interesting task for Mariner Mars '71. R e l a t e d structures in Elysium seem to a p p e a r in Mariner 6 p h o t o g r a p h 6F40.

FIo. 9. Portion of Mariner 7 Far Encounter photograph 7F75, with contrast enhancement, of the white streakiness in the vicinity of Nix Olympica. Note how the streaks seem to follow elevation contours (cf. Fig. 6).

2(i6

CARL SAGAN, JOSEPH VEVERKA, AND PETER GIERASCH

Because of the large diurnal variation in surface temperature on M a r s - p e r h a p s >IOOOK for an equatorial latitude (Morrison el
(as

are

sea

breeze

will(Is

on

Earth) when the t e m p e r a t m ' e difference has persisted with one sign tbr tile longest available time. Morning observations tend to be <)bscured by tile so-called dawn haze. We pr<)pose here t h a t the late afternoon I)rightenings t h a t have /)een ret)orted in such areas as Elysium, Amazonis, and Hellas are connected with late afternoon e n h a n c e m e n t of relief winds, i>roducing a low-altitude dust pall. The t r a n s p o r t process may be reversible I:etween locM dust sources and sinks, because the directions of the slope winds reverse at night when the surface is colder than the atmosphere, An alternative explanation in terms of water percolation through the epilith has been proposed and criticized by L e o v y et al. (l (,171 ). P L A T E A U AND BASIN W I N D S

In Paper I velocities of some 90 m sec are calculated at the periphm'ies of large elevations and depressions, when the relief is comparable to a scale height. The direction is clockwise a r o u n d plateaus and counterclockwise within basins in the northern hemisl)here: with an opposite sense in the southern Eemisphere. These winds. as is indicated ill Table 1, a u g m e n t the seasonal winds of tim general circulation on the poleward slopes ()f elevations and on the e(luatorward slopes of depressions. Three l)rovinces where, a c c o r d i n g t o t)res'ent information, the elevation ditt)rences in or near a plateau or basin are COlllparable to a scale height are Hellas, Elysiunl. an(l Amazonis-Candor. The free flow winds on s u m m e r slopes, and on winter slopes and fiats, at nonequatorial latitudes can range t~'om 90 to 140m sec I (Nee Table I) much more t h a n (be minimum velocity to initiate grain t r a n s p o r t on the surface.

Accordingly these locales should, at the a p p r o p r i a t e season, he major sources of dust. On the other hand the wind patterns are constrained into circular arcs. concentric a b o u t the elevation minimum (n' maximun~, and thus may be p r e v e n t e d from contributing t/,) planet-wide dust s t o r l | l s.

Evidence for such motions is difficult to find. The (mlv interesting instance in the ease of Elvs£um is a S W motion of a cloud between April 22 and 23, 1!1211 (Martin and Baum. 1969: sheet No. 3). It was then sui'nmer in the northern tlemisphere. In the case of ttellas, tigure 15 on page 47 of Antoniadi (1930) suggests a northeasterly motion a r o u n d Hellas between ()ctoher 10 and ()ctober 13, I ,(t24. It was early sununer in the southern hemisl)tmre. Ill short, at the pre,:ent time t h e r e seOlllS t o b e lie evidence to COll~irlll o1' deny the existence of circular motions a r o u n d depressions of plateaus. Of these three areas only Hellas has been examined at high resolution from Mariner spacecraft. Of all high resolution photographs t a k e n of Mars, only Hellas is found to be crater-free (Sharp et al., 1971). We propose t h a t these two circumstances are causally connected. If the highest winds on Mars occur within Hellas. t h e n aeolian erosion should be most efficient t h e r e ; and the possibility arises t h a t the rubbing down of crater walls and the filling in of crater b o t t o m s by windbh)wn dust is the origin of the crater-free terrain in Hellas. An Mternative possibility is that. 1)ecause of the large wind velocities in Hellas. a dust cloud was raised at the time of the Mariner 6 and 7 e n c o u n t e r which just filled the basin. The Mariner 6 and 7 e x p e r i m e n t e r s have mentioned this I)ossibility in passing 1)ut rejected it, perhaps because no r e a s o n foI: 1)referential obscuration in Hellas was known t',) them (Sharp e! al., 1971). Rapid dav-to-(tav changes in the albedo of Hellas are known and have ah:eady been ascribed to (unspeeitied) meteorological p h e n o m e n a (Boyce, 1971). The s u m m e d basin and global winds are predicted to I)e a m a x i m u m (~ 14(i m see 1) a t the northern edge of the

CONSEQUENCES OF MAI%TIAN WINDS

Hellas basin in southern winter. Mariner '69 arrived in early southern spring, when the dust pall may have been maximum. Mariner Mars '71 is arriving in southern summer, when basin but not global winds should be strong (~90m see 1). It is possible t h a t southern summer winds will not be lifting much dust, and t h a t some features in Hellas will become visible as the mission progresses. Of special interest is the dark central feature Zea Lacus, reported in early but not in recent observations. Dollfus (1969) argues t h a t the polarimetrie characteristics of Hellas at the time of encounter were in no way anomalous, and therefore t h a t a basinfilling cloud is unlikely. But it is not clear t h a t a low-lying dense dust cloud could not give photometric and polarimetric characteristics closely similar to those of the presumptive surface of Hellas itself a t phase angles _ 40 °. Another immediate implication of these ideas is t h a t at least at the appropriate seasons both the Amazonis-Candor and the Elysium plateaus should also appear to be crater-free. Thus the hypothesis t h a t a craterless appearance is due to plateau and basin winds, and the questio,1 of whether craterless terrain is due to preferential erosion or to preferential obscuration, may be settled by the Mariner Mars 1971 Orbiters. An additional anomaly which applies to Hellas, Elysium, and, to a lesser extent, to Amazonis-Candor, is color. These regions tend by all visual accounts to be pinker than adjacent Martian bright areas. Unfortunately no photoelectric confirmation exists. I f this effect is real, it may again be connected with the special wind regimes of these regions. A fine material raised in other bright areas of Mars can easily be distributed over significant fractions of the Martian disk i~1the Stokes-Cunningham fallout times (Paper II), and will experience even more distant lateral transport if there is additional turbulent support. Hence there should be a tendency for compositional homogeneity over the entire surface of Mars. These three bright areas thus require a special explanation. T h e concentric wind patterns of Hellas,

267

Elysium, and Amazonis-Candor tend to prevent access to their interiors by low altitude dust-carrying winds ; it is possible t h a t the color differences are connected with the preferential presence, because of limitations on lateral transport, of very fine particles in these regions: many blueabsorbing minerals, such as goethite, are known to become pinker as the particle size is decreased. I t would seem t h a t the pink appearance requires some particle transparency even in the blue, implying particles smaller than roughly lOt~. Alternatively, the pink hue may be due to white-saturated red, caused by the presence of white frost or cloud patches. TOPOGRAPHIC CONTROL OF WINDS

The possibility t h a t yellow clouds preferentially move through lowland areas and are deflected when they encounter highlands was raised by Sagan and Pollack (1968). They used observations of such deflections to deduce the high altitudes of certain dark areas. We here use observed motions and the radar topography to demonstrate the existence of at least some topographically controlled winds on Mars.

Example 1." Motions of Yellow Clouds Along Syrtis Major from Isidis Regio to Lybia towards Hesperia In his book, de Vaucouleurs (1954) gives four instances of such motions (1907, 1911, 1926, 1941). Slipher (1962) documents four more such motions (1920, 1924, 1943, 1958). These were discussed by Gifford (1,(}64). de Vaucouleurs (1959) also gives a description of the 1.958 event in which a yellow cloud appeared over Moeris Lacus and Lybia and then moved on above Amenthes into Hesperia. These motions are clearly topographically controlled, the flow being along the edge of the Syrtis Major-Aeria plateau. There appears to be an avoidance of the region around [230 °, 0 °, Gomer Sinus]. This may meal1 t h a t the radar-high region around (250 °, 0 °) may not extend far into Hesperia. I f Hesperia is lower than the equatorial edge of Amenthes and Gomer Sinus then winds would be funneled

268

CARL SAGAN. J O S E P H V E V E R K A , AND P E T E R GIERASCH

into it a n d t h u s would explain the o b s e r v e d cloud motions. H o w e v e r , it is also possible t h a t the entire Mare T y r r h e n u m - H e s p e r i a Mare C i m m e r i u m region is a high p l a t e a u , a n d t h a t the yellow clouds are swung a r o u n d b y a generally easterly circulation b e t w e e n latitudes - 3 0 ° a n d 50 ~'S. These motions t e n d to occur in s o u t h e r n s u m m e r . H o w e v e r from o b s e r v a t i o n s of the 1956 d u s t s t o r m it seems t h a t a t high southern

latitudes during s u m m e r the circulation is westerly. T h u s the general easterly swing of yellow cloud p a t h s o v e r H e s p e r i a m u s t be due to t o p o g r a p h i c funneling.

Example 2: The Margeritifer-Ninus Regio~, The t o p o g r a p h i c m a p shows a definite t r o u g h between longitudes +15 ~' and +45". In 1922 Slipher o b s e r v e d a large

F~G. 10. Drawing by E. C. Slipher (1922) of cloud motions, interpreted here as elevation-avoiding. Observations were made on July 9 (upper I.). July 10 (upper r.), July 11 (lower 1,), and July 12 (lower r.), 1922.

CONSEQUENCES OF MARTIA~ WINDS

269

FIG. 11. O b s e r v a t i o n s b y C a p e n (1971) of m o t i o n o f a yellow cloud i n t h e 1956 d u s t s t o r m w h i c h followed t h e p a t h of S l i p h e r ' s 1922 cloud. O b s e r v a t i o n s were m a d e o n Sept. 10 (1.) a n d Sept. l l (r.), 1956.

yellow cloud drift through this region (Fig. 10). This occurred again during the 1956 dust storm (de Mottoni, 1969; Capen, 1971). Observations of this motion made b y Capen on September 10 and 11, 1956 are shown in Fig. 11. The funneling of the clouds around the tip of Margeritifer Sinus is clear, and suggests that the elevation trough at (+35 °, 0 °) extends around the back of Margaritifer Sinus. Related observations of topographical funneling of yellow cloud motions during the 1956 dust storm are described b y Sagan and Pollack (1967b). ORIGINS OF Y E L L O W CLOUDS

Evidence is overwhelming that yellow clouds are most common near perihelion, and occur mostly in the southern hemisphere (Antoniadi, 1930; de Vaucouleurs, 1954; Capen, 1971 ; etc.) Furthermore most of the extensive yellow clouds originate in a relatively small part of the southern hemisphere. Capen (1971) writes: "Two generalizations can be drawn from the historical reports of large yellow clouds. First, they are more likely to be seen when Mars is near perihelion. Second, they show a preference for that part of the southern hemisphere extending from longitude 270 °

westward to 70 ° [Argyre-Noachis-Hellespontus-Iapygia]. In particular, the region of Hellespontus (near longitude 325 °) is repeatedly mentioned." From Table I we see that dust devils occur preferentially when insolation is large, and should preferentially occur in summer and equatorial flats. Accordingly we suggest that the generation of Martian yellow clouds near perihelion is due to dust devil activity. The 1971 encounter of the Mariner Orbiters with Mars occurs a few months after a perihelic opposition. It is therefore possible that dust devil activity, occurring on a large scale on Mars, might be detected directly. Because of the high eccentricity of the Martian orbit, the insolation varies b y a factor of 1.45 between perihelion and aphelion. It is possible that this variation runs through the critical flux for large scale generation of dust devils. Wells (1967) gives histograms of the frequency of yellow clouds versus the heliocentric longitude of Mars (7) for both hemispheres. These show that yellow clouds are most frequent in the southern hemisphere. The southern hemisphere curve shows a well-defined maximum near ~ = 4 0 ° (perihelion is at ~ = 3 3 5 °, and summer solstice is at ~ = 357°), and a

;27(I

CARL SAGAN~ JOSEPH VEVERKA, AND PETER GIERASCH

m i n i m u m near aphelion. The n o t h e r n hemisphere histogram is fairly flat except for a slight m a x i m u m near ~ - - 2 6 ( ) ~ (northern late summer) which p r o b a b l y represents the only true n o r t h e r n hemist)here yellow cloud component. Ahnost eertMnly m a n y of the other clouds in this Mstogram are clouds which originated in the southern, and drifted into the northern hemisphere. The Wells histograms eontMn no information on the importance of tim yellow eh)uds. Generally speaking, yellow clouds which originate in the northern hemisphere are small, localized, and insignificant. However, m a n y <)fthe sc)uthern hemisphere yelh)w clouds develop into l)lanet wide disturbances. Thus at the present time the southern henfisphere is acting as a source of yelh)w cloud material, and the northern hemisphere, as a sink. The most likely areas to be denu(ted of line particles are wind-swel)t slopes. Assuming t h a t such areas are uniformly distributed over Mars, the present net removal of small p a r t M e s from the southern hemisphere to the northern hendsphere should have the effect ()f prefbrentially denuding slopes in the southern hemisphere. This may explain tile i)resent preponderance of dark areas in the southern hemisphere. I)ue to precession the role of the hemispheres is reversed e v e r y 5 z IO t years (Leighton and Murray. 1(.t66). Thus it, is possible t h a t there are times when the net flux ()I' small i)artMes is from the northern hemisl)here to the southern hemisphere. At such times the distribution of clark areas m a y be opposite to what it is t o d a y , in other words, the l)resent distri1)ution of dark and bright areas on Mars m a y t o s o m e e x t e n t change with time. folh)wing the t)recession of the Ma,rtian e(luillOxes.

AI:BEDO ] ) I F F E R E N C E S BET\V EEN BRIGHT ANt) ])ARK AREAS

(kltts et al. ( 19 71 ) r e m a r k that,, c o n t r a r y to the lunar ease, heavily cratered uplands

on Mars do not consistently have a higher albedo. This circumstance can most readily be understood in t e r m s of aeolian t r a n s p o r t

of fines on Mars. Similar remarks apply to their conclusion from Mariner 6 and 7 photograph): that topographical control of albedo exists, with bright material in

lowlands. Our comments above on the origins of bright clouds have a nM, urM application to the n a t u r e of the all)edo differenees on Mars. Areas which are preferentiMly d e n u d e d of fine p a r t M e s should be systematically dark. From the previous discussion we see t h a t such areas t e n d to be slopes, and we therefore i)rediet a correlation of slopes with low visual albedo. This has already been mentione(l as a possibility in Paper II. The radar elevation d a t a now show a statistieall 3 significant correlation of this sort (IAneoln Laboratory, 1970). Because of the (.(>mplexity of the prevailing wind p a t t e r n we (to llOt e x p e c t f o h a v e a o i l e - t o - o i t e c o r r e :

lation between stee l) slopes and h)w all)edos. But a general t r e n d is alttieiliate(l. The steepest area of large lateral e x t e n t on Mars, Syrtis Major, is also the darkest area of large lateral extent. This interI)retation is based upon the assuml)tion t h a t the f l m d a m e n t a l differ enee between l)right and dark areas tion tbature in the ultraviolet reflection spectrum of Mars (Wallace, 1!)70: Barth and Herd, 1971 ), and although the monomer of carbon suboxide is one t)ossible explanati(m of this feature, ozone is a more likely one. The q u a n t i t y of ozone

CONSEQUENCES OF MAI¢TIAN WINDS

d e d u c e d is in good accord w i t h t h a t predicted from photochemical equilibrium in a carbon dioxide a t m o s p h e r e on Mars (Belton and H u n t e n , 1968). Now for significant oxidation to occur, there must be efficient erosion, so t h a t new material will be exposed to the oxidation source. B u t oxidation itself is a weathering agent, and t e n d s to generate fines. Material which can be m o v e d b y the winds on Mars can be more fully oxidized t h a n material which cannot. F u r t h e r m o r e , because small particles have larger ratios of area to volume t h a n large particles, we e x p e c t t h a t the fine mobile particles on Mars should be more oxidized t h a n the larger immobile particles. Accordingly we e x p e c t Martian bright areas to be more oxidized on the whole t h a n Martian d a r k areas because of aeolian t r a n s p o r t . An explanation of the difference between bright and d a r k areas in t e r m s of particle size therefore remains an a t t r a c t i v e possibility. I t is possible t h a t the ozone will be d i s t r i b u t e d according to the oxidation state of the underlying material; it should be preferentially present over the polar caps which are already fully oxidized c a r b o n dioxide and water. There should be smaller ozone a b u n d a n c e s over the partially oxidized bright areas, and even less ozone over the less oxidized d a r k areas. The findings of Bai~h and H o r d (1971) of the preferential presence of ozone over the polar caps is consistent with these views. I t would be interesting to compare the detailed high resolution ozone distribution established b y the Mariner Mars 1971 Orbiters with detailed albedo maps.

SEASONAL AND SECULAR CHANGES

F r o m the foregoing discussion we can u n d e r s t a n d at least one category of seasonal changes on M a r s - - t h o s e which occur because of winds in regions where there exist a d j a c e n t slopes and fiats. The strongest large-scale winds will occur in fall and winter : b u t the greatest difference between winds on slopes and winds on fiats will occur in spring and s u m m e r (cf. Table I). Therefore the m a x i m u m

271

seasonal changes are e x p e c t e d in spring and summer. T h e y occur because of the t r a n s p o r t of fine suspendible particles between fiats and slopes. I n this picture there is almost no difference between fiat areas which are at high elevations and flat areas which are at low elevations. A similar picture was derived in P a p e r II, b u t there it was imagined t h a t the fine particles were cycled over a year between d a r k highlands and bright lowlands. The analysis proposed here differs little from t h a t in P a p e r I I if we replace the word " h i g h l a n d s " there for slopes and the word "lowlands" there for flats. I n s u m m e r there exist obstacle, slope thermal, and Coriolis winds which denude slopes of fine particles and deposit these fines on fiats, t h e r e b y darkening the slopes and brightening the fiats. I n some fiats the fine particles which arrive in spring and s u m m e r m a y be no smaller t h a n the particles already there, in which case there will be no noticeable brightening because of no i m p o r t a n t increase in multiple scattering. On the other hand, the d a r k areas must d a r k e n because the particles which are r e m o v e d b y the winds in spring and s u m m e r must on the whole be smaller t h a n the particles which cannot be m o v e d b y the winds. Absolute p h o t o m e t r y of Martian seasonal changes would shed considerable light on the mechanism of seas
"272

CARL

SAGAN~

JOSEPH

VEVERKA,

s l o p e s at t h e s a m e l a t i t u d e . T h e p r o m i n e l t t role w h i c h H e l l e s p o n t u s a n d I ) e s p r e s s i o H e l l e s i ) o n t i c a p l a y a c t o r ( l i n g to classical

d e s c r i p t i o n s of t h e s e a s o n a l d a r k e n i n g . m a y ~gain be c o n n e c t e d w i t h small l a t e r a l e x t e n t s a n d s t e e p slopes. T h e (:ase o f Svrtis Major. which has s t e e p sh)pes b u t n<)~ small lateral e x t e n t , is discussed in detail below. T h e possible com~eetion of a l b e d o c h a n ges w i t h small lateral e x t e n t s is i m p o r t a n t . an(1 d e s e r v e s f m t h e r exlfloration. S~ mall
AND

PETER

GIERASCH

be c a p a b l e of c h a n g i n g t h e d i s t r i b u t i o n o f surface m a r k i n g s o~t Mars, a n d smMl d a r k a r e a s c o m p l e t e l y s u r r o u n d e d by I)right a r c a s should be more likely to u n d e r g o secular c h a n g e s t h a ~ m o r e e x t e n sive d a r k areas. T h e o b s e r v a t i o n s seem t o s u p p o r t this correlation. When more d e t a i l e d t o p o g r a p h i c d a t a is in h a n d it; would 1)e v e r y i m p o r t a n t t,'~) t e s t w h e t h e r t h o s e a r e a s which c h a r a c t e r i s t i c a l l y undergo secular ('hanges (for e x a m p l e . Solis La(;tts a,nd T h o t h - N e p e n t h e s ) are also a r e a s of sballow slopes, as would statisticallv be e x p e c t e d fl'om w i n d b l o w n d u s t mo(tels of secular changes. ( ' u t t s el (d. (1(.t71), while g r a n t i n g t h a t s o m e s e a s o n a l a n d secular (~hanges m a y b(, due to win(tMow~t dust, r e m a r k t h a t "del)osition f r o m s u s p e n s i o n m a y 1)e inade(luate as an e x p l a n a t i o n of the details of a l b e d o I)ounda.ries. lit is difficult t() i m a g i n e how t h e g r a d m f l settling of susp e n d e d ' d u s t ' c<)uld c r e a t e s h a r p a l b e d o

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274

('ARL SAGAN. JOSEI~H Y E V E R K A , A N D PETER GIERASCH

b o u n d a r i e s a n d r e f l e c t t o p o g r a p h i c control." But if the winds which carry the dust are under strong topographic control, this difficulty tends to disappear.

d i m i n i s h e s r a p i d l y in w i d t h n e a r ~ :2(')0 a n d b e y o n d . F o r M a r s a p h e l i o n o c c u r s at, ~7 - ] 55 °, a n d p e r i h e l i o n a t ~1 - 335 ~. T h u s the narrowest aspect of Syrtis Major o c c u r s d u r i n g w i n t e r in t h e n o r t h e r n h e m i s p h e r e , a n d its w i d e s t a s p e c t , d u r i n g northern summer. The o t h e r iml)<)rtant l ) o i n t stresse not specifically deal with the superiml)osed secular changes. The topography of the Syrtis Major r e g i o n is s h o w n in F i g . 13. T h i s m a p is based on the radar elevation data obtained a t L i n c o l n L a b o r a t o r y (1970). T h e m a p shows that Syrtis Major corresponds to the e a s t e r n s l o p e o f a high f l a t p l a t e a u w h i c h

NEASONAL C H A N G E S IN NYRTIS M A J O R : A 1)RELIMINARY (~ASE STU DY

S y r t i s M a j o r is one . I t t h e n

TA B I , E 1 I ()hlSEIgVFD ~'VINI) MOTIONS IN THE NYIUI?[S ~ I A J ( ) R AREA a

Mars date (Southern Year E a r t h date Hemisphere)

Latitude

Longitude

907 911 924 926 941 943 958 1873 1937

() t(> 2 0 (): (o 4() 15 t() 0 10' t() 3 0 30 < t() 4 0 ' (): to- 2 0 20 t() 2 5 15 (() 15 15 t(> 15

270 to 220' 2 7 0 t()21()" 280: to 290 ° 270:1<)255 270 ~ t<)40' 270 t<) 240: 265 t ( ) 2 3 0 3151'to 305 295' t() 270:

29-30 Jul 20 21 Apt l l 18 Oct 2 7 Aug 9 ]0 Aug 14 15 May 25 26 Oct 3 1 J u l / l Aug 12 28 Nov 31,Jul/10Ang 3 5Oct 3 4Aug 12 15()ct 29 31 Jul 24-25Ma 5 II 12 Feb 2 5 May 10 12 Feb

l)rift Path

Average Speed (m,,se(')

Lil)ya to M. Cimm('rimn 5 l A b y a t o Eri(hmia (i ]sidis tl. t() Libya 1() Libya t o A u s o n i a 25 Libya to Phaetho)itis 5 Libyat() Hosperm, II lsidis It. t()Hesp(~ria 11 Aeria 7 Atria t() Syrtis Maj<)r 3

" The last (mt,ry refers to a "white" cloud and is from d(' Vaueoulem's (I 954); all oth~,l' t~rltl'i(,s 1'(,~[(~1" () "yellow" chmds and are fl'om ({ifl~)rd (1964).

275

C O N S E Q U E N C E S OF MARTIAI~ W I N D S

TABLE I I I MEAN SEASONAL WINDS AND VISITAL AP/'EARANCE OF SYRTIS MAJOR AREA

Mars longitude

Northern Hemisphere

Wind type

Mean speed (m/s)

Frequency of yelh)w clouds

Visual appearance of Syrtis Major

296' to 4 5 230 ° to 232

Autumn-Winter Smnmer

A B

5 25 3-7

High Low

Minimum wi
Season

extends west t h r o u g h Aeria. The bright region l,ibya is a nearly flat area a b o u t 6kin below the level of this plateau. Gifford (1964) has given a compilation of most yellow cloud motions detected on Mars up to 1961. The wind velocities derived from his d a t a set b y Gifford are not in good a g r e e m e n t with those derived

b y B a u m a n d Martin (1(.)71): b u t this m i g h t conceivably be because Gifford's d a t a are directed t o w a r d yellow clouds a n d B a u m a n d Martin's t o w a r d white clouds. Gifford's list contains eight cloud motions of interest here. These are reproduced in Table I I , to which has also been a d d e d one " w h i t e " cloud m o t i o n reported

Fro. 14a.

276

CARL SAGAN, J O S E P H V E V E R K A , AND P E T E R G I E R A S C H

b y de Vaucouleurs (1954). The implied wind motions fall clearly into two categories : 7'ype A: (7 examples) occur between ~/ : 2 9 6 and ~/ 4 5 . t h a t is during n o r t h e r n a u t u m n and winter. T h e y are characterized b y average wind speeds of 5 to 25m/see and are obviously controlle
of T y p e B motions is p r o b a b l y explaine(t b y the fact t h a t such winds are relatively dust-fl'ee compared to those of T y p e A. However, an observational selection effe(.t may also be present since it is much more dil~cult to observe Mars during northern s u m m e r t h a n during n o r t h e r n winter. The two deduced prevailing wind conditions in the area are summarized in 'Fable 111, along with the behavior of Syrtis Major according to Antoniadi (1930). (See also Fig. 14.) I)ust clouds seem to be most c o m m o n in the area during n o r t h e r n a u t u m n and winter when T y p e A conditions prevail. Possibly this is because it is t h e n southern spring and s u m m e r and there is good evidence t h a t it is during t h i s s e a s o n and in this hemisphere t h a t most dust is picked up. Certainly Table II indicates t h a t there

FIG. 14. Apparent t,opographie control of albedo discontinuity at the western boundary of Syrtis Major. Two versions of Mariner contrast-enhanced photograph 7F9 l.

CONSEQD'E:N'CES OF MAI~,TIA:bl WINDS

is much less dust in the atmosphere around Syrtis Major during Type B conditions t h a n during Type A conditions. Type A winds carry considerable dust which t h e y deposit along the eastern slopes of Syrtis Major making it much narrower and decreasing its contrast. The effect according to Table III should be a m a x i m u m between ~ = 296 ° and ~ = 45 ° . It is significant t h a t Antoniadi notes t h a t Syrtis Major begins to narrow soon after ~ = 290 ° and reaches its minimum width near = 15 °. During northern summer, Type B winds prevail. T h e y must carry little or no suspended dust, since dust clouds are seldom observed in the area at this time. Blowing off the Aeria plateau towards the east, t h e y denude the eastern slopes of Syrtis Major. Such winds are apparently import a n t near ~ - 2 3 0 ° (Tables II and III). Syrtis Major then expands east into Libya and its darkness is enhanced by the removal of small, bright particles. Significantly, according to Antoniadi, Syrtis Major achieves its maximum width near 230 ° . One implication of the above explanation is t h a t the increased width and darkness of Syrtis Major should be accompanied by the e n h a n c e m e n t of Moeris Lacus and Nepenthes-Thoth, while these dark regions should become indistinct as Syrtis Major becomes narrower and more diffuse during the northern winter. This prediction seems to be borne out by the observational record (Antoniadi, 1930; Slipher, 1962). Continuing studies of Mars by 1971 orbiters and their follow-ons should permit similar types of analyses to be made of all areas on Mars undergoing seasonal changes by windblown dust. We do not mean here to exclude the possibility t h a t there are areas undergoing seasonal changes for other (e.g., biological) reasons. We merely stress t h a t there is no compelling evidence for such alternative mechanisms. ACKNOWLEDGMENT W e are i n d e b t e d to P a u l F o x , S t e p h e n Ginsborg, a n d H u g h P h i b b s for the careful

277

c o n s t r u c t i o n a n d p a i n t i n g o f t h e Mars T o p o g r a p h i c a l Model, a n d to C o n w a y L e o v y for reading the manuscript. This research w a s supp o r t e d i n p a r t b y N A S A J P L C o n t r a c t 952487 a n d in p a r t b y N A S A G r a n t N G R 33-010-098.

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('A]~L S A ( I A N , J O S E P t t V E V E R K A ,

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