VOL. 78, NO. 26 JOURNAL OF GEOPHYSICAL RESEARCH Variable Features on Mars, 2, Mariner 9 Global Results C. Sacan, J. Veverka, P. Fox, R. Dusiscu, R. Frexcu, anp P. GIERASCH Laboratory for Planetary Studtes, Cornell University, Ithaca, New York 14850 L. Quam, J. LepErBERG, E. Levinruar, R. Tucker, anp B. Eross Artificial Intelligence Laboratory and Department of Genetics, Stanford Medical School Stanford University, Stanford, California 94305 J. B. PoLLtack NASA Ames Research Center, Moffett Field, California 94035 Systematic Mariner 9 monitoring of the space and time distribution of Martian bright and dark markings, the streaks and splotches, indicates a range of global correlations. The time- variable classical dark markings owe their configurations and variability to their constituent streaks and splotches, produced by windblown dust. Streaks and splotches are consistent wind direction indicators. Correlation of global streak patterns with general circulation models shows that velocities ~50 to 90 m/sec above the boundary layer are necessary to initiate grain motion on the surface and to produce streaks and splotches. Detailed examples of changes in Syrtis Major, Lunae Palus, and Promethei Sinus are generally consistent with removal of bright sand and dust and uncovering of darker underlying material as the active agent in such changes, although dark mobile material probably also exists on Mars. The generation of streaks and the progressive albedo changes observed require only threshold velocities of about 2 m/sec for about 1 day at the grain surface. We propose that the dark collar observed following the north polar cap in its retreat is produced by the scouring of bright overlying dust from the polar peripheral ground by winds driven by the temperature differences between frosted and unfrosted terrain. The stability of bright streaks and the variability of dark streaks and splotches, as well as their contrast, can be the result of size differences JULY 10, 1973 of the constituent particles. One of the principal objectives of the Mariner 9 mission was to examine, at high resolution and extended time baseline, the surface albedo variations on Mars. The preliminary results of this investigation have been presented by Sagan et al. [1972; here called paper 1]. The time- variable Martian dark areas and representative semitone areas were found commonly to be resolved into two kinds of fine structure: streaks and splotches. Most streaks emanate from eraters, although some begin at positive relief features. Bright streaks tend to be long and narrow; dark streaks, shorter and broader. Typical streak lengths are tens of kilometers. Splotches are irregular markings that exhibit Copyright © 1973 by the American Geophysical Union. a significant tendency to be located inside craters, often asymmetrically against a crater wall. Larger splotches may wash over crater ramparts onto adjoining terrain. A large variety of splotches and streaks have been systematically observed during the mission. In many cases, all three photometric angles were held nearly constant so that true variations in the albedo of these. markings could be sepa- rated from changes in shadows and the effects of the surface scattering function. These ob- servations were designed to detect relative albedo changes within the field of view. No conclusions are based on absolute photometry. Major variations in splotches and dark streaks were uncovered with a characteristic time scale for variations of <2 weeks. Variations on a 4163 4164 SacAN ET 4t.: Mariner 9 Mission time scale of 1 day were sought: such rapid variations must be relatively uncommon, as none were discovered. No variations of bright streaks, either in production or dissipation, were found. In several places, most notably Syrtis Major, the configuration of dark and bright streaks corresponds remarkably well to the classical earth-based configuration of the dark area that the streaks constitute. Observed time variations in the distribution of such streaks correspond to regions in which the albedo variations were previously observed from earth. Paper 1 proposed that the distribution and time variation of streaks and splotches are the esuses of the classical seasonal and secular vari- ations of Martian albedo markings. Two principal hypotheses were propoged in pre-Mariner 9 days to explain these variations: biology and windblown dust. Although con- vincing evidence against biological explanations are not forthcoming, mainly because a wide range of properties can be proposed for hypo- thetical Martian organisms, the evidence of paper 1 is strongly in favor of windblown dust producing streaks and splotches and their time variations, A range of explanations of the streaks and splotches was mentioned in paper 1. For example, bright streaks might be produced by bright dust trapped in negative relief dust sinks (such as crater bottoms) in the waning stages of the dust storm, and subsequently deflated by strong gusts. Or fine dust deposited uniformly by a dust storm may subsequently be stirred up, suspended, and blown away because of collisions with saltating grains everywhere but in the lee of crater walls, where the wind velocities are low. Examples of both of these cases on a smaller seale are known in Antarctic dry valleys [Aforris et al., 1972]. Alternatively, dust darker than the mean albedo of a given area might be layered down by the wind from a prevailing direction everywhere but in the lee of crater ramparts. But in these cases, as with other explanations of the streaks, they will point in the direction of the wind flow. Thus it was proposed in paper 1 that the streaks may be natural wind vanes and possibly anemometers placed on the Martian surface. In this paper, we present global maps of the streaks, streak-splotch correlations, and a com- parison of wind flow patterns deduced from these maps with results on the general circulation of the Martian atmosphere. We derive a new estimate of the wind velocities necessary 10 initiate dust movement on the Martian surface, exhibit detailed examples of variations in three recions of Mars, and present new conclusions on the mechanisms of wind-initiated albedo changes. GLopaL STREAK Alaps The entire surface of Mars was mapped for streaks, both bright and dark. Mapping was based primarily on Ozalid mosaics of wide- angle pictures prepared by the Astrogeology Branch, US. Geological Survey. These pictures are a convenient source, allowing easy mapping of streaks that traverse wide-angle picture boundaries, but they also limited our effective minimum detectable streak length to about 10 km. There are many smaller streaks on narrow-angle pictures, but Mariner 9 did not provide adequate narrow-angle cover- age of Mars to permit useful high-resolution streak mapping. The mosaics cover the time interval between revolutions 101 and 222, corre- sponding to L, = 320° to 353°, or late summer in the southern hemisphere. Our results apply only to this season. Quite different results may apply to other seasons, inasmuch as only the appearance of dark streaks, never the removal of dark streaks nor the appearance of bright streaks, was observed during this time interval. Such a situation cannot continue indefinitely if Mars is to maintain the general appearance to earth that it has exhibited for more than a century. On the other hand, the frequency of alteration of streaks observed must represent some inte- gration of streaks produced recently and streaks produced at some more remote times, perhaps of the order of 1 year in the past. Streaks were mapped by four coauthors of this paper, and there was significant overlap among their counts to guarantee no major personal systematic errors. The relative number of marginal streaks proved to be few, primarily eases on the border between short streaks and long splotches spilling over crater walls. The final streak maps were carefully proofed against the original compilations. In the present study, we are concerned only with the weather vane, not with the hypothesized ancmometer, aspect of the strenks, Accordingly streak lengths are Sac displaved on two seales: the short ; spond to streaks <60 km long; the correspond to streaks >60 km streaks are represented by solid a: streaks by dashed arrows. The re: played in Figure 1 in Lambert cc Mercator maps. There are area: number of parallel streaks was si given small area that representa’ streak on maps of this scale woul! impossible (cf. paper 1, Figure 1] regions of great arrow density, o sentative sampling of streaks is i) loss in generality for streak dire from this convention. Figures 2 and 3 show the streaks projection on earth-based albedo : 9 topographic maps, respectively. 7 of the north polar hood and an ap; paucity of streaks in the south region for the Z, of these observ: reason for the blank areas in th the absence of streak maps for r of —65°. In Figures 2 and 3, only tl streak direction within each 10° square is shown. Where there are tw directions, both are shown. As is the rosette diagram for the Solis ] (Figure 4), one or two prevailing d almost always a good approxima there is a notably high density of grid square, it is represented by a Wind directions have been plotte bright and dark streaks, which in s give concordant and in other region directions, probably because they produced at different times. IMPLICATIONS OF THE STREAK We have previously shown (pape 16) that the configuration of Sy corresponds well to the distributi stituent dark streaks. In Figure 55 plot the distribution of streaks in of Solis Lacus, a well-known se secular variable. The earth-based Solis Lacus, circa 1969, after the servatory cartography, also is sh sidering the variability cf this f agreement between the classical cc and the locus of streaks is excellent with our previous conclusions. We p ION ian atmosphere. We derive a new the wind velocities necessary to movement on the Martian surface, led examples of variations in three fars, aud present new conclusions hanisms of wind-initiated albedo Groat STREAK Maps surface of Mars was mapped for h bright and dark. Mapping was ily on Ozalid mosaics of wide- es prepared by the Astrogeclogy . Geological Survey. These pictures lient source, allowing easy mapping that traverse wide-angle picture but they also limited our effective etectable streak length to about here are many smaller streaks angle pictures, but Mariner 9 vide adequate narrow-angle cover- < to permit useful high-resolution ning. The mosaics cover the time veen revolutions 101 and 222, corre- L, = 320° to 353°, or late summer yern hemisphere. Our results apply season. ‘erent results may apply to other smuch as only the appearance of :, never the removal of dark streaks ypearance of bright streaks, was uring this time interval. Such a nnot continue indefinitely if Mars iin the general appearance to earth exhibited for more than a century. -r hand, the frequency of alteration sybserved must represent some inte- treaks produced recently and streaks t sume more remote times, perhaps of 1 year in the past. vere mapped by four coauthors of and there was significant overlap ir counts to guarantee no major tematic errors. The relative mumber streaks proved to be few, primarily e border between short streaks and hes spilling over crater walls, The’ maps were carefully proofed against compilations. In the present study, corned only with the weather vane, we hypothesized ancmometcr, axpect Accordingly strenk lengths are rs CLINE SaGaANn ET AL.: Mariner 9 Mission displayed on two scales: the short arrows corre- spond to streaks <60 km long; the long arrows correspond to streaks >60 km long. Dark streaks are represented by solid arrows, bright streaks by dashed arrows. The results are dis- played in Figure 1 in Lambert conformal and Mercator maps. There are areas where the number of parallel streaks was so great in a given small area that representation of each streak on maps of this scale would have been impossible (cf. paper 1, Figure 16). In such regions of great arrow density, only a repre- sentative sampling of streaks is indicated. No loss in generality for streak direction results from this convention. Figures 2 and 3 show the streaks in Mercator projection on earth-based albedo and Mariner 9 topographic maps, respectively. The existence of the north polar hood and an apparently real paucity of streaks in the south circumpolar region for the L, of these observations is the reason for the blank areas in the north and the absence of streak maps for regions south of —65°. In Figures 2 and 3, only the prevailing streak direction within each 10° x 10° grid square is shown. Where there are two prevailing directions, both are shown. As is indicated in the rosette diagram for the Solis Lacus region (Figure 4), one or two prevailing directions are almost always a good approximation. Where there is a notably high density of streaks in a grid square, it is represented by a thick arrow. Wind directions have been plotted for both bright and dark streaks, which in some regions give concordant and in other regions discordant directions, probably because they have been produced at different times. IMPLICATIONS OF THE STREAK Maps We have previously shown (paper 1, Figure 16) that the configuration of Syrtis Major corresponds well to the distribution of con- stituent dark streaks. In Figure 5 we similarly plot the distribution of streaks in the vicinity of Solis Lacus, a well-known seasonal and secular variable. The earth-based outline of Solis Lacus, cirea 1969, after the Lowell Ob- servatory cartography, also is shown, Con- sidering the variability of this feature, the agreement between the classical configuration and the locus of streaks is excellent, consistent with our previous conclusions, We propose that 4165 marked secular changes in Solis Lacus [e.g., Antoniadi, 1930, p. 140] are due to extraordinary wind regimes, redistributing fine dust in this area. Other factors being equal, small dark regions surrounded on all sides by bright areas should be more susceptible to eolian secular changes [Pollack and Sagan, 1967]. Ultimately a reconstruction of several different wind re- gimes, a kind of eolian stratigraphy, should be possible from data such as that exhibited in Figure 5. We return to this subject. else- where. In addition to specific eases, such as Syrtis Major and Solis Lacus, we see from Figure 2 that the global distribution of streaks corre- sponds well to the general configuration of dark areas as viewed from earth. In these figures there are many cases where two different flow directions are in evidence. We interpret these as the remnants of the flow regimes in the Martian atmosphere at two different epochs, probably two different seasons. It algo is possible that a subset of flow directions belongs to the great 1971 dust storm and is not typical of wind patterns in the absence of global storm systems. In the latitude band between 20°N and 20°S (regions MC-S to MC-23) there is a clear tendency for the flow to be from the NE north of the equator and from the NW south of the equator. This pattern corresponds to the zonally averaged surface wind calculated by Leory and Mintz [1969] for southern hemisphere summer. It also appears in the wind field maps derived from the Mariner 9 infrared interferometer spectrometer (Iris) experiment (Figure 6). The Tris fields sre determined from the pressure-temperature profiles obtained by in- version of the infrared emission spectrum. The results above the surface boundary layer dis- played in Figure 6 were derived by J. A. Pirraglia (cf. Hanel et al., 1972; Conrath et al., 1973] from temperatures during the dust storm. An approximate dynamical theory is used. It is especially important for our purposes that: (1) topography is neglected, and (2) the results are most uncertain close to the equator, Winds at the equator are probably overestimated. The general configuration of the flow pattern is probably correct, however, and the agreement between the caleulated winds and the streak wind 4166 Sagan ET aL.: Martner 9 Mission 85e NORTH SS Sem NORTH et ee NORTH er a —-— ‘ 60? . : - a 4 5 . 550° \ 50. : ‘, : a o 4 5 S00. g AQ go. = 14, S| age . 450 Se. . - 2 — \ 40 : 400 4 ° \ . ~t 4 25e Jy LS \ : 950: Ne 4 x } ie" nae “SL ° 29s : 44 = P20 Vi 1006 ° g0° i Tage 150° 140) 90 SOUTH soe MC-2 : OS: i . NORTH S50 . ' i a sm, NORTH " | Oop ’ Bn ee : Q * B00. : ' S50 R : oy 500 [ £ 2 450 ~~ Wo, o 250, : 300. ° . o Oo rm pe F5QE OP Sage 330° 320° SOUTH SOUTH MC~4 MC-5 * NORTH “te. NORTH cane, S06 4 S00. * *, ’ : S50f \ Sgoe 4 f 4 : , i ~ { iy 3007 { £ é 450f 400, ° f SGo0fF 300 > 26° “2708 260° SOUTH MC-6 2208 “Sipe 200° SOUTH MC~?7 Fig. 1. Streak maps of Mars. Solid arrows represent dark streaks; dashed arrows represent bright streaks. The arrow length is about four times the streak length, but is approximate. The shortest arrows shown (2° of latitude long) represent streaks <60 km in length. Regions of the planet north of 50°N could not be mapped for streaks because of obscuration by the polar hood. The absence of streaks in these regions, evident in Figures 1 and 2, may not be real. wn NORTH BQO TT ee 7 25e- 20°- a 15° eae 10°- se OF 180° 170° 160° 15C SOUTH MC-8 NORTH 30°: eco — 25° 20°- WEST aN WEST z 8 eerie te oO o repent aeaes orientations in the latitude b: remarkably good. At equatorial latitudes the ca ure strongest and are practical higher latitudes their magnitude. the direction varies during a Pirraglia’s calculation isolates t steady winds from the diurnally - SION NORTH T 7 \ x ‘ Va Y “poy a= \ oo / \ 3 a LEE mp Sf a Es 90° 100° 90° g0° SOUTH MC-3 oo ORE ee et Qh Lae we, . ee 0 340° 330° 320° SOUTH MC-5 a awe A gs - ss 790° 220° zige 200° SOUTH MC-7 streaks; dashed arrows represent reak Jength, but is approximate. reaks $60 km in length. Regions ‘s because of obscuration by the in Figures 1 and 2, may not be SAGAN ET aL.: Martner 9 Misstox 4167 NORTH 30° pr BR ren 25°F : r : 20° : b - B ; : Bisel a = 5 13 : 7 L + 1o°- : 5° 2 L : f 2 OF a L L : bad : . - 190° 170° 160° 150° 140° SOUTH MC-8 NORTH 30°1 r + —~ [ : b - b ~ 25°F : b 4 F i 20° : 4 - 5 ik 2 F “ =2 = 7: . is b “ 4 10° 3 : a : Ser te “ : Cie a 7 - , : ort bowl : ae ba ; 90° gO° 70° 60°" Y * SOUTH MC-10 NORTH FOF rr ee rere t : 23° : if L 2005 : oes “5 2 Sr ” < i - 10°F oy - be Sl AY eo Z wad var ' “ fie Yi? / z ort 7! { : ° —/ , 340° 336° «3.208 ‘ SOUTH MC-12 . NORTH goer ER oe m rey 1 25° “at 202 “ 2 ee 7 ape wa yse> a“ Ie zr 12 ~ / 4 10° : ” : ’ 3 or . j : 4 Orr : 4 Cod 130° 120° 1109 100° 90° SOUTH MC-9 NORTH ee tp eee } “ ; 3 i : { ! 4 ‘ 1 vs 4 a 4 Y (oak w \ i , iQ = wy Uf, a8 IU FG i \ fae fo 4 PONY ¢ , ; a : iif 4 40° 30° 20° 10° oe SOUTH MC-11 NORTH 30° oe — er 95°- 20°. & : = 15°. 28 yo°: , 5: ce os 210? 270° Fig.l. (continued) orientations in the latitude band -£20° is remarkably good. At equatorial latitudes the caleulated winds are strongest and are practically steady. At higher latitudes their magnitudes are less and the direction varies during 2 diurnal evele. Pirraglia’s calculation isolates the lurge-xeale steady winds from the diurnally varying winds. We know that there is a range of other winds expected on Mars, including slope and obstacle winds driven by the large clevation differences [Gierasch and Sayan, 1971. Sagan et al., 1971; Blumsack, 1971] and winds driven by the large temperature gradient between frosted and anfrosted polar ground [Leovy ef al. 1973]. We might expert such winds to dominate at 4168 SAGAN ET aL.: Martner 9 Mission NORTH NORTH 30° eeenee: Ses ose ae re or ey 30°07 T T T ; Oe : : | 25°F a | 25° t a : t , 4 20°7 ’ a ay # “at j - 1 6 » a “os ie “1B wa : rs a n” 4 fy ; << = os , “3S ju ; e : 4 j | 2 3 ; t , 3 ot, , , 3 7 270° 260° 250° 240° 230° SOUTH MC-14 NORTH a : orf | ~ : : 4 “5c 4 -5ee t : i : : °° 7 -10°- 1 =10°r 4 10°: 1 - t y : e & : 3 i- WY Lysol ‘, 24 -15e- dy 2 a . : < 5 : 4 < : i t “20° z -20°- 6 OS ‘ 1 . -25°[ SI ~25%F 1 t P C i ~30°" fee fe 930% EROS 189° 170° 160° 150° 140° 130°77 Ce 110° 100° 90° SOUTH SOUTH MC~16 MC-17 NORTH 2 opt TIT ITT TT TAT TTT TT?) 4 + : { : roy | : j 4 1 j j & in & i, = ig = 1s i | { } ; 4 2 | 7 j \ NS : } vo 30° 20° 10° 0° SOUTH. MC-19 Fig. 1. (continued) higher latitudes, and the steady component that Pirragha calculates to dominate at low latitudes. The streak maps (Figures 1 to 3) support this contention. The flow indicated at high latitudes does not correspond to that predicted either by Pirraglia or by Leovy and Mintz. An interesting regularity at higher latitude is the indication of a steady easterly flow component at latitudes 20° to 40°S. This is the latitude range and initial direction of flow of the great 1971 global dust storm, and it is possible that this flow component is a marker of that storm. Other peculiarities may be connected with SAG NORTH oe - a ~5° : -10°. ™e, th * -} oO s BS) =29°- WEST NORTH on my 38 ~ * Ly af ae _ % - as o “ yt a oe a ge es 150935 \ SOUTH MC-24 topography. There is the appeara flow away from a region centet 110°W. This is the area of Tharsis region on Mars. The flow mar! wind transport downhill here, altt <— TT T EAST EAST Obes dd ee a ° ‘ation of a steady easterly flow at latitudes 20° to 40°S. This is range and initial direction of flow 1971 global dust storm, and it is . this flow component is a marker L. uuliarities may be connected with SaGan ET au.: Martner 9 Mission 4169 “10°. an WEST 4 ua 3 ae \ “30°F 270° SOUTH . MC-22 NORTH awe Ny ae ~ oO wa - e c ok “ & % 7 “ ot $ “ ro oe eee 1508 Peo Wb 18S 352 e SOUTH Ge MC-24 Fig. 1. topography. There is the appearance of radial flow away from a region centered at 5°N, 110°W. This is the area of Tharsis, the highest region on Mars. The flow markers suggest wind transport downhill here, although this is NORTH 0° oo Lay Sl ltpeee _ I, “T oy i ve -5°. soe. 7 : “15° a 28 2 & =20° -25°. -30%° Fee eee eee 310° 300° 2909 280° 270° SOUTH MC-2) NORTH ee - 1 ] : : 4 as ig s “008 1902 180° “SOUTH MC-23 NORTH cee Se age. > ee Se oo = FL me or ~ ~ ye ‘. . oS % Pa Loe a = (continued) not the predicted flow direction in the vicinity of such elevations in current theories during daytime. Such downhill flows are common on earth at night, however, owing to radiative eouling of the ground. A divergence or bifur- 4170 SAGAN ET AL.: NORTH ae X : 7 MS \ oe . : Lhe > : Zz 20° 7 & ae \ a, I> ae ER OTB es 5 a SOUTH MC-26 NORTH as eaten —— 0° ; 3 e. . oe : ww” os % . 28 . vw % ao *y 7 j 8 / 3S: Co “ _ + Olea a “ee? < 38° 270° 2558 el SOUTH We MC-28 Fig. 1. cation of the flow patterns near 20°S, 110°W, seems to be connected with the presence of rough terrain there (Figure 1c); the winds appear to move to avoid the rough region. The preceding results provide us with a new estimate of the threshold velocity necessary to initiate grain motion on the Martian surface. There is a modest variation among various recent estimates of the critical velocity V above the surface boundary layer necessary to initiate grain motion on the surface. Sagan and Pollack [1967, 1969; see also paper 1] estimated this velocity at about 65 m/see for a 15-mb surface pressure level, about SO m/see for 10 mb, and about 110 m/sec for 5 mb. Golitsyn [1973] proposed that these values may be reduced by about 38066 by introducing sharp roughness gradients. Hess [1972], recalculating the problem, derives V for an S-mb surface pressure to be between 38 and 60 m/see, depending on Martner 9 Mission NORTH we Tr pa 7 _ ~N ~ “ ae — 23 ; “, . ; oe ey . we > oe _ “ : Th “2 es ae “oe ~ ey ‘ “ys “eo 330° 5S SOUTH 5000 MC-~27 NORTH ee ~ ae 4, os — — 2 - Oh -\ Zh an . c F Oh ie 3% = : os a “A > x / 2 wh YN yf ‘> - : : S spr ‘ sa ao fo : Oh i _ Ps Sees S10 SS 18" 210° 1950 ~~! 2 SOUTH Se MC-29 (continued) the velocity distribution function through the boundary laver. Yet another independent esti- mate by Gierasch and Goody [1973] is about 30 m/sec for a surface pressure of 5 mb. The range among these models is about a factor of 2 for comparable ‘surface pressures for the threshold velocity V,, to initiate grain motion at the ground but 4 for the velocity due to differences rises to a factor of almost above the boundary layer among the models in the assumed functional form of the velocity dis- tribution through the boundary layer. Dis- tinguishing among these results is a matter of some importance for understanding the generation of dust storms and eolian transport on Mars. It also has a more practical im- portance: Lower velocities present no problem to a lander mission such as Viking, whereas the higher velocities pose grave hazards. The existence of a 40° wide equatorial Fig. 2. Prevailing streak direct Lowell Observatory earth-based a areas. latitude band in which the streal follow strikingly the mean general with significant deviations outside indicates that the global wind veloci borders of the band are approxir velocities appropriate for initiating ¢ ment on the surface. From the I SR! Te Tomo ton ott marcia ie Fig. 3. Prevailing streak direction Marh De meere ee * SION NORTH Cee ~~ ae 4 oY : NN f a ; J : Z a a“ “fs ve a = o Ve 4 ye 545° “330° 3 age SOUTH 5000 MC-27 NORTH _ 5 ’ af ad. " . 325° 210° ges T SOUTH Ss MC-29 distribution function through the ver. Yet another independent esti- rasch and Gveody [1973] is about r a surface pressure of 5 mb. The - these models is about a factor of arable “surface pressures for the locity V,, to initiate grain motion vd but rises to a factor of almost elocity above the boundary layer ences among the models in the ictional form of the velocity dis- rough the boundary Jayer. Dis- among these results is a matter portance for understanding the f dust storms and eolian transport t also has a more practical im- ower velocities present no problem mission such as Viking, whereas ‘alocities pose grave hazards. ence of a 40° wide equatorial SaGan ET AL.: Mariner 9 Mtsstow 4171 Fig. 2. Prevailing streak directions (ay eraged over 10° by 10° squares) superimposed on a Lowell Observatory earth-based albedo map of Mars. Thick arrows indicate heavily streaked areas. latitude band in which the streak patterns follow strikingly the mean general eirculation, with significant deviations outside this band, indicates that the global wind velocities at the borders of the band are approximately the velocities appropriate for initiating grain move- ment on the surface. From the Iris results Fig. 3. Prevailing streak directions (averaged over 10° by 10? x Mariner 9 topographic map of Mars. (Figure 6), these velocities range from V = 50 to YO m/sce above the surface boundary layer. Were topography introduced, somewhat higher velocities would be implied [Gleraseh and Sagan, 1dv1}. Because grain transport typically occurs on time scales of ~10 davs rather than ~1 day (this paper), it is not the mean wind of the Hares) stiperlmposed on the SAGAN ET AL! as — =I lo WEST SOUTH Fig. 4. Rosette diagram of the wind streaks in the Solis Lacus region shown in Figure 5. The number of dark snd bright sireaks is plotied as a function of azimuth. general circulation, but the high-velocity tal of the distribution function of the general circulation winds that must be operative. These velocities He in the middle of the range summa- Manninen 9 Misston rized above and derived from quite diferent con- siderations, Moreover, a recent revision of the wind velocities at these altitudes at the start of the dust storm, for the numerical circulation models of Leory and Mirtz [1960], raises these values to 40 to GO m,see at equatorial latitudes [Leovy et al.. 1973]. Velocities derived fram the wavelengths of lee wave clouds ‘Leocy et al. 1972} are of this sume order. Porar Winbs As we have mentioned. few wind indicators ure present ut very high latitudes, both because of obscuration and possibly because of efficient scouring of fine particles trom the polar regions. The high sploteh density in the south circum- polar region in some sense compensates for the region's low streak density: there is a marked ubsence of streiks in the must heavily splotched polar regions. Nevertheless. there are a few places both in the uorth (Mare Acidalum, MC-4) and in the south (Mare Australe, MC- 25) where many streaks are directed away from the poles. This is hardly an mvariable high- latitude phenomenon (there is also a westerly circumpolar component), as imspection of Figure 1 (MC-24 to MC-29) clearly shows; but it nevertheless seems noteworthy. The most straightforward explanation would be in terms of the polar cap temperature gradient winds toby Fig. 5. Streak map of the Solis Lacus region of Mars superimposed on an earth-based albedo map of the urea. Solid arrows represent dark streaks; dashed arrows represent bright streaks. The arrow length is appoximately four times the length of the streak. Where a num- ber of similar streaks occur close together, only one is shown with a number to indicate how many such streaks there were. Craters with short. incipient durk streaks are mapped as circles with short arrows in the direction of the incipient tail. Saga? postulated hy Leory et al. [1973]. fleeing streaks in Mare Acidalinm are because this mare ix one of the few at these latitudes for which seazor are expected at the observed xer possible that seasonal albedo variatic latitudes are driven by such polar most likely mechanism would he removal of bright overlying dust, re darker material underneath, as pos several varieties of seasonal changes | Pollack, 1967, 1969; Sagan et al. 16 1}. A related phenomenon may be the 3 collar. This dark band, surrounding : ing the northern ice eap in its sumr to the pole, has been reported by m observers [eg., de Vaucouleurs, 195- 1973]. Because of possible psvchopl contrast effects, and especially beea interpretation of the northern moistened ground, the very exister collar has been called into question years, Mariner 9 extended mission -pl inne eee eee Ee Fig. 7. The north polar cap of Mz late spring in the northe aN I derived from quite different con- oreover, a recent revision of the -at these altitudes at the start of n, for the numerical circulation ry and Mintz [1969], raises these » 60 m/see at equatorial Jatitudes 1973]. Velocities derived from the fF lee wave clouds [Leovy et al., is sume order. PoLaR WINDS mentioned, few wind indicators very high latitudes, both because and possibly because of efficient e particles from the polar regions. tech density in the south cireum- 1 some sense compensates for the freak density; there is a marked aks in the most heavily splotched Nevertheless, there are a few in the north (Mare Acidalium, . the south (Mare Australe, IC- any streaks are directed away . This is hardly an invariable high- oymenon (there is also a westerly component), 4s inspection of C-24 to MC-29) clearly shows; eless seems noteworthy. The most ‘do explanation would be in terms cap temperature gradient winds perimposed on an earth-based dashed arrows represent bright h of the streak. Where a num- with a number to indicate how k streaks are mapped as circles SAGAN ET at.: Martner 9 Misston All: postulated by Leovy et al. [1973]. The pole- fleeing streaks in Mare Acidalium are interesting because this mare is one of the few dark areas at these latitudes for which seasonal changes are expected at the observed season. It is possible that seasonal albedo variations at high latitudes are driven by such polar winds. The most likely mechanism would be the eolian removal of bright overlying dust, revealing the darker material underneath, as postulated for several varieties of seasonal changes [Sagan and Pollack, 1967, 1969; Sagan et al.. 1971; paper 1): A related phenomenon may be the north polar collar. This dark band, surrounding and follow- ing the northern ice cap in its summer retreat to the pole, has been reported by many visual observers [e.g., de Vaucouleurs, 1954; Dollfus, 1973]. Because of possible psychophvsiological contrast effects, and especially because of the interpretation of the northern collar as moistened ground, the very existence of the collar has been called into question in recent years. Mariner 9 extended mission -photography Pa vba ee bee ee xp e ee ee ew Spo eee, Aen = as =: iodd. = ENNNNN 7 yen where il feurvy ebanvs wpe ea eee ay seen Fig. 6. Iris wind fields determined from pres- sure-temperature profiles of the Martian atmo- sphere. The season is summer in the southern hemisphere. The wind direction is into the grid points; a wind veetor equal in length to one grid spacing corresponds to a velocity of 80 m/sec (courtesy of F. A. Pirraglia). has demonstrated unambiguously (Figure 7) the existence of the north polar collar. Under the circumstances, the report by earth-based visual observers that the collar follows the retreating polar cap toward the pole must now be taken seriously. Fig. 7. The north polar cap of Mars, showing the prominent dark polar collar. The season is Jate spring in the northern hemisphere (MTVS 5019-40, DAS 13317330). +174 Both the existence of the collar and its retreat ean be understood in terms of polar winds, deflating s thin, bright surface Iayer of dust in the immediate vicinity of the northern polar cap edge. But why did Mariner 9 find no evidence for a south polar collar? If the south collar visibility indeed follows the curve of de Vaucoweurs [1954, Figure 31], peak visibility corresponds to EL, -~ 210°. Mariner 9 did not observe Mars near this L,. Another curious circumstance its that the southern cireumpolar streaks are, almost with- out exception, dark streaks, whereas the northern streaks, und especially the Mare Acidalium streaks, are largely bright during Mariner 9 obzervations. One possible explanation of some of these phenomena is the following: The steep tempera- ture gradient between frosted and unfrosted terrains at the edge of the retreating polar ice cap produces strong winds. These winds de- flate fine, bright surficial dust, uncovering the AT SaGaN ET AL.: Martner 9 Mission dark splotehy terrain that follows the waning periphery of the eap. The transported bright material is layered down somewhat equator- ward, producing bright circumpolar. streaks. High polar winds are also consistent with the probably eolian etch pits seen exclusively in polar regions. CORRELATIONS OF STREAKS AND SPLOTCHES We also have mapped the distribution of erater splotches over the entire Martian surface, using the techniques and cautions described above for the streak mapping program. Splotches spilling over crater ramparts and splotches unconnected with craters were not mapped. Streaks and splotches in five repre- sentative regions of Mars are displayed in Figures 8 through 12. The convention for representing streaks is deseribed above. Splotehes are represented by sketches within erater boundaries showing the approximate configurations of the splotches. Only craters EAST id Fig. 8. Splotch-streak map of the MC-10 region. This is the area surrounding Lunae Palus. All craters larger than about 50 km in diameter and all dark crater splotches are shown. Solid arrows indicate the directions of dark crater tails; dashed arrows, bright. crater tails. The ur- row length is approximately four times the length of the tail. The shortest arrows shown are 2° (of latitude) and represent crater tails <60 km in length. MAGA? 4 A 30° TOT TTT TTF L . fe Se 5 a“, 7 25 20%-) = oO Cc - +e Ost 0 a = L mr et ” wey r “ ee * oO.’ 1 Oe L eK oY « a L NS sy . oa \ oO b YS L “0 on biti por dipiua | eee 220° ais 210" Fig. 9. Splotch-stre OITA -5*% r © L ae” 3 2 15th = L . , # i r SA : \. ‘4 ; -20%- ‘ = ‘ \ L OX ; be RN SN ss 25th ON ww RAN * LL ‘} “300 diy Pe 35° Soe Fig. 10. Sploteh-str ION y terrain that follows the waning the cap. The transported bright ayered down somewhat equator- cing bright cireumpolar. streaks. vinds are also consistent with the lan etch pits seen exclusively in INS OF STREAKS AND SPLOTCHES ave mapped the distribution of ies over the entire Martian surface, ‘chniques and cautions described the streak mapping program, illing over crater ramparts and connected with craters were not eaks and splotches in five repre- rions of Mars are displayed in hrough 12. The convention for streaks is described above. 2 represented by sketches within daries showing the approximate s of the splotches. Only craters Trp Tr ety rr rt EAST e area surrounding Lunae Palus. ‘rater splotches are shown. Solid ows. bright crater tails. The ar- ~The shortest arrows shown are 30° SaGan et au.: Mariner 9 Mission NORTH 4175 est TTT TT Pt T vy T T TTT tt Dog EAST o° Lou as 2to* Fig. 9. Splotch-streak map of the MC-15 region Cerberus-Elvsium. NORTH -5o a % 15° TTT ITT TTT Tt T T EAST ° C Ll 1,16 4 2, pera ta tt 145° hoe 105* 100° s° SOUTH MCI? Sploteh-streak map of the MC-17 region Tharsis-Daedelia. 4176 SaGan ET au.: Mariner 9 Missiow o NORTH TOD FIO iS 5° (e [e 6 -5° ° | @ re ® OHO Po -10° ny @ @ \ a \ - : nN uM Ww \ rity 8h who \ AY @ raat eo oT . “a = aN \ ” v \ T “gl Tr oe T T © Ghesee” = T T T T Re 6 eo Oo eo O ae” Lins a” 42 30° 270° 265° 265° Fig. 11. Splotch-streak map of the MC-22 region Mare Tyrrhenum-Hesperia. NORTH a - - “ LL. * c sac ee L aoe oO © or NO e @ , @ oO © S + 2 2 @o ° -15*| WEsT EAST 2 e sy @ xu a L : 4 Roy ‘ 4 -30° putt tr a M1 TAiy piss pp tt a Preys a bt 225° eer 2is?s, AW 210° “205% 200° 195° wor * ies° sO” Ny SOUTH ew, N00 meas ‘ ‘ Fig. 12. Splotch-streak map of the MC-23 region Mare Cimmerium-Zephyria, SaGa larger than about 50 km are sho: that splotches tend to be localized face of the containing crater (cf. There is, at least in many cases, for splotches in adjacent craters to in the same face of their respect This face tends to be the direct which the wind has been blowing, 4 the predominant nearby streak wir indicators. AS is usual in our present ignorance surface processes, there are two ¢ of this correlation, each the obve other. In the first view, the winds dark material into the craters, wl accumulated against the leeward ra the second view, the winds have pi deflated bright material off the leewai of the craters, exposing the underly material, possibly basement rock. T! streaks would then bear a closer : to Saharan sand streamers, describe: [1963] as ‘relatively thin, ribbon- or accumulations of sand downwind fro source areas or from topographic ec Where long, straight, parallel and by relatively sand-free strips, the: indicate consistency and direction moving winds.’ Accordingly, we postulate some Mars in which dark mobile mate crater floors is overlain with briy material, perhaps as a consequenc or global dust storms. Subsequent the crater produces long, bright streaks emanating from the erater, a ally exposes underlying dark mate: interior leeward side of the crater, { winds, possibly from another direc can deflate the exposed dark material. incipient dark tails. Dark splot associated dark tails are known, e.g., i [paper 1, Figure 27]. The developme crater tails from craters with pre-exis tails is also known, eg., in Hesperia Figure 23]. These views are consi the conclusions that we will dra detailed study of albedo variations selected regions, Syrtis Major, Lw and Promethei Sinus. In other cases splotchy material may be bedrock dark streaks may be produced by d overlying bright material. © Tyrrhenum-Hoesperia. }:..'. F fs roe PPT - . pitt dk wr at lad © of eC @ °4 ° 4 eo O Oo ef Oo, - SO. | wks g 4 | tay at tu yi 195° so * 165° 18” p Cimmerium-Zephyris. SAGAN ET AL.: MARINER 9 Mission 4177 larger. than about 50 km are shown. We see that splotches tend to be localized toward one face of the containing crater (cf. paper 1). There is, at least in many cases, a tendency for splotches in adjacent craters to be localized in the same face of their respective craters. This face tends to be the direction toward which the wind has been blowing, according to the predominant nearby streak wind direction indicators. AS is usual in our present ignorance of Martian surface processes, there are two explanations of this correlation, each the obverse of the other. In the first view, the winds have blown dark material into the craters, where it has accumulated against the leeward ramparts. In the second view, the winds have preferentially deflated bright material off the leeward ramparts of the craters, exposing the underlying darker material, possibly basement rock. The resulting streaks would then bear a closer relationship to Saharan sand streamers, described by Smith [1963] as ‘relatively thin, ribbon- or banner-like accumulations of sand downwind from localized source areas or from topographic constrictions. Where long, straight, parallel and separated by relatively sand-free strips, they serve to indicate consistency and direction of sand- moving winds.’ Accordingly, we postulate some cases on Mars in which dark mobile material within crater floors is overlain with bright mobile material, perhaps as a consequence of local or global dust storms. Subsequent deflation of the crater produces long, bright downwind streaks emanating from the crater, and eventu- ally exposes underlying dark material on the interior leeward side of the crater. Subsequent winds, possibly from another direction, then ean deflate the exposed dark material, producing ineipient dark tails. Dark splotches with associated dark tails are known, e.g., in Bosporus {paper 1, Figure 27]. The development of dark erater tails from craters with pre-existing bright tails is also known, e.g., in Hesperia [paper 1, Figure 23]. These views are consistent with the conclusions that we will draw from a detailed study of albedo variations in three selected regions, Syrtis Major, Lunae Palus, and Promethei Sinus. In other cases, the dark splotehy material may be bedrock, and the dark streaks may be produced by deflation of overlying bright material. Syrtis Mason At Mariner 9 resolution, the classical albedo feature Syrtis Major is outlined by a concen- tration of bright and dark streaks. The bright streaks dominate near the western (stable) edge of Syrtis Major. The dark streaks outline the eastern, variable edge (Figure 13) of Syrtis Major (paper 1). Telescopic evidence suggests that the eastern -boundary of Syrtis Major varies seasonally [Antoniadi, 1930]. Since the end of the 1971 dust storm, Mariner 9 photography has revealed a gradual darken- ing of Syrtis Major at resolutions of the wide- angle camera (Figures 14 to 16) and the narrow- angle camera (Figures 17 to 20). This darkening resulted largely from the appearance, develop- ment, and merging of dark crater tails of the type seen in Figure 19, which are characteristic of Syrtis Major. In other areas, dark tails do not have the patchy, discontinuous appearance of the Syrtis Major tails. This nonuniform patchiness of the tails and their tendency to shed tangentially from topographic protuber- ances such as crater walls suggest that they are produced by eolian erosion of extensive, but very thin, deposits of bright material, resulting in the exposure of dark, wind-resistant, under- lying material. Radar evidence suggests that the region of Figure 13 is a relatively smooth surface, sloping from west to east (left to right on Figure 13), making it a likely locale for eolian transport. We assume that, after the 1971 dust storm, mutch of this slope was covered by a thin laver of bright dust. Subsequent winds, blowing pre- dominantly downslope, have scoured off this material, especially in regions where wind speeds are intensified by topography. A close exami- nation of the high-resolution narrow-angle pictures (Figures 17 to 20) is especially in- structive in this context. Many instances of dark tails or sections of dark tails shedding off craters can be seen. This is especially evident in Figure 20. We postulate efficient microtraps for the dust scoured off in the above manner. Our model predicts that Svyrtis Major will continue to darken until tr is affected by either a local or global dust storm. The contrasts involved between the dark and bright portions of Svrtis Major are small, being typically about 10 (for the albedo markings in Figure 18). 4178 Sacan Er at.: Martner 9 Missto~ Fig. 13. General view of Syrtis Major. This wide-angle picture, centered at 138°N, 283° W, is ubout 360 km across (MTVS 4186-72, DAS 07147383). As in other regions of Mars, white streaks exist in Syrtis Major but have not been seen to change (Figure 21). Their nature (erosional or depositional) and time of origin (before, during, or after the dust storm) remain enigmatic. Luwae Parts Another region studied lies close to the classical albedo fenture Limae Palns, sus- pected of being both a seasonal and secular variable by telescopie observers [Poeas, 1961]. Mariner 9 photography resolves the region Into a wide, deep channel dissecting relatively smooth surrounding plains (Figure 22). Although no albedo changes in these plains have yet been detected in Mariner 9 pictures, strong localized albedo changes have been found within the channel itself (Fignre 23). At least three separate areas were scen to darken during the mission. The largest af these is ahout 20 by 70 km in size. Only a slight change is evident at wide-angle camera resolution between revo- lutions 125 and 160, but a very pronounced darkening occurred sometime during the 39-day interval between revolutions 160 and 238 (Figure 23). A high-resolution view of the floor of the channel, including a section of the largest of the three darkening areas, is shown in Figure 24. This area is outlined in Figure 22, At the resolution of the narrow-angle camera, it is clear that darkening occurred within the channel between revolutions 125 and 160 (Figure 25), although this iz not evident at wide-angle camera resolution (Figure 23). The two views shown in Fignre 25 do not overlap exactly and are not aligned with rcexpect to each other. They are therefore difficult to compare directly. To faeilitate the compurison, three arrows are shown in each view. The tap two point to a pair of dark, round spots: the bottam arrow points to a characteristie bend jin the small ss SaGan channel] seen in the figure. The view « was obtained on revolution 125: th right, 17 days later on revolution change that has occurred in this regic evident in Figure 26, which shows scaled, similarly projected, and alg of the region surrounding the three Figure 25. The two dark round spc 1 km) can be seen just below the cer Fig. 14. Darkening of Syrtis 4 three rows is about 270 km acro proximately to the upper half of tion 233, Stanford picture differen to right): revolution 233. revohitic revolufion 155, revolution 430, reve Q191, 102105-6-7). The lust row shi Left to right: revolution 153, re 283.9°W; width = 140 km (Stanfc photometric zeometry is closely si on revolution 430. ure, centered at 138°N, 283° W, 3 OF 147383). camera resolution between revo- nd 160, but a very pronounced urred sometime during the 39-day reen revalutions 160 and 238 ution view of the floor of the ling a section of the largest of the ng areas, is shown in Figure 24. outlined in Figure 22. At the the narrow-angle camera, it Is next section, The contrast between nd dark splotches in this region is gions of special interest are desig- ttery in Figure 2s: their behavior Miriner § mission is documented 29 to 34. Figure 29 shows the ; vo . 2 dave fin « ; ~+ : 2 : : - > in fess then 13 days (.c., sometime Fig. 26. Region of the two dark spots and the small channel shown in Figure 25. Left: olttions 99 and 126) of a 10-km revolution 125. Middle: revolution 160. The two views, similarly scaled and projected. were This region, resembling a leaf or aligned relative toorach other and differenced picture element by pierure clement to give the pierre difference ar righy, Note the ckakening in the viemtty of the two dark spots. Centered subsequently remained unchanged, eee ‘ ; ee . ee : ~ at D2O°N, 665° Ws wilth = 30 km (Stanford AIL Picture Product STN 0166. 042904). surrounding areas were undergoing SAGAN ET AL! Mariner 9 Missio~ Fig. 27. 71 750 km across. The solid outline shows the area of Figure 28 (MTVS 42 Fignre 300 shows some of the significant changes that took place in region Bo after revolution 126, (Unforninately, not observed on revolution 99) this region was . In this region the splotch within the erarer changed slightly between revolutions 126 and 179 and. signifi- cantly between revolutions 181 and 220, In region D, however, changes occurred only between revolution IST and 220 (Figure 32), At the time af the fermation of the leaf in region A, a strong ehange occurred in region C (top row of Figure 31). The change consisted in a general grawth of the dark splotch both by the extension of the points on the scalloped edge and by the outward displacement of the amorphous cdec, Na subsequent changes were observed. A sudden change in region Eo was observed between revolutions IS) and 220 (Figure 33), resulting in the extension of the points along the Wide-angle camera view of Promethei Sinus, centered a c 269° W, and about -9, DAS OSOOSOS3S). 8, ll scalloped edge of the splotch. A similar change took place in region F at the same time (Figure 34). A pattern of =wift changes, relatively interrupted by long periods of quicseence, seems typleal of Promethei Sinus. It is significant that regions A and C, which changed drastically hetween revolutions 99 and 126, did not partic- ipate in the major changes that affeeted the entire area between revolutions 181 and 220, A saturation effect seems to be involved. This major change affected surrounding arcas, even at a seale detectable in low-resolution, wide- angle pictures (Figure 35). Winp TunNeL ANALoGs oF Martian Dust Transport A set of experiments posstbly relevant to Miatian eolian dust patterns was conducted: by Hertaler (19660, b| in the MeDonnell Aircraft SacGan Corporation's 14-foot environmental Ambient conditions were 5.35-26.7 mb and room temperature, Two specimens ing Martian surface material were used flour with particle diameters largely between 1 and 100 ym, and a white su with particle diameters Inreely rangi 100 to 700 um. Piles of these powders em thick were placed on emooth a plates at the orifice of a wind tun i velocities measured were at an altitud the surface boundary laver and ranged m/sec. The resulting morphology of wi ; dust apparently depended much more : : cle size and laver thickness than up i velocity, ; In Figure 36 are shown hefore a ' photographs of a layer of sugar sand ex these winds for about 100 min. Here a Fig. 28. is about 80 km across and is centered following figures (MTVS 4213-12, DAS Narrow-angle camera view ered at 71°S, 269° W, and about (MTVS 4211-9, DAS 08008038). re of the xplotech. A similar change .reaion F ar the same time (Figure of swift changes, interrupted by ne periods of qnieseence, scems ‘omethoi Sinus. It is significant that nd ©. which changed drastically vations 99 and 126, did not partic- s major changes that affected the setween revolutions IS1 and 220. A freer seems to be involved. This © affected surrounding arcas, even letectable in Jow-resolution. wide- = (Fieure 85). TUNNEL ANatogs OF MARTIAN Dust Transport experiments possibly relevant 10 an dies pattertis was conducted by aie A) te the MeDornell Aireraft SAGAN ET AL.: Mariner 9 Mission 4187 Corporation’s 14-foot environmental simulator. Ambient conditions were 5.385-26.7 mb pressure and room temperature. Two specimens simulat- ing Martian surface material were used: a silica flour with particle diameters largely ranging between 1 and 100 um, and a white sugar sand with particle diameters largely ranging from 100 to 700 pm. Piles of these powders about 1 em thick were placed on smooth aluminum plates at the orifice of a wind tunnel. The velocities measured were at an altitude within the surface boundary laver and ranged up to 40 m/sec. The resulting morphology of windblown dust apparently depended much more on parti- cle size and layer thickness than upon wind velocity. In Figure 86 are shown before and after photographs of a laver of sugar sand exposed to these winds for about 100 min. Here a marked Tae Sa sat Fie. 28. Narrow-angle camera view of the r embayment is produced by the removal of bright material, revealing underlying dark ma- terial; the wind direction is into the figure, and the extent of the remaining bright material has decreased because of removal. Figure 37 shows a nearly identical experiment with a layer of silica flour exposed to these winds for about 100 min, but with the base plate inclined at an angle of 20°, Long streamers are produced opposite the direction of wind flow, possibly because of the backward gravitational displace- ment of particles. The scalloped material in Promethei Sinus resembles Figure 36 much more than Figure 37. Figure 38 shows the results of an experiment performed on sugar sand for less than 10 min, but at velocities at the surface significantly above the threshold velocities for grain motion. The resulting pattern, hke that in Figure 36 egion outlined (solid) in Figure 27. The pleture is about 80 km across and is centered at 69.6°S, 233.1°W. Lettered regions are pictured in the following figures (MTVS 4213-12, DAS 08079893). 4188 SAGAN ET AL.: Mariner 9 Mission Fig. 29. Changes in region A. Top row (left to right): revolution 98. revolution 126. Stanford picture difference: revolution 126 minus revolution 99. Middle row: revolution 126. revolution 179. revolutions 179 minus 126. Bottom row: revolution 181. revolution 220 revolutions 220 minus 181. The window is about 20 km across, and is centered at 70.1°S J , 253.3°W (Stanford AIL Picture Products STN 0167, 050609, 050610, 050611). but unlike that in Figure 37, resembles Pro- methei Sinus. The difference in scale is about 10°, and so the following comparison of wind tunnel and Mar- tian eolian morphologics must be treated with some caution. From the optieal depth of the great 1971 dust storm, the precipitated thick- ness of atmospheric particulates is easily esti- mated to be about 1 mm. Thus the thin lavers being removed in the wind tunnel experiments are not inappropriate to the Murtian ease. Ag time passes in Figure 37 it IS- minus 179. Fourth row: he window is about fH km across ‘ours STN OL73, 061105. 061106, SAGAN ET AL: Mariner 9 Mission 4191 Fig. 32. Changes in region D. Top row (left to right): revolution 126. revolution 181, revolutions 181 minus 126. Middle row: revolution 179, revolution 181, revolutions 181 minus 179. Bottom row: revolution 181. revolution 220. revolutions 220 minus 181. The window is apout 20 km across and is ventered at 69.3°S. : 0173. 061101, 061102, 061103). seen, while corresponding developments of bright features have not been observed. However, as was pointed out in paper 1, such a trend cannot continue indefinitely if Mars is to maintain its mean albedo. Comparison with Mariner 6 and 7 results and Viking orbiter observations may uncover development of bright material in the complementary seasons. We suspeer that both bright and dark materiale are transported on Mars, Rates or Dust TRANsport From the foregoing data. we derive some simple results on the rates of lateral transport of dust on Mars at times other than during major dust storms, The material transport in the leaf region of Promethei Sinus amounts to 277W cSranford AIL Picture Products STN about 2 km in time intervals <19 davs. Taking a threshold velocity of about 2 m see. the pre- vailing winds transporting the mobile dust must tdeww for abour IO see fabout 15 min) in <19 dave, Sume of the streaks are several hundred > long. Ti we assume that streak ma- xilomeret rorial is transported only at V,,. this imphes occasional strong prevailing winds of this veloe- itv, with no signifeant dissipative currents for about 1 day out of several hundred. If the dust iz carried higher in the boundary layer, the time scale for a pronounced directionality of strong winds can be even less. The continuity of the streaks, unaffected by local topography, arenes for norleeal transport. possibly high in the boundary laver; the remarkable straight- ness of the streaks argues for high winds with a 4192 SaGaw ET av.: Martner 9 Mission Sewer eereerenenRe RUDE eeRTERECEE ; oe ; Fig. 35. Changes at wide-angle re revolution 179. revolution 181. revolu tien 220. revolutions 220 minus 179, 72.4°S, 267.2°W: its outline is show STN 0174, 061306. 061307). Fig. 33. Changes in region E. Top row (left to right): revolution 126, revolution 181, revolutions 181 minus 126. Middle row: revolution 179, revolution 181, revolutions 181 minus 179. Bottom row: revolution 181, revolution 220. revolutions 220 minus 181. The window is about 45 km across and is centered at 69.8°S, 252.5°W (Stanford AIL Picture Products STN 0173, 061112, 061113, 061114). Fig. 34. Changes in region F are shown in the bottom row. Left to right: revolution 181, revolution 220, revolutions 220 minus 81. This window is about 40 km across and is centered at 69.6°S, 251.8°W (Stanford AIL Picture Product STN 0173, 061104). The top row shows changes in regions B,D. E over the sume time interval. Left to right: revolution 181, revolution 220, revolutigns 220 minus 181. The top window is xbout 60 km «across, and is centered ut 69.6°3, ($40 m/sec) for 100 min: (a) before 253.1°W (Stanford AIL Picture Product STN 0166, 042905). Hertzler, 1966a, b]. Fig. 36. Photographs of a layer | SION Fig. 35. ing regions. Top row (left to right): revolution 179, revolution 181, revolutions 181 minus 179. Bottom row: revolution 179, revolu- tion 220, revolutions 220 minus 179. The window is about 220 km across and is coniered at T2A4°S, 267.2°W; its outline is shown dashed in Figure 27 (Stanford AIL Picture Products STN 0174, 061306, 061307). revolution 126. revolution 181, Jution isi, revolutions 181 minus 1s 220 minus 181. The window is nford AIL Picture Products STN ow. Leit to right: revolution 181, nit 40 km across und is centered at SLLOE). The top row shows changes , tg-tvaliilion, 131, revoliiioy 2 Fig. 36. Photographs of a layer of sugar ‘sand’ erposed to simulated Martian winds across, end is centered at 69.6°R, (40 m/sec) for 100 min: (7) before; (b) after. Wind dircetion is into the picture [from Hertzler, 1966a, b}. lis 4194 Sagan ET au.: Martner 9 Mission strong prevailing character for about 1 day, not an unlikely requirement. On earth saltating particles can induce creep in surface grains many times their own diam- eter. For Mars the diameter ratio should be even larger. Thus small bnght particles can induce creep in large dark particles. On earth, typical sandstorm creep velocities are about 1 em/sec [Bagnold, 1954]; on Mars they should be correspondingly larger. Thus, during 1 day on Mars, creep of about 1 km is possible. It is curious, therefore, that the same velocities for the same periods of time can account by salta- tion for the production of bright streaks and by creep for the motions of dark material in places like Promethei Sinus. Hertzier [1966D]} estimated eolian transport rates under typical Martian conditions when the threshold stress is exceeded. Typical values range from 3 x 10% gem” sec* to 3 x 10° ¢ em™ see, Thus a layer of dust 1 em thick could easily be removed in 1 Martian day under conditions of moderately high winds, a result quite consistent with that just derived on the time scale for the generation of streaks. These numbers imply typical dust removal rates for an area the size of the leaf in Promethei Sinus of about 100 tons/sec, most of which, being in saltating particles, will be immediately rede- posited. Deflation of dust out of gravitational poten- tial wells such as craters probably requires velocities >V.,; in this case also transport times are reduced. The high values of V., deduced here and elsewhere have important consequences for eolian erosion rates on Mars, as is discussed in a separate publication. The contrasts that we find between adjacent Fig. 37. Results of a wind tunnel experiment similar to that in Figure 36, but with the base plate inclined at 20° to the wind. Wind direction is from right to left {from Hertzler, 19662, b]. Fig. 388. Results of a wind tur sand was exposed for ~10 min velocity needed to initiate eran Hertzler, 19660, b]. bright and dark terrain. <20G. n either to composition differences, size differences, or to both. The two may indeed be connected through weathering of small particles [Su 1971]. The apparent generation of brigh: some Martian areas only as a re: global dust storm (paper 1) sue bright streaks require exceptionally | tics for their formation. Accordine bright: streak particles would be - below the most easily saltated yp. Le, “KO wm (ef. paper 1). This exphuns the otherwixe puzzling fac eases of the development of dark - splotches have been uncovered since of the great 1971 storm, but no ¢: SSION 38 x 10° g em™ sec’ to 3 x 10% ¢ Thus a layer of dust 1 em thick ‘be removed in 1 Martian day under of moderately high winds, a result stent with that just derived on the for the generation of streaks. These nply typical dust removal rates for size of the leaf in Promethei Sinus of tons/sec, most of which, being in yarticles, will be immediately rede- 1 of dust out of gravitational poten- such as craters probably requires >Var; in this case also transport reduced. The high values of V., ere and elsewhere have important es for eolian erosion rates on Mars, ssed in a separate publication. trasts that we find between adjacent o that in Figure 36, but with the from right to left [from Hertzler, SaGAN ET AL! Fig, 35. Maniner 9 Mission 4195 Results of a wind tunnel experiment similar to that in Figure 36. Here the sugar sand was exposed for ~10 min to wind velocities significantly in excess of the threshold velocity needed to initinte grain movement. Wind direction is from mh: to lett {from Hertzler, 19661, b1. bright and dark terrain, 20°. may be due either to composition differences. to particle size differences, or to both, The two differences may indeed be connected through preferential weathering of small particles [Sag 1971]. The apparent generation of bright streaks in some Martian areas only us a resilr of the m octal. global dust storm (paper 1) suggests that bright streaks require exceptionally high veloci- ties for their formation. Accordingly, typical bright streak particles would be significantly below the most easily saltated particle size, Le. K100 gm (ef. paper 1). This deduetion explains the otherwise puzzling fact that many eases of the development of dark streaks and splotches have been uncovered since the settling of the great 1971 storm. but no esses of the development of bright material have been de- tected. The high stability of the br is expected! for an array of particles with thre mn n Var bret the bright streak regions mist be free of larger ! olf velocities far above the minir mrticles with threshold velocities near the ninimam V.,: saltarion of sich lareer particles would, by momentum exchange, ser the smaller particles of the bright streaks into suspension. Likewise dark areas are darkened when lirger salrating particles eject smaller bright fines whith are then carried off in suspension, Acknowledgments, We are grateful to R.A, Hand and J. A. Pirraglia for permission to repro- dace here some of their Tris atmospherie cir- vilition results: to W. Green, J. Seidman. PR. Ruiz, and AL Sehwartz for invaluable help with the metare processing: to Ro Beeker for interfacing: 4196 to A. T. Young, B. A. Smith, C. Leovy. and J. McCauley for useful discussions; and to the sclen- tists and engineers of the Jet Propulsion Lab- oratory who made this mission possible. All pic- ture differencing in this paper was performed by Lynn Quam and the staff of Stanford’s Artificial Intelligence Laboratory. REFERENCES Antoniadi, E. M., La Planete Mars, Hermann, Paris. 1930. Bagnold, R. A.. The Physics of Blown Sand and Desert Dunes, Methuen, London, 1954. Blumsack, §. L.. On the effects of large scale temperature advection in the Martian atmo- sphere, carus, 13, 429, 1971. Bovce, P. B., and D. T. Thompson. A new look at the Martian violet haze problem. 1, Arabia- Syrtis Major, Zcarus, 16, 291, 1972. Conrath, B., R. Curran, R, Hanel, V. Kinde, W. Maguire, J. Pearl, J. Pirraglia. J. Welker. and T. 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(Received February 7. 1973; revised March 9, 1973.) ee