The significance of bright spots observed during the 1971 Martian dust storm

ICARUS 31, 1 4 6 - 1 5 6

(1977)

The Significance of Bright Spots Observed during the 1971 Martian Dust Storm HI('HARD E. ] ) ' A L L I Department qf Geological Sciences, Browt~ University, l'ro*,Mence, Rhode Island 02912 lleceived July 12, 1976 ; revised Set)l ember 14, 1976 Mariner 9 photographs of the southern hemisphere of Mars taken during the 1971 planetwide dust. storm display circular bright spots al a tilne when all near-surface features were totally obscured. Correlating the posit |(ms and diameters of these spots with 1opography shows that they (~(irl'est)ond to m'aters. Abi)ul half of all lhe large cr'tters in Ihe study area were brightened. The associated craters arc large and fiat-floored, have significant riln uplift, and contain dark splotches im their th)ors. The depth/diameter rela|ionship of the brighl Sl)l)t craters is comparable to lhal ot'a planet-wide sample, l)epth Inay not be important in selectively brightening (:ertMn or:tiers. The visibilily i)f bright spots in A-camera photogr'~phs is strongly dependent on Ihe wavelength of lhe filler used during exposure. II ix proposed that bright spots result from the lmfllil)le scallering of incidenl light in dust (.hinds entrained within craters during dusl slorms. The appcaran('e ()f the dusl (.Iou(ls is a fun(qiim of the awfilabilily of a dust, supply and, perhaps, air turbulen('e generaled by winds flowing over upraised rims and rough (:rater floors. Bright spots persisl during lhe final stage [)t" the t)lanet-wide dust storm. If bright Sl)Ols are (his! clouds, Ihi.~ l)crsislen('e (temonslral.es thai cra,er inlerim's are the last. regi
seen d u r i n g t h e e a r l i e s t t)art (,f t h e m i s s i o n W(T(~ a ll(Rtr-sur|'~(.e |)h(,lmtnenon (~o|l[Jll(,(t to c r a t e r i n t e r i o r s ; (2) d e t e r n f i n e w h e t h e r t h e associat('d ('raters c o u l d be d i s t i n g u i s h e d f r o m t h e g e n e r a l p o p u h t t i o n of (:raters on t h e basis ()f m()rph(dogy ; a n d (3) propos(, :~ m ( , e h a n i s m for b r i g h t e n i n g . T h e a r e a ch()s(m for s t u d y ()f b r i g h t s p o t a(:tivity (Fig. 2) wt~s d e t e r m i n e d b y t h e l)h()tographi(~ c()v(*rage of tim A - c a m e r a ()n l{(~vs. S a n d 10, o r b i t s d u r i n g which b r i g h t s p o t s were m o s t ('ollspi(~uous. Th(, g r o u n d t r a c k s a n d vi(,wing ge(mletri(,s ()t' t h e s e tw() passes w(,re v e r y similar, a n d p ( , r m i t t e d obs(,rw~timl of t h(, s a m e b r i g h t s p o t s set)ar~ted b y a 2 4 - h o u r int(,rwd. T h e area. is m a p p e d g(,ologically ( M u t c h ct al., 1976) as m()derat(,ly It) d ( , n s d y crat(,r(,d uplantts. Num(,r()us b r i g h t :rod d a r k surf act' s t r e a k s , as w(,ll as d a r k sl)l()t(~hes w i t h i n c r a t e r s , ,tr(, 146

Copyright © 1977 by Acmtenfic Press, Inc. All rights of reproduction in any form reserved.

ISSN 0019 1035

FIG. ]. Typical stretched and high-puss filtered A-camera picture displaying bright spots. This photogruph was taken on Rev. 10 from a range of 7843 km through the orange filter. Local 51artiun time is 10:30 AM. Indicated is a prominent bright spot which corresponds to the 170-km Molesworth basin located at -27.5, 210.5 °. 5iariner 9 DAS 1 955 835.

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Fro. 2. LoeMion of study area. Heavy lines are boundaries of study area. Topographic contours are given in kilometers above the 6.1-mbar reference geoid for Mars. Source: Atlas of Mars M 25 m 3RC, 1:25 000 000, 1975. present which indicate significant eolbm through the orange and green filters during activity (Sagan et al., 1972). The topog- Revs. 8 and 10. Each spot center was r a p h y generally dips northward from "~ digitized with respect to the picture corner broad dome in Eridania to the contact coordinates. The diameter of each spot was between the cratered units and the Elysium measured in numbers of pixels (picture volcanic plains along the northern border elements) whose equivalent length in kilometers on the surface was also known. of Aeolis. l~atitude and longitude coordinates were e~tlcu|'tled for c~tch spot center and maps C()IH/ELATION WITH T()P()( ~RAPIt Y w('r(' I)rep'~,red ()f ,~eah,d ('ircles whi('h ('()uhl N i n e t y different bright spots w(,re i(h,n- be ()verlaid ()n the Ae()lis and Erid~mh~ tiffed on MTVB f~)rmat photographs t~ken photom()s~dcs,

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F~o. 3. Shaded circles represent craters which been have identified as displaying interior brightening during a dust storm. Maps include all craters greater t h a n 20 k m in diameter.

Each of the 90 bright spots corresponds well in position and diameter with a crater (Fig. 3). Figure 4 (Rev. 78) demonstrates that brightening over craters in the study area persisted for the duration of the dust storm. Revolution 78 coincides in time with the dissipation of the active part of the dust storm in late December as determined by Hartmann and Price (1974). MOI{PHOLOGIC CORI{ELATIONS The following morphological characteristics of the cr,~ters associated with bright spots were compared with those for the planet-wide crater population and study area population using the Brown University Mars Data System (Arvidson et al., 1974):

diameter, rim uplift, dark splotch occurrence (Fig. 5). In each case crater statistics were compiled in 10-kin-diameter intervals. Figure 5a shows the diameter distribution for each crater population. The study area appears to be a representative sample of the Martian crater population in this regard. However, the diameter frequency distribution of the bright spots is of different form, and, in fact, peaks in the 30-40-kin interval. It might seem that the weak contrast and mottled appearance of dust storm pictures could effectively hide small bright spots in the background noise. However, the presence of a few clearly defined small spots at diameters where the general crater population frequency should

F,(~. 4. llevoluti(m 78 p h o t o g r a p h of eratered unils in t h e s o u l h e r n t.ropies ( a k e n in viole( light with l he A - c a m e r a h ' o m a r a n g e of 7019 kin. T h e s l u d y ~u'ea is ,)n the n o r t h e a s t e r n limb with t h e M o l e s w o r t h bas}n indicated. Inter('rater regions are b e g i n n i n g to clear as t h e dust. s t o r m dissipates. T h e two 3 0 :~M. 1)AS 4 398 210. ~u'cuale sT)'eaks are residutd images h.om previous limb pi(,lm'es. Local Martia)~ time is a b o u t S <

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BRIGHT SPOTS DURING 1971 DUST STORM be approaching u maximum, as well as the consistently larger frequencies of the 30-80km diameter spots, could also suggest that there exists a threshold diameter associated with brightening. A corollary to these results is that nearly all the craters associated with bright spots have flat floors, a feature which occurs at diameters greater than 10 kin, but becomes dominant at 50 km (Cintala et al., 1976a). Figure 5b shows the frequency of rim uplift for craters in each diameter interval. The incidence of raised rims is always higher for brigbt spot craters than for either of the two other populations. Plotted in Fig. 5c is the frequency of dark splotches, or irregularly shaped markings on the crater floor with distinctly lower albedos than the surrounding topography. They are almost always present in craters associated with bright spots. There seems little doubt that dark splotches are a product of eolian activity (Sagan et al., 1973). Cutts and Smith (1973) have identified a dark splotch on a crater floor at B-frame resolution as a dune field (Fig. 7). Finally, crater depth was considered a parameter important to the bright spot problem. Determining the depths of Martian craters directly from Mariner 9 photography is not possible, because of the very low contrast of the surface. However, topographic profiles of the Martian surface, including 11 bright spot craters, were derived from the ultraviolet spectrometer experiment (Barth et al., 1974). Figure 6a is a plot of the post-dust-storm depths of the bright spot craters versus diameters. A least-squares fit has been drawn through the points to indicate the depth/diameter relationship. Drawn for comparison is a similar curve for a large sample of Martian craters derived by Cintala et al. (1976b). Although the scatter is large, the two curves appear qualitatively similar. The interpretation of the UVS data depends upon the assumption that the ultraviolet albedo of crater floors is not significantly

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FIG. 5. (a) Diameter frequency distributions of the three crater populations. (b) Raised rim frequency distribution. (c) Dark splotch frequency distribution. different from that exterior to the craters. The dark splotches demonstrate that the albedo in visible light does differ between surface materials exterior and interior to craters, but not by more than a few percent. If the ultraviolet albedo differences follow the visible albedo differences, than at least to a first approximation bright spot craters are neither deeper nor shallower relative to their diameters than the general crater population. The depths of bright spot craters probably range from ½ to 2 kin. Figure 6b shows, for each diameter interval,

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the percentage of craters in the study area which are bright spots. Since craters of presumably equivalent depths are not uniformly brightened, crater depth may not be i m p o r t a n t in controlling the appearance of bright spots. As an independent test of this proposition, again c(msider Fig. 2. The total regional relief in Aeolis is more than 3 kin, e(msiderably in excess of the deepest bright, spot crater. Yet no systematic changes in brightness from s(mth to north in pictures taken over Aeolis during the dust storm are discernible. In summary, craters which appear as bright spots during an obscuring (lust. storm are likely to be large and fiat-floored with raised rims. Consistent with the presence of dark splotches, they are also likely to be sites of eolian activity and, perhaps, r(,servoirs of dust. I)ISCUSSION Although limb brightness profiles derived from the Mariner 9 television experiment indicate that, dust was stirred to heights (ff 50 km or mor(, during the 1971 dust storm (Leovy et al., 1972), the strong topographic correlation of bright spots with craters over hmg periods of time suggests

that bright spots are ncar-surfaee phenomena. The high incidence of rim uplift and dark splotches, especially if most of the splotches are dune fields, may suggest that surfaee roughness contributes to the mechanism by which bright spots are created. Wind tunnel experiments have demonstrated that air moving over scale models of bowl-shaped craters breaks into turbulent flow as a result of crater rim uplift (Greeley et al., 1974). During Revs. S and 10, the A-camera was shuttered through a tilter wheel so t h a t the (,xposures were cycled through four segments (,f the visit)le spectrum. The [)right spots w('re most clearly delineated . n the photographs exposed through the hmgest wavelength filter, i.e., orange light. Bright spots nearly disappear into the background pall when phot()graphed in violet light. This effect can be seen in Fig. S which shows tw() suecessiw, A-frames t~ken h,ss than 1 nfinute apart on ll('v. 10. The sam(, thr(,e spots (m ea('h photo are indicated for c.mparison. The wavelength (h,pendclmC of bright spot visibility is r('miniscent of th(' t)oorly m~derstood "violet haz(," ('fleet on telescopic ,)bservation of M~u's (Thompson, 1972). However, no common mechanism which explains vioh't haze and bright Sl),~l visibility is

:FIG. 7. Dark splotches and at least one solution to the problem of their origin. Lower right is an A-camera view of several craters located on the northern edge of Eridania which were correlated with bright spots. A typical dark splotch is indicated b y the arrow. Picture width is approximately 285 km. Upper right is an A-camera view of craters in the central Noachis M C quadrangle, where the dark splotches have the same general appearance as in the study area. Picture width is about 380 kin. Left is a B-camera view ( ~ 1 0 X better resolution) of the splotch indicated by the arrow in the upper right picture. Centered at -47.5, 329.5 °. Picture width is 35 kin. Clockwise from left: DAS 9 807 424, I)AS 5 598 983, DAS 6 462 643.

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:hr¢)~gh orange filter, Rev. 10, Frame 30. DAS 1 956 675. Frame 29 was a B-camera picture.

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BRIGHT SPOTS DURING 1971 DUST STORM implied. I t is m o s t i m p o r t a n t to point out t h a t the " s t r e t c h i n g " control, or enhancem e n t of contrast, was identical for b o t h pictures. Thus, the wavelength dependence effect is real and not an artifact of picture processing. T h e reason for this effect remains to be investigated. CONCLUSIONS Thus, a plausible model for the origin of bright spots over craters m i g h t go as follows : 1. Large craters which h a v e significant rim uplift and either an exterior or interior dust supply are likely to be sites of bright spot generation. 2. D u s t clouds become entrained within these craters. Wind tunnel experiments suggest t h a t turbulence introduced b y dustladen winds blowing over crater r a m p a r t s and rough floors generate dust clouds. 3. T h e n u m b e r density of particles within these dust clouds becomes significantly high so t h a t multiple scattering of incident light p r e d o m i n a t e s within them. 4. T h e overlying a t m o s p h e r e acts primarily as a single-scattering layer, attenuating the backseattered light from the dust clouds (bright spots). L e e r y et al. (1972) find t h a t the limb brightness profiles are consistent with a single-scattering layer, at least in the upper, thin p a r t of the atmosphere. As displayed in Fig. 4, craters arc the last regions to clear during a dust storm. T h e y are also likely to be sites of ongoing eolian activity. T h e m e c h a n i s m for brightening described a b o v e could also explain the spectacular bright clouds observed in the Valles Marineris canyon complex during the waning stages of the dust storm, assuming t h a t sufficient winds were blowing through the canyon at this time. T h e r e m a y be other contributions to brightening over craters during a dust storm. P e r h a p s intrinsic p h o t o m e t r i c differences exist between surface materials inside and outside

]55

the crater. Such differences would be emphasized during a dust storm, particularly if a thin veneer of bright dust were swept clear of the intercratcr substrate. I n a n y case, meteorological ground t r u t h and continued observation of M a r t i a n variable features from orbit will be necessary to expand our understanding of eolian phen o m e n a on Mars. ACKNOWLEDGMENTS The author is indebted to T. Mulch, who suggested the study and supplied ample encouragement. He also wishes to thank C. Sagan, J. Veverka, and R. Greeley for useful discussions and criticism. REFERENCES ARVII)S()N, l{.., MUTCH, T., AND JONES, K. (1974). Craters and associated aeolian features on Mariner 9 photographs : An automated data gathering and handling system and some preliminary results. The Moon 9, 105-114. BARTtt, C., HORD, C., STEWART,A., LANE, A., DICK, M., SCHAFFNER,S., AND SIMMONS,K. (1974). In Ultraviolet Spectrometer Experiment--An Atlas of Mars: Local Topography. Laboratory for Atmos-

pheric and Space Physics, Univemity of Colorado. CINTALA, M., HEAD, J., AND ~IUTcH, T. (1976a). Characteristics of fresh Martian craters as a function of diameter: Comparisons with the Moon and Mercury. Geophys. Res. Lett. 3, 117-120. CINTALA, ]~'I., HEAD, J., :kiD MUTCH, T. (1976b). Martian depth/diameter relationships: Comparison with the Moon and Mercnry. Prec. 7th Lunar Sci. Conf. 3, 3575-3587. CUTTS, J., AND SMITH, 1{. (1973). Eolian deposits and dunes on Mars. J. Geophys. Res. 78, 41394154. GREELEY, |~.., IVERSEN, J., POLLACK,J., UI)OVICH, N., AND WHITE, B. (1974). Wind tunnel studies of Martian aeolian processes. Prec. Roy. Soc. London, A 341,331-360. H:~RTMANN, W., .iNn PalCE, M. (1974). Mars: Clearing of the 1971 dust storm. Icarus 21, 28-34. HESS, S. (1973). Martian winds and dust clouds. Planet. Space Sci. 21, 1549-1557. LEovY, C., BRIGGS, (]., YOUNG, A., SMITH, B., POLL.kCK,d., SHIPLEY,U., :iNn W1LDEY,R. (1972). The Martian atmosphere: Marnier 9 television experiment progress report. Icarus 17, 373-393.

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.~It"r(;lt, T., AItVID~(~N,11., llg\D, .I., .l~xl':.~, K., ~nD SAUNI)I~'RS, S, (1976). The Geoloff!t of Mac.~. Prin~'elon Univ. Press, Princeton, N..1. PIItlt:~_GLIA, ,l., ANt) CONI~,ATII, B. (1974). M:~,riian tidal pressure and wind fields oblained fr~ml Ihe Mariner 9 infrared spectroscopy experimenl. J. Atmos. Sei. 31, 318-329. SAGAN, C., VEVERKA, J., Fox, P., l)u,~tst:l~, I~., FRENCH, }{., (~IEItASCH, P., QUkM, L., I,n,)rZm~Er%

,l., LI,]VINTHAL, l']., TUCKI,.Ht, l(., ANi) I']R'.)SS, I~.

(1973). V:~riabh~ fe;dures -t~ M:trs. 2. MtLrim~r 9 ghd)al |'esull:..l. Geophg,s. l?e,s. 78, 4163 4196. SA~.~N, C., VEVERK:~, ,1., lg)X, P., I)VI~I~CH, R., I,EI)EltltEItG, .1., LEVINTHAI,, E., (~UAM, L., Tt;~:l,~Elt, R., t'(~Lt,.~('l% J., ANJ9SmlTti, B. (1972). V~riable feutures on Mar,'~: Preliminary Muriner 9 t,elevision results. Icarus 17, 346-372. TH()MPS()N, 1). (1972). Brief history of t,he M~rtian "violet haze" problem. Rev. Geophys. Space Phys. 10, 919-(.)33.