ICARUS 17,346-372(1972)

Variable Features on Mars: Preliminary Mariner 9 Television Results
CARL SAGAN, JOSEPH VEVERKA, PAUL FOX, AND R'USSELL DUBISCH
    Laborntory for Planetary Studies, Cornell University, Ithaca, New York 14850
   JOSHUA LEDERBERG, ELLIOTT LEVINTHAL, LYNN QUAAM,
                 AND ROBERT TUCKER

   Department of Gen,etics, Stanford University Medical School and
Artijkial Intelligence Laboratory, Stanford University, Stanford, California 94305
            JAMES B. POLLACK
     ames Research, Center, Moffett Field, California 94035

                           AND

BRSDFORD A. SMITH

Department of Astronomy, New Mexico State Unioersity, Las Cruces.
              Xew Mexico 88001

Keceived May 22, 1972

Syst,ematic Mariner 9 photography of a range of Martian surface features,
observed with all three photomet,ric angles approximately invariant, reveals
t,hrcc general categories of albedo variations: (1) an essentially uniform contrast
enhancement due to the dissipation of the dust storm; (2) t,he appearance of
splotches, irregular dark markings at least part,ially related t,o topography ; and
(3) the development of both bright and dark linear streaks, generally emanating
from craters. Some splotches and streaks vary on characterist'ic timescalcs -2
weeks ; they have characteristic dimensions of kilometers to t,ens of kilomet~crs. The
loci of these feat,ures appear in some cases to correspond w-e11 to the ground-based
alhcdo markings, and t,he integrated time variation of splotches and streaks is
suggested t,o produce the classical "seasonal " and secular albedo changes on Mars.
The morphology and variability of streaks and splotches, and the resolution of at
least one splotch int,o an extensive dune system, implicates windblown dust as the
principal agent. of Mart,ian albodo differences and variability.

        INTRODUCTION           some may, nevertheless,  contain infor-

That the bright and dark albedo mark-  mation on contrast changes. In addition
ings of Mars a,re time variable has been  to the wide range of visual reports,
known for more than a century. Contrast  photographic (Slipher, 1962) and phot,o-
changes and, in many cases, attendant  metric (Focas, 1961) evidence of albedo
color variations, were reported by Xchia,-  changes exists, An impression of a pro-
pa,relli, Lowell, Antoniadi, de Vaucouleurs,  gressive contrast change, proceeding in
Focas, Dollfus, and many others. The  the spring hemisphere from the vaporizing
color variations may be a psychophysio-  cap toward and across the equator has been
logical phenomenon m which the eye/brain  reported (see, e.g., Antoniadi, 1929; Focas,
system attributes to a neutrally colored  1962) and has been given the name "wave
region adjacent to a brilliantly colored  of darkening" by de Vaucouleurs (1954).

region a color complementary to the  A statistical analysis (Pollack et al., 1967)
                     of Focas' data shows that, while some
brillia,nt hue. Thus, while reports of color  correlation exists between the season and
changes should not be taken at face value,  the latitude of observed contrast enhance-
Copyright 0 1972 by Academic Press, Inc.            346
All rights of reproduction in any form reserved.


MARTIAN VARIABLE FEATURES                 347

men&, the connection is far from one-to-
one. Beside the seasonal changes, which
are largely but not exclusively contrast
enhancements between adjacent bright
and dark areas, there are also a multitude
of reports of secular cha'nges (see, e.g.,
Antoniadi, 1929; Slipher, 1962), the non-
periodic and highly unpredictable varia-
tion in the boundaries between adjacent
bright and dark markings-usually des-
cribed in terms of the appearance or
disappearance of a new albedo feature on
the planet. The characteristic time scale
for seasonal changes to develop (features
several hundred kilometers across) is
reported by ground-based observers to be
several days (Dollfus, 1968).

Seasonal and secular variations on Mars
have been att'ributed both in the scientific
and in t'he popular literature for more than
a century to the growth and decay of
Martian vegetation, a conjecture imposs-
ible to disprove from the Earth or from
Martian orbit because the hypothetical
Martian organisms can have a wide range
of conceivable at,tributes. An alternative
detailed interpretation of these changes
ha,s been proposed (Sagan and Pollack,
1967, 1969; Sagan, Veverka, and Gierasch,
197 1). The observed contrast, variations are
int)erpreted in terms of t,he alternate
deposition and deflation of windblown
dust' having detectable contrast with
respect to ba,sement, material. In this wind-
blown dust' model some seasonal effects are
ant,icipated because t'he winds should be
seasonally variable; and the relat'ion of
such changes to topography is to be
expect'ed? both because of winds induced
by. topography (Gierasch and Sagan, 1971)
and beca,use of topography act,ing as a sink
or source of transport,able material. Winds
above the surface boundary layer required
t,o move part,icles on the surface a,re
calculat,etl to be many t,ens of meters/
second.

The Jlariner 9 mission provides an ideal
opport,unitj- to examine 3~1art~ian variable
feat'ures. The resolution is improved by a
factor of 100 t'o 1000, so t,hat, the mechan-
ism of t,he cha,nges may conceivably be
revealed directly. The high resolut'ion of
the Mariner 9 observat#ions permit's us,
   12

for example, to test the hypothesis that a
given change is due to the dissipat'ion or
formation of a long-lived obscuring cloud-
a hypothesis extremely difficult to test
with ground-based resolution. Moreover,
ground-based restrictions on observable
lat'itudes, longitudes, and seasons are to a
considerable degree relaxed, because of the
long operating lifetime of Mariner 9.

In the original conception of the two
spacecraft Mariner `7 1 mission (see Masur-
sky et al., 1970), the orbit of one spacecraft
was optimized for variable features invest'i-
gations. To avoid misinterpreting bright-
ness variations due to changing lighting
conditions as intrinsic albedo changes,
the spacecraft was to arrive over the same
area of Mars with all three photometric
angles close to constant every several days
-the time scale for seasonal changes
reported by ground-based observers. With
the failure of Mariner 8 this strategy was
necessarily abandoned, and Mariner 9 was
placed into an orbit representing a com-
promise among various competing uses of
the science systems. With the post trim
orbital period just over 12 hr the Mariner 9
groundtrack drifts only about So/day.
Accordingly the same region can be
viewed on successive days with the
illumination, viewing, and phase angles
varying by <lo', a value quit,e accept'able
for our purposes. We can, however: do
much better by waiting 19 days. In t'hat,
time, a particular groundtrack will have
drifted halfway around Mars ; the grormd-
track of a zenith pass will be close t*o that
of a nadir pass 19 days earlier, and vice
versa. Such picture pa.irs can reproduce
the light,ing!!vie~ving angles to 2-X", and
the phase angle to 5-10". The contjrol of
the three photomet'ric angles was decidetl
upon because the ext'ent of albedo raria-
tions,  particularlp at, high resolutionj
was of course not previously knowil. Most
of the albedo changes discovered were
in fact so striking t#hat, t)he,v would have
been observed even without careful control
of photSometric angles.
Through the end of the initial data
taking on revolution 262, 17 variable
featsure target' areas were systemat~ically
observed, a8s indicated in Fig. 1. These


x VF25
ARCAOlA

CNRONIUM

,VFI9

MERIDIAN1 SINU                     P
                        i?
                E
                      2
                       2

13
ISMENIUS
LAWS

  vF15 X
PROPONTIS

x VF6
ARGYRE

I)EPRESSIO HELLESPONTICA

XVFl7 NOVISSIMA TnYLE

VF3X

x VFl6

xVFI


MARTIAN VARIABLE FEATURES                 319

FIG 2. Five views of the same region in Thyles Mom (753, 16O'W) as it appeared on revolutions
18, 78, 113, 152, and 191 (top left to bottom right). These are sections taken out of Mariner 9 wide
angle -4 frames. Approximate width = 290km. (STN 0163:040909, 10, 11, 12).

areas represent' a wide range of albedos,
positions, topographies, and elevations.
The? include regions reported in t'he
cla'sslcal literature to be highly time
variable and regions reported to be quit,e
stable. Unt,il the failure of the filter wheel
on revolution 118, some of t'hese regions
were observed in three colors and in three
polarizations, with the polarization vector
stepped in 60" increments. ARer revolution
11 H all X-camera observations were made
with an effectively orange polarization
filter. All high resolution B-frame photo-
graphs have been obt,ained with a yellow
filter. In addition to areas specifically
designated as variable features target,s,
overlapping mapping frames provide an
important additional source of variable
feat,ures information,  with turna,round
t,imes of 19 days.

In the timeSinterval between the decav
of the dust storm and t)he end of th;
nominal mission on revolution 262 a very
wide array of surface albedo variations

have been discovered. The systematic
character. of these changes, particularIS
in cases where there are many successive
observations and changes observed over
only a small fraction of the frame, clearly
show that the observations reported here
are not due to any progressive phot,o-
metric deterioration of the camera systems.
Other, especially at'mospheric, variations
are reported elsewhere (Leovy et rzl., 1972).
All phot,ographs which accompany this
article have been cont,rast enhanced b)
computer processing.

DISSIPATIOX OF ,~TMOSPHERIC DUST

At least, three general categories of
albedo variations were discovered b)
Mariner 9: (1) an essentially uniform
contrast enhancement wit*h time over
broad regions, coinciding with the deca!
of the dust storm ; (2) the appearance of
irregular da,rk markings which we here
call splotches ; and (3) t,he development of


350                           CARL SAGAN ET AL.

variable linear features to be described
below which a're called streaks or tails.
An example of category 1 changes is shown
in Fig. 2, a sequence of photographs of the
Thyles &Ions region at (75"S, 16O"W).
The progressive nature of the contrast
enhancement, and t'he correlat#ion of these
changes with the time scaJe of dust settling
as determined by a variety of techniques
(Masursky et al., 1972; Hord et al., 1972:
Hanel et al., 1972; Neugebauer et al., 1972)
clearly indicate such changes to be a conse-
quence of the dissipation of the great
dust storm. A characteristic settling time
scale of weeks implies particle sizes >
some tens of microns (Pollack and Sagan,
1969; see also Leovy et al., 1972). In
addition, there is a range of cases which are
clearly due to the drifting of a small cloud
over an area with attendant obscuration

and subsequent, improvement of the detec-
tivity of surface features (see, e.g., Leovy
et al., 1972). Leovy et al. exhibit a dust,
storm which left the underlying area
darkened in its wake with a characteristic
time for albedo change of <I9 da?s,
which they point out is consistent with
t#he windblown dust model (Sagan and
Pollack, 1967, 1969). The array of parallel
curvilinear streaks extending 1000 km
southwest'ward from South Spot', as ob-
served in t'he early phases of the Mariner 9
mission (Masursky et cd., 1972), also proved
to be t'ime va'riable.

At least some areas of Mars are predicted
to have long-lived and recurrent dust
palls above them related t'o the t'opography
(Sagan, Veverka, and Gierasch, 1971).
produced by, for example, slope and
obstacle winds, and it is conceivable that

FIG. 3. Section of A frame showing t,he region of Depressio Hellrspontica (60'S, 3457V) on IT\-olu-
tion 153. Approximate width = 1201rm. (IPL 490: 134154).


iK4RTIAK VA4RIABLE FEATURES                  351

some small-scale changes reported below
ma)- be due to the long-term presence
follo\ved by the rapid dissipation of such a
localizetl cloud. However, such processes
appear to us meteorologically implausible,
particwlarly in cases where t'he hype-
t'hetjical c~loutl must) be absolutely stai,ion-
ary for periods of weeks or mont,hs and
then dissipate rapidly. In most cases the
distinction between changes induczd bJ
the dissipat,ion or motion of overlying
clouds and changes in the actual surface
rnat,erial is apparent: In the former case,
bot(h t,he dist,ribut,ion of albedo and visible
topographic det'ail vary, and topographic
features are not well-defined ; in t,he latt,er
case, the topography is well-defined and
only the albedo distribution varies.

SPLOTCHES

Nany of the cla'ssical dark areas of Mars
are revealed, when viewed wit'h Mariner !I
resolution, to have a mott,led appearance.
For example, Depressio Hellespontica is
resolved into a bright cratered terrain
generouslv sprinkled wit'h dark irregular
splotches (Fig. 3). Splotches have charac-
terist*ic dimensions between several kilo-
meters and several tens of kilometers;.
They often appear to lie within craters
and sometimes seeIn to wash over crater
walls. Rplotches seem to bc cont'rolled by
topography since t)hey are common near
the inside and out'side of crater walls and
along what8 appear t,o be ritlges or scarps.
The t'rue contjrast of a' typical splot,ch with

respect to its surroundings is some 10 or
15%. The larger splot'ches and splotch
complexes a'rc several hundred kilometrrs
in laWa dimensions. Thes- t\\-o facts
impljr that the largest splotches shoultl be
visible under ideal seeing conclit ioils by
observers on Eart'h. It. is notable that' ob-
servers such as Pocas (1061) have rcportctl
the Martian dark areas t'o br composed of
irregular detail which FOCRS culled "tlark
nuclei."

A preliminary comparison of Mariner (i
and 7 topography of Mars, obt'ained in 196Cbj
with Mariner 9 phot'ographv of Mars in
the same regions: obtained in 1071/1Oi1",
indicates the appearance and development
of splotches in this time interval. \\`e
have found many cases of large scale
development of splot'ches in time scales
bebween several months and <i" n-eeks.
We have found no clear cases of the
development8 of splotches on a time scale
of 1 day. Thus, the characteristic dimen-
sions,  cont'rast#s,  and variational time
scales of splotches are consistent with the
observations of Focas. \Z'e have not yet
sought agreement8 bet'ween t!le positions
of the da,rk nuclei of Focas and of the large
splotch complexes uncovered by Mariner I).

Data from different orbits have been
processed.  in a search for variat'ions,
at the =\rtificial Intelligence I,aborat,ory of
Stanford Universit,y. In an interactive
procedure describctl by Levinthal d nl.
(1973), t*he regions common to t)he two
pictures are isolat)etl, calibratrtl, scaled,
rectified, and t'hen tlifferencetl, pi& ure

FIG. 4. Two views of Depression Hellcspontia (WS, 345'1V). Left: revolution 153. Right:
revolution 227. Shown are sections of A frames. =Ipproxirnate kvidth = 3G0 km. (STK OlG3 : 040906).


352

CARL SAGAN ET AL.

FIG. 5. Two views of Pandorae Fretum (%"S, 335%`). Left: revolution 108. Right : revolution 188.
Shown are sections of A frames. Approximate width = 16Okm. (STN 0162: 01109).

element by picture element. The differ-
enced pictures should be featureless in
cases where no changes have occurred,
but' should strikingly display regions of
variation where they occur. Figure 4
displays a region in Depressio Helles-
pontica, nea'r (55"S, 346"W), as it appeared
on revolution 153 (at left) and revolution
227 (at right). Much of the intercrater
area has darkened considera'bly and some
splotches within craters have changed.
The area involved in the change is approxi-
mately 400 x 600km. A biological inter-
pret,ation of this variation implies that a
major bloom of organisms occurs in 38 days
over an enormous area. On the windblown
dust model fine dust has been deposited
over the entire area and was subsequently

.

scoured off by strong minds. As in man:*
other cases presented below, the biological
explanation cannot be excluded, but the
windblown dust explanation appears to be
a more natura,l interpretation of the
observed va,ria,ble features.

Figure Fi exhibits changes in splotches
in and around two crat'ers in Pandorae
Fretum, a, seasonal and secular variable
in the classical literature. The larger crat,er
is about 60 km across ; t'he time interval
between the two views is 38 days. Such
variations of crat,er-connected splot#ches
are typical in our data. In Fig. 6 is an
example of anot,her typical change ob-
served in splotch areas--the development
of dark areas along topographic features
such as ridges and crater walls. The region

FIG. 6. Comparison of A-frame sections showing a region of Pyrrhac Regio (25'S, l5W). Left:
revolution 137. Right: revolution 176. Approximate width = 130km. (STN 0160:03010G).


MARTI- VARIABLE FEATURES                 353

is Pyrrhae Regio. Sections of A-frames are
shown ; the linear dimension is about
125km, and the t,ime interval between the
two views is 19 da,ys. In t'his case, dark
material has appeared against an arcuate
topographic fea,ture which may be the
remnant of an ancient cra,ter wa,ll. In
addit'ion, da,rk material has also appeared
along the rim of the smaller crat'er below.
Hypothetical Martian vegetat,ion might
,grow  preferent'ially  along t,opographic
fea,tures-for example, to insure stabiliza-
tion against winds. In the windblown dust
model, we are observing eit,her the deposi-
tion of da,rk mat,erial by winds blowing

generally at right angles t,o the ridges or
the scouring of bright fines by winds
blowing along the ridges (cf. Bagnold,
1960, pp. 142-3).

In addition to splotch variat'ions at A-
frame resolution shown in t,he last, three
figures, we have also uncovered cases of
very major changes at B-frame resolution.
Figure 7 displays a low-resolution vien
of t'he region between Th\-les Collis and
Depressio Magna at about (70'S, %Ct'\!~).
The splot'ches of interest are those intli-
cated near the center of the frame be!twren
the two craters. Figure 8 shows a high
resolution photograph of this intrinsicall)

FIG. 7. \Vidr-angle (A-cmncra) view of the region bctwccn Thylcs Collis and Deprcssio Mngna
clear (7O"S, 259"W) on revolution 179. The arrow indicaks the area of the next figure. Approximate
width = (IPL 1283:220256).


354                         CARL SAGAN ET AL.

FIG. 8. High-resolution (B-camera) view of the area out,linrd in Fig. 7 at about (70"s. 254'W).

Not,e espociaily t,he dark ieaf-ahap& area just,
width = 75km. (IPL 359:080348).

high-contrast region. We note especially
t,he scalloped boundary between bright
and dark mat,erials, which is strongly
suggestive of aeolian t,ra,nsport, a point
to which we return later. On the basis of
this photograph, however, it' is discult
to decide whether it is bright or dark
material which is being blown. Figure 9
is a comparison of t,he center of t,he region
shon-n in Fig. x on revolut'ion $19 and on
revolution 112. In this period of 13 days
an entire albedo marking, shaped like a
spearhead or a leaf, suddenly appeared.
In adtlit,ion to t,he a,ppearance of the leaf-
shaped appendage on its stem, the boun-
dary of one of t)he dark areas at, left moved
to the right. In a period of ~13 days a
l@km dark area has appeared discon-
tinuously and remained essent,ially un-
changed in appcaranrc for many months

below t)he crater. Revolution 179. Approximate

thereaftx, a variation suggest,ive of sat,ura-
tion phenomena. In biological models,
the surface goes from no or sparse cover of
organisms to saturat'ion surface density
in -1 week. On the windblown dust model,
most, of the transport,able matx?al which
can be removed or deposited is removed or
deposited in -1 week, in t,he case of removal
exposing t,he undcrl~ing material of COD
trssting albedo. The observations can be
underst'ood with either bright or dark
transportable material.

An apparentlv tSypical dark splotch
area is seen in F&q. 10, an X framcl. HOJV-
ever, a high-resolution B frame (Fig. 11)
of the dark splotch \vit,hin the crater at
bottom right in Fig. 10 reveals that the
dark splotch is a tlune firl~l. It, is cert'trinly
not' true t'hat' all dark splotches are dune
fields, but t,his c~xamplc proves t'hat dark


MARTIAN VARIABLE FEATURES

355

FIG. 9. Comparison of the appearance of the central area of Fig. 8 on revolution 99 (top left) and
on rerolution 126 (top right). ARcr the two views were similarly scaled and projected, the I-iew at
top right, was aligned and registered to that at top left, pictare clement by picture element. The11 the
two views lvere differmccd. again picture element, by picture element. Shown at bottom left is tho
99-126 difference; at, hot,torn right, the 126-99 (reverse) difference. Approximate width = 45km.
(STN 015R:Ol%%Ol).


356                          CARL SAGAN ET AL.

Flc. 10. Wide-angle (A-camera) view of a splot)ch field in Hellespontus near (46'S, 33OTT) on
revolution 112. X'ote especially the large splotch inside the crater at lower right. Approximate I\-idth
= 430km. (IPL 1863:112847).


MARTIAX VARIABLE FEATURES                 357

FIB 2. 11. High-resolution (B-camera) view of the dark splotch referred t'o in the previous figure,
reves tling a dune field near (48"S, 33O"W). Revolution 229. Approximate width = 65km. (MTVS

4264: 167.

material is efficiently moved by Mart'ian
winds. Sand dunes on Mars were suggested
by Gifford (1964), and further discussed
by Belcher et al. (19'71).

STREAKS

While some classical variable features on
Ma'rs are resolved into splotchy terrains
as indicated above, other are revealed
by Mariner 9 to be composed of a new
category of Mart'ian albedo features not
reported by classical observers, arrays
of streaks. Figure 12 shows one such region,
Hesperia, the somet'ime dusky or semitone
region between Mare Tyrrhenum and Mare
Cimmerium which seems to be made up
of bright streaks associated with craters.

A credible explanation of such a,rrays
of long parallel streaks emanating from
crat'ers is that fine bright dust, transported

into the craters in the waning stages of t*he
dust storm (see below) was subsequently
blown out by high velocity winds having
a prevailing dire&ion. In this view, the
streaks are downwind of the crat#ers.
The length and half-angle of t,he streak
may be some measure of t'he velocity of the
wind which deflated the crater, If this
conjectureis corroborated, there arenat'ural
urind direction indicators and conceivably
anemometers laid out on t'he Mart&
surface.

Bosporus, a da'rkish area near Solis
Lacus, is resolved into a host of dark
cometlike tails or streaks, again associated
mit,h craters and again reflecting a pre-
dominant direction (Fig. 13). Some of
these dark streaks are more tha'n 50 km
long, yet remain very narrow (-5km)
throughout t,heir length. They are too small


FIG. 12. A-camera view of a region in Hesperia (23%. 242%`). Resolution 128. Approxima
width = 400km. (MT\% 4155:84).

te

li 'IG. 13. A-camera view of a, region in BosporUS (34% 62"W). Revolution 129. Approxim
wid .th = 400km. (MTVS 4156:72).

ate


MARTIAN VARIABLE FEATURES                 359

to be individually resolved by ground-
based observers. A comparison of Figs.
12 and 13 indicates that dark streaks
tend to have a larger half-angle than
do bright st)reaks, a.lthough a class of
very long dark narrow streaks also exists.
Three explanations of dark stresks suggest
themselves. The first possibility is that
dark streaks represent the wind shadows
of crater rampart's and that the dark streaks
can be understood in terms of bright
material deposited by a dust storm on
underlying dark material everywhere but
downwind of craters. This explanation
seems t'o us unlikely, because of cases
where new da,rk tails have developed
while the relative contrasts between older
da,rk tails and their surroundings remain
unaltered. The second possibility is that
dark ma'terial is deflated downwlnd from

the crater bottom. The third possibility
is t'hat turbulence downwind of the crater
scours bright fines preferentially in t'hat
area, although we have no underst,anding
of t'he dynamics of such a process. Mixed
classes of st'reaks exist. Figure 14, for
example, shows a set of bright streaks
with dark borders in Syrtis Major.

Syrtis Major exhibits a very large surface
density of streaks, bright, dark, a,nd mixed.
Some dark streaks are irregular and
morphologically quite different from those
occurring a,nywhere else on the planet
(Fig. 15). Because of the verv steep slopes
on Syrtis Major, higher winds may be
more frequent there than in most other
surfa'ce regions of the planet (Gierasch
and Sagan, 1971). It ha's been proposed
that' two different varieties of winds
occur in Syrtis Major, one blowing along

FIG. 14. A-camera view of a region in Syrtis Major (4"N, 295"W). Revolution 153. Approximate
widt,h = 500 km. (IPL 490 : 184752).


360                        CARL SAGAN ET AL.

FIG. 15. A-camera, view of another portion of Syrtis Major (13"N, 283'1V). Revolution 155.

Approximat,e midt,h = 540km. (IPL 488: 100143).

the slope, the other blowing across the
slope (Sagan et al., 1971). Such strong
crosswinds could conceivably produce the
irregular streaks observed.

In Fig. 16 are plotted all apparent
bright', dark and mixed albedo streaks in
Xyrtis Major, superposed on a map showing
the classica, boundaries of S@s Major.
It, is apparent that the loci of streaks
correspond rather closely to the con-
figuration of Syrtis Major as seen from
Earth.

The evident aeolian origin of the streaks
immediately suggests that the classical
seasonal and secular albedo variations
report'ed for such areas as Syrtis Major are
clue to t,he production and dissipation of
bright and dark st,reaks. We have observed
many cases of variations in dark streaks
during the course of the mission. Figure 17
is a comparison of st,reaks within Syrtis

Major in revolutions 1% and 233. The
configuration of irregular dark streaks
is clearly variable on a time scale of 38
days. According to Ant#oniadi (1929) it
is the eastern flanks of SyrGs Major which
exhibit the greatest seasonal variations
(Fig. 18). It is notable (Fig. 16) that this
flank is a region of high-density dark
variable streaks.

In Fig. 19 are shown t'wo views of the
Daedalia region near Solis Lacus. The
first, obt,ained on revolution 115, shon-s
s cratered terrain wit,h very few dark
strea,ks. By revolution 1!)5, shown on the
right-hand side of Fig. 19, the area had
darkened considerably due t,o t)he appear-
ance of numerous dark wide-angle st,reaks
behind most, of t,he craters. Figure 20
shows a closeup view of an individual
crat-er in t,his area behind which a dark
tail about 50 km long appeared in 38 days.


MARTIAK VARIABLE FEATURES                 361

FIG. 16. Streak map of Syrtis Major. The full a,rrows represent dark streaks. dashed arrows bright
streaks, and dash-dotted arrows bright streaks with dark borders (cf. Fig. 14). Circles with short
arrowheads ropresent craters with rudimentary streaks. Whenever numerous streaks of one type
occur in a small area, only one streak is shown, along with the number of such st'reaks in the area. The
streak length is exaggerated by a factor five.

FIG. 17. Two views of a streaked area within Syrtis Major (15"N, 28"W). On the left: revolution 155.
Right: revolution 233. Shown are section of A frames. Approximate width = 140km (STN 0164:
041505).


Q


MARTIAN VARIABLE FEATURES                363

FIG. 19. Two views of a region in Daoda,lia (25"S, 125"W). Left: revolution 115. Right: re\-olution
195. Shown are sections of A frames. Approximate width = 340 km. (STN 0162 : 040105).

FIG. 20. Two views of a crater near the center of Fig. 19. Again on the left: rc\-olution 115: ott the
right: re\-olution 195. Shown are sections of A frames. Approximate width = 100km. (STS (0162:
040107).

FIG. 21. A comparison of t,he appearance of a region in Mare Erythraeum (25'S, 35"W) on revolution
133 (left), and revolution 211 (right). Shown are sections of A frames. Approximate width = 160km.
(STN 0162:040110).


364                         CARL SAGAN ET AL.

FIG. 22. Twm vie\+ s of a region in Hcsperia (26"S, 245"W) shon-illp tlw stnhility of bright
tails. Left : rcvollltion 128. Right : rerolut~ion 167. Shown are sections of AI frame. Approxinlatc~
width = 90knl. (STN Ol~iO:O30409).

A similar case is the comparison in
Fig. `1 of a region in Mare Erythraeum
at the left on revolut,ion 133 and at the
right, on revolution 211. A new dark tail
as well as several other dark regions have
developed in this time interval.

Ground-based observations of Mars,
made at' New Mexico Sta'te University
Observatory, showed that Hesperia was
dark and indistinguishable from the adjac-
ent maria Tyrrenum and Cimmerium in
September 197 1, before the dust storm. But'
Hesperia was noted to be appreciably
brighter when the dust cleared in cJanuary
1972. The bripht8ening of Hesperia took
place during the dust storm, when the
surface of &rs was hidden from view. The
1,falarincr !) high resolution appearance of

Hesperia (Fig. 12) suggest's that2 this bripht-
ening is due t,o the generation of numerous
bright streaks, or possibl)- the removal of
previously coexisting dark st'reaks.

Our observations show the white streaks
to be verv st'able. Figure 22 shows little or
no variation in the bright streak configura-
tions in Hesperia over a period of 38 clays.
Lat'er, however, in a time interval of 19
da,ys (Fig.  23) crat#er-associated dark
streaks developed, while in the sa,me inter-
val no variation whatever is evident in the
bright' streaks. These cha'nges illustrate
the general trend for bright tails to
be much more sta,ble t,han dark tails. \Vhile
we have circumstantial evidence, from
ground-based  observat)ions  ment)ioned
above; for the development, of new bright,

FIG. 23: Two views of a region in Hesperia (25"S, 245"W) showing t'lic stablit,>- of white
tails and the appearance of dark tails. Left: revolution 128. Right: revolution 206. Shown are
sect,ions of A-frames. Approxirnatc width = 220km. (STN 0164:041501).


?MARTIAN VARIABLE FEATURES                 365

tails, we have so far failed to notice any
new bright tails appearing or existing
bright t'ails changing anywhere on Mars
during the course of the mission. On the
other hand, numerous changes in dark
st'reaks on a time scale ~19 days have been
uncovered. The general trend since the
termination of the dust storm has been for
more dark streaks to appear and for exist-
ing dark streaks to increase in size. The net
effect is that dark areas have increased at
the expense of adjacent bright areas.
Such a trend cannot continue indefinitely,
and it mav be that by Eart,h summer of
1972, Mariner 9 will observe a stat,istical
reversal of t,his trend.

The development of dark streaks in
Hesperia (Fig. 13) by a process which
produces no discernible changes in the

bright st'reaks emanating from the same
craters is extremely int,eresting and implies
that the process which produces dark
streaks requires lower wind velocities than
the process which alters bright streaks,
at least in this area.

Figure 24, an A-frame view of a'n area in
Tharsis, is charact,erized by a wide assort,-
ment of bright' streaks. No variations in
the configuration of these streaks have
been observed through late in the mission.
Figure 25 shows, in high resolut'ion, t)he
complex st,ructure of part of the wedge-
shaped bright streak complex near right
cent,er of Fig. 24. The straightness of these
strea,ks, the longest of which extends for
SOkm, the precision with which the>
join each other, not transgressing to the
opposite side of the t,ra,nsverse streak,

PIG. 24. A-frame view of a region in Tharsis (IO'S, 107%`). Revolutjion 156. Approsilnate widt~h =
4lOlun. (IPL 1108: 150842).


I PIG. 25. High-resolution (B-camera) view of a complex streak pattern near the center of Fig.
at ( :llYOS, 105?4W). Revolution 236. Approximate width = 45km. (MT\% 4271;31).

I +G. 26. High-resolution (B-camera) view of a group of bright streaks near the left-center of Fig
at ( ,9?5S, lOSY4W). Revolution 234. Approximate width = 45km. (MTVS 4269:46).

.24,


MARTIAN VARIABLE FEATURES                 367

and their time invariability together pose
an interesting problem. There is a definite
suggestion here, as elsewhere, of infor-
mation which will lead to an aeolian
depositional stratigraphy, since it is im-
probable that all the streaks in Fig. 25
were formed simultaneously. Figure 26
displays a high-resolution view of the area
near left center of Fig. 24 where a bright,
transverse streak crosses what appears to
be faults in the vicinity of ma,ny other
bright, streaks. The origin of the streaks
in an evident topographic feature is
reminiscent of other cases of topographic
control (e.g., Figs. 5 and 6) and suggests
that t,opographic irregularit,ies other t,han
craters may be repositories of fine mat'erial
which later is wind-transported from these
topographic features.

Some of the dark tails in Bosporus
(Fig. 13) or in Daedelia (Fig. 10) may be
deflat#ionarv in character ; t)hat is, resulting
from scouring of fines downstream of
craters or ot'her topogra,phic irregularities.

.

However, other dark streaks, such as those
shown in Pig. 17, mav be depositional in
                  Y
character. This is an A frame of a region
in Cereberus nea'r Elysium. The upper
crater, about 30km across, can be imer-
preted as the source of dark material
carried downwind to produce t'he intense
dark tail. In the process, part of the rim
of the smaller crater appears to have been
covered by dark material. Not,e a sha'dow
zone behind the smaller crater where no
dark material was deposited. This is
another case from which aeolian deposi-
tional stra'tigraphic relationships are expec-
ted to emerge. We know of no ot,her dark
streak so similar in shape to bright
streaks.

DISCCSSION

A variety of classical time-variable
bright and dark areas on Mars are found
t'o be constituted of, and have their
variations cont'rolled by, the distribut,ion

FIG. 27 : Wide-angle A-frame of a region in Cerberus, near (S'N, 191"W). Revolution 179. Approxi-
mate height = 435km. (MTVS 4207 : 89).


368                         CARL SAGAN ET ,4L.

and appearance of two categories of
features: splotches and streaks. There are
apparently only dark splotches, but both
bright and dark streaks. Both are connec-
ted with topography, part,icula'rly craters,
ridges, scarps, a,nd faults. Some cases,
possibly intermediat,e between streaks
and splotches, also exist,, and it may be
that the essential distinction between the
two types of feat'ures has to do with
wind velocity. The parallel arrangement
and streamlined shape of the streaks
points very strongly t'o an aeolian origin.
A given variation m a. streak or splotch
requires the deposition or removal of only
a thin layer of material, easily sequestered
in craters or elsewhere.

The serrat,ed interface between bright
and dark areas in some splotches (see, e.g.,
Fig. 8) is also very reminiscent of aeolian
transport. When a longitudinal pile of

rock flour of particle size in the 100~
range characterist'ic of the Martian surface
(Pollack and Sa'gan, 1969 ; Neugebauer
et al., 1971) is exposed to not a,typical
(Sagan et al., 1971) Martian wind velocibies
-lOOm/sec, a pattern like that in Fig. 8
is formed (Hertzler, 1966a). While the
homology transformation must scale over
a factor of lo', the wind tunnel results are
provocative and support' t,he aeolian origin
of splotches. The resolut,ion (Fig. 11) of
some dark splotches into dune fields
demonstrat,es the aeolian character of at
least some splotches.

The bright streaks are most, readily
underst'ood if we assume that, in the waning
stages of the dust st'orm, fine particles,
sett,ling through t,he atmosphere, are
preferentially t,rapped in topographic fea-
t'ures such as craters by lat,eral t~ransportj.
Deflation of craters by subsequent, high-

ilc:. 28. A portion of a, global A frame taken during the waning stages of the 1971 dlwt stc
wing n~mwro~~s bright craters. The dark spot at lower left is a camera defect. Rcvollltion
me ccntcr (:35'S, 29"W). Approximate width = 200krn. (IPL 930: 222549).


MARTIAN VARIABLE FEATURES                 369

velocity winds produces the parallel array
of strea'ks. The appearance of bright
craters towards the end of the dust storm
(Fig. 28) is consistent with this idea,
although some contribut,ion t,o higher
albedo follows from the longer slant path
through a dusty atmosphere above crat'ers
t,han above neighboring terrain.

Thus, while an excellent case can, in
our opinion, be made for t'he aeolian origin
of both splotches and streaks, there are
many detailed quest,ions which remain
unanswered: Is a given dark splotch or
st,reak due to the deflation of overlying
bright matserial or t,he deposition of dark
material? If the latt,er, what is the source
of the dark material? Why do some dark
streaks have an appearance as in Fig. 13,
while ot,hers (Fig. 87) resemble closely the
form of bright streaks? Is a given topo-
graphy-related dark region due to t,he
preferential removal of overlying bright
material downwind of the obstacle or to
bhe wind shadowing of downstream area.s
upon deposit'ion of bright material on the
surroundings? In serrated interfaces be-
t,ween bright and dark material (cf. Fig. 8),
which boundary of the dark splotch
(dendritic protrusions or a,morphous edge)
is in motion? Many of these questions will
be approached in work now underway-
in a wide range of sta'tistical studies, in the
pursuit of st(ratigraphic relations, and in
frames where several wind indicators,
among them serrated edges, streaks, and
splotches, are simultaneously present.

While detailed statistical studies are
now in progress, preliminary results indi-
cat'e no progressive latitudinal dependence
of the observed albedo changes. If this is
typical on Mars, and not a consequence of
condit8ions following a' major dust storm,
then the phrase "wave of darkening"
is a misnomer. Nevertheless, there is a real
bime-dependence of albedo changes, which
were much more apparent all over Mars
after revolution 200 than at any other time
during the mission.

The evidence present,ed above suggests
the transport of both bright and dark
material on Mars. Yet the transport of
bright material must, in the long term,
dominate. In a plot of mean particle radius

versus threshold velocity for init,iating
particle motion within the surface velocit)
boundary layer (or, equivalently as dis-
played in Fig. 29, t,he corresponding
velocity above the bounda,ry layer), there
exist,s a minimum in t,he curve correspond-
ing to a most easily transported particle
size. Larger particles a,re more immobile
because the lift varies with the square of
the particle dimensions while the mass
varies with t,he cube. Smaller particles are
more immobile because t'hey are unaffected
by the t,urbulent eddies producing the
motion. For Martian conditions the most
readily t'ransport'able particles are in the
2oop range (Sagan and Pollack, 1967,
1969). However, such particles saltating
along the surface exchange momentum
with the surface and splay out smaller
suspendable particles, not' easily 1iRed
by the winds directly. The net' result is t(he
preferential injection into the atmosphere
of small part'icles which fall out slowly
according to the Stokes-Cunningham equa-
tion or even more slowly if turbulent
support exists. For single composition
models, the sma,ller particles are (because
of multiple scattering) expected to be
brighter t'han the larger particles. Dark
tails developed in Hesperia by processes
which did not affect the preexisting
bright tails (Fig. 23). In t'he filling of
cra'ters by dust fallout, the smallest,
particles arrive last. These particles, de-
flated by poststorm high-velocity winds,

FIG. 29. The curve labeled V shows t'he velocit,y
above the surface boundary layer required to
move particles of radius a on the Martian surface.
(After Sagan and Pollack, 1967.) Also shown is
the temperature dependence of the velocity of
sound V,, and of 0.9 V,.


370                         CARL SAGAN ET BL.

would be difficult to move once layed
down in bright streaks (Fig. 29). But
deflation of larger dark crater particles,
nearer to 200~ radius, could still be
accomplished by winds of lower velocity
which leave the bright streak particles
unmoved.

However, single composition models,
while they provide a convenient starting
point, are geologically oversimplified. It
may be (Sagan et al., 1971) that, because of
the greater area to volume ratios of small
part'icles and the presence of oxidizing
compounds such as ozone in the Martian
atmosphere, there is a statistical con-
nection between particle size and com-
position. The smaller particles would be
more oxidized. The Mariner 9 IRIS
results (Hanel et al., 1972) suggest a
variety of siliceous minerals on the Mart'ian
surface, and the existence of major summit
calderas indicates in another way that the
planet has been differentiated (McCauley
et aZ., 1972). It is possible that' the bright
and dark material corresponds to different
atmospheric exposure ages of basaltic or
other minerals. As long as the planet
remains geologically active we would no
more expect a complete homogenization
of such components on Mars than we do
on t,he Earth. This view evokes in several
ways the "volcanic-aeolian" hypothesis
of Martian albedo feat,ures (McLaughlin,
1955), which now appears remarkably
prescient, in at, least some of its features.
An additional possibility is that, all wind-
blown constituent,s may not have the same
density. Small particles of volcanic tuff
with a large empty fracbional volume or
nonspherica,l particles such as mica flakes
may be much more readily transportable
than solid or spherical part,icles of the
same dimensions. In a given aeolian
transport event, we expect both salt'ation
of coarse particles and suspension of fines.

Wind velocities -lOOm/sec are deduced
from lee wave clouds (Leovy et nZ., 1972)
and from the IRIS atmospheric tempera-
ture structure via the thermal wind
equation (Hanel et al., 1972), as well as
from a variety of theoretically expected
wind regimes (Sa,gan et al., 1971), lending
some support to the ca,lculations exhibit'ed

in Fig. 29. Such high-velocity sandblasting
in the thin Martian atmosphere should
result in aeolian erosional processes much
more efficient than those common on
the Earth. Because of the lower atmos-
pheric temperature and the larger mean
molecular weight of the Martian atmos-
phere the velocity of sound on Mars is
less than on Eart'h. Velocities required
to just initiate particle motion (Fig. 29),
correspond to velocities of N 7Omlsec
above the boundary layer, or Mach 0.4.
Considerably higher velocit,ies mar be
expected, at least at some places and times.
Mars therefore presents an unusual possi-
bility : that wind velocities in t'he transonic
range (between Mach 0.9 and 1 .O) occasion-
ally exist. The erosional, depositional, and
deflationary effects of transonic met.eor-
ology, if it exists, remain entirely un-
explored.

CONCLUSIONS

We have found splot'ches and streaks
as the albedo fme st,ruct#ure of Martian
bright and dark a,reas. The superposition
of splotches and streaks seems to det,ermine
the configurat'ion of ground-based albedo
markings. Both splotches and streaks vary
on characterist'ic time scales of the order of
2 weeks. The integration of such variations
is suggested to be the cause of t'he classical
seasonal and secular variatJions, alt~houph
no clear dependence of changes on latitude
or season is found in these preliminary
results. Aeolian t,ransport' of fine particles
appears clearly to be implicat,ed in the
morphology and variat,ion of splotches
and streaks. We suggest that bot'h bright
and dark material is laterally transported
on Mars.

We thank all tllose at the Jet, Propulsion
Laboratory whoso collective effort,s led to the
success of this remarkablo mission; and to our
fellow members of the Mariner 9 television team
who accommodat,ed the variable surface features
segment of the mission into the picture budget.
John McCarthy, Director of the Artificial


MARTIAN VARIABLE FEATURES                371

Intelligence Laboratory (AIL) at Stanford
University, was instrumental in making possible
the calibration, registration, scaling, and picture
differencing protocol used in the present paper.
B. Eross, L. Wasserman, M. Noland, S. Saunders,
and T. Kirby rendered specific assistance. We
also thank William Green and the staff of the
Image Processing Laboratory at JPL for picture
processing. 1sVc are grat'eful t,o J. Cutts and
C. Leovy for constructive reading of an earlier
draft of this paper. This research was sponsored
by N&kSA, through JPL. AIL was also supported
by NASA grant NGR-05-020-508 and by ARPA
grant SD-183.

R,EFERERCES

ANTONIADI, E. M. (1930). "La Planktc Mars."
Hermann. Paris.

BAGNOLD, R'. A., (1960). "The Physics of Blown
Sand and Desert Dunes." Methuen, London.
BELCHER,D.,VEVERKA,J.,ANDSAGAN,C. (1971).
Mariner photography of Mars and aerial
photography  of Earth: Some analogies.
Icarus 15, 241-252.

DE VaUCOULEURS, G. (1954). "Physics of the
Planet Mars." Faber and Faber, London.
DOLLFUS, A. (1968). Private communication.
FOCAS, J. H. (1961). Etude photometrique et
polarimctrique des phenomene saisonniers de
la plan&c Mars. Bnn. &trophys. 24, 309-
323.

Focas, J. H. (1962). Seasonal evolution of the
fine structure of the dark arcas of Mars
Planet. h'pce ,%ci. 9, 371-381.
GIEK~~S~YT, P., AND SAGAN, C. (1971). A prelimi-
nary asscwxmnt of Martian wind regimes.
Icarus 14, 312-318.

GIFFORD. F. A. (1964). The Mart,ian canals
according to a purely aeolian hypot,hcsis.
Icarrts 3 1 `:opl ?i
       ,. *..
HAXEL, H.. COXRATH, B., Hovls, W., KUNDE,
\'.. Lo\T->L~N, I'., MAC'GUIRE, IV., PEARL, J.,
PIKR.4~:LIA,   %I .,  PRABH~~AKA, C.,   AND
S~~HL.~('H~IAY. B. (1972). Investigation of t,he
Mart ian en\-iroiimtnt by infrared spectroscopy
on Illariiicr 9. Icarus 17. 423-442.
HERTZTXK, 1P. G. (1QGGa). Particle behavior in
a simu1atc.d Xbirtian environment. McDonnell
Corporation Rrport E 41 I;.
HERTZLER, It. G. (1966b). Brhavior and charac-
teristics of simulatzed Martian sand and dust
storms. JIcDonncll Corporation Report E 720.
HoR~,C. \\Y.,B~4~~~,C. A., STEXYART,I. A.,BND
Law:, A. L. (1972). Mariner 9 ultraviolet
spectromctci experiment, : photometry and
topograplry of Mars. Icarus 17, 4233456.

LEOW, C. B., BRIGGS, G. A., YOUNG, A. T.,
SMITH, B. A., POLLACK, J. B., SHIPLE:T, E. pi.,
AND ~VILDEY, R. L. (1972). Mariner 9 TV
experiment: progress report on studies of
Mars atmosphere. Icarus 17, 373-393.
LEVINTHAL, E.C., GREEN, W. B., CUTTS. ,J. A.,
SBNDER,M. J., SEXDMAN. J.B.,YouNG, A.T.,
dND SODERBLOX, L. A. (1973). Mariner 9-
image processing and products. Icarus 18,
in press.

MASURSKL', H., BATSON: R'., BORGESON, W.,
&RR, M.? MCCAULE~~, J., MILTON, D.,
WILDEY, R'., WILHEL~\IS, D., MURRAI-, B.,
HOROWITZ, K., LEIGHTON, R.. SHARP, K.,
THO~WPSON, W.. BRIGCS, G., CHANDFXSSOS,
P., SHIPLEY, E., S~GAX;, C.. POLL~~K. J.,
LEDERBERG, J., LEVINTHBL, E., H~RT~IAN-s,
W., MCCORD, T., SMITH, B., DA\-IES, M.,
DE VAUCOULEURS, G., ASD LEOVX-, C. (1970).
Television experiment for Mariner Mars 197 1,
Icarus 12, 10-45.

MASURSKY, H., BATSON, R. M.,McCA~LE~-. J.F.,
SODERBLOA~, L. A., WILDEY. R. L., &RR,
M. H., MILTON, D. J., WILHELMS, D. E.,
SMITH, B.A., KIRBY, T. B., ROBINSOS.J.C.,
LEOVY, C. B., BRIGGS, G. A., DUXB~RY. T. C.,
ACTON, C. H., Mc~nsl-, B. C.. CUTTS, J. A.,
SHARP, R. P., S~IITH, S., LEIGHTON. R. B..
SAGAN, C., VEVERKA, J., NOLAND. M.,
DE VAUCOULEURS, G.. DAWES. M., AND
YOUNG, A. T. (1972). Mariner 9 television
reconnaissance of Mars and it's sat,ellites:
Preliminary results. Science 175. 294-304.
MCLAUGHLIN, D. B. (1955). Changes on Mars, as
evidence of wind deposition and volcanism.
Astronom. J. 60, 261-270.

NEUGEBAUER,MUNCH G.,KIEFFER, H.. CHASE.
S. C., JR., ANU MINER, E. (1972). Mariner 1969
infrared radiometer res<dts : t~cmperatures and
t,hermal properties of the Martian slirface.
dstrorz. J. in press.

Po~~acr;,.J. B., GREENBERG.E.. AND S.~GAS, C.
(lQG7). A statistical analysis of the Martian
wake of darkening and related phenomena.
Planet. .Ypacc Sci. 15. 81'imm817.
SACaN, C., .~ND Po~~aclc, J. B. (1967). X
~vinclbl0w-n dlist motlel of Martiau surface
fcaturrs and s~awnd cliangc~. Smithsonian
Astrophysical Observatory Spceial Report
255.

SAGAN-, C., AND Po~~acs, .J. B. (1969). \Vind-
blown dust on Mars. Natwe (Low~o~c) 223,
791-794.

SAGAN. C., VEVERI~A, J., AND GIERIRCH, P.
( 197 1). Obser~~ational consequences of JIart ian
wind regimes. Icarus 15. 253.
SLIPHER, E. C. (1962). Mars. Low-cl1 Observatory.
Flagstaff, Arizona.


372

CARL SAGAN ET AL.

Discussion

MAROV: You suggest that particle size differences are responsible for contrasting
  brightness of streaks and splotches. What differences in particle contrasts
  with color did you observe?

SAGAN: The three color filters carried on the A-Camera are not different enough
in effective wavelength to provide much color information. Also, the filter
wheel stuck in one filter position soon after the dust cleared making subse-
  quent color measurements impossible.

Manov: Are the particle size differences required to explain the contrasts of the
  streaks consistent with your earlier paper?
SAGAN: Yes. The size differences required is smaller than a factor of ten.
MARO~: The Mars 2 and 3 spacecrafts observed increasing in bright,ness contrasts
  of from about 10% at 0.7pm to about 20% at 1.4mn.
YOUNG : As a way of calibrating your eye when studying the Mariner Mars picture,
  tho shadows of the two dust spots on the vidicon face plate have a contrast of
  6% at all wavelengths.

MCCORD : The differences between the bright and dark regions cannot be explained
  by particle size differences alone. The spectral reflectivity differences and
  particularly the absorption band differences in the reflection spectra for
  bright and dark areas require that compositional differences exist, such as
differences in the amount of oxidized basalt as John Adams and I have sug-
  gcsted. Particle size differences may also occur, of course.
SAGAN: Yes. Real composit'ional differences are possible, and we have suggested
that chemical composition and particle size are correlat,ed (C. Sagan, J.
  Veverka, P. Gicrasch. Icarus 15, 253, 1971).
MCCORD: There is, in fact, a photo-oxidation mechanism which a graduate
  student of mine, Bob Huguenin, has discovered which explains nicely how the
  surface of Mars could have been oxidized to produce contrasting bright and
  dark material of different degrees of oxidation. It also turns out that the more
  oxidized material is more easily broken into small grains.
IRVISE: Are there any bright dune fields? Could the darkness of dune fields bc
  due to a shadow effect?

SaGAN : No. The brightest part of the dune fields are darker than the surrounding
   areas.

HART~VANN: The classica, dark areas do seem to have dark background material
  in addition to the dark streaks. Is t,his what you seo also?
SAGAN: Yes. The dark background may be left over from previous tails.
RUNCORS: What is the scale of the dune fields? The entire field and one dune to
   another?

SAGL4N: About 5Okm across the field and about 2km crest-to-crest, for the dimes.