Mariner 9 Television Reconnaissance of Mars
and Its Satellites: Preliminary Results

Abstract. At orbit insertion on I4 November 1971 the Martian surface was
largely obscured by a dust haze with an extinction optical depth that ranged from
near unity in the south polar region to probably greater than 2 over most of the
planet. The only features clearly visible were the south polar cap, one dark spot
in Nix Olympica, and three dark spots in the Tharsis region. During the third
week the atmosphere began to clear and surface visibility improved, but contrasts
remained a fraction of their normal value. Each of the dark spots that apparently
protrude through most of the dust-filled atmosphere has a crater or crater com-
plex in its center. The craters are rimless and have featureless floors that, in the
crater complexes, are at different levels. The largest crater within the southerntnost
spot is approximately 100 kilometers wide. The craters apparently were formed
by subsidence and resemble terrestrial calderas. The south polar cap has a regular
margin, suggsting very flat topography. Two craters outside the cap have frost
on their floors; an appurent cruter rim within the cap is frost free, indicating pref-
erential loss of frost from elevated ground. If this is so then the curvilinear
streaks, which were frost covered in 1969 and are now clear of frost, may be
low-relie.( ridges. Closeup pictures of' Phobos and Deimos show that Phobos is
about 2.5 i 5 by 21 2 1 kilometers and Deimos is about 13.5 t 2 by 12.0 & 0.5
kilometers. Both have irregular shapes and are highly cratered, with some cra-
ters showing raised ritns. The satellites are dark objects with geometric albedw

of 0.05.

The Mariner 9 spacecraft arrived at
Mars on 14 November 1971 during a
planetwide dust storm of unusual in-
tensity. While forcing the postpone-
ment of most geologic and cartographic
objectives, the storm has provided an
unparalleled opportunity to examine at
close range a phenomenon connected
with Martian meteorology, topography,
depositional and erosional processes,
and variable features.

On 22 September Earth-based ob-
servers (I) recorded that a bright yel-
low cloud had developed over Noachis,
in the midsouthern latitudes of Mars.
It spread rapidly over the rest of the
planet, and in a little more than 2
weeks the entire visible globe was cov-
ered by dust; even the south polar cap
had disappeared from view of telescopes.
By the fifth week the dust storm had
reached its peak, exceeding all previous-
ly observed Martian storms in obscura-
tion, area1 extent, and duration. Over
the next several weeks the atmosphere
of Mars showed a gradual clearing,
and the south polar cap reappeared.
Upon is arrival Mariner 9 found the
obscuration still severe.

As Mars rotated in the view of the
approaching spacecraft, three preorbital
science (POS) sequences of photographs
were taken, giving total global coverage
of the dust-shrouded planet. Only five
distinct features could be seen-the
south polar cap and four dark spots
(Fig. 1). One of these has been identi-
fied as Nix Olympica, and the other
three (provisionally labeled North,
Middle, and South Spots) have been
identified with a group of features that

Table 1. Camera filters. The effective wave-
length is for sunlight.

Effective
wave-
length
(km)

Camera       Filter type

Wide angle
Wide angle
Wide angle
Wide angle
Wide angle
Wide angle

Narrow angle

Orange
Green
Blue
Violet
Polarization*
Yellow
(minus blue)
Yellow
(minus blue)

0.610
0.545
0.477
0.414
0.565
0.560

0.558

*Three filters with axes spaced in 120" steps.


become bright (the "W-cloud") (2)
during the Martian afternoon at cer-
tain seasons. The rest of Mars was
veiled by heavy, but regionally variable,
atmospheric dust, and it was apparent
that Mars was not the same planet for
which this mission had been planned.

Gradual clearing began during the
third week of orbital operations. The
degree of obscuration varies from re-
gion to region. The south polar region,
for example, has consistently been
clearer than other parts of the planet.
However, as of this writing (mid-
December), Mars is still heavily ob-
scured.

The dust storm has had a pro-

nounced effect on the original mission
plan (3), which included both global
surveillance of Mars and contiguous
high-resolution mapping sequences near
periapsis. The mapping sequences were
soon found to be ineffective: Dust ob-
scuration varied widely, but the map-
ping sequences could not be made suf-
ficiently flexible to take advantage of
the relative clarity of some regions.
Also, periapsis was located close to the
evening terminator, where the more
diffuse illumination at the surface se-
verely degrades surface contrast. It was
necessary to direct the high-resolution
photography to the few places where it
would be effective. Therefore, a new

Fig. 1. (a) Telescopic photograph of Mars
taken from the earth by E. C. Slipher dur-
ing the 1956 dust storm (22). The dark
spot at the top is an example of several
such temporary features observed when
the planetary surface has been largely
obscured by dust. (b) Computer-enhanced
picture of Mars taken by Mariner 7 in
1969 (photograph number 7F74). (A)
Nix Olympica with its ringed appearance
and (B, C, D) three aligned craterlike
features that make up part of the "W-
cloud" complex are shown. (c) Slightly
enhanced picture of the same region taken
by Mariner 9 during POS 2 about 1%
days before insertion (Image Processing
Laboratory roll 147, 113230). The small
south polar cap shows at the bottom of
the planet's disk. A dark spot identified
with Nix Olympica and three aligned dark
spots that lie on the western part of the
"W-cloud" complex are visible. The very
low contrast is due to obscuration by the
planetwide dust cloud. (d) Filtered mo-
saic of the four dark spots, taken 1 day
later: the streaks run 1000 km south of
South Spot (Jet Propulsion Laboratory P-
12676). The bright wings on either side
of each dark spot are artifacts of the
filtering. (e) Filtered picture of South
Spot, taken on Rev 24 about 13 days later
than (d); the streaks extending south of
South Spot are no longer visible (IPL roll
909, 123836). The streaks reappeared in
subsequent photographs.

mission plan,  the  "reconnaissance
mode," was developed and implemented
on revolution (Rev) 26, the earliest op-
portunity. This mode included on each
revolution, global coverage, from which
specific targets in relatively clear areas
could be located, and two groups of
four  high-resolution   frames   each
(tetrads), which could be placed on
such targets.

The Mariner 9 spacecraft is equipped
with two television cameras (3)) similar
to those flown on Mariners 6 and 7.
The wide-angle camera has a field of

lIIrllIIlIlrIrrl1
8o 4                                          15 km
        0     4     8      8     4     0     4

         Distance normal to cap edge (km)

Fig. 2 (left). Profiles of relative brightness across the edge of
the polar cap, measured with the narrow-angle camera (yellow
filter). The relatively sharp rise represents the discontinuity at
the edge of the polar cap. Th gradual decrease in brightness on

Wide-angle camera

l--l-
Narrow-angle camera

either side of the edge results from light scattering in the dust above the polar area.   Fig. 3 (right). Photographs of the limb
of Mars taken on Rev 16. The view on the left (JPL 4024-13) was taken with the wide-angle camera and a violet filter; that on
the right (JPL 4024-10) was taken with the narrow-angle camera and a yellow filter. Note the detached layer of haze about
1.5 km above the apparent limb of the planet. The limb region is located at 20"s latitude and 102"W longitude.


DL7  Wave Cloud       NUmbW           + A Nix Olympia    0 South Spot

                  Indicote$               B North Spot      E Coprater Streaks
0 Narrow-Angle limb Frame  Revolution              C Middle Spot     F Radar Depreuion

Fig. 4. Mercator map showing the approximate locations of wave clouds visible in violet and blue frames taken with the wide-angle camera. The lines show the orientation of the
wave bands. The circles show the positions of narrow-angle frames of the limb. The numbers near the lines and circles designate the revolutions on which the observations were
made. The areographic features, Nix Olympica (A ), the three dark spots (B, C, D), the bright streaks in the Eos-Ophir region (E), and the radar depression near JZos (F), were ob-
served in Mariner 9 photographs.


Fig. 5 (left). Detached layer of haze located near the terminator on Rev 29 is about 30"N latitude, 14O"W longitude. The
thin layer is about 30 km above the extrapolated apparent limb. The picture was taken with the wide-angle camera and violet
filter (JPL 4039-19).  Fig. 6 (right). Terminator wave clouds in violet wide-angle frame on Rev 18 (IPL roll 813, 164208).
The wave crests are approximately parallel to the terminator, and the wavelength is about 40 km. The locations of the waves

are shown in Fig. 4.

view of 1 lo by 14' and employs eight
interchangeable filters (Table 1). The
narrow-angle camera has a field of view
of 1 .l" by 1.4" and is equipped with a
fixed yellow filter. Thirty-one pictures
are taken on each revolution of 12
hours.

Caution must be used in interpreting
the details visible in the pictures ac-
companying this report. Ordinarily, the
contrast of visible detail on Mars is
some tens of percent. However, the
1971 dust storm has drastically reduced
contrasts. In order to make visible any
detail other than the polar cap and the
four dark spots, it has been necessary
to increase the contrast of the Mariner
9 pictures by much more than the usual
photographic factor of 2 or 3.

Some of the pictures reproduced here
(for example, Figs. 12c and 13) have
been photometrically "stretched" so
that a small brightness range (usually
about 20 percent of the original range
of the data) is expanded to cover the
full range from white to black. This
technique (4) is limited by the strong
limb-to-terminator brightness gradient
(see Fig. lc). It is not practical to in-
crease the contrast more without first
removing the average brightness gradi-
ents across a picture. This is accom-
plished by subtracting a running mean
(for example, 17.5 consecutive picture
elements or samples along each line)
from the signal level at each sample in
the original data. Thus, features much
larger than 175 samples in width are
removed from the data. The remaining
small-detail modulation, typically of the

order of a few percent, can then be ex-
panded to cover the full range from
white to black. In this way, the contrast
of small detail can be increased by a
factor of about 50. The process
amounts to high-( spatial) -frequency fll-
tering followed by severe "stretching";
for brevity, we call the products "fil-
tered."

It is essential to bear in mind the
enormous exaggeration of contrast in
such filtered pictures. Most of the detail
in the Mariner 9 pictures actually has
an order of magnitude less contrast
than the weakest telescopic details seen
on Mars from the earth. Some examples
may help in understanding this con-
trast exaggeration and certain artifacts.

1) Most of the filtered pictures show

a prominent pattern of diffuse vertical
stripes, caused by electronic noise on
the spacecraft (for example. Fig. 11).
The peak-to-peak amplitude of this
noise is typically a few tenths of 1
percent, but it is easily visible because
it has been amplified some tens of times.

2) Reseau fiducial marks on the vidi-
con are about three samples across and
are almost black. When a rsseau falls
within the range of a running mean, it
lowers the mean by about 2 percent.
When this lowered mean is subtracted
from a sample near the reseau, the
sample appears to be about 2 percent
brighter than the mean. Thus, on fil-
tered pictures, each reseau is flanked
by two bright lines (of combined length
equal to the length of the running

Fig. 7. (a) Mariner 9 (JPL 4019-25) and (b) Mariner 7 (7N17) views of the south
polar cap. The dark markings in the Mariner 9 wide-angle view of cap are correlated
with the light markings in the Mariner 7 narrow-angle frame. The Mariner 7 image has
been rectified and then enhanced. Although (b) shows strong contrast, the entire area
actually was covered by frost.


mean) about 2 percent brighter than
its surroundings. In most prints these
bright lines are almost at the white level
(for example, see Fig. 11).

3) Both wide- and narrow-angle pic-
tures show small circular blemishes,
about 40 samples in diameter,  which
are the penumbral shadows of  small
dust specks on the vidicon faceplates.
The contrast of the most prominent
shadow is 5 percent in the wide-angle
camera and 0.5 percent in the narrow-
angle camera. These shadows are invisi-
ble on the raw pictures. (In filtered
wide-angle pictures, such as Figs. 11
and 12e, they sometimes appear black,)

4) A prominent feature of many pic-
tures is a residual image of an earlier
limb picture (see Fig. 12~). This resid-
ual is about 10 percent in brightness in
the first picture after the limb frame

and decays slowly: it is about 5 percent
on the next frame, 3 percent on the
next,  and so on (continuing greater
than 1 percent for many frames). For
example, the wide-angle pictures from
Rev 2 showed prominent limb residuals
from  the last preorbital sequence.
These residuals have about 1 percent
peak contrast.

In many processed photographs these
artifacts-the streaks about the reseaus,
the shadows of the dust specks, the
limb residuals,  or even the vertical
noise-are more prominent than the
images of real features on Mars. which
shows that the contrast in the scene is
less than that of the artifacts. Features
of such low contrast would be invisible
by telescope or on conventional photo-
graphs of Mars.

Photometric

function.   The limb

Fig. 8. Details of the south Polar cap. Patterns in the subliming frost cap are recorded
in the five narrow-angle frames. The locations are indexed in the accompanying wide-
angle frame. The maximum dimension of the narrow-angle frames is about 100 km
on the surface of Mars. Frames A (JPL 40.56-G) and E (JPL 4056-62) are from
Rev 46, C (JPL 4045-71) and D (JPL 4048-65) from Rev 38, and B (JPL 4033-45)
from Rev 25. The wide-angle frame (JPL 4056-65) is from Rev 46; a residual image
of the polar cap is faintly visible, displaced toward the upper right corner.

darkening of the planet is evident in
preorbital data. If we assume that the
limb darkening can be described by a
Minnaert function (5, 6). the darken-
ing parameter k is uniquely related to
the position on the disk of maximum
brightness, if it is assumed that the
view is orthographic (constant phase
angle over the disk). This assumption is
not quite met, but the angle subtended
by the planet is less than 3" in the pic-
tures taken with the wide-angle camera
just before orbit insertion. These pic-
tures cover the full range of color filters
in quick succession, the maximum
value of k occurs on the terminator side
of the subsolar point for orange, yel-
low, and green light, and on the limb
side for blue and violet. The corre-
sponding values of k progress regularly
from 1.12 to 0.93 with decreasing
wavelength, for an assumed mean phase
angle of 58". The differences from a
Lambertian surface (k = 1.00) are
small but significant; the limb is clearly
bluer than the center of the disk.

Since the surface features are gen-
erally hidden by the dust storm. the
quasi-Lambertian limb darkening is not
surprising, although data from Mariner
4 (5) indicate much lower values of k
(0.7 to 0.8). Lambertian surfaces can
be approximated in the laboratory by
powdered materials of low refractive
index that produce high-order multiple
scattering. Thus, the present values of
k near unity are consonant with an
optically thick scattering atmosphere.
Unlike the isophotes of a quasi-lamber-
tian sphere, which are ellipses centered
on the subsolar point, the Martian iso-
photes are elongated toward the poles-
consistent with a significant contribu-
tion from single scattering. Because the
single scattering is nonconservative, it
represents an important fraction of the
net albedo from multiple scattering. The
weak absorption in the scattering is
consistent with the scattering material
being dust raised from the surface.

Visibility of slirfme features. The
most prominent features in the POS
pictures are the south polar cap and
the four dark spots. The dark appear-
ance of these spots is particularly strik-
ing, as they are usually observed to be
at least as bright as their surroundings.
However, a dark feature of relatively
high contrast, which may have been
South Spot, was reported during the
1924 dust storm (7), and other tempo-
rary dark spots have been noted when
the surface of Mars has been obscured


by dust (Fig. la). The maximum con-
trast (8) of the spots was 20 percent
for Nix Olympica and 10 percent for
South Spot, from Mariner 9. The con-
trast of the three aligned spots de-
creased as they rotated toward the
early afternoon limb, but Nix Olympica
lost contrast toward the limb on only
one of the two POS sequences. This be-
havior suggests that the extinction opti-
cal depth T (9) above fhe three aligned
spots was about 1, but it may have
been significantly less over Nix Olym-
pica.

Because the south polar cap showed
the highest contrast of any surface fea-
ture, and because it was observed fre-
quently, it was a useful target for
measuring variations in atmospheric
extinction. It does not exhibit diurnal
brightening as the spots sometimes do.
Figure 2 shows typical profiles of rela-
tive brightness across the edge of the
cap, from narrow-angle frames. The
shape of these profiles-a sharp jump
in brightness with gradually decreasing
variations on each side of the jump-
may be due to a sharp-edged cap that
appears somewhat diffuse because of
atmospheric scattering of the radiation
reflected from both sides of the surface
albedo boundary (10). An alternative
interpretation of the features of these
brightness profiles, in terms of gradual
variations in the intrinsic surface albe-
do on both sides of the cap edge, seems
less likely. With the model of a sharp-
edged cap, we derive a contrast of 14
percent at the cap edge on Rev 11.
The normal contrast of the cap edge
in yellow light is about 90 percent (II).
The contrast reduction of the Rev 11
narrow-angle picture suggests that T was
about 1, reduced to the zenith, over
the cap at that time. The contrast of
the cap showed little variation during
the first 2 weeks of the mission but
began to increase during the third week.
By Rev 34 a narrow-angle picture
showed significantly more contrast than
the Rev 11 frame, and continued con-
trast increases have been observed on
later revolutions. Evidently, T decreased
significantly during this time.

Nix Olympica, the three spots, and
the south polar cap were the clearest
regions on the planet; other albedo fea-
tures of large size also were faintly
visible, mainly in the southern hemi-
sphere. However, the disappearance of
almost all of the classical albedo fea-
tures and of many craters suggests that
T was 2 or more over most of the

planet. Craters, appearing as features
of high albedo, can be seen in many
pictures taken at high angles of the
sun. In general,  craters and other
albedo features were observed better
in orange light than in violet. Some
relatively low areas also appear bright,
for example, the long, bright streaks
in the regions of Ophir and Eos (Fig.
12).

Except in Nix Olympica and the dark
spots,  surface relief (in contrast to
albedo variations) could not be seen
during the first 2 weeks. This can be
attributed in part to very strong diffu-
sion of solar radiation by the haze, so
that surface illumination was nearly iso-
tropic. During the third week, surface
relief could be seen more readily, espe-
cially near the south polar cap. This
observation supports our conclusion
from the contrast data for the polar cap
that the optical thickness of the haze
was diminishing during this period.

Structure of the limb. Figure 3 shows
typical limb structure in overlapping
narrow- and wide-angle frames. The
features shown-a gradual decline in
brightness at the limb, a well-defined
gap, and a thin elevated layer of haze
-were observed in most frames of the
apparent limb, with typical scales of 10
km for the region of brightness decline

Fig. 9. Mariner 9
photographs display-
ing the gradual dis-
appearance of the re-
sidual polar cap. At
the top   are   two
Mariner 9 wide-angle
pictures of the polar
cap acquired about 9
days apart on Revs 11
(left) and 29 (right).
These   two  images
have been rectified to

a polar stereographic
projection. The rela-
tive distortion between
the two projections is
due to uncertainties
in  the preliminary
pointing data. The cap
is approximately 450
km in diameter in the
earlier  image. The
white arrows indicate
common areas that
have suffered signili-
cant defrosting over
         

the Y-day perlod. A residual image of the cap can be seen to its right in the left-hand
picture. At the bottom are two Mariner 9 narrow-angle frames from Revs 11 (left)
and 34 (right); these show changes in the cap over 11V'z days. The area of the narrow-
angle frames is indicated by black arrows in wide-angle frames. Since certain fine details
can be seen in both images, the difference in the appearance is real and not solely the
result of varying obscuration. Detailed patterns parallel to the main frost-free corridor
are emerging in the later version.

and 1.5 km for the gap between the
limb and the elevated layer. The first
dimension suggests that the scattering
particles responsible for the decline in
brightness are distributed with the scale
height of 10 km. The narrow-angle pic-
tures taken north of 15 ON showed more
complex structure, often with several
layers and with horizontal variations in
the layers, but all the narrow-angle
frames of the limb showed some de-
tached layer structure. The locations of
these pictures through Rev 35 are
shown in Fig. 4. In addition, four nar-
row-angle frames showing limb struc-
ture were taken south of 65"s. The de-
tached layer shown in Fig. 3 is brighter
in the violet wide-angle frame than in
the yellow narrow-angle frame, in con-
trast to the region of brightness decline,
which is distinctly brighter in the yel-
low frame. A photometric four-color
sequence including the limb near the
south polar cap showed that the planet
and the regions of brightness decline
were about as red as the normal Mars,
which, together with the scale of 10 km,
suggests that the lower scatterers are
dust particles. The elevated haze was
white or slightly blue and is evidently
composed of different particles, possibly
condensates. Elevated thin layers of
haze were also visible at, or just


beyond, the terminator on the limb
(Fig. 5).

Atmospheric waves. Figure 6 shows
apparent atmospheric wave structure
near the terminator just east of Elysium.
Several large wave trains have been
seen on the terminator in pictures
through Rev 50. Typical wavelengths
are 40 km. These features are plotted
in Fig. 4. Waves were observed in one

wide-angle frame taken with a blue fX-
ter; in all other cases they were seen in
violet light. None was apparent in
orange frames, even when the overlap-
ping violet frames showed waves in the
same region on successive days. The
brightness variation across a given wave
and the increasing contrast near the
terminator (Fig. 6) suggest relief varia-
tions in an atmospheric scattering layer,

Fig. 10. Narrow-angle views of craters in the dark spots, rectified on the basis of
preliminary orbital data. (a) Crater complex in rJix Olympica. Arcuate scarps con-
tinuous with the scallops on the wall indicate that the floor is not fully obscured.
Smaller craters to the east and south may be superposed, unrelated features (IPL roll
882, 025127). (b) Crater complex of North Spot. The central crater with terraced walls
apparently truncates several subsidiary craters (IPL roll 882, 030159). (c) The South
Spot crater photographed on Rev 28 (IPL roll 882, 022534 and 023951). A concen-
trically fractured zone extends about one crater radius out from the crater edge; the
surface beyond has a complex subradial texture. Several rimless craters in the smooth
zone appear to be aligned along the arcuate fractures. Troughs composed of numerous
coalescing craterlets and grooves extend to the north-northeast and south-southwest of
the main crater. Small rimmed craters to the west may be impact craters unrelated to
the regional structure. (d) South Spot viewed on Rev 32, 48 hours after the previous
view (IPL roll 882, 033019 and 033901). The differences are due to real changes in
the dust cloud or in the lighting and view paths through the dust. The disappearance
of the radial facies on the northwestern rim suggests that other apparently smooth
areas, such as the crater floor, actually may be concealed by atmospheric dust.

but the waves also could be made visi-
ble by variations in the optical proper-
ties of the scattering la!,er. Their visibil-
ity in violet but not in orange. together
with the crisp detail. suggests that these
waves are not seen in the to? of the
main atmospheric dust layer but most
likely occur in a higher, bluer layer,
possibly of condensates. The occur-
rence of thin, bright clouds near the
terminator (Fig. 5) supports this inter-
pretation. The preferred orientation of
the wave crests. parallel to the termina-
tor, is apparently real.

Shorter waves (5 km long) were oh-
served in a few narrow-angle framej
(yellow filter). mainly near the termi-
nator. Some features suggestive of
waves were also observed in POS nar-
row-angle frames. The curvilinear
streaks that extend nearly 1000 km
southwest from South Spot were seen
only at low angles of the sun (see Fig.
1, b-d). During a 90-minute period in
POS 3 these features exhibited large
changes. They may be atmospheric
waves in the upper part of the dust
layer, controlled by the elevated topog-
raphy kno,vn from Earth-based radar
(12) to lie to the east. The wave struc-
ture seen in the }ellow frames i; evi-
dence for some vertical structure in the
dust, at least locally.

Interpretations. The three aligned
dark spots are located at or near
points that are high in Earth-based
radar profiles. Crater floors are seen as
brighter objects than their relatively
higher surroundings, although no uide-
spread correlation between crater5 and
higher albedos was noted in the 1969
data (13). The long depressions in the
Ophir-Melas Lacus region are brighter
than their rims. Thus. certain elevated
regions (12) are relatively dark. and
certain regions that are lower than their
surroundings appear bright. The ap-
parent connection between bright and
dark areas and elevation differences
during this dust storm raises the pos-
sibility that some classical seasonal and
secular variations of features have been
due to differential obscuration of areas
at different elevations (24). In addition.
local dust storms could produce local
obscuration (1.5), even when no global
dust storm is in evidence.

The consistently better visibility of
the high areas may be due to concen-
tration of the dust in widespread low
layers through which the high terrain
protrudes, or to there being less atmo-


sphere above the high terrain if the
dust is uniformly mixed. There is evi-
dence for both hypotheses. The wide
distribution of the surface obscuration
and the apparent correlation between
brightness and elevation, as well as the
scale-height variation of the dust in limb
frames, indicate thorough mixing of the
dust. Regional nonuniformities in ob-
scuration are indicated by the relative
clarity of the south polar region, the
complex structure of northern limbs,
and perhaps by the clarity of Nix Olym-
pica, which may contradict the altitude-
clarity relationship. Radar data do not
show this feature to be a particularly
high object (12).

The height of features on the limb
is not known because the limb of the
solid planet cannot be seen. However, if
the dust is well mixed we can infer ,a
height. A well-mixed dust layer with
T value of 2 would not show a bright-
ness decrease on the limb below heights
at which the density is about l/40 of
the surface density. This implies that
the lower part of the region of bright-
ness decline is between 3 and 4 scale
heights (30 to 40 km) above the sur-
face. On this basis, the thin detached
layer would be near 60 `km, at a pres-
sure between 0.1 and 0.01 mb. At
least some of the wave clouds near the
terminator may be occurring in this
high layer. The nearest terrestrial ana-
logs to a scattering layer in this pres-
sure range are noctilucent clouds, which
are sometimes apparent near or beyond
the terminator during the summer at
high latitudes.

Surface features of the south polar
region. One objective of the Mariner 9
television experiment is to monitor the
retreating south polar cap and to com-
pare the exposed surface features with
their frost-covered appearance recorded
by Mariner 7 in 1969. These observa-
tions should provide evidence about the
nature and origin of the features and
terrains peculiar ,to the south polar re-
gion. The problems involve the past and
present thickness of the 0% ice, the
possible presence of other frozen vola-
tiles, the control of the distribution and
thickness of the frost by regional topog-
raphy.  and the geological processes
associated with polar phenomena that
may have uniquely modified that to-
pography.

Owing partly to the high intrinsic
contrast between the cap and the
adjacent ground, it has been possible

Fig. Il. Linear scarps, possibly a product of block faulting, west of Solis Lacus. As in
almost all Mariner 9 pictures, the crater floors appear featureless. In this narrow-angle
frame from Rev 42 the three dark circular spots are shadows of dust specks on the
vidicon faceplate (JPL 40.55-75)

for Mariner 9 to monitor frost distribu-
tion throughout the dust storm. Correla-
tion with the Mariner 7 pictures shows
that the residual cap lies within the
region of peculiar curvilinear features
previously seen (16) near the south
pole. Figure 7 shows the south polar
cap as viewed by Mariner 7 late in the
Martian southern winter and by Mari-
ner 9 during the Martian southern sum-
mer. The planimetric similarity of the
curvilinear patterns in both views is
evident. However, in the 1969 picture
the streaks are bright, being highlighted
by frost, whereas they are dark in the
Mariner 9 pictures. It is now evident
that these curvilinear patterns are
permanent topographic forms that con-
trol the outline of the residual cap as
it retreats. It was observed in 1969 that
local topography, principally craters,
strongly controlled the detailed form of
the margin of the retreating cap near
60'S latitude. Parts of the boundary
and interior of the retreating cap, as
seen in the Mariner 9 pictures, are re-
markably regular and uniform and,
therefore, appear to contain few craters.
The topography seems to be limited
primarily to curvilinear ridges or
troughs, or both, perhaps of a highly
subdued character.

Figure 8 presents selected high-reso-

lution pictures that illustrate the variety
of patterns displayed in the subliming
cap. Whereas the low-resolution view
gives the impression of a roughly cir-
cular pattern of conformal curvilinear
features,  the high-resolution images
show a much more irregular pattern,
although the individual dark streaks are
quite uniform in width. One ring-
shaped feature, probably a frost-free
crater rim (see Fig. 8B), suggests that
the positive relief may have defrosted
there more rapidly than in adjacent
areas. If this is true, the curvilinear
streaks are most likely a complex of
ridges in low relief. However, shallow
scarps or troughs cannot be excluded
by the available data.

The two wide-angle frames in Fig.
9 illustrate part of the significant dis-
appearance of frost that occurred be-
tween Revs 11 and 44 (an interval of
16% days). Two high-resolution views
in the figure show details of the disap-
pearance over 11% days. As only a few
centimeters of solid CO, could possibly
sublime in this time, a very thin de-
posit covering a surface of extremely
low relief is indicated in the areas that
are clearing. It is still possible that
thicker deposits exist elsewhere in the
residual cap. The high-resolution frame
acquired on Rev 34 suggests topograph-


ic control of the sublimation along  cap is related to past or present thick
small, shallow, irregular curvilinear fea-  ice deposits. Continued monitoring to
tures paralleling the main dark band.   record changes of detail combined with
At present, there is no way to de-  quantitative processing and manipula-
termine whether the unusual topography  tion of all the relevant photographic
at the site of the residual south polar  data may permit at least some intelli-

Fig. 12. Evidence of topographic control of atmospheric features. (a) The Aurorae
Sinus region (center) displayed a discrete bright cloud during the 1956 dust storm. A
similar discrete cloud appeared over Eos during the early stages of the 1971 dust
storm (I). (b) Radar elevation contours (in kilometers) in the Eos region show that
the brightest knot in the cloud lies near the deepest depression. The area shown cor-
responds with (c) and (d). (c) Eos-Ophir cloud complex as observed by Mariner 9
during POS 3 on I3 November 1971. The background IS entirely obscured by atmo-
spheric dust. The arcs are residual images of the limb from previous frames. The area
shown corresponds to the area in the box in (a) (IPL roll 529. 252424). (d) Portion
of a rectified-Mariner 7 photomap of the same legion as (c), with the n&&ally visible
dark markings. [From J. A. Cutts et al. (13)] (e) Narrow-angle mosaic of the western
part of the cloud, photographed on 2 December. Some surface detail can be seen outside
the sharp boundaries of the cloud. The area corresponds to that in the left-hand box
in (c) (IPL roll 905; 201449, 202414, 203341, 204.508). (f) Mariner 7 frame (6N7)
showing the scarp-bounded valleys that delimit the eastern end of the cloud in the
Eos region. These valleys contain the chaotic terrain discovered in 1969. The area

gent speculations as to the history of
this unique portion of the Martian sur-
face.

Geology. The atmosphere has been
clear enough for geologic study of only
a few areas, principally those indicated
by radar and occultation data to be at
high elevations, or those at far south-
ern latitudes.

Perhaps the most significant pictures
were taken in three of the four dark
spots viewed in the preinsertion se-
quence. Nix Olympica, North Spot, and
South Spot were photographed with
narrow-angle frame tezrads; each con-
tained a crater or a crater complex.
The Nix Olympica (Fig. 10a) and
North Spot (Fig. lob) features are
each composed of four or five inter-
secting craters that have floors at dif-
ferent elevations and appear to have
formed successively, The visibility of
the terrace edges indicates that the
craters are not filled by dust; the ap-
parent smoothness of the floors may
be real, or the floors may be ob-
scured by local dust. The craters are
rimless or have very low rims; sharp
ridges like those that surround most of
the lunar craters are strikingly absent.
On the highest-resolution photographs,
those of South Spot, features less than
1 km in dimension can be distinguished
(Fig. 10, c and d).

South Spot is a single, approximately
circular structure, about 100 km across,
that in its entirety is unlike any known
terrestrial or lunar crater or any Mar-
tian crater observed on previous Mar-
iner missions. The crater does resemble
some large lunar craters in having a
flat floor and completely terraced walls;
it is distinctive in that a zone with a
smooth surface broken into arcuate
horsts and grabens concentric with the
crater immediately surrounds the rim.
Fresh, large lunar craters typically have
coarse, hummocky topography imme-
diately outside the crest of the rim that
grades outward into radial ridges and
grooves with low relief. Here the hum-
mocky facies is absent, and a radial
pattern is visible only beyond one
crater radius from the edge of the
crater. The elements in this zone are
short and lobate, unlike the narrow,
gradually tapering ridges radial to lunar
craters. On the rim are several small
(1 to 5 km in diameter) rimless cra-
ters, some of which appear to form
arcuate chains. Major valleys extend
to the northeast and southwest of the

shown corresponds to that in the right-hand box in (c) and the box in (d).       main crater (perhaps not coincidentally


Fig. 13. Field of sinuous features near the
south pole, in a wide-angle photograph
from Rev 16 CJPL 4024-201. One feature
apparently transects the rim of the crater
A, which suggests that the features were
formed by surficial processes.

in the direction of alignment of North
Spot, Middle Spot, and South Spot).
These valleys consist of many coalesc-
ing irregular craterlets and sinuous or
linear grooves, and are another feature
not closely matched in terrestrial or
lunar craters.

The morphology of the craters in
the three spots indicates sulbsidence.
The most prominent features of large
subsidence on the earth are volcanic
calderas, which are caused by the with-
drawal of magma, principally by erup-
tion (17). Some calderas lie on the
summits of broad volcanic shields, are
accompanied by rimless pit craters on
the upper flanks and lava flows on the
lower slopes, and show multiple arcu-
ate faults similar to those `of South

Fig. 14 (left). Computer-enhanced photo-
graph of Deimos obtained on Rev 25 at a
distance of 8830 km and a phase angle of
68" (IPL roll 752, 144007).   Fig. 1.5
(right).  Computer-enhanced frames of
Phobos. (a) View obtained on Rev 31 at
a distance of 14,440 km and a phase angle
of 77" (IPL roll 792, 2203 12). (b) View
obtained on Rev 34 at a distance of 5550
km and a phase angle of 56" (IPL roll
820, 002025).

Spot, though rarely as well developed.
The largest terrestrial calderas are ir-
regular multiple structures with out-
lines and dimensions comparable to
those of the Nix Olympica and North
Spot features. A single, symmetrical
caldera as large as the South Spot fea-
ture would be extraordinary on the
Earth. An alternative hypothesis, that
these craters are solely of impact ori-
gin, is difficult to reconcile with the
absence of walls between component
craters, the different levels of their
floors in Nix Olympica and North
Spot, and the lack of markedly ele-
vated rims and hummocky rim deposits.
Nevertheless, the continuing debate on
the origin of many craters on the moon,
where atmospheric processes neither
cause surface modification nor degrade
the viewing, indicates the advisability
of caution in this preliminary interpre-
tation, particularly since what we have
seen may be only small parts of large
structures. The fact that these features
are elevated to a level of visibility when
the rest of the planet is obscured sug-
gests that they may not be typical of
Mars as a whole. If the sharpness of
the features and the smoothness of the
surrounding terrain is not an effect of
the conditions of viewing or image
processing, these spots may be geologi-
cally young features compared to the
heavily cratered areas observed in 1969
(18).

As the dust cleared elsewhere, some

terrain became visible that exhibits
more linear structural patterns than
were recognized in the 1969 pictures.
For example, west of Solis Lacus the
surface is broken by irregular subpar-
allel scarps (Fig. 11). The pattern is
suggestive of block faulting on the
earth;  its true nature may become
more apparent as broader areas be-
come visible.

A pattern of light streaks in the re-
gion from Eos to Ophir, observed con-
tinuously since the POS sequences, is
shown in Fig, 12. The brightest knot
corresponds to a bright yellow cloud
first viewed in photographs taken from
the earth on 27 and 28 September, a
few days after the 1971 dust storm be-
gan; a similar cloud was se:n in the
same location during the 1956 dust
storm (Fig. 12a). Radar data show
that in this region the dark areas (such
as Aurorae Sinus) are high, while the
bright regions are lower. The bright
knot lies near a deep depression that
is 6 km lower than the mean terrain
(Fig. 12b; Fig. 4, point F). A com-
parison of the 1971 and 1969 Mariner
pictures reveals that the bright mark-
ings obscure the dark markings that
are normally visible in the region (Fig.
12, c and d) and suggests that the
marking is an effect produced by a
greater amount of dust in or above a
system of valleys. It is not clear wheth-
er the cloud is a true concentration of
dust or merely an apparent effect of in-


55        60
Picture elements

creased path length through a more
uniformly dust-laden atmosphere. Fig-
ure 12e is a narrow-angle tetrad in
which part of the streak, and its scal-
loped, sharp boundaries, can be seen.
Figure 12f shows that the eastern end
of the cloud complex lies in the valley
system containing the chaotic terrain
discovered in 1969.

As in 1969, the south polar region
presented features of types not seen in
other southern latitudes. One of the
more notable of these is a field of sinu-
ous markings trending approximately
north-south (Fig. 13). The identity of
the pattern in views taken days apart
shows that the markings are fixed fea-
tures and not cloud or dust billows.
One feature extends without deflection
over what appears to be the rim of a
crater. which suggests that the features
have been produced by material trans-
ported over the surface with only slight
control by underlying topography.

Sntcllitc astronomy. Preorbital and
orbital photographs of the Martian sat-
ellites, Phobos and Deimos, have pro-
vided new  information about their
orbits, shapes, sizes, albedos, and sur-
fact morphologies. To photograph Pho-
bos and Deimos at close range it was
necessary to obtain improved orbits.
The first corrections to the orbital data
were based on 21 pictures taken during
the POS sequences that contain Phobos
or Deimos together with stars. The
ephemerides are being improved by in-
corporating pictures taken in orbit. A
first-order analytic theory including
zonal harmonics up to fourth order is
used to model the motion of Phobos
and Deimos (19). The initial condi-
tions were obtained by fitting data ob-

I
65

J
70

Fig. 16. Brightness
traces across the two
prominent craters on
Deimos  (see Fig.
14). From left to
right, the trace pro-
gresses from near
the sunlight limb to
near the terminator.
The ordinate is the
difference in bright-
ness between a trace
across one of the
craters   and   that
across an uncratered
part of Deimos. The
image used to obtain
the brightness values
was not photometri-
cally decalibrated.

tained from Wilkins' theory (20). From
the processed television data up to 28
November, the position of Deimos was
determined with a standard deviation
of 20 km. Since there were only five
Phobos pictures, its position was ob-
tained only to 100 km. Corrections to
the mean orbital elements indicate that
Phobos is approximately 2" ahead of
its expected mean anomaly, while Dei-
mos is about lo behind. The inclina-
tions of the orbital planes of Phobos
and Deimos relative to the equatorial
plane of the earth differ by 0.5' from
those expected.

Figures 14 and 15 show the first
closeup views ever obtained of these
satellites. Both are irregular in shape,
as expected for bodies too small for
their gravitational fields to impose a
spherical shape. The bottom side of
Deimos displays an interesting indenta-
tion. Similarly, an indentation appears
on the right side of Phobos in Fig. 15b.
Phobos is estimated to be 25 & 5 km
long (e'ast to west) and 21 + 1 km
wide (north to south); Deimos is 13.5
+- 2 km long and 12.0 i 0.5 km wide.
The nominal values are based upon the
correction appropriate for a spherical
object viewed at the observed phase
angle. The larger limits of error for the
length measurements reflect the un-
certainty in allowing for the unillum-
inated portions of the body.

Combining the above dimensions with
Kuiper's visual estimates from the earth
of the magnitudes of the satellites (6),
we infer a geometric albedo of 0.05
for both satellites, with a formal uncer-
tainty of about 25 percent. Such a low
albedo puts these two satellites among
the darkest objects in the solar system,

equivalent to the darkest parts of the
lunar maria.

The picture of Deimos (Fig. 14) re-
veals two clear craterlike depressions
near the terminator and a third dusky
spot, which may be a crater. The cra-
ters are about 1.4 km in diameter, and
their topography can be inferred from
the patterns of brightness near and in
the craters. Figure 16 shows a bright-
ness profile through each crater in the
direction from the limb to the termi-
nator. The ordinate is a monotonic
function of brightness but is not neces-
sarily a linear function, since the pic-
tures have not been photometrically de-
calibrated. To partially cancel the fall-
off of light toward the terminator, the
data were normalized to the values on
a parallel line across the smooth sur-
face between the two craters. The shad-
owed, depressed interior for each cra-
ter is shown, and each crater is seen
to have a bright outer rim on the ter-
minator side. The presence of an outer
rim is consistent with an impact origin
for the craters. In the negligible grav-
ity field of the satellites, little of the
fragmental ejecta would settle near the
crater, so that the rims correspond to
the raised bedrock rims of terrestrial
craters. These rims indicate some de-
gree of plastic response. A more brit-
tle response, which could depend upon
the velocity of impact as well as the
material of the satellite, would cause
spalling of the outer shells of the satel-
lite over a broad zone around the
crater, around the antipodal point, and
possibly even over the entire surface
(21). While extensive spalling is not
evident. some of the angular irregu-
larities may have originated through
such a process.

Many craters also appear on the
pictures of Phobos. Particularly strik-
ing is the 5.3-km crater near the bot-
tom (south pole) in Fig. 15a and near
the center in Fig. 15b. The impact that
produced this crater is close to the
largest impact Phobos could have sus-
tained without fracture and disruption.

Preliminary measurements indicate
that the crater density on the two satel-
lites is within about a factor of 3 of
that corresponding to a surface satu-
rated with craters. On the surface of
,Mars itself, the density of craters of
similar size is roughly two orders of
magnitude less. Thus. Phobos and
Deimos serve as "control surfaces" for
Mars and indicate that substantial ero-
sive processes have acted to remove


ancient Martian craters. Phobos and
Deimos can be regarded as old, rela-
tive to Martian surface structures on
the kilometer scale.
HAROLD MASURSKY, R. M. BATSON
J. F. MCCAULEY, L. A. SODERBLOM
              R. L. WILDEY

U.S. Geological Survey,
601 East Cedar Avenue,
Flagstaff, Arizona 86001
      M. H. CARJX, D. J. `MILTON
            D. E. WILHELMS
U.S. Geological Survey,
354 Middlefield Road,
Menlo Park, California 94025
     B. A. SMITH, T. B. KIRBY
             J. C. ROBINSON
Department of Astronomy,
New Mexico State University,
Las Cruces 88001
               C. B. LEOVY

Department of Atmospheric Sciences,
University of Washington,
Seattle 98105

   G. A. BRIGGS, T. C. DUXEXIRY
          C. H. ACTON, JR.
Jet Propulsion Laboratory,
California Znstitute of Technology,
Pasadena 91103

    B. C. MURRAY, J. A. CIJTTS
     R. P. SHARP, SUSAN SMITH
Division of Geological and Planetary
Sciences, California Institute of
Technology, Pasadena 9ZZO9
              R. B. LEIGHTON

Division of Physics, Mathematics, and
Astronomy, California Institute of
Technology

C. SAGAN, J. VEVERKA, M. NOLAND
Laboratory for Planetary Studies,
Cornell University,
Ithaca, New York 14850
    J. LEDERBERG, E. LEVINTHAL
Department of Genetics, Stanford
University School of Medicine,
Stanford, California 94305
  J. B. POLLACK, J. T. MOORE, JR.
Space Sciences Division,
Ames Research Center,
Moffett Field, California 94305
           W. K. HARTMANN
ZZT Research Institute, 60 West
Giaconda Way, Tucson, Arizona 85704
              E. N. SHIPLEY
Bellcomm, Inc.,
955 L'Enfant Plaza North, SW
Washington, D.C. 20024
          G. DE VAUCOULEURS
Department of Astronomy, University
of Texas, Austin 78712
              M. E. DAVIES

Rand Corporation, 1700 Main Street,
Santa Monica, California 90401

References and Notes

1. T. B. Kirby and I. C. Robinson, Sky T&SC.
42, 264 (1971).

2. E. Pettit and R. S. Richardson. Publ. Asrron.

sot. Pac. 67, 62 (1955).

3. H. Masursky, R. Batson, W. Borgeson, M.
Carr, J. McCauley, D. Milton, R. Wildey,
D. Wilhelms, B. Murray, N. Horowitz, R.
Leighton, R. Sharp, W. Thompson, G. Briggs,
P. Chandeysson, E. Shipley, C. Sagan, J.
Pollack, J. Lederberg, E. Levinthal, W.
Hartmann, T. McCord, B. Smith, M. Davies,
G. de Vaucouleurs, C. Leovy, Icarus 12, 10
(1970).

4. A. A. Mikhailov,, in The Moon, Z. KopaI
and Z. K. Mikhatlov, Eds. (Academic Press,
New York, 1962), p. 3. Our "stretched"
pictures correspond to the Russians' "photo-
metric cross-sections."

5. A. T. Young, Icarus 11, 1 (1969). The dark-
ening parameter k is defined in terms of the
brightness B, incidence angle i, and emission
angle c by B = B,(cos i)"(cos E)~-I, in which
the parameters B, and k are both functions
of phase angle in general.

6. D. L. Harris, in Planets and Satellifes, G. P.
Kuiper and B. M. Middlehurst, Eds. (Univ.
of Chicago Press, Chicago, 1961), p. 289.
1. E.-M. Antoniadi. La Planete Mars 1659-1929
(Librairie Scientifique Hermann, Paris, 1930).
8. Contrast is defined here as the difference in
brightness of the dark and light areas divided
by the brightness of the dark area.
9. Optical depth is the natural logarithm of the
reciprocal of the atmospheric transmission
  factor. Extinction is the sum of attenuation
caused by scattering and by absorption.
10. D. J. Van Blerkom, Zcorus 14, 235 (1971).
11. This is based an an average of 90 percent

for Mariner 7 red and green photographs at
  a different season but with similar illumina-
tion and on a measurement of 60 percent in
a Lowell Observatory plate at the same
  season and illumination. The latter value is
  expected to be somewhat below the true value
because of the small cap size in the image.
12. G. H. Pettengill. C. C. Counselman, L. P.
Rainville, I. I. Shapiro, Asrron. 1. 74. 461
(1969); A. E. E. Rogers, M. E. Ash, C. C.
Counselman, I. 1. Shapiro, G. H. Pettengill,
Radio Sci. 5, 465 (1970); R. M. Goldstein,
W. G. Melbourne. G. A. Morris, G. S.
  DOWllS,  D. A. O'Handley. ibid., p. 475;
G. H. Pettengill, A. E. E. Rogers, I. I.
Shapiro, Science 174, 1321 (1971); G. S.
Downs, R. M. Goldstein, R. R. Green, G. A.
  Morris, ibid., p. 1324,

13. J. A. Cut&, L. A. Soderblom, R. P. Sharp,
B. A. Smith, B. C. Murray, 1. Geophys. Res.
76, 343 (1971).

14. C. Sagan and J. B. Pollack, Narrrre 223, 791
  (1969).

15. C. Sagan, 5. Veverka. P. Gierasch, Zcorus
15, 253 (1971); P. Gierasch and C. Sagan,
ibid. 14, 312 (1971).

16. R. P. Sharp, 8. C. Murray, R. B. Leighton,
L. A. Soderblom, J. A. Cuts. 1. GeOphJ's.

Res. 76, 357 (1971).

17. H. Williams. Univ. Calif. PI&I. Geol. Sci. 25,

239 (1941).

18. B. C. Murray, L. A. Soderblom, R. P. Sharp
J. A. Cutts, J. Geophys. Res. 71. 313 (1966),
19. K. Aksnes, paper presented at the 8th annual
  seminar on Problems in Celestial Mechanics

University of Texas, Austin, 1970.
20. G. A. Wilkins, in Man&s of rhe Earfh and
Terrestrial Planets. NATO Admnced Study

Znstirrrre, Lmiversir; of Newcasrle-rrpon-Tyne,
1966 (Interscience, London, 1967). p. 17.
21. D. E. Gault and J. A. Wedekind, J, Geophyr.
  Res. 74, 6780 (1969).

22. E. C. Slipher, The Photographic Story of
  Mars (Northland  Press,  Flagstaff,  Ariz.
  1962), p. 142.

23. We thank E. M. Shoemaker and A. P.
Ingersoll for reviewing the manuscript. We
also thank R. Tyner and S. Reed for making
mosaics of the pictures during mission opera-
tions. This work was performed for the Jet
  Propulsion Laboratory, California Institute
of Technology, under NASA contract NAS 7-
  100.

21 December 1971