ICARUS 22, 205-211
(1974)
Ozone
and the Polar
C. A. BARTH
AND
Hood of Mars
M. L. DICK
Departwaent of A&o-Geophysics
and Laboratory for Atmospheric
and Space Physics,
University of Colorado, Boulder, Colorado 80302
Received January 21, 1974
Ozone co-appears with the clouds of the polar hood in the winter hemisphere of
Mars, but each is variable from day to day and location to location. Both the
appearance of ozone and the polar hood clouds correlate with the temperature
of the atmosphere which varies from day to day and location to location. A cold,
clean, dry atmosphere is conducive to the formation of ozone.
Ozone was first measured on Mars over
the south polar cap by the Mariner 7
ultraviolet spectrometer (Barth and Hord,
1971). At the time of this measurement,
the season in the Mars southern hemisphere
was early spring. From the extensive
observations made from Mariner 9, the
amount of ozone over the polar caps was
found to vary depending on the season.
Its abundance is greatest following the
winter solstice and smallest after the
summer solstice, with a systematic variation during the intervening seasons (Barth
et al., 1973).
In addition to its appearance over the
polar cap, the Mariner 9 observations
show that ozone also appears in association with the polar hood (Barth et al.,
1973). During the fall and winter seasons,
the polar hood and ozone are both present
in the latitude region poleward of 40”N.
While there is a systematic seasonal variation in the a,mount of the polar hood ozone,
there are day to day and location to location variations that are much greater in
magnitude. In this report, detailed observations of the co-appearance of the polar
hood and ozone will be presented. The
observational evidence is drawn from several of the Mariner 9 experiments : the ultraviolet spectrometer
(Lane et al., 1973;
Barth et al., 197’S), the infrared interferometer (Hanel et al., 1972a,b; Conrath
et al., 1973), and the television experiment
Copyright
0 1974 by Academic
Press, Inc.
All rights of reproduction
in any form reserved.
Printed in Great Britain
(Masursky et al., 1972; Leovy et al., 1972;
Leovy et al., 1973). Earth-based observations of the polar hood will be briefly
described before the spacecraft observations are presented.
EARTH-BASED OBSERVATIONS
Since at least 1907, earth-based observers have recognized the presence of
two polar phenomena on Mars (Slipher,
1962). In the spring and summer, the polar
cap appears as a deposit on the surface.
During the fall and winter seasons, the
polar hood is seen as a cloud mantle.
Further, earth-based observers have recognized that the polar hood varies in detail
from day to day (Slipher, 1962).
A recent study of plates obtained over
the past 6Oyr shows that the north polar
hood extends southward to 50”N latitude
during the time period shortly after L, =
180’ (planetocentric longitude of the sun,
see American
Ephemeris,
1972) until
shortly after L, = 0”: i.e., the northern
fall and winter (Baum and Martin, 1973).
The southern boundary of the polar hood
is variable and often reaches latitude
40”N.
MARINER OBSERVATIONS
Mariner 9 observations
of the north
polar region of Mars began in mid-winter
when the north polar hood extended
205
206
BA~TH AND DICK
southward to the 40-50°N latitude region.
During this period, extensive fields of
wave-clouds were observed by the Mariner
television experiment (Leovy et al., 1972).
These cloud systems moved from day to
day and the cloud coverage over a particular area was variable.
The co-appearance of ozone and the
clouds of the polar hood is illustrated in
Fig. lb. In the upper portion, a mosaic
of four television pictures shows an extensive winter cloud system. Below is a
rectilinear plot of latitude and longitude
of the area covered by each of the television
pictures. This particular picture sequence
started just north of 30°N latitude and
extended to north of 60°N latitude: the
longitude was approximately centered on
180 ° . An interpretation of the cloud cover
present in the television pictures is given
in the lower part of the figure. South of
approximately 42°N latitude, the television picture shows surface features.
To the north, a wave system appears:
at first the clouds are thin but northward
of 50°N latitude, t hey become quite thick.
Still further north, the wave system ends,
and thin, diffuse clouds apparently extend
to the northern edge of the picture.
Superimposed on the picture outline
is the observation track of the ultraviolet
spectrometer. At the time of shuttering of
the television picture, the ultraviolet
spectrometer is aimed at the center of the
television picture, but subsequent maneuvering carries the observation track
through the area covered by the television
picture, often in a complicated manner.
In this figure, the width of the track is
used to depict the amount of ozone measured b y the ultraviolet spectrometer.
Starting at the b o t t om of the picture outline, the thin line indicates the amount of
ozone is less than the detection limit of
the spectrometer, 3/~m-atm. Following the
track north over the cloudless region,
ozone remains undetectable. Moving north
over the cloud bank, measureable ozone
appears at approximately 47°N latitude
and the amount increases continuously
with increasing latitude. At approximately
58°N latitude, 177°W longitude, near the
northern edge of the cloud bank, the maxi-
mum amount of ozone, 60tzm-atm , is
measured. In this example, the clouds of
the polar hood appear northward of 47°N
latitude and ozone appears in the same
region at the same time.
This same location was observed with
Mariner 9 instruments on three successive
days. Figure la shows television pictures
and ozone measurements made one Mars
day earlier than those shown in lb: Fig.
lc shows the result of observations made
one Mars day later. The three days of
observation are of a region including longitude 180°W between latitudes 32°N to
58°N. The Mars season was near the end of
winter, just a few days before the spring
equinox; L s = 352 +. In Fig. la, the television pictures show a cloud system moving
in from the west. On the next day, Fig. lb,
the cloud system extends across the picture
frame from longitude 190°W to 170°W.
The cloud system is gone in Fig. lc, having
moved presumable to the east. The ozone
measurements in the lower half of the
figure show t h a t at a particular location,
58°N, 177°W for example, the amount of
ozone on the first day was moderate, on
the second day the amount had doubled,
and on the third day an intermediate amount of ozone was measured between the
two previous day's observations. Thus, at
the same location on Mars, both the cloud
cover and the amount of ozone vary from
day to day.
In the observations presented so far,
the clouds of the polar hood and ozone have
several common characteristics. T hey both
appear in the winter hemisphere poleward
of approximately 45 ° latitude. The amount
of either of them is variable, both spatially
and temporally. However, a detailed
examination of the observations shows t h a t
a quantitative relationship between the
reflectivity of the clouds and the amount
of ozone does not exist. In the data presented in Fig. lb, the amount of ozone over
the cloud bank gradually increases from
its southern edge near 47°N to 57°N where
the maximum amount appears. Also, the
reflectivity of the clouds varies dramatically because of their wave nature, while the
magnitude of the ozone abundance varies
smoothly even though the spatial resolu-
OZONE AND THE POLAR HOOD OF MARS
207
j
60"
50 °
40 °
180°
(a)
LONGITUDE (lOdeg.)
(b)
(e)
Fro. 1. P h o t o m o s a i c s o f a region in t h e M a r s n o r t h e r n h e m i s p h e r e o n t h r e e successive d a y s d u r i n g
l a t e w i n t e r , L s = 352 °. T h e a m o u n t o f ozone m e a s u r e d is r e p r e s e n t e d b y t h e t h i c k n e s s o f t h e line
s u p e r i m p o s e d o n t o t h e r e c t i l i n e a r p r o j e c t i o n of t h e T V p i c t u r e s i n t h e lower p o r t i o n s of t h e figure.
P i c t u r e s a r e : (a) M T V S 4246-68, D A S 9198464; M T V S 4246-76, ]:)AS 9198604; M T V S 4246-92,
DAS 9198884; (b) MTVS 4248-72, DAS 9270354; M T V S 4248-80, ]gAS 9270494; M T V S 4248-88,
DAS 9270704; MTVS 4248-96, ]:)AS 9270774; (c) M T V S 4250-64, ]:)AS 9342244; M T V S 4250-72,
DAS 9342384; MTVS 4250-80, ]:)AS 9342594.
tion of ultraviolet spectrometer is comparable to the cloud wave structure. The
general co-appearance of polar hood clouds
and ozone, coupled with the lack of quantitative correlation in their magnitudes,
suggest t h a t one is not the cause of the
other, but t h a t the cause of both needs to
be sought elsewhere.
Temperature profiles of the lower atmosphere of Mars were obtained by the Mariner
9 infrared interferometer spectrometer
(Hanel et al., 1972a,b; Conrath et al.,
1973). Analysis of the spectral shape of the
15Fro (667cm -1) band of carbon dioxide
gives the temperature at discrete pressure
levels between 10rob. and .lmb. In Fig.
2, temperatures measured at the 5rob
level (near the surface) are combined with
television pictures of cloud formations
and
ultraviolet spectrometer
ozone
measurements. In the right half of the
figure, a mosaic of three television pictures
344
561
,
336
THIN
,
328
~
/
!
31LOUDS
320
C)S
312
344
"'~ 3 6 - - -
336
LONGITUDE (DEG)
'
46-
56-
328
320
312
FIe. 2. P h o t o m o s a i c of g r o u n d a n d cloud f o r m a t i o n s a r o u n d t h e c r a t e r L y o t , t o g e t h e r w i t h ozone c o n c e n t r a t i o n s a n d a t m o s p h e r i c t e m p e r a t u r e s .
T h e left h a n d side outline is a r e c t i l i n e a r p r o j e c t i o n of t h e p h o t o g r a p h s on t h e r i g h t . T h e a m o u n t of ozone m e a s u r e d is p r o p o r t i o n a l to t h e t h i c k ness of t h e viewing t r a c k ; t e m p e r a t u r e s are g i v e n in °K. T h e p i c t u r e s are: I P L roll 6050, D A S 8119198, 8119408, a n d 8119478, Ls -- 344 °.
.J
I-F-
bJ
C3
¢3 56
(9
LU
56'
C~
209
OZONE AND THE POLAR HOOD OF MARS
shows a region north of 36°N latitude
during late winter. These pictures have
been rectified to fit a Lambert conformal
projection. The topographical feature at
51°N, 331°W, is the crater Lyot. A
system of wave clouds exists to the east of
this crater. South of 45°N latitude, the
atmosphere is clear, ground features can
be seen clearly. North of approximately
52°N, thin diffuse clouds exist. In the left
half of the figure, an outline of the television pictures is given together with an
interpretation of the location of the wave
cloud system and the thin clouds. Superimposed on this outline is the observation
track of the ultraviolet spectrometer
and the infrared interferometer spectrometer. These two instruments are boresighted and view the same region on the
planet simultaneously. The width of the
line is again indicative of the amount of
ozone. South of 40°N latitude, no ozone was
detected. At the furthest point north along
the observation track, the maximum
amount of ozone was measured. Temperatures along the observation track are indicated by portions of isotherms. In general,
the temperature decreased along the track
to the north.
The interrelationship of ozone, temperature, and clouds may be explored by
following the observation track in the
left half of Fig. 2. Starting at low latitudes,
no ozone is observed until the track crosses
the 190°K isotherm. Between 40°N and
50°N, ozone is detected in a region where
no clouds are present. As the track proceeds north, the amount of ozone increases
while the temperature drops first through
180°K and then below 170°K. Since the
condensation temperature of a few /xm
of precipitable water is 180°K while the
condensation temperature of 6.1mb of
carbon dioxide is 148°K, the clouds in
this television mosaic are most likely made
up of water ice crystals (Hanel et al.,
1972b; Lane et al., 1973). The variation
in ozone is smooth while the observation
track crosses the crater, the wave cloud
bank, and the region of diffuse clouds.
The correlation is clearly with temperature
and not with the nature and opacity of the
clouds.
Another example of the interrelationship of the amount of ozone, the temperature, and cloud formations is shown in
Fig. 3. In this case, the crater Mie is the
topographic feature responsible for the
wave cloud system near 48°N latitude.
To the north of the wave cloud system is a
bank of thin clouds, while still further north
the clouds are either very thin or snow is
present on the ground. Along this observation track, the temperature at the 5mb
level drops very low, becoming less than
150°K north of approximately 52°N latitude. Near 40°N latitude, the ozone
abundance starts increasing, before the
observation track crosses the wave cloud
bank. The maximum amount of ozone
occurs over the region of thin clouds and
not over the wave cloud system. Ozone
decreases in abundance after the track
has passed to the north of the thin cloud
system. Once again, the variation in
ozone amount is smooth while the changes
in the character and brightness of the
clouds are dramatic.
DISCUSSION
Ozone and the clouds of the polar hood
both appear in the winter hemisphere and
both are variable. Both the appearance
of the ozone and of the clouds are caused,
in part, by the temperature of the atmosphere dropping below some low value. The
cloud morphology is complicated by atmospheric motions and water vapor content,
i.e., Martian polar region weather. Ozone
appears when the atmosphere is cold and
dry with or without the presence of clouds.
It may be t h a t the removal of water vapor
from the atmosphere allows ozone to
appear or it may be the removal of some
other condensable t h a t is critical. Both
ozone and its precursor, atomic oxygen,
are very reactive; hence they reach their
greatest abundances when the atmosphere
is very clean.
The observation of the polar hood on
Mars may be used as an indicator of the
appearance of ozone in the polar region,
but only as an indirect indicator. The
appearance of the polar hood clouds means
the temperature has dropped below the
56'
240
]
S
250
j
J /
220
I
210
/ " - - - - 180"
250
LONGITUDE (DEG)
56
36
200 240
i/J
48
220
210
200
FIG, 3. Photomosaie of the ground and cloud regions around the crater Mie, t o g e t h e r with ozone and t e m p e r a t u r e m e a s u r e m e n t s of t h e same
:egion. The pictures are: I P L roll 6070, DAS 8982934, 8983144 and 8983214, Ls -- 350 °.
..J
<Z 4 8 -
bJ 5 4
C~
C~
VERY
-]
=
L'~
OZONE A N D T H E POLAR HOOD OF MARS
condensation temperature corresponding
to the w a t e r v a p o r c o n t e n t of the atmosp h e r e a n d t h e cold, d r y a t m o s p h e r e is
v e r y c o n d u c i v e t o t h e f o r m a t i o n o f ozone.
ACKNOWLEDGMENT
The comparison of the data from several of the
Mariner 9 experiments was possible only through
the unselfish cooperation of the Mariner investigators. Many of the ideas presented here were
developed through interaction with that group.
During the operational phase of the Mariner
mission, T. E. Burke and A. L. Lane of the Jet
Propulsion Laboratory first recognized the
interrelationship between the ultraviolet spectrometer ozone measurements and the infrared
interferometer spectrometer temperature profiles.
Tile illustrations presented here were prepared
by Robert McDonald of the University of Colorado. This research was sponsored by the National
Aeronautics and Space Administration.
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