The International Ultraviolet Explorer detected the Hartley bands of ozone in the spectrum of Mars. Seasonal observations show a variation in the north consistent with the measurement of Mariner 9. Observations during Martian late fall in the south were also made

Barth, C. A.

Conway, R. R.

Durrance, S. T.

Lane, A. L.

English

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SEASONAL OBSERVATIONS OF MARS
Robert R. Conway, Samuel T. Durrance, and Charles A. Barth
Department of Astro-Geophysics and
Laboratory for Atmospheric and Space Physics,
University of Colorado, Boulder, CO 80309
A.L. Lane
Jet Propulsion Laboratory,
California Institute of Technology,
Pasadena, CA 91103
ABSTRACT
IUE has detected the Hartleybands of ozone in the spectrum of Mars.
Seasonal observations show a variation in the north consistent with the
measurements of Mariner 9. New observations during Martian late fall in
the south were made.
INTRODUCTION
Ozone was discovered on Mars by the ultraviolet spectrometer on board the
Mariner 7 spacecraft (Barth and Hord, 1971), and was subsequently measured
extensively from the Mariner 9 orbiter (Barth et al., 1973; Lane et al.,
1973; Barth and Dick, 1974; Wehrbein, 1979). Those measurements demonstra-
ted variations in both its vertical and global distribution. The total
ozone amount was found to be a maximum during winter over the north pole
but slowly decreased throughout the spring. In the south it was absent
during the Martian midsummer season but appeared again in late summer.
Daily variations were also observed and were occasionally associated with
the presence of clouds. The general conclusion of these observations was
that ozone is present when the Mars atmosphere is cold and dry. The photo-
chemical theory of ozone, which considers the role o_ water vapor, supports
this conclusion.
The Mariner measurements were restricted to northern winter-spring, and
southern summer. In this report we describe a technique of using the IUE
satellite to observe ozone on Mars, and present the results of a prelim-
inary analysis on some of the data acquired.
TECHNIQUE
Ozone is detected by observing its absorption of sunlight in the wavelength
region from 2100 to 2800 _ where the Hartley bands have a peak cross section
of i x lO-l?cm 2. Because of its restricted global distribution on Mars, a
successful detection with IUE depends on the ability to restrict the instru-
ment's field of view to that region of the planet where ozone is present.
As shown in Figure i, the angular diameter of the planet as seen from earth
has varied between 5 and 13.8 arc sec since April of 1979. The optimum
observing time is at opposition when the 3 arc see small aperture includes
only about 5% of the bright disk. Ideally the small aperture would be
33
https://ntrs.nasa.gov/search.jsp?R=19810017361 2020-03-21T12:29:24+00:00Z
placed over the winter pole where Mariner found ozone to bemost abundant.
However Figure i shows that the aspect b_ M_rs also varies considerably,
withthe sub-Earth point moving between -25 v latitude. During the February
opposition the northern hemisphere faced the Earth and provided the opportu-
nity to observe the northern mid to late spring, Under these same condi-
tions Mariner observed between 5 and I0 _-atm. of ozone (l_-atm. = 2.68 x
I0-Is molecules cm-S).
The drift rate of Mars with respect to the stars and its position were
calculated in spacecraft-centered coordinates. Even though Mars had a
maximum magnitude of -0.3 it was possible to track using the center-of-
light lock after placing the planet in the large aperture. The pattern of
scattered light in the Fine-Error-Sensor (FES) image was monitored to
verify the tracking. During an observing session a pattern of five expo-
sures were made as shown in Figure 2. Displacements were made in ecliptic
coordinates by first slewing the spacecraft 4.5 aTc sec in the desired
direction from the center-of-llght point and then moving directly to the
small aperture. The orientation of Mars shown in Figure 2 is for March 15,
1980. In January, before the opposition, the terminator was to the west of
the planet and its north pole was 10 deg closer to ecliptic north. Between
January and April the phase angle varied between -30 and +30 deg.
Observations were made on September 25, 1979 and on January 23, March 15,
and April 8, 1980 with exposure times varying from 9 to 75 see. These
exposures were made with the long wavelength camera in low resolution using
both the large and small apertures. For the observations with the small
aperture a i0 sec exposure gave good signal without saturation.
OBSERVATIONS
From the set of observations made in the pattern shown in Figure 2, the
relative reflectance of the various regions of Mars was calculated as
follows. The center-of-llght observation from each session was taken to
represent the equatorial region and was used as the standard spectrum.
Each of the other four observations were divided by this spectrum and the
ratio was normalized to unity in the 3000 _ region. The extracted net
spectra computed by the IUE data processing operation were used.
Figures 3 and 4 show the relative reflectance spectra for the wavelength
region from 2400 to 3000 _. In each figure the ratio for the northern
region is on the right and that for the southern region is on the left.
The data were smoothed by an ll-point running average before the ratio was
calculated. In all of the southern ratios and in the northern ratio for
January, the smooth curve plotted over the data is the absorption spectrum
of ozone normalized to the data.
On the basis of this analysis, we conclude that these observations have
detected ozone in the Mars atmosphere. The observations of the northern
region in January and March occurred when the Martian seasons were mldand
late spring respectively. They show a positive detection in mldsprlng and
34
the absence of ozone in late spring. This is the same behavior observed by
Mariner 9 over the north pole. The observations of the southern region are
for Martian mid to late fall. These IUE observations contain new information
since Mariner 9 did not oSserve the south during those seasons.
REFERENCES
i. Barth, C.A. and C.W. Hord (1971). Science, 173, 197.
2. Barth, C.A., et al. (1973). Science, 179, 795.
3. Lane, A.L., et al. (1973). Icarus, 18, 102.
4. Barth, C.A. and M.L. Dick (1974). Icarus, 22, 205.
5. Wehrbeln, W.M. et al. (1979). Icarus, 38, 288.
35
NORTH
WINTER SPRING SUMMER
+25- - 15.0
-12.5
>
+ 1J SUB-EARTH ::0
POINT L")
ANGULAR GO
m
. lO - _ DIAMETER _ C)(:3
U,I
D
'_" 0 2.5
UJ OOE3 270= 90° LS
:::)
i--
I- -10-<
.J
-15-
-20 -
SUMMER FALL WINTER
SOUTH
I I ! I I X I XlX I I I
APR I MAY 29 JUL 28 SEP26 NOV 25 JAN24 MAR 24 MAY 23 JUL 22 SEPT20
i. Variation of the latitude of the sub-Earth point on Mars with Martian
season from April of 1979 through September of 1980. The apparent
angular diameter of Mars is also shown.
_ MARCH 15, 1980
MARS NORTH POLE'
N
/
/s
EQUATOR
p
S
ECLIPTIC
2. Aspect of Mars as viewed from Earth on March 15, 1980. Small circles
indicate locations of small aperture for each of five exposures.
36
NORTH/EQUATOR SOUTH/EQUATOR
JANUARY 23, 1980 JANUARY 23, 1980
1.2 - 1.2 -
0 1.0 1.0 " ^
rr 0.8 0.8
0.6 - 0.6 --
0.4 I I I 0.4 I I I
2400 2600 2800 3000 2400 2600 2800 3000
WAVELENGTH (_)
3. Ratio of north and south observations to equatorial observation for
January 23, 1980 session. Smooth curve is ozone absorption spectrum.
NORTH/EQUATOR SOUTH/EQUATOR
MARCH 15, 1980 L MARCH 15, 1980
1.2 _ _ 1.2
!
0 1.0 V - _ 1.0I--
.<
n-
0.8 - 0.8 -
0.6 - 0.6 -
0.4 I I I 0.4 I I I
2400 2600 2800. 3000 2400 2600 2800 3000
WAVELENGTH (_)
4. Ratio of north and south observations to equatorial observations for
March 15, 1980 session. Smooth curve is ozone absorption spectrum.
37


Seasonal observation of Mars

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