Mariner IV radio occultation measurements of Mars upper atmospher

Cain, D. L.

Drake, F. D.

Eshleman, V. R.

Fjeldbo, G.

Kliore, A. J.

Levy, G. S.

English

NASA Technical Reports Server

PRELIMINARY RESULTS OF THE MARINER IV RADIO OCCULTATION MEASUREMENTS
OF THE UPPER ATMOSPHERE OF MARS*
G. Fjeldbo and V. R. Eshleman
Stanford University
Stanford, California
A. J. Kliore, D. L. Cain, and G. S. Levy
Jet Propulsion Laboratory
Pasadena, California
F. D. Drake
Cornell University
Ithaca, New York
The radio occultation experiment conducted with the
and telemetry system of the Mariner IV space-
as provided new and precise information on the
physical properties of the Martian atmosphere (Ref. 1).
The purpose of this paper is mainly to discuss that part
of the occultation data analysis that is related to the
upper atmosphere of Mars. This portion of the study is
important because:
1. It helps us form a more complete picture of how the
Martian atmosphere changes with the altitude (that
is, the temperature profile, number density profiles,
etc., together with the controlling physical and
chemical processes).
2. The precision with which one can determine the
refractive index, and therefore the number density,
in the lower neutral atmosphere depends on how
well one can separate the refractive effects caused
by the ionized and the neutral regions of the
atmosphere.
When an electromagnetic wave propagates through an
ionized medium, its wavelength is longer than it would
be in free space. Figure 38 illustrates how this effect
caused a decrease in the two-way phase path as
Mariner IV moved in behind the Martian ionosphere.
*Presented by G. Fjeldbo. The principal investigator for the
Mariner IV radio occultation experiment was A. J. Kliore.
267
https://ntrs.nasa.gov/search.jsp?R=19660022188 2020-03-16T20:42:09+00:00Z
MARS
× STATION II DATA
• STATION 12 DATA
o STATION 42 DATA
TIME, sec
02 30 O0
IO 20 30 40 50 0 I0 20 30 40
x _
x x
x x
5O
x x
x •
• ®
®
02 31 O0
0
Fig. 38. Ionospheric phase change vs time, during immersion
I0
0 eJ
-3 (.9
Z
-4 <Z
-1-
-5 (')
<
-7 T
(3_
-8
LIJ
-10 i
(3_
-II _
le g
Data from three different JPL tracking stations are
shown in this figure.
It can be seen from Fig. 38 that the main ionospheric
layer on Mars caused an ll-cycle phase decrease-or an
apparent position change of the spacecraft of about 75 cm
in the direction of the Earth ()_ _ 13.6 cm). Starting at
about 02 h 30" 20% another change of only about 4 cm was
observed at each of the tracking stations, and it is be-
lieved that this was caused by a minor layer in the lower
ionosphere.
From the ionospheric phase path data one can deter-
mine the electron density profile of the Martian iono-
sphere (Ref. 2). The preliminary electron density profile
shown in Fig. 39 was computed from the data taken as
the propagation paths probed through the ionosphere
above the bright area Electris. The solar zenith angle was
about 70", and the Martian local time was early after-
noon. Electris is in the southern hemisphere of Mars at
latitude 50 °, and it was winter there at the time of the
measurement.
The peak electron density is seen to be of the order of
10 _ cm -:_ at a height of 120 km. The plasma scale height
at the top side is about 24 km. Both the small scale height
and the low altitude of the main layer show that the
Martian atmosphere is considerably colder than pre-
viously anticipated.
250
t
E
"I-
(,..9
LLI
"r
2OO
150
IOO
50
o
I I I i t
2-104 4-104. 6.10" 8.104 105
N, el/cm 3
Fig. 39. Electron number density vs altitude above
Electris. Martian local time, 1 p.m.
Measurements were also made during occultation exit.
In that case the radio energy probed through the night-
time ionosphere above Mare Acidalium in the northern
hemisphere of Mars. The signature of the night-side
ionosphere appears to be very weak, however, and about
all we can say at present is that the measurements sug-
gest that the electron density on the night side was down
by at least a factor of 20 from the peak density measured
on the day side.
The problem is of course that sensitivity to these low
electron densities is not very great at S-band frequencies.
268
FJELDBO_ AL.
However, this low sensitivity is by no means inherent in
the radio occultation method when this technique is seen
from a broader point of view. Thus, a somewhat different
scheme has been suggested-in which two or more har-
monically related frequencies may be used to probe the
atmosphere of Mars (Refs. 3 and 4). Choosing the lowest
frequency at a few hundred Me/see would provide a
more sensitive measurement of ionospheric electron
densities. In addition, the two-frequency technique has
the advantage that one can separate the effects of the
electrons and the neutral molecules because of their dif-
ferent dispersive properties.
a brief discussion will be given of the tempera-
some of the reactions thought to be important in
the upper atmosphere of Mars.
Carbon dioxide is apparently the main constituent in
the lower atmosphere of Mars (Ref. 1). In addition, there
may be some nitrogen and argon. At a higher altitude,
CO2 is dissociated into CO and O by the solar ultraviolet
flux shortward of about 1700 A (Ref. 5).
Atomic oxygen is lighter than CO, N2, A, and CO2; thus
the number density of O is not expected to decrease as
rapidly with altitude as do the number densities for the
other constituents. Hence, above some altitude one might
expect atomic oxygen to become the main constituent.
Some of the preliminary models we have made for the
Martian atmosphere have O + as the main ion at and
above the electron density peak. These models have a
temperature at the top side of the ionosphere of about
90*K-which corresponds to an electron scale height of
24km.
Bates has shown that radiative recombination of O ÷
w_lectrons takes place somewhat faster at low than at
h111Ftemperatures (Ref. 6). In spite of this temperature
effect, it does not seem possible to explain the loss rate
in the Martian ionosphere on the basis of radiative re-
combination of O ÷. It is therefore necessary to assume
that molecules of minor constituents take part in the loss
processes. One can, for instance, consider the following
possibilities:
O ÷ + N2 _ NO + + N (1)
0 ÷ + 02--> 02 ÷ + 0 (9.)
O + + CO2_ 02 + + CO (3)
where the molecular ions may lose their charge quite
easily through dissociative recombination with electrons.
The two first reactions are relatively well known and
are thought to play an important role in the Earth's iono-
sphere (Refs. 7 and 8). The last reaction, brought to our
attention by E. E. Ferguson, should also be considered
because the lower atmosphere of Mars contains large
amounts of CO2.
In models where O ÷ is the principal ion and where one
of Reactions (1)-(3) is the main loss process, there is a
Bradbury-type layer (such as the Earth's F2 layer) at
120 km altitude. The number density of atomic oxygen
is of the order of 109 cm -3 at the peak of this layer. If
Reaction (1) is assumed to be the main loss process
(k _ 10 -1_ cm3/sec), one finds that there would have to
be large amounts of N2 present in order to account for
the loss rate, which is of the order of 102 cm -3 sec -1. In
fact, a rate coefficient of 10 -12 cm3/sec requires that N2
should have a higher abundance than CO2 in the lower
atmosphere of Mars, at least ff it is assumed that the
lower atmosphere is mixed. However, CO2 must be
the main constituent in the lower atmosphere in order to
explain the occultation data and the spectroscopic mea-
surements (Ref. 1). Therefore, it is tempting to look for
other alternatives.
If Reaction (2) is assumed to be responsible for the loss
of O +, one finds that with k m 10-11 cm3/sec it would be
necessary to have on the order of 20% O2 at the peak of
the main ionospheric layer in order to account for the
loss rate. The spectroscopic observations indicate that
there is less than 0.1% O2 in the lower atmosphere of
Mars (Ref. 9). However, the O2 distribution in the
Martian atmosphere is complicated by the fact that
atomic oxygen, produced by photodissociation of carbon
dioxide, associates to form 02. Molecular oxygen might,
therefore, still be of some importance for the loss of O +,
at least in the lower portion of the Martian ionosphere.
At present, the most promising model appears to be
one in which O ÷ mainly loses its charge through Reaction
(3). With a reaction rate coefficient of the order of 10 -9
cm3/sec (Ref. 10), one finds that it is necessary to have
on the order of 106 CO2 molecules/cm 3 at 120 km altitude.
This appears to be within reach of what can be supplied
through upward diffusion.
Another approach one might take in constructing
models for the Martian atmosphere is to assume that a
considerable portion of the oxygen formed through the
269
MARS
dissociation of carbon dioxide has been lost. (If this
could take place to such an extent that CO + became the
principal ion, then the main ionospheric layer would be
of a Chapman type. The temperature on the top side,
corresponding to a molecular weight of 28, would be
about 150 ° K. The ion CO + could lose its charge through
dissociative recombination with electrons.)
There are two ways that O could conceivably be lost:
escape into space or loss of O: through oxidation pro-
cesses involving the surface crust. The first process is not
expected to be important because the temperature is too
low (Refs. 11 and 12). The second process would require
diffusive and convective transport of oxygen from the
dissociation region down to the surface. However, such a
transport would probably not have too important an
effect on the CO/O ratio in the upper atmosphere of
Mars, since both constituents probably would be trans-
ported downwards at approximately the same rate. There-
fore, it appears that we are left with atomic oxygen as the
principal constituent in the Martian ionosphere.
One of the main difficulties in identifying the ion loss
processes in the Martian ionosphere is the uncertainty in
the reaction rate coefficients, particularly with respect to
temperature dependence. Thus, further studies of these
effects might require revision of the preliminary con-
clusions in this paper.
The low temperatures obtained from the occultation
data appear to be in reasonable agreement with atmo-
spheric models where CO2 is the main constituent at
ground level. This conclusion is based on calculations
utilizing the work of Bates, Chamberlain, and others,
which shows that COx, CO, and O are more effective in
radiatively cooling the upper atmosphere than is
molecular nitrogen, which was previously believed to be
the main constituent (Refs. 12, 13, 14, and 15).
Most of the clouds and hazes in the Martian atmo-
sphere are observed at altitudes below 30 km (Ref. 16).
However, there is also some evidence of violet haze
layers around 100 km altitude. Urey and Brewer
have suggested that this might be caused in part
which absorbs in the ultraviolet and blue. The occulta-
tion experiment strengthens this suggestion, since CO + is
quite certainly one of the important ions in the upper
atmosphere of Mars.
Figure 40 illustrates how the atmospheric mass density
varies with altitude. With a Bradbury-type ionospheric
layer where the principal ion O ÷ is losing its charge
through one of Reactions (1)-(3), we find that the mass
density at 120 km altitude should be of the order of
2.10 -14 g/cm 3. However, the mass density could be two
orders of magnitude higher if the main ionospheric layer
were a Chapman layer with CO ÷ as the principal ion.
18o
16c
14C
12C
E
._- lOC
I
__ 80
t--
60
40
20
o
MARS-MEAN
NASA
EARTH MODEL
16'" 16's 16'_ I_' 16'° 16_ 16_ iJ 16_ 16_ 16" 16_
P, g Icm 3
Fig. 40. Atmospheric mass density vs altitude
270
I
L
FJELDBO ET AL.
This difference in interpretation results in the uncertainty
indicated by the error bar shown at 120 km altitude.
The most interesting point illustrated in Fig. 40 is that
the mass density distribution shown for Mars and for the
Earth do not cross each other at higher altitude. Previ-
ously the opposite was believed to be true, and the argu-
ment was that since the gravity is less on Mars than on
the Earth, the scale height has to be larger.
The question of how the mass density varies with alti-
tude in the Martian atmosphere has also some practical
implications. If one wants to put a satellite in orbit
Mars, one needs to know how the atmospheric
varies with altitude. Most previous models re-
quired an orbit altitude of several thousand kilometers
for a satellite with 50 years lifetime, but the occultation
results suggest that a few hundred kilometers may be
sufficient.
Table 10 summarizes the more striking features of the
upper atmosphere of Mars.
Acknowledgment
The work reported in this paper was sponsored by the
National Aeronautics and Space Administration under
contracts NGR 05-020-065 and NsG-377, and NAS 7-100.
Table 10. Summary of preliminary results for the
upper atmosphere of Mars
Property Value
Maximum electron density, el/cm:
Altitude of maximum, km
Electron scale height above maximum, km
Temperature at the top side, °K
O principal ion
CO principal ion
Preliminary estimate of the atmospheric mass
density at 120 kin, g/cm:
Maximum electron density, el/cm s
Imme_ion _
9.0 -1-1.0 X 104
120 ±5
24 "4-3
2 X 10-"to2 X 10 -_
Emersion b
<5 X 10:
• Over Electris at 50 ° S latitude, 177 ° E longitude, early afternoon, winter,
solar zenith angle 70 ° .
UOver Mare Acidalium at 60 ° N latitude, 36 ° W longitude, night, summer,
solar zenith angle 106 ° .
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272


Preliminary results of the Mariner IV radio occultation measurements of the upper atmosphere of Mars

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