REDERIVATION OF THE MGS RADIO OCCULTATION MEASUREMENTS
IN THE MARTIAN SOUTH POLAR WINTER REGIONS USING MRO-MCS
TEMPERATURE CLIMATOLOGY.
K. Noguchi, M. Shimomura, Faculty of Science, Nara Women’s University, Nara, Japan (nogu@ics.narawu.ac.jp), A. Kleinböhl, D. Kass, S. Piqueux, Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, CA, USA.

Introduction

Data and Method
The MGS radio occultation data set [Tyler et al., 2001],
which can be obtained from the NASA Planetary Data
System (PDS), includes more than 20,000 profiles during four Martian years from Mars Year (MY) 24 to 27.
The data includes altitude, temperature, pressure and
air number density. We took into account the measurement uncertainty of temperature which was provided in
the original PDS data set, and added the uncertainty of
temperature at each level when comparing with the saturation temperature [Hu et al., 2012, Noguchi et al., 2017]
to exclude the wrong detection of supersaturation.
We focused on the southern polar night regions (60◦ S–
90◦ S), where the decrease of CO2 is large due to the
condensation of CO2 onto the polar cap. We followed
the method proposed by Noguchi et al. [2014] to estimate the change of CO2 volume mixing ratio (VMR)
in the polar nights. We utilized the Ar measurement
by the Gamma Ray Spectrometer (GRS) onboard Mars
Odyssey [Sprague et al., 2012] at two latitude bands
(60◦ S–75◦ S and 75◦ S–90◦ S) and regarded the first band
as the representation of 67.5◦ S, and the second as 82.5◦ S.
Then, we interpolated the Ar VMR between 67.5◦ S and
82.5◦ S. As for the region north of 67.5◦ S, we extrapolated Ar VMR assuming that Ar VMR was constant
(1.6%) at 60◦ S through a Martian year. Between 82.5◦ S
and 90◦ S, we assume the constant VMR same as the

0.9
CO2 VMR

Radio occultation (RO) measurements can be used to
obtain vertical profiles of temperature and pressure in
a planetary atmosphere assuming that its atmospheric
composition and a temperature at the uppermost altitude
of the measurements are known. In the present study,
we consider the change of the atmospheric composition
in the Martian polar regions in polar night, where supersaturation and condensation of CO2 frequently occur
[Kieffer et al., 1977]. We utilize the zonal-mean temperature climatology obtained by the Mars Climate Sounder
(MCS) onboard Mars Reconnaissance Orbiter (MRO) in
order to update the vertical profiles of temperature and
pressure obtained from Mars Global Surveyor (MGS)
RO measurements.

1

0.8
60.0S
67.5S
75.0S
82.5S-90.0S

0.7
0

90

180
Ls [deg]

270

360

Figure 1: Time series of CO2 VMR estimated from the GRS
Ar measurements for the south polar region of Mars.

VMR of 82.5◦ S for each Ls. Finally, we converted the
Ar VMR into CO2 VMR as follows:
χCO2 = 1 − χAr (1 + f ),

(1)

where f is the ratio of the standard VMR of N2 to the
standard VMR of Ar (=2.7%/1.6%) [Withers, 2010]. Figure 1 shows the time series of CO2 VMR estimated from
the GRS Ar measurement mentioned above. The minima of CO2 VMR occurred around Ls=120◦ and the
lowest value was approximately 78% at 82.5◦ S.
In the original MGS RO data set, temperature at the
uppermost altitude of the measurements, T u , was fixed
to several typical values around the altitude of 40 km.
In the present study, we used a temperature climatology
based on MRO-MCS observations [Kleinböhl et al., 2017]
for T u . The problem is that there is no overlapping period between the MGS and MRO observations. Thus,
we have to apply data from different MYs of MRO-MCS
to MGS-RO. The winter season in the southern hemisphere ranges in Ls=0◦ –180◦ , which is less influenced
by dust and less interannually variable than the dusty
season (Ls=180◦ –360◦ ) [Kass et al., 2016]. Therefore,
we simply averaged the temperature profiles obtained by
MCS in MY29–33 to make a zonal climatology of temperature. We gridded the zonal averages in five degrees
of Ls and latitude bins.
Hereafter, we label the T u of the original MGS-

A study on the CO2 supersaturation in the Martian polar nights
-60

(a)

25

3021K38A
LON315.750 LAT-81.442 Ls126.04 LTST14.215 MY26

20
10

5
0
-5
-80

-10

Pressure [Pa]

10

-70

Tdiff [K]

Latitude [deg]

15

100

-15
-20
-90

Torg
TMCS

-25
0

90

180
Ls [deg]

270

120

360

u
u
Figure 2: The difference of TM
CS from Torg . The black curve
indicates the border of polar night.

(b)

130

140
150
Temperature [K]

160

170

9174I12A
LON281.254 LAT-66.955 Ls159.00 LTST9.778 MY24

Results
In the present study, we focus on the effect of the replacement of T u ; we make comparison between Torg
and TM CS . The samples of the RO temperature profiles
rederived are shown in Figure 3. In Figure 3(a), supersaturation occurred in TM CS at several levels around
100 Pa, where no supersaturation was seen in Torg . The
u
u
difference between TM
CS and Torg was more than 20
K, and such an overestimation of T u caused the underestimation of the occurrence of supersaturation. The
other way around, the underestimation of T u caused the
overestimation of the occurrence of supersaturation as
is shown in Figure 3(b).
Figure 4 shows the histograms of CO2 supersaturation found in the rederived temperature profiles. In the
u
u
case of TM
CS lower than Torg shown in Figure 4(a), the
total number of CO2 supersaturation detected in TM CS
increased by 14% from Torg . Conversely, the total number of CO2 supersaturation in TM CS decreased by 19%
u
u
when TM
CS higher than Torg shown in Figure 4(b). As
expected from the results in Figure 3, the overestimation
and underestimation of T u result in the underestimation
and overestimation of CO2 supersaturation, respectively.

Pressure [Pa]

10
u
RO dataset as Torg
and the T u from the MRO-MCS
u
temperature climatology as TM
We also refer to
CS .
u
the whole temperature profiles rederived with Torg
and
u
TM
as
T
and
T
,
respectively.
The
difference
org
M CS
CS
u
u
u
of TM
CS from Torg is shown in Figure 2. The TM CS
u
tends to be larger than Torg outside the border of polar
night and smaller inside the border of polar night. The
difference reaches 25 K in both cases, which suggests a
large influence of the detection of CO2 supersaturation
u
when using Torg
for the derivation of RO temperature
profiles.

100

Torg
TMCS
120

130

140
150
Temperature [K]

160

170

Figure 3: Samples of temperature profiles (Torg in blue and
TM CS in red) rederived by using MGS radio occultation data
u
u
u
for the cases of (a) TM
CS lower than Torg and (b) TM CS
u
higher than Torg . The error bars for temperature are plotted
by six points. The black curve indicates CO2 saturation curve
for CO2 VMR = 90.8%.

Conclusion

The present study updated the MGS RO temperature
profiles by considering the change of CO2 VMR in the
Martian southern polar nights and utilizing the MROMCS temperature climatology as T u , which was fixed
to several typical values in the original MGS RO data
set. The replacement of T u resulted in 14% increase
of the total number in the detections of CO2 supersatuu
u
ration when TM
CS lower than Torg , and 19% decrease
u
u
when TM CS higher than Torg . This indicates that the
assumption of T u is important for the estimation of CO2
supersaturation and would also affect the discussion of
CO2 condensation over the polar cap on Mars.

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(a)

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Acknowledgements
This work was initiated during a stay of K. N. at JPL
funded under the JPL Science Visitor Colloquium Program. Work at the Jet Propulsion Laboratory, California Institute of Technology, is performed under contract
NASA. The authors are grateful to David P. Hinson and
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Node of NASA PDS (https://pds-atmospheres.
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