論文内容要旨

論
氏
名
文
内
容要旨
青木翔平
提出年
平成 25 年
学位論文の
Study of Trace Gases in the Martian Atmosphere by Infrared Spectroscopy
題
(赤外分光観測による火星大気微量成分の研究)
名
論文目次
Abstract
Acknowledgement
1. General Introduction
1.1. Methane cycle on Mars
1.1.1. Methane on Mars
1.1.2. Hydrogen Peroxide on Mars
1.2. Water cycle on Mars
1.3. Limb observations at Mars atmosphere
1.4. Purpose of this thesis
2. Seasonal variation of H2O2 observed by MEX/PFS
2.1. Instruments
2.1.1. Planetary Fourier Spectrometer (PFS) onboard Mars Express
2.1.2. Data sets and treatment for H2O2 study
2.2. Method of analysis
2.3. Results
2.3.1. Detection of H2O2 and its seasonal variation using 379 cm-1 band
2.3.2. Verification using 362 cm-1 band
2.4. Discussion
2.4.1. Comparison with previous studies
2.4.2. Constraint of CH4 sink
2.5. Summary
3. Spatial and Temporal variations of HDO/H2O ratio
3.1. Observations
3.1.1. Ground-based observations with SUBARU/IRCS
3.1.2. Coordinated joint observation by MEX/PFS
3.2. Method of analysis
3.2.1. Retrieval of H2O and HDO abundances from SUBARU/IRCS data
3.2.2. Retrieval of H2O abundances from MEX/PFS data
3.3. Results and discussions
3.3.1. Longitudinal distribution at Ls=52° (northern spring)
3.3.2. Latitudinal distribution at Ls=52° (northern spring)
3.3.3. Latitudinal distribution at Ls=96° (northern summer)
3.4. Summary
4. Development of Limb radiative transfer model
4.1. SARTre
4.2. JACOSPAR
4.3. Test simulations with Mars atmosphere
4.4. Summary
5. Conclusions
Appendix A. Retrieval of instrumental line shape of IRCS
Appendix B. Calculation of absorption coefficients
Bibliography
Mars, our neighbor planet, has attracted a lot for interests as a possible planet exiting life. In the year 2004,
small amount of methane (CH4) was discovered in the Martian atmosphere. Since CH4 production by atmospheric
chemistry is negligible and its photochemical lifetime in the atmosphere is estimated to be several hundred years,
the presence of CH4 requires its recent release from subsurface reservoirs, and the source of CH4 could be either
biological or geological activity. However, the source of CH4, which is one of the most puzzling aspects on Mars,
is still open questions. During the last decade, sporadic releases of CH4 (tens of ppb) were detected by remote-
sensing observations with orbiters and Earth-based telescopes. These observations suggested that the lifetime of
CH4 were the order of months or days, requiring a strong sink of CH4. The known processes to destroy CH4 in the
Martian atmosphere are UV photolysis at high altitude (> 50 km) and gas phase oxidation by OH at low altitude
(0–50 km). In the terrestrial case, more than 85% of CH4 loss is explained by atmospheric oxidation caused by
OH. However, measurements of such hydrogen radicals in the Martian atmosphere are very limited and further
observations are indispensable to constrain the CH4 loss. We investigated abundances of hydrogen peroxide
(H2O2) in the Martian atmosphere through Planetary Fourier Spectrometer (PFS) onboard Mars Express. It is well
known that H2O2 plays a key role in oxidizing capacity of the Martian atmosphere. However, only a few studies
based on ground-based observations can be found in the literature. Here we present the first analysis of H2O2
using a space-borne instrument. We detected H2O2 on Mars through its absorption around 379 cm-1. In order to
detect the weak absorption of H2O2, a sensitive data calibration was performed, and a large number of spectra was
selected and averaged. We made three averaged spectra for different seasons obtained over relatively low latitudes
(50°S-50°N) during five Martian years (MY27-31). H2O2 were detected from the all spectra. The following
abundances (± σ) were retrieved: 31 ± 10 ppb at Ls = 0°-120°; 57 ± 20 ppb at Ls = 120°-240°; and 29 ± 15 ppb at
Ls = 240°-360°. We found the H2O2 mixing ratio increased during the summer-autumn season. We also made use
of another band of H2O2 around 362 cm-1 ranges. Although this spectral range has larger uncertainty due to the
contamination of weak H2O band and side-lobe of the instrumental line shape, we could confirm that the mixing
ratio of H2O2 increases during the summer-autumn season, as found from the 379 cm-1 bands. The retrieved H2O2
mixing ratios with PFS agree with the values reported by the few previous ground-based observations. Moreover,
our results prefer the seasonal variation predicted by General Circulation Model (GCM) with photochemistry
developed at Laboratoire de Météorologie Dynamique (LMD) (Lefévre et al., 2008). It suggests that rate
coefficient of the H2O2 formation (2HO2→H2O2+O2) considered in the GCM model is the most accurate. Finally,
we estimated CH4 lifetime near the surface based on the retrieved H2O2 mixing ratio. The lifetime was several
decades in the shortest case. It revealed that the atmospheric oxidation could not explain the observed short
lifetime of CH4. Other processes are required to explain the short CH4 lifetime.
In addition to organic molecules like methane, water is an essential element for life. Although Mars seems to
be devoid of water except for the polar caps, recent explorations by spacecraft have found that water exists as
water vapor in the atmosphere, and water ice in the clouds, on the surface, and under the ground. Water on Mars
circulates via sublimation-condensation process. However, from the currently available observations, it is difficult
to discriminate several different physical mechanisms for the global distribution of the atmospheric H2O in Mars:
i.e., atmospheric dynamics, sublimation from and condensation to polar cap ice or ice clouds, and release from
sub-surface reservoir. Here, the physical processes in water cycle on Mars, especially sublimation-condensation
process, have been identified through mapping of D/H ratio in water vapor. The HDO/H2O ratio was retrieved
from ground-based observations by high-dispersion echelle spectroscopy of Infrared Camera and Spectrograph
(IRCS) at SUBARU telescope, and their support observations by Planetary Fourier Spectrometer (PFS) onboard
Mars Express (MEX). The observations were performed at middle of the northern spring (Ls=52°) and beginning
of the summer (Ls=96°) in Mars Year 31. The averaged HDO/H2O ratios were obtained to be 4.1 ± 1.4 (Ls=52°)
and 4.6 ± 0.7 (Ls=96°) times larger than the terrestrial Vienna Standard Mean Ocean Water (VSMOW), which is
consistent with previous observations. However, the HDO/H2O ratio appears a large seasonal variation especially
at high-latitudes due to interaction between the polar cap and atmosphere, as expected by a GCM simulation. We
found that the HDO/H2O ratio increased from 2.4 ± 0.6 VSMOW (Ls=52°) to 5.1 ± 0.7 VSMOW (Ls=96°) over
the polar region (70°-80°N). It could be explained by a preferable condensation of HDO vapor at Ls=52° and a
preferable sublimation of HDO vapor at Ls=96° via sublimation-condensation process due to difference in their
vapor pressures. We concluded that our results identified the condensation process of water vapor, which was
released from the edge of seasonal CO2 polar cap at Ls=52°, and the sublimation process of water ice polar cap at
Ls=96°. We also found lower HDO/H2O ratios at equatorial regions (HDO/H2O = 3.5 ± 2.2 VSMOW at 0°N) at
Ls=52°. This result would imply the condensation of water vapor as equatorial cloud belt (ECB). In addition, we
investigated geographical distribution of HDO/H2O ratio at the northern spring over the longitudinal range
between 220°W and 360°W in order to constrain the surface-atmosphere interaction at low-latitudes. Although we
confirmed that longitudinal distribution of water vapor exhibits a local maximum around Arabia Terra (~330°W)
as reported by space-borne observations, no significant longitudinal distribution of HDO/H2O ratio is appeared
over this area. It suggests that the release from sub-surface ice is not dominant.
Vertical profile of methane and water vapor are also crucial for searching the source and sink of methane and
understanding of water cycle on Mars. Space-borne limb observation is a powerful tool to investigate the vertical
profiles. There are available datasets of limb measurements by several instruments, however, quantitative analysis
of the data is very limited due to the lack of radiative transfer models with multiple scattering terms for Mars limb
observation. Therefore, we developed limb radiative transfer codes for the Martian atmosphere. We applied the
two codes, SARTre (Medrok, et al., 2007) and JACOSPAR code (Iwabuchi and Suzuki, 2009). They were
originally developed for the terrestrial atmosphere and we adapted them to the Martian atmosphere. SARTre
calculates the contributions due to multiple scattering by assuming local plane parallel atmosphere at individual
grid points along observing line of sight. At the each local plane-parallel atmosphere, SARTre drives DSIORT
package which is widely used in data analysis of remote-sensing data. On the other hand, JACOSPAR calculates
the contributions due to multiple scattering by backward-propagating Monte Carlo method. The Monte-Carlo
calculation with dependent sampling to simultaneously measurement signals at thousands of wavelengths allows
us to achieve very fast calculation. Development of such two codes based on completely different algorithms
allows us to demonstrate their validation. The comparisons demonstrate that the outputs of the two codes are
reasonable, and these codes can be applied to the data analysis of limb observations in the Martian atmosphere.
論文審査の結果の要旨
本研究は、火星の生命存在環境の決め手となる有機物・水の存在量とその変動解明に資する複
数のトピックスからなる。
(1) Mars Express (MEx) 衛星に搭載された Planetary Fourier Spectrometer (PFS)を用いて火星
大気の過酸化水素(H2O2)量を調べた。H2O2 はメタンなど有機物の酸化消失指標であり、本研究は初
の衛星観測調査となる。観測された季節に応じて 3 つの平均スペクトルを作成し、全てから H2O2
の検出に成功。得られた H2O2 量はそれぞれ、31 ± 10 ppb (Ls = 0°-120°;北半球春-夏), 57
± 20 ppb (Ls = 120°-240°;北半球夏-秋), 29 ± 15 ppb (Ls = 240°-360°;北半球秋-冬)で
あり、北半球の夏-秋にかけ最も量が多い。これらは、数例の先行地上観測結果と概ね一致し、ま
た、Laboratoire de Météorologie Dynamique で開発された General Circulation Model (GCM)に
よるシミュレーション結果とも整合する。同モデルで考慮している H2O2 生成過程(2HO2→H2O2+O2)の
反応係数が他のモデルより正確であることを示唆する。観測された H2O2 量は近年観測にかかって
いるメタン消失速度を説明できないことを明瞭に示した。
(2) 火星の水は、水蒸気・氷雲、地下・極冠氷として存在する。本研究では、水蒸気同位体比
(D/H 比)の変動から、この「水循環」での凝結-昇華プロセスの特定を試みた。水蒸気 D/H 比は、
HDO と H2O の飽和蒸気圧の違いから起きる「凝結時の同位体分別」によって減少する。SUBARU 望遠
鏡/近赤外分光計 IRCS と MEx 衛星/PFS の同時観測から、D/H 比の全球平均値は春で地球大気標準
値(VSMOW)の 4.1±1.4 倍、夏で 4.6 ± 0.7 倍であり、また「水蒸気 D/H 比が極冠と大気の相互作
用の影響で大きく変動」する事を明瞭に示した。季節変動は、HDO 分子の選択的凝結による HDO 枯
渇と、HDO 分子の選択的昇華による HDO 増量によるもので、春には極冠縁で放出された水蒸気が
雲・季節極冠として再凝結、夏では極冠が昇華したことを示唆する。また、低緯度領域での雲ベ
ルト形成に伴う凝結による HDO 枯渇も示唆した。
(3) 高度分布を知るには周回衛星リム観測が重要だが、要する大気放射モデルが無く定量的解析
は未だ途上にある。本研究では、地球大気用放射モデルである SARTre code (Mendrok et al.,
2007)と JACOSPAR code (Iwabuchi and Suzuki, 2009)を火星大気版に改良した。鍵となるのは、
大気中エアロゾルの多重散乱の取扱いで、双方異なるスキームで計算を行っている。開発の結果、
双方のリム観測放射輝度値は互いに 5%以下となり、今後の計算速度や評価精度等用途に応じた
「リム観測データ解析」を可能とした。
これらの成果は自立して研究活動を行うに必要な高度の研究能力と学識を有することを示して
いる。青木翔平提出の博士論文は，博士（理学）の学位論文として合格と認める。

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