Variations of the Martian middle atmosphere and their influence on composition in the upper atmosphere

概要

博士論文 (要約)

Variations of the Martian middle atmosphere and
their influence on composition in the upper atmosphere
(火星中層大気の変動と上層大気組成に及ぼす影響)
Present-day Mars is a cold and dry planet, however, the evidence of enrichment of heavy isotope
ratios and morphology on the surface suggests that Mars once had a warm and wet environment that could have
retained plenty of liquid water and a dense atmosphere. How much volatiles and what kind of species escaped to
space are the important key to understanding the drastic change in the evnironment throughout the history of
Mars. The impact of external forcing (e.g., solar extreme ultraviolet (EUV) and solar wind) on the atmospheric
escape has been investigated comprehensively. Recently, it has been reported that a significant variation in the
thermosphere, which is suggested to be affected by the variability in the lower and middle atmosphere. However,
the impact of the variability in the lower and middle atmosphere on the compositions in the thermosphere and
ionosphere has not been measured directly. Furthermore, the variability of composition in the thermosphere and
ionosphere might affect the composition of escaping species since the thermosphere and ionosphere are the main
reservoirs for escape. In this thesis, we focus on the impact of the variability in the middle atmosphere on the
compositions in the thermosphere and ionosphere. The variability of atmospheric compositions in the
troposphere, mesosphere, and lower thermosphere was investigated by Nadir and Occultation for Mars
Discovery (NOMAD) solar occultation (SO) aboard the ExoMars Trace Gas Orbiter (TGO). We also utilized
simulations by a 1-dimensional photochemical model and the Global Environmental Multiscale Mars
(GEM-Mars) model to interpret the observed variations in the middle atmosphere and discuss the connection
between the middle and upper atmosphere. The neutral and ion compositions in the thermosphere and ionosphere
were investigated by Neutral Gas and Ion Mass Spectrometer (NGIMS) aboard Mars Atmosphere and Volatile
Evolution (MAVEN). Furthermore, their possible impact on escaping ions was discussed.
First, we derived the CO/CO2 profile from 75 and 105 km altitudes by NOMAD SO using the
equivalent width technique. Observed CO/CO2 profiles were applied to infer the variability of the eddy diffusion
coefficient in that altitude range. Since the eddy diffusion coefficient can be regarded as the efficiency of
atmospheric mixing, its variability could change the altitude of the compositional boundary called homopause.
The observed CO/CO2 ratios are smaller at the perihelion and in the summer hemisphere. The observed CO/CO2
profiles are reproduced by the 1-dimensional photochemical model when we assumed that the eddy diffusion
coefficient increases with altitude. Our result suggests a two times larger eddy diffusion coefficient and efficient
vertical mixing near the perihelion in the southern hemisphere.
Second, we retrieved the CO volume mixing ratio (VMR) from 20 to 120 km altitudes by the
measurement of NOMAD SO using a radiative transfer code. Based on the comparison between the measured

CO VMR and prediction by the GEM-Mars model, the distributions of the CO VMR were interpreted by the
transport of the atmosphere. We found the seasonal variation in the latitude-altitude distribution of the CO VMR,
which was interpreted by the change in the meridional circulation. The change of CO number density from the
aphelion to the perihelion suggests the intensified circulation in the perihelion, as well as the inflation of the
atmosphere. In addition, we found that the enhanced CO VMR at the morning terminator in the northern winter
hemisphere is caused by the increase in CO number density at that local time. The local time variations of
temperature and vertical wind predicted by the GEM-Mars model suggest that the semi-diurnal tide would
modify the local time distribution of CO VMR. For the impact of the dust storm on the CO VMR, the decrease in
CO VMR was reported in previous literature, however, we found that the decrease in the CO VMR in the
mesosphere and lower thermosphere during a dust storm event is restricted in the polar regions. Our
measurement showed that the decrease in CO VMR is caused by the increase in CO2 density at that area. This
suggests that the intensified circulation could heat the atmosphere and reduce the CO VMR during the dust storm
event, which is a mechanism newly proposed in this thesis to reduce the CO VMR during a dust storm event,
apart from the photochemistry between CO and OH.
Third, we investigated the variability of composition in the thermosphere (CO2, N2, and O) and
ionosphere (CO2+, O2+, O+, and N+) between ~150 and 250 km altitudes. Time variation in neutral species at 200
km altitude showed that the number density increases around the perihelion and decreases around the aphelion,
which is correlated with the seasonal variation of density in the lower thermosphere. The vertical structure of ion
species showed that the CO2+, O2+, and O+ densities at 180 km altitude vary by factors of ~5.0, ~2.9, and ~1.3,
respectively. The variations in ion number densities are basically reproduced by the photochemical equilibrium
theory. It showed that composition in the ionosphere is correlated with the combination of seasonal variation in
the thermosphere and solar EUV variation. In addition, we found that the O, O2+, and O+ number densities are
decreased by ~40%, ~40%, and ~50% at 180 km, respectively, during the regional dust storm event at Ls = 342 346 in MY 33. Such decreases in ion densities are also qualitatively explained by the photochemical reactions. In
order to discuss the effects on escaping ions, the change in the CO2+/O+ and O2+/O+ ratios around a pressure level
of 8×10-8 Pa was investigated, and we found that they vary by factors of ~3.0 and ~1.3, respectively.
These results clarified the following connections between the middle and upper atmosphere. The
increase in the eddy diffusion coefficient near the perihelion in the middle atmosphere leads to an increase in the
homopause altitude near the perihelion, as observed in previous literature. The inflation of the atmosphere and
intensification of the circulation in the middle atmosphere near the perihelion also cause the increase in the
homopause altitude and decrease in the mixing ratio in the lower thermosphere. The compositions in the
thermosphere and ionosphere vary in response to that in the lower thermosphere due to the variations in the
homopause altitude, the solar EUV flux, and dust storm events. Several-ford variations in the ionospheric
composition around a pressure level of 8×10-8 Pa suggest that the composition of escaping ions can vary by the
connections to the lower and middle atmosphere as well as by external forcing. ...