AN OVERVIEW OF MARS OBSERVATION PLANNED BY THE
MARTIAN MOONS EXPLORATION (MMX)
K. Ogohara, Faculty of Science, Kyoto Sangyo University, Kyoto, Japan (ogohara@cc.kyoto-su.ac.jp), H.
Nakagawa, Graduate School of Science, Tohoku University, Sendai, Japan, S. Aoki, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Japan, T. Kouyama, Artificial Intelligence Research Center,
National Institute of Advanced Industrial Science and Technology, Tokyo, Japan, K. Masunaga, T. Usui, Institute of Space and Astronautical Science, JAXA, Sagamihara, Japan, N. Terada, Graduate School of Science,
Tohoku University, Sendai, Japan, T. Imamura, Graduate School of Frontier Sciences, The University of Tokyo,
Kashiwa, Japan, M. Imai, Faculty of Science, Kyoto Sangyo University, Kyoto, Japan, MMX Mars System
Sub-Science Team

Introduction:
Mars is the planet most similar to the Earth in the
solar system in that it has a solid surface and an appreciable amount of atmosphere with moderate temperatures, and that there is evidence of liquid water
activity, volcanic activity and possible habitability.
In recent decades many Mars missions have been
conducted to reveal the planet’s evolution and the
current climate. Tough the current Mars is a dry and
cold world that includes a significant amount of frozen surface/subsurface water resources, geological
evidence obtained by the spacecraft suggests that
Mars may have sustained a wet and warm world in
the past under the influence of a greenhouse effect
(e.g., Ramirez and Craddock 2018). The transition to
the current climate should have been driven by a
combination of various physical/chemical processes,
such as degassing, escape of volatiles to space, loss
of the intrinsic magnetic field, exchange of water and
other volatiles between the atmosphere and the crust,
photochemistry, radiative transfer and atmospheric
circulation. A possible example of strong coupling
between different processes is the recent discovery
of the apparent response of escaping ion fluxes to the
upward transport of dust and water in the lower atmosphere (Heavens et al. 2018). The growing global
interest in the history of the Martian surface environment motivated us to develop a Mars observation
plan in the MMX (Martian Moons eXploration) mission aiming to disentangle the coupling among different processes, thereby assessing the roles of shortterm elementary processes in the atmosphere and in
the circum-Mars space in the long-term evolution of
the Martian environment
The Martian Moons eXploration mission:
MMX is the third sample return mission of
JAXA (Japan Aerospace Exploration Agency) (Kuramoto et al. 2021). The spacecraft will conduct
close-up observations of the Martian satellites Phobos and Deimos, collect samples from Phobos, and
bring them back to the Earth to reveal the origin of
the two moons. In addition, MMX will conduct remote observations of the Martian atmosphere and

surface and in situ measurements of volatiles escaping to space during the 3-year Mars orbiting phase.
We take advantage of MMX’s unique orbit around
Mars: the equatorial high-altitude orbit will enable
continuous remote monitoring of short-timescale
phenomena in the atmosphere and increase opportunities of the measurement of low-energy ion outflows in the shadows of Mars and Phobos. The samples brought back to the Earth are expected to contain particles ejected from the Martian surface by
asteroid impacts, from which information on early
Mars will be retrieved. Combining this broad range
of information will allow for an integrated understanding of Mars system that consists of Mars, its
moons, and the circum-Mars space. For example, the
transport of water across the atmosphere from the
surface to space will be tracked with an infrared imaging spectrometer and an ion spectrometer, and this
knowledge of water transport, combined with isotopic fractionation analysis of returned Mars samples,
will allow us to develop a scenario of the environmental evolution of Mars. The spacecraft will be
launched in 2024, put into a Martian orbit in 2025,
stay in orbit around Mars until 2028, and return to
the Earth in 2029.
Currently, quite a few Mars missions are ongoing: Mars Odyssey, Mars Express (MEX), Mars Reconnaissance Orbiter (MRO), Curiosity rover, Mangalyaan orbiter, Chinese Tianwen 1 mission, Mars
Atmosphere and Volatile Evolution (MAVEN), ExoMars Trace Gas Orbiter (EMTGO), and InSight
lander. The Hope orbiter planned by United Arab
Emirates has started remote observations of the Martian atmosphere from a high-altitude equatorial orbit
since 2021. Mars sample return (MSR) mission is
also currently under consideration to bring back
samples from Mars (Beaty et al. 2019). The Mars
observations in MMX are complementary to those
missions in several aspects, such as the observation
wavelength, the spatial resolution in global imaging,
the sensitivity to low energies and the mass resolution in ion measurements, and the sample return to
the Earth.
Mission definition regarding the Martian sur-

Figure 1 A trajectory plan of MMX (http://www.mmx.jaxa.jp/en/gallery/)
face environment:
We are motivated to plan MMX by the growing
global interest in the evolution of the Martian environment as mentioned in the “Introduction” section.
This is reflected in one of the scientific goals (SGs)
of MMX. The first of SGs (SG-1) is related to studies on the Martian moons and the understanding of
the planetary system formation and material
transport in the solar system (Kuramoto et al. 2021).
The second scientific goal of the MMX (SG-2) is:
• From the viewpoint of the Martian moons,
clarify the driving mechanism of the transition of the
Mars-moon system and add new knowledge to the
evolution history of Mars.
What is crucial for studies on the evolution of the
planetary surface environment is to directly observe
the history as geologists have reconstructed the history of the terrestrial environment from rock or sediment samples. Another important approach is to
understand the phenomena seen in the atmosphere
and circum-Mars space today and infer the past from
them. Thus, SG-2 is broken down into three medium
objectives (MOs). Ogohara et al. (2022) provides an
overview of the relationship between the SG, MOs
and related science themes. The first MO is associated with the surface processes of the airless small
body (i.e., Phobos) and the other two MOs cover the
Mars environmental evolution:
• MO-2.2: add new findings and constraints on
the history of changes in the Martian surface,
• MO-2.3: constrain the mechanisms of material
circulation in the Martian atmosphere affecting the
transitions in the Martian climate.

One of the methods to achieve MO-2.2 is to restore the paleoclimate changes from Martian samples
that have been accumulated on the Phobos surface
(Hyodo et al. 2019). The other is the estimation of
the past atmospheric escape based on the present
observations. They are expected to be achieved by
analyses of Martian samples on Phobos returned to
Earth and observations of the atmospheric escape
(Usui et al. 2020). MO-2.3 covers understanding of
the efficient upward transport processes required for
efficient H2O escape from the top of the atmosphere.
MO-2.3 is expected to be achieved by continuous
full-disk monitoring of Mars with visible and infrared wavelengths. Observations of distributions of
dust, ice clouds, and water vapor with higher temporal and spatial resolutions than the previous observations are necessary for MO-2.3.
Mars observation:
Orbit. MMX will travel in a cruising orbit to
Mars for 1 year, observe Mars and its moons for 3
years from 2025 to 2028, and eventually bring back
collected samples to the Earth after a 1-year journey
(Fig. 1). During most of its 3-year mission, MMX
trajectory will be in Quasi-Satellite Orbit (QSO)
around Phobos, which is very similar to Phobos orbit
around Mars: nearly equatorial and circular, with a
period of 7h40’ and distance of about 9,376 km from
the Mars center (about 6000 km from the Mars surface). In operation on Mars Moon proximity, remote
sensing for atmospheric phenomena and in situ
measurement for atmosphere escaping are continuously conducted on QSOs as a home position
throughout the mission from Mars Orbit Insertion
(MOI).
Mars observation by MMX from the Martian
equatorial plane (inclination of~1°) has several as-

sets. This circular orbit is relatively close to Mars
when comparing to the areostationary orbit. MMX
orbital motion (0.013°/s with the period of 7h40’)
will be faster than the rotational period of Mars
(0.004°/s). As a consequence, the difference of rotation rate drifts the subsatellite point around 0.009°/s.
In 7h40’, subsatellite point shifts ~248° eastward.
After three orbits of 7h40’ (~23 h), subsatellite point
has shifted of ~744°=2 × 360°+24°, so MMX is located above a longitude 24° east of that three orbits
ago. Quasi-areostationary orbiters are innovative
platforms for space-born Mars science, as has been
geostationary satellite imagery which revolutionized
Earth weather forecasting from 1970s. The first
equatorial orbiters will blaze a trail for the continuous remote sensing of the Martian atmosphere. Remote sensing from high orbit will enable to work in
conjunction with satellites in polar or eccentric lowaltitude orbits, which provide truly global coverage
and synergistic perspective. From the equatorial
plane, it is however noted that the apparent visible
disk of Mars takes up a field of view of about~43°.
The coverage reaches up to approximately 68° latitude. In addition, the equatorial orbit of MMX will
essentially increase opportunities to directly measure
the low-energy outflow of ionospheric ions from
Mars in the inner magnetosheath, which has been
poorly understood so far.
Instruments. The mass spectrum analyzer
(MSA) is a scientific instrument onboard the MMX
spacecraft, which consists of an ion energy mass
spectrometer and two magnetometers (Yokota et al.
2021). The ion spectrometer measures distribution
functions of low-energy (<10 s keV) ions and their
mass/charge distributions using a top-hat electrostatic energy analysis and a linear-electric-field time-offight mass analysis. The fluxgate magnetometers
measure the vector magnetic field of the solar wind,
which is occasionally perturbed by Mars and possibly Phobos in their neighborhood. The MSA experiment is designed to perform in situ observations of
secondary ions emitted from Phobos, water ions
generated from the Phobos torus if exists, and escaping ions from the Martian atmosphere with monitoring the solar wind. The mass resolution of M/ Δ
M~100, because the MSA will measure heavy ions
and their isotopes coming from the Martian atmosphere to estimate the amount of the atmospheric escape. MMX will be located in the Mars shadow
(eclipse) at approximately half of the year. In these
periods, MMX will measure escaping ions in the
magnetotail. In other periods, MMX will observe the
solar wind. Seasonal variation of the MMX orbit
with respect to the magnetotail can allow us to observe the longitudinal–latitudinal structure of escaping ions at 6000 km distance from Mars.
Optical radiometer composed of chromatic imagers (OROCHI) is designed for capturing spectral
features of Phobos from ultraviolet to near-infrared
wavelengths with 7 wide-angle bandpass imagers

and 1 panchromatic imager. Detailed information of
design and performance of OROCHI is introduced in
Kameda et al. (2021). The field of view (FOV) of
each imager is > 1 rad and the instantaneous FOV
(iFOV) is 0.4 mradian. Because of the wide FOV,
OROCHI can capture whole disk of Mars in its FOV
from the Phobos altitude with the pixel resolution of
2.4 km. Since the orbit of MMX is similar to the
Phobos orbit, which is almost a circular orbit around
Mars, OROCHI will observe Mars from almost same
altitude (~6000 km), and thus it will provide images
with a stable spatial resolution. The dataset from
OROCHI is similar to those from a geostationary
weather satellite whose spatial and temporal resolution are the order of 1 km and 10 min, respectively.
Also, thanks to its wide FOV, OROCHI can observe
a wide range of local time from morning to evening
in one observation. It will also give a lot of information about local time dependence or morning–
evening symmetry/asymmetry. The center wavelengths of the OROCHI bandpass filters are 390 nm,
480 nm, 550 nm, 650 nm, 730 nm, 860 nm, and
950 nm, and designed signal-to-noise (S/N) ratio for
spectral analysis (i.e., band ratios) is 100 (Kameda
et al. 2021). For Mars observation, the blue and red
bands will be mainly used for monitoring atmospheric dynamics. Dust and water ice clouds can be
roughly distinguished based on a difference in
brightness between the two bands. The high SN ratio
is enough to resolve the difference in the photometrical properties between dust and water ice (c.f. Bell
et al. 2009). The near-infrared band (950 nm) will be
also important to connect the OROCHI images and
MIRS data since the wavelength is commonly used
by both instruments, then more detailed spectral
analysis will be possible.
Telescopic Nadir imager for GeOmOrphology
(TENGOO) is a panchromatic (370–850 nm) imager
with a quite high spatial resolution, <6 μ-radian/pixel,
which is 60 times higher than that for OROCHI. The
high spatial resolution allows to observe the Martian
surface with a resolution of 35 m/pixel from the altitude of Phobos, thus we can analyze sub-km scale
atmospheric phenomena from TENGOO images.
TENGOO’s FOV is 0.82×1.1°, which corresponds to
86.5×115.5 km region in one TENGOO FOV when
observing Mars from the QSO. Same as OROCHI,
the most significant advantage of TENGOO observation is that TENGOO can conduct a sequential observation for several hours which is critical for understanding short timescale atmospheric phenomena
on Mars. Tanks to the high spatial resolution of
TENGOO, we may track sub-km scale dust flows,
and then we will be able to retrieve a local wind distribution in the FOV of TENGOO by tracking modifications of clouds and dust events.
MMX InfraRed Spectrometer (MIRS) is a pushbroom imaging spectrometer in the near-infrared
spectral range (0.9 to 3.6 µm). MIRS detector is a
2D array providing an image of a strip in one spatial

direction with the spectrum of each point in the spectral direction. A second spatial direction is provided
by the instrument line of sight in the along-track
direction of MMX. MIRS FOV is ±1.65° and while
targeting a spatial resolution of 10 km on Mars,
MIRS expected geometric resolution on Mars from
QSO is ~2 km so spatial binning can be implemented
to increase signal-to-noise ratio and decrease data
volume. MIRS has a spectral resolution of about
20 nm and a targeted signal-to-noise ratio above 100
for features of interest. Further detailed information
on the instrument can be found in Barucci et al.
(2021). MIRS will also perform limb observations to
obtain information on Martian atmosphere at high
vertical resolution.
MIRS observations will be used to constrain
transport processes for dust and water in the Martian
atmosphere, observations of the equatorial to midlatitude distributions of dust storms, water ice clouds
and water vapor. CO2 (and thus pressure) will be
monitored through CO2 2.0 µm band. Water vapor
will be monitored on a daily basis using its 2.6 µm
band (Maltagliati et al. 2008) and water ice clouds
through their spectral features between 0.9 and
3.6 µm (Vincendon et al. 2011). MIRS will be able
to detect CO and O2 dayglow with their 2.35-µm and
1.27-µm bands, respectively. MIRS will also provide
monitoring of the atmospheric dust content and of
water adsorbed in the surface regolith.
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