Host: Aqib Khan; Text: Lan Man; Photo: Ahmed Mohamed Reda; Video: Xu Lei; Reviewer: Deng Qingyun, Li Xuerou, Stephen C. McClure
About the speaker:
Koji Matsumoto is a researcher in National Astronomical Observatory of Japan (NAOJ). He got his doctoral degree from University of Tokyo, and works in the research field of planetary geodesy. His research topics include Development of a VLBI mission instrument for SELENE-2, and he is planning to pursue research on the internal structure of the Moon and the planets.
About this English GeoScience Café session:
At 20:15, 19th December 2018, Professor Koji Matsumoto attended English GeoScience Café No.30 and presented an overview of Martian Moon eXploration (MMX) geodesy. At the beginning, the host gave a quick preview the session and introduced the speaker. Professor Koji Matsumoto expressed his appreciation to EGSC and started his talk. The presentation was divided into three parts as follows:
Part1: Solar System, MMX Geodesy, and the Martian Moons
Part2: MMX Mission Objectives and Status
Part3: Phobos Shape Modeling and Internal Structures Modeling
Fig.1. A Solar System visualization
Part 1: Solar System, MMX Geodesy and Martian Moons
1.1 Review of Solar System：
Professor Koji Matsumoto gave a review of solar system using video software. As he explained, the solar system includes the Sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and many asteroids. The software, moves and can speed up and down the visualization. Professor Matsumoto showed the movements of planets and asteroids in the same scale. The demonstration aroused the interest of the participants.
1.2 Introduction of MMX Geodesy：
Professor Matsumoto briefly introduced MMX Geodesy. As he said, MMX stands for the Martian Moon eXploration. An Overview of MMX Geodesy
was actually, the same title as their work shown the day before, focusing on geodesy. Professor Matsumoto mentioned that this presentation would be a more of a introduction to the MMX mission but still focused on geodesy.
Fig.2. Logo of MMX
1.3 Martian Moons Phobos and Deimos：
Fig.3. Phobos and Deimos
After a short tour of solar system and an introduction to MMX, Professor Matsumoto gave an overview of the two Martian Moons, Phobos and Deimos. Comparing pictures of the two small satellites of Mars, he pointed out that the average radius of Phobos is about 11 km and the average radius of Deimos is smaller of 6 km. and really smaller than the Earth’s Moon, with the average radius more than 1,700 km. As Professor Matsumoto said, the density of Phobos is about 1.9 g/cc and Deimos is 1.5 g/cc. The albedo of Phobos and Deimos are also small (0.07), noting that it is extremely dark in space.
Part2: MMX Mission Objectives and Current Status
2.1 MMX Mission Objectives：
In this part, Professor Koji Matsumoto introduced the initial motivation for the MMX mission. As he explained, there were many Mars missions driven by interest in “what its surface environment used to be”. Their key questions were, for example, “what is the history of the Mars surface environment”, “how did the atmosphere loss happen” and “what was driving the climate change”. He said, driven by these questions, many Mars missions were sent to Mars including an orbital robot named Curiosity.Comparing images of the old Mars and current Mars, the question arose, “was there water on Mars in ancient ages?”
Fig.4. Old Habitable Mars and Current Mars
Professor Matsumoto followed up the question with further discussion. From an early age solar system image, he marked the snowline between Mars and Jupiter. He said that the water will freeze beyond the snowline but evaporates inside the snowline, so it cannot be in the liquid form or ice form, the protoEarth or protoMars therefore should have been bone dry. This led to a key question of a different type, “how was water delivered to Mars”, which is a part of the big question, “how was water delivered to rocky planets enabling the habitability of the solar system”.
Fig.5. Solar System in Early Age
Professor Matsumoto explained that the delivery of water, volatiles or organic compounds from outside the snowline enabled or changed the rocky planets to be habitable. Small bodies played the role of delivery capsules. The dynamics of small bodies around the snowline in the early solar system was the issue that needed to be understood. Then he introduced the next question, “which, among the seven objects in the inner-solar system, should we explore to address this key question”.
Fig.6. Seven Objects in the Inner-Solar System
Since Mars is at the gateway position of the rocky planet region witnessing the transportation process, Professor Matsumoto said that MMX proposed to go to the Martian Moons, Phobos and Deimos, which would be categorized as asteroids, if they were not Martian Moons in ancient ages. These Martian Moons might have been delivery capsules of water in the early solar system. Then he explained two leading hypotheses for origin of Phobos and Deimos: the captured asteroid hypothesis and giant impact hypothesis. Both have own basis in the data.
With this foreshadowing, professor Matsumoto said that MMX mission was set up to answer these questions. As he said, MMX is a mission to Phobos and Deimos planned by JAXA, a space agency in Japan. The main objective is to reveal the origin of the Martian Moons and then to make a progress in understanding of planetary system formation and the primordial material transport around the border between the inner and the outer part of the early solar system. They also have a sub objective, which is to observe processes that have impact on the evolution of the Mars system from a new vantage point and to advance understanding of Mars surface environmental transition.
Fig.7. Two Hypotheses for Origin of Martian Moons
Professor Matsumoto described how MMX will get samples from Phobos surface and bring them back to Earth, and the significance of these returned samples for solving question of the origins of the Martian Moons. If their origins were as captured asteroids, then a sample analysis would characterize a capsule that was in its way to deliver water and organic compounds to the inner-solar system. In the case of giant impact origin, a sample analysis would reveal that the samples were mixture of Mars materials and impactor object materials.
2.2 MMX Mission Current Status：
Professor Matsumoto said that status of MMX is a JAXA pre-project, and not officially funded yet but he hoped the project or mission will be promoted as an official approved project. The MMX mission will be launched around 2024 and it will do remote sensing of Phobos and Deimos and get samples from Phobos surface. The mission duration will be five years. As Professor Matsumoto explained, it will take one year to go to Mars and the mission will stay around Mars for three years. It will take one year to get back to the Earth. In the proximity phase, MMX will take a Quasi Satellite Orbit (QSO) to Phobos and try to make a landing and stay there to collect samples for several hours. Some flybys will be made for remote sensing of Deimos.
Then, Professor Matsumoto talked about some research progress of MMX mission.
He showed the MMX trajectory in inertial frame and rotating frame. In figure 8, the purple line is the Mars direction, the yellow line is the Sun direction and the light blue line is the Earth direction. In the Phobos-Sun fixed frame, there are three cases for spacecraft movements, including high altitude, middle altitude, and low altitude cases. In the Phobos-Mars fixed frame, the spacecraft arbitrarily orbits around Phobos. He also showed Phobos from MMX in QSO-Mid and explained that in the Sun-Phobos fixed frame, the spacecraft will make one revolution in four days. Over the four days cycle, during the first two days the system can see the day-side of Phobos, while during the other two days the system cannot see the day-sides but only the night-side.
Fig.8. MMX Trajectory in Inertial Frame and Rotating Frame
The spacecraft configuration, as Professor Matsumoto said, is based on a wide range of trade-off studies. In one study, the configuration of spacecraft systems and major specifications were preliminarily defined. Then he talked about seven nominal science payloads, MEGANE, OROCHI, MacrOmega, MSA, TENGOO, LIDAR and CMDM. The geodetic contribution is to provide shape models, constructed from images taken by narrow-angle cameras and LIDAR ranging. To observe rotation and gravity is the secondary contribution, informing us about the moments of inertia and explore the internal structures. Precise orbit determination is crucial, and geodetic products are also of importance for landing site selection.
Part 3: Phobos Shape Modeling and the Internal Structures Modeling
3.1 Phobos Shape Modeling：
In this part, Professor Matsumoto used a supposition to explain moments of inertia and interior structures. He showed two bodies, the surface of which are the same but the internal structures are different. As can be seen from figure 9, the left body has a crust and the body on the right has another shape with a core inside. Professor Matsumoto said, if these two bodies are considered as spinning tops, the way they rotate will differ from each other.
Fig.9. Two Bodies with Different Internal
Through the equation in case of Phobos, Professor Matsumoto told that there is some deviation from the uniform rotational state. Since orbit of Phobos around Mars is eccentric, Phobos’ orbital speed is changing does not always show exactly the same face to Mars. Consequently, Mars exerts a time-variable gravitational torque on Phobos that modifies the rotation rate. The corresponding changes in the rotation angle are called as forced libration in longitude. He explained the variables and their current observations in detail, but the moments of inertia are still unknown . However, by using the shape model and assuming a homogenous density distribution inside the martian moon, the moments of inertia and as well as the libration amplitude can be calculated.
3.2 Phobos Internal Structures Modeling：
As Professor Matsumoto said, if the mass distribution inside Phobos was not homogeneous, for example, if water ice was concentrated near the surface or the center, a deviation of MOI will be observed from the value for homogenous interior. The MOI differences (MOI) with respect to the homogenous Phobos are calculated for some cases. There are two assumptions, one is that Phobos has a tri-axial ellipsoidal shape, the other is that Phobos has a two-layer structure and their boundary also has the similar ellipsoidal shape for which the libration amplitude of 1.15 degrees being consistent with the observed value. Then he described a simple 2-layer model of water ice inside and water ice outside as can be seen from figure 10.
Fig.10. Water Ice Inside and Outside
Finally, Professor Matsumoto summarized that in order to detect the possible non-homogeneity, a few percent accuracy is required for both the libration amplitude and the degree-2 gravity coefficients. Low-altitude gravity observations above topographic features such as Stickney crater will help us estimate the near surface density, to constrain estimations of the Phobos interior.