太空|TAIKONG 国际空间科学研究所 - 北京 ISSI-BJ Magazine No. 10 June 2018

LUNAR AND PLANETARY SEISMOLOGY IMPRINT FOREWORD

太空 | TAIKONG In the last few years, during different planetary seismology. Especially ISSI-BJ Magazine meetings, discussions took place on since although the data available possible international cooperation dates back from the Apollo missions on planetary science, especially in the 70s, and there are several Address: No.1 Nanertiao, in the field of lunar seismology. papers reviewing past results on Zhongguancun, Haidian District, These discussions were related lunar seismology, the ISSI-BJ forum, Beijing, China to the possibility of joint scientific however, was rather to focus on the Postcode: 100190 experiments on Chinese lunar prospects of new science and future Phone: +86-10-62582811 Website: www.issibj.ac.cn missions or European space projects. programs, as there is still much It was subsequently suggested that to do in terms of new science. In

Authors the scientists who are interested this sense, the proposed topic has in this topic should have closer a very high added value for ISSI- Philippe Lognonné (IPGP, France), Wing Huen Ip (NCU, GIA, Taiwan), contacts to pave the way for future BJ, especially with the success of Yosio Nakamura (UTIG, USA), collaboration when the opportunities Chang’e-3, and even more for the Wang Yanbin (SESS, PKU, China), Mark Wieczorek (LAGRANGE/ arise. With this in mind, the ISSI-BJ future Chinese Lunar/Planetary OCA, France) Executive Director, Prof. Maurizio missions, which may consider having William Bruce Banerdt (JPL/ Falanga, has been invited to visit Prof. such instruments onboard. The ISSI- Caltech, USA), Raphael Garcia Ip Wing Huen at the National Central BJ Science committee members (ISAE/SUPAERO, France), Patrick Gaulme (MPS/MPG, Germany), University, and, subsequently, he have positively recommended the Jan Harms (GSSI, Italy), Heiner visited Prof. Wang Yanbin, Peking forum proposal for implementation in Igel (LMU, Germany), Taichi Kawamura (IPGP, France), Martin University, and Peimin M. Zhu at 2017. Knapmeyer (DLR, Germany), the University of Geosciences in Brigitte Knapmeyer-Endrun (MPS, Wuhan and its group. After these This was the tenth science forum Germany), Shaobo Qu (HUST, China), Daoyuan Sun (USTC, discussions, which were including successfully organized by the China), Chi Wang (NSSC, CAS, also Prof. Philippe Lognonné, they International Space Science Institute China), Lin Xu (NSSC, CAS, China), Jinhai Zhang (IGG, CAS, decided to submit an ISSI-BJ forum in Beijing (ISSI-BJ). ISSI-BJ forums China), Peimin M. Zhu (CUG, proposal, i.e., from a bottom-up are informal and free debates, China) approach. There was no doubt that brainstorming meeting, among some Editor the addressed science topic is very twenty-five high-level participants on Anna Yang interesting and prospective: making open questions of scientific nature. a foresight exercise on the future of the study and understanding of This two-day Forum was held the internal structure, evolution and between January 11-12, 2017 and was designed to provide a Front Cover present activity of the Lunar and

A nearly full moon taken from the Apollo 8 spacecraft at a point above 70 degrees east longitude. (Credit: NASA)

2 太空|TAIKONG brainstorming opportunity to discuss the and scientific analysis that may place the most important science developments in lunar and planetary seismology project and “lunar and planetary seismology” and how China in a central position due to its unique to achieve them technologically, by bring objectives and technology. together experts in both research and instrumentation. During the two-day forum, This TAIKONG magazine provides an special attention and discussions were given overview of the scientific objectives and to the post Apollo key science goals, as well the overall list of proposed seismometer as some of the key technological issues, experiments, including instrumentation and the next steps for future projects. In discussed during the Forum. total, over 30 leading scientists from eight countries participated in this Forum. I wish to thank the conveners and organizer of the forum Philippe Lognonné, Ip Wing Huen, The participants recognized the very high Yosio Nakamura, Mark Wieczorek, Wang scientific value for innovative lunar and Yanbin, as well as Michel Blanc who chaired planetary seismology project especially for the forum and welcomed all participants at the future, and raised constructive comments that time as the ISSI-BJ executive director. and suggestions. They recognized that the Special thanks are given to the ISSI-BJ staff, Chinese Lunar and Deep Space Exploration Lijuan En, Anna Yang, and Xiaolong Dong, program is innovative and challenging. for the active and successful organization of This offers significant opportunities for the forum. Let me also thank all those who cooperation through project coordination participated actively in this stimulating forum and contributed in writing this magazine.

Prof. Dr. Maurizio Falanga

Executive Director

Beijing, May 2018

太空|TAIKONG 3 INTRODUCTION

Forum Overview

Planetary seismology is on Earth, and provided the seismology on other bodies not only the best tool to impact rate of meteoroids of the solar system, such determine the internal on the Earth-Moon system. as Mars, Venus, Mercury structure of planets, but it and small bodies. This also enables us to monitor The first goal of the assessment was made not the tectonic activity of Forum was to review only for the ongoing missions planets, to determine the the achievements of the in development, but also impact cratering rates of Apollo seismic experiment, in terms of seismic waves planets, and to quantify almost forty years after and source modeling, with acoustic sources of planetary the termination of ALSEP, specific focus on differences atmospheres. The Apollo with a special focus on the between Earth and planets; Passive Seismic Experiment analyses made in the last such as impact processes, (PSE), conducted as a fifteen years, and to identify scattering of waves in a component of the Apollo the science goals of a new high-Q crust, and interior/ Lunar Surface Experiment post-Apollo seismic return atmosphere coupling). Package (ALSEP), was not on the Moon. The Forum only the unique example did not only focus on the This FORUM gathered of a successful seismic lunar interior, but also together multidisciplinary experiment on a terrestrial addressed science goals key scientists from different body other than Earth, but associated with the use countries with the objective also one of the few examples of the Moon as a platform of to not only review the of seismic data used by the enabling the detection of unique heritage of the lunar science community more gravitational waves, which seismology experiment than forty years after their is made possible by its very and data sets, but also to acquisition. The experiment low background seismic contribute to the achievement successfully led to the noise. of a deeper scientific discovery of the crust and understanding of future core of the Moon, quake The second goal of the planetary seismology where mechanisms never observed Forum was to review the international coordination perspective of planetary and collaboration is needed.

4 太空|TAIKONG The Chinese Lunar Exploration Program

The China’s Lunar by working in the 200 km orbiter, the Chang’E-2 Exploration Program (CLEP) high orbit above the Moon’s orbiter was launched is divided into three phases surface for more than 1 year, on October 1, 2010, by named “circling round the to detect the topography working in the 100 km high moon”, “landing on the and geomorphology, mineral orbit above the Moon’s moon” and “returning from composition and the first surface. Its spatial resolution the moon” before 2020. microwave detection of the has been significantly Up to now, Chang'e-1, moon, and high-energy increased compared to Chang’e-2 and Chang’E-3 particles and low-energy that of Chang’E-1, and the have been successfully ions near the moon. It was images of the moon with a launched (Figure 1). controlled to impact the resolution ratio of 7 m were moon and completed the obtained. It also obtained the The Chang’E-1 orbiter was preset science mission in images of topography with launched on October 24, 2009 on March 1. As a a resolution of 1.3 m at the 2007 carrying a total of 8 backup to the Chang’E-1 landing site for Chang’E-3, sets of scientific payloads,

Figure 1: The roadmap of China Lunar Exploration Program. The Chang’E-5 probe was going to be launched in November 2017, which has been delayed until 2019.

太空|TAIKONG 5 so it was called the guide by a moon-based ultraviolet and poles of the moon. satellite of Chang’E-3. Since telescope (LUT). This makes Implemented missions are then, the Chang’E-2 satellite China the third nation to mainly limited on the surface conducted an extended have achieved soft landing of the moon. And some mission, especially with the and patrol on the moon after seismic data is acquired close flyby of asteroid Toutatis the United States and the by the Apollo program to obtain a 10 m-resolution former Soviet Union. more than 40 years ago, image. By implementing therefore, there is a lack the Chang’E-1 and The Chang’E-5 probe was of understanding of the Chang’E-2 satellites, China going to be launched in composition and structure of has made a breakthrough November 2017. However, the deep part of the moon. in the development of the its launching has been The South Pole-Aitken (SPA) key technology of lunar delayed until 2019 due basin is the largest and circulation exploration. The to a rocket failure. China oldest recognized impact aerospace engineering will launch an automatic basin on the moon, but it system of China in lunar sampler, which will land on has never been detected in exploration has been set up the nearside of the moon in a site. Thus, there is a lack of preliminarily. new area far from the Apollo understanding of the moon’s and Luna missions’ sampling earliest impact history. Chang’E-3 was successfully points to collect lunar rock China’s lunar exploration launched on December and soil samples, and return program aims these 2, 2013, realizing soft- them to the Earth for further important scientific issues in landing and patrol detection. analysis. The selected the future. At the same time, Chang’E-3 made a lot of landing site is located at the use of lunar platforms progress in terms of the one of the youngest lunar and in-site utilization of geological history of the surface areas, therefore, resources will be considered Imbrium basin, and revealed through the isotopic dating in the future. that large-scale volcanic of these returned samples, activities in the region can we shall modify crater dating The Chang’E-4 last much later. It was the first methods and provide a mission includes a time to carry out observation more accurate time scale for telecommunication relay in the earth's plasmasphere the evolution of the surface satellite, two mini satellites in a global scale meridian of the moon and terrestrial around the moon, a lander view by the Extreme Ultra- planets. and a rover, which will be Violet (EUV) camera, and launched in 2018, and soft- monitored variable stars, So far, human’s probe has landing and patrol detection bright active galactic nuclei never landed on the far side will be carried out for the

6 太空|TAIKONG first time on the Aitken basin exploration of the moon, mission is mainly to collect on the far side of the moon. to obtain information on lunar samples in the South At the same time, the low the topography, mineral Pole and SPA, and return frequency radio noise on composition and elemental them to the Earth for further the far side of the moon is composition, and volatile analysis. This mission will used to observe the low- content of the moon use the Chang’E-6 backup frequency radio astronomy. by means of cameras, of Chang’E-5 to carry out, spectrometers, neutron & which will reveal the early Systematically considering gamma spectrometers, X-ray impact history and deep the major scientific issues of spectrometers and mass material composition of the the moon and the utilization of spectrometers. Information moon. The third mission the lunar resources, Chinese on the structure of the South mainly aims at the in-site scientists and technical Pole from shallow to deep utilization of the resources, experts have proposed level is obtained by means and carries out moon-based an overall plan through of radar, seismograph and scientific observation on the continuous exploration to magnetometer to reveal the lunar surface. It will carry out preliminarily build a research chemical composition and an in-site rare-gas extraction station on the lunar South structure of the deep part test in the lunar soil, and a Pole by implementing 3-4 of the moon. Water (ice) in series of experiments on missions during the period the permanent shadow area small terrestrial ecosystems of 2020-2030. was detected in site to reveal on the lunar surface in order the content, distribution to provide technical support The first mission will and source of water and for future manned lunar carry out in the South volatiles on the surface landing and lunar resource Pole a comprehensive of the moon. The second utilization.

Goals and Challenges of Planetary Seismology

The early 1960s not instrumentations and the Oscillations [41] and the only revolutionized Earth occurrence of the M=9.5 first attempt to deploy seismology with the first mega earthquake in Chile seismometer on body observations of Earth in 1960, but also projected other than Earth, with free oscillations, made seismology in the solar the Ranger 3-4-5 landers possible by both new steps system, with both the first (Figure 2), all equipped in long period seismic observations of the Solar with an autonomous

太空|TAIKONG 7 seismometer in a survival seismometer, precursor of tide, but also gravitational module. None of the three the Apollo Lunar Network, waves. See [47] for a more missions launched in 1962 deployed progressively by all detailed review of Planetary succeeded in this first extra- landing missions (Figure 3), seismology. terrestrial endeavor. The first including the last one, Apollo success was the Apollo 11 17, with a gravimeter which The science motivations in first human landing mission, was not only designed for the seismic exploration of with the solar-powered monitoring moonquakes and the Sun, planets and small

Figure 2: Ranger lander during its integration Figure 3: Apollo seismometer on the Moon. at the Jet Propulsion Laboratory, in 1961-1962. The Apollo seismometer is protected by a The Ranger seismometer was located in the top Mylar cover, which goal is to stabilize the sphere, made of balsa wood, and was designed ground temperature. The power system to survive the crash of the sphere, ejected before and data transmission system is beneath the impact of the Ranger on the Moon surface. the seismometer. 5 seismometers were None of the missions succeeded. From [40]. successfully deployed by the Apollo with the Apollo mission 11, 12, 14, 15 and 16 and with the Apollo 11 powered by solar panels, all seismometers operated with nuclear power source until September 1977. From [39].

8 太空|TAIKONG bodies of our solar system geophysical technic is the are required to confirm our are related to key questions only one able to illuminate understanding of the Earth- of the formation of planets the interior of planets and Moon formation scenario. and of their geological to extrapolate in depth evolution over billions years. the mineralogical and Further in the Solar system geochemical measurements are Venus and Mars. Venus The Moon is the relic of performed with surface is almost Earth like in size, debris, generated by the or crustal rocks, including but has been unable to store impact of a Mars-sized (or magmatic ones which its carbon dioxide in the crust possibly slightly smaller) sample the planet deeper which lead to a catastrophic planet on the early Earth. than its crust. atmospheric greenhouse The consequences of this effect and furnace surface impact on the evolution Lunar seismology in this temperature of more than of Earth are not yet fully context is the best tool to 450°C. Mars, in contrary, has understood, but the Earth is measure the size of the not been able to maintain nevertheless accumulating Lunar core. This size is likely the habitable environment several features different to the only parameter of the which existed 4 billions the four terrestrial planets of Earth-Moon system which years ago as demonstrated the solar system, Mercury, provides us, more than 4.4 by the several NASA and Venus, Earth and Mars. The billions years after the lunar ESA Mars orbiters and Earth is indeed the only one impact, the impact angle of NASA landers and rovers conjugating plate tectonics, the proto-Moon on Earth. in the last 40 years. The surface stable liquid water, Lunar seismology, especially interior structure of these active volcanism and strong after the Grail mission, two planets is however magnetic dynamo. What will be the best tool to get still largely unknown. Only is the importance of the details of the Lunar crust, the mean density of Venus proto-moon impact in this and especially the volume of is known, and even if the striking planetary evolution anorthositic crust extracted geodetic and gravimetric which provided to Life all from the post-impact molten data on Mars are much habitability conditions for its Moon. This again will provide better, they provide only evolution toward complex key constrains on the impact the mean density, moment living species? Why is energy partition between inertia and tidal response the Earth the only planet Earth and Moon. Even if of the planet. These three with long duration and still the Apollo seismic network parameters are enough active plate tectonics? The provided a first sketch of the only for constraining the answers to both questions Lunar interior, as developed core size and state, crustal request seismology, as this in section 3 and 4, new data thickness, type and depth

太空|TAIKONG 9 of mantle discontinuities, but non-clearly located However, most of these as well as mineralogy and volcanic activity. scientific goals still remain temperature of all these unsolved 60 years after layers. Even with naturally Last but not least, there are the beginning of space returned samples (for Mars giant planets of our solar exploration. Seismic signals SNCs) or robotic sample system, especially Jupiter are just so vague. Most of return mission, seismology and Saturn, which formed the Moonquakes and lunar is required for providing not so rapidly that they attracted impacts generate waves with only the final state of these most of the hydrogen left by ground displacement smaller planets after their accretion the Sun in the proto-planetary than one nanometer, with and differentiation (through nebula. Here as well, the expected core phases 20 the measurement of the size most exciting questions are times smaller. Marsquakes of their core and thickness of deep inside their surface. are possibly larger, but the crust) but also their present Again, the unknowns are surface temperature of the thermal state and deep the mass of their core, as planet varies by more than mineralogy composition (by the later is controlling the 60°Celsius and strong winds correlating the measured primordial gravitational blow on its surface. Yet, Mars seismic velocities with attraction generating the is almost a paradise when mineralogical constrains hydrogen nebula collapse compared to Venus, where from both sample analysis or the deep transition the surface temperature and high pressure laboratory phase, which control their exceeds 450°C. Installing experiments). Further steps present dynamic and future a seismic station on another can even be imagined with evolution. And here also, planet is therefore a long seismic network or seismic seismology is the only way -term and costly effort. This waves imaging, which to get access to these succeeded with the Apollo could (like the Earth global hidden worlds, that time with program, but mostly due seismic network) image the remote sensing technics. It to the strategic importance convective patterns in the can be either implemented of Apollo during the cold mantle beneath volcanic on Earth telescopes, as war between the USA and province, either on Mars, those monitoring the free the Soviet Union. On the where high resolution oscillations of Jupiter, or in otber hand, opportunities imaging has identified young Space, as demonstrated for Mars concretized in volcanic calderas and lava by Cassini-Huygens, which real missions only three flow with very few impact detected in Saturn rings times, and unfortunately cratering, or on Venus where subtle feature associated started with two failures atmospheric di-oxide sulfur to ring resonances with the in 1976 and 1996. Viking demonstrates a still ongoing Saturn free oscillations. failed for seismology with

10 太空|TAIKONG only one sensor deployed after launch after losing its November 2018 is therefore successfully on the Mars' orbiter, and surface small the next opportunity for surface, but with a lack of stations with penetrators, planetary seismology in seismic detection due to all four equipped with the 20 years periodic cycle the non-capability of the seismometers. The launch of seismological launch lander to deploy the sensor of InSight, described in windows in the solar system. to the ground. And 20 years section 5a took place in May later, Mars 96 failed shortly 2018. Its landing on Mars in

Figure 4: Viking seismometer on the deck of the Viking -2 lander. The exobiology driven goals of the Viking project prevented the lander from providing the necessary resources for a deployment on the Mars surface. Out of the two Viking landers, only the seismometer on Viking-2 operated nominally. The signals were corrupted during the day time by the lander's vibration due to the wind, while the sensitivity of the instrument was not high enough to record signals during the night. From [1] .

Figure 5: One of the Mars 96 small stations during the integration of the OPTIMISM seismometer by IPGP/INSU engineers. The station was expected to stay on the Mars surface to protect the instrument which had been designed for the very low power delivery of the small Radio-Thermal Generators of the station. Sensitivity at 1Hz was about 1 ng, i.e. 10-8 m/s2 in ground acceleration. The mission was lost shortly after launch in November 1996. From [43].

太空|TAIKONG 11 THE APOLLO SEISMIC EXPERIMENTS

What started the lunar shallow seismic structure did observe meteoroid and planetary seismology, from signals generated by impacts, but initially we did essentially, were the Apollo a thumper and mortals and not recognize them on the seismic experiments, recorded with a linear array seismograms because they conducted nearly half a of three geophones, was looked so much different century ago, from 1969 to conducted at the Apollo 14 from what we see here on 1977. The Apollo missions, and 16 landing sites. Lunar the Earth from any impulsive in which the U.S. astronauts Seismic Profiling Experiment source. We learned later that landed on the Moon, (LSPE), to derive somewhat this was due to the significant for the first time on any deeper structures with difference in seismic signal extraterrestrial body, also signals generated by transmission through the provided us an opportunity to explosive packages highly fractured near-surface deploy seismic instruments detonated at distances up layers of the Moon with on the Moon to record to about 3 km and impact of very low intrinsic seismic seismic signals, with which Lunar Module at about 9-km attenuation. The discovery to infer the internal structure, distance recorded on a 2-D of deep moonquakes at a both static and dynamic, of array of four geophones, was depth half way to the centre the Moon. Passive Seismic conducted at the Apollo 17 of the Moon, which occur Experiments (PSE), with a landing site. LSPE was also synchronously to the tides 3-component long-period turned on from time to time caused by the Earth and (medium-period in current to record natural seismic the Sun, was a big surprise. standard) and a single- events. At the Apollo 17 Another surprise was the component short–period landing site, Lunar Surface shallow moonquakes of very seismic sensors, were Gravimeter (LSG) to detect high frequency content that conducted at the Apollo 11, gravitational waves also occur in the upper mantle. 12, 14, 15 and 16 landing functioned as a short-period They are much fewer than sites (Figure 3), and all but seismometer and detected other events but include the Apollo 11 instrument natural seismic events till it some of the most energetic recorded the seismic data was turned off in 1977. events observed. The real continuously till the data cause of these events is transmission was terminated Most of what we observed still debated. Yet another at the end of September, during the Apollo missions surprise was the observation 1977. Active Seismic was a series of big surprises of thermal moonquakes that Experiment (ASE), to derive to us. As expected, we occur near the surface of the

12 太空|TAIKONG moon clearly responding to after a continuous detection keep working on acquiring the large diurnal temperature of moonquakes, is helpful more data of higher quality. changes. to move forward towards resolving the mystery Furthermore, a new tool What did we learn by setting of both earthquakes called CWPAR (Clipped up seismic stations on and moonquakes, Waveform Pick-up and the Moon? Of course, as using synchronous data Restoration, available from planned, we learned how acquisition from both the IRIS) is developed to restore the interior of the Moon Earth and the Moon. Such a the clipped waveforms looked like, at least as a seismic array would begin to [97], which could rescue first approximation. We be set up around 2020-2030 some moderately saturated also learned that there still in the SPA basin via China's waveforms . Rescuing parts are seismic activities inside lunar exploration program. of precious Apollo data the Moon. However, there Thus, an important message would greatly enhance the were many things that were is: Expect the unexpected! useable waveform data unexpected. We found archives. out that there are types of The initial analyses of the seismic events that are not Apollo seismic data were normally observed on the done with computers and Earth. We also found out analysis techniques available that the way seismic signals more than four decades ago. transmit through the Moon Since then, there have been is quite different from the tremendous improvements way they do here on Earth. in both of these areas, and This kind of situation may new discoveries are being apply whenever we go to made with the old data. another planetary body with These new results are an environment different discussed in the following from the Earth. Furthermore, section. On the other hand, these findings also provide the Apollo data, the only new perspective to extraterrestrial seismic data understanding our own we now have, are very limited Earth. A seismic station in scope, both spatially and array on the lunar surface, temporally. Thus, we must

太空|TAIKONG 13 LUNAR SOURCES AND LUNAR STRUCTURE AS SEEN BY APOLLO

Lunar Sources

The Moon is often seen seven years of the Apollo seismic signals with the as a tectonically dead seismic network operation LP instruments, and many terrestrial body. The Apollo were however able to more events seen on the seismometers during the detect more than 12,500 SP instruments remain

Figure 6: Seismic activity at Apollo 14 seismic station. The number of events per year is shown, for the different type of lunar events. Grow zone is corresponding to amplitudes which are hidden on Earth by the microseismic noise. These amplitudes are those of the seismic waves at 2 sec of period, with detection threshold of the Apollo instruments as low as 10-10 m/s. Reprinted from [46]

14 太空|TAIKONG uncatalogued. Figure 6 from the release of stresses in the determination of shows the statistics of the associated with the global the cristal lunar thickness. detection on the horizontal thermoelastic cooling of the But many natural impacts component of Apollo 14 Moon, and show similarities were also detected, as the station per year, based to the Earth's intraplate Moon has no atmosphere on the seismicity catalog earthquakes which also to protect its surface from made by Y. Nakamura, exhibit much higher stress the flux of meteorites. They from Univ.ersity of Texas. drops (for a given seismic constitute about one fifth of This is therefore almost moment) than plate tectonic the detected events (1742 5 events per day, in term earthquakes. of the 9315 identified and of detection. Most of the classified events) on the events have however very The other moon seismic Apollo seismic network, and small amplitudes and could signals were much more the strongest ones have not be detected on Earth exotic with respect to amplitudes comparable to at such distance, because Earth standard. The Moon the largest moonquakes. of the Earth microseismic very low seismic noise if Figure 7 shows the signals noise. giving indeed a window recorded for one of the to seismologists to detect largest natural impact which The moonquakes the most seismic sources never or very mass is estimated to 25-30 comparable to earthquakes weakly observed on Earth. tons and impact energy to are the shallow moonquakes Controlled impacts of the 5-6 Tera Joules, equivalent which occur at depth up Saturn IVB upper stage or to 1.2-1.45 kiloTons of TNT. to 200 km. 28 events were of the Lunar Ascent Vehicule These large energies are detected during the Apollo are the first example. The 15 linked to the very high impact operation, but, in contrary tons SIVB upper stage was speed of the interplanetary to Earth, they do not show in addition tracked by NASA meteorites when they hit obvious correlation with when it was impacting the the lunar surface. The mean surface features on the Moon at about 2.5 km/s and value is about 20 km/s, but Moon. On Earth, the largest provided almost controlled 15% have a speed of 30 events will have body waves seismic sources to km/s or more. In addition, magnitudes ranging from seismologists, with known these impacts make ejecta 4.8 to 5.5, and generate location and time. P and S during the crater formation, high seismic frequency wave arrival times where which generate, through [63], which denotes a very therefore directly providing momentum conservation, fast rupture process with the average seismic velocity an extra push on the lunar high stress-drop. They are of the lunar crust, and surface, leading to a source interpreted as resulting provided the best constraints almost twice larger than the

太空|TAIKONG 15 Figure 7: Apollo Records of the large natural impact occurring on November, 14, 1976, recorded at the Apollo stations. Data are shown for the long-period seismometer (LPX, Y, Z) plus the vertical axis of the short-period seismometer (SPZ) in ground velocity. The mass of the impact has been estimated to about 25-35 tons, assuming an impact velocity of 20 km/s. The lunar globe was taken from the LROC observation of NASA (http://photojournal.jpl.nasa.gov/catalog/PIA14011) and Apollo stations and deep moonquake nests were added by the authors. Figure reprinted from [44].

16 太空|TAIKONG one related to the impact The advantage of impacts tracking failure) for which momentum transfer only. is the reduced number of the precise impact location In contrary to the shallow focal parameters, limited is also now known from moonquakes however, the to geographical location LRO imaging (e.g. [93]). As seismic source, related to and time. Moreover, for an alternative to the artificial the impact shock wave artificial impacts these impacts, Lognonné et al. propagation in the near three quantities are known [49], Yamada et al. [94] surface lunar regolith, is of with a high precision, when proposed recently to monitor relatively low frequency, with the impacts can be radio- the flashes generated by the shock wave propagation tracked. This was the impacts (e.g. [66], [78], [9]) time ranging from 1/3 sec to case for most of the Lunar on the near side and even about 3 sec for the largest impacts (with the exception the far side [55] in order to impacts. of the SIV-B 17 impact, obtain impact data for future which suffered from a radio- lunar seismic experiments.

Notes on the observational Study of Lunar Impact Flashes

Ground-based telescope years [3], [67], [66], [95], complemented by the list of observations of lunar impact [96], [66]. More recently, 61 impact events reported flashes which mechanism Suggs et al. [79] produced by Rembold et al. [69] based was first suggested by a summary of the detections on the observations made Melosh [54] can provide a of 126 lunar impact flashes with the 35-cm telescope convenient way to monitor (with R- magnitude ~ 7.5 stationed in Socorro, New the fluxes of inter planetary - 10) obtained between Mexico. In Europe, new meteoroids and small bodies 2006 and 2011 by using activities have also been (~100 g – 10 kg) in the near- the twin 35-cm telescopes initiated. These include the Earth space environment. of the Automated Lunar SPOSH project for Smart Even though the observation and Meteor Observatory Panoramic Optical Sensor times are limited to the new (ALAMO) at NASA/MSFC Head for observations of moon phase, the effective in Huntsville, Alabama. transient optical flashes on detection area is large (~ Their statistical results the nightside of the Moon 1.3x106 km2). Successful showed that the meteoroid [64], [65], and the NELIOTA measurements have been impact rates peaked at project of the European obtained by a number of major meteor stream. This Space Agency for a 1.2- research groups over the important data set has been m telescope [7] that has

太空|TAIKONG 17 just made the first flash meteoroid bombardment body-wave magnitudes as temperature ever [8]. There can provide very useful low as 1.6. Their epicentre is also a plan to establish a information on the precise is very deep, with source lunar impact flash telescope times and locations of depths between 700-1200 (LIFT) in Xinjiang, China, moon-quake sources for km and more surprisingly, as a component of an lunar seismology. An attempt deep moonquakes are international monitoring was made to introduce a originating from well located network. This field is therefore seismometer experiment sources, which can repeat in a rapid expansion phase on the lunar nightside for and repeat similar quake probably because of the the Chang’E 4 mission months after months. increasing attention to the with the relay spacecraft The number of known establishment of research at the L2 point serving as source regions for deep facilities on the lunar surface the observation station. moonquakes is currently by several space-faring Even though the proposed estimated as ~250 [61], [62] nations. experiment was not and the most active source accepted, mainly because located at depth of about The orbital observations of the time constraint, 850 repeated 320 times a of the LADEE spacecraft there are really no technical Deep Moonquake (DMQ) showed clearly that the lunar showstoppers. We believe during the Apollo mission. dust cloud and exospheric that such a project will be Deep moonquakes originate environment are closely realized in near future. therefore from regions connected to the impact that appear to undergo effects of the interplanetary Deep moonquakes are the repeated failure, giving rise meteoroids [11], [81], [80]. last example of very atypical to sets of moonquakes In addition, the comparative quakes with respect to with similar waveforms and study of the ALAMO record the Earth standard, and occurrence times not only of the impact flashes and constitute about 60% of periodic but related to the the high-resolution imaging the quakes detected on tides generated by Earth data of small craters from the Moon, and therefore on the Moon. However, the the Lunar Reconnaissance to more than 7000 quakes relationship with the tides Orbiter (LRO) camera has over the almost 8 years of and explanation of the DMQ led to a new understanding operation. Most of the Deep has been a longstanding of the generation and Moonquakes are however puzzle for seismologist. evolution of lunar regolith very small in magnitude, DMQ respond for example to [77]. Last but not the least, and the smallest reported several cycles of the Earth- the monitoring of optical moonquakes correspond Moon orbit but also to more transients produced by to terrestrial events with subtle cyclic features of the

18 太空|TAIKONG Earth’s, Sun’s, and Moon’s detected on the moon and by one order of magnitude orbits. The most recent the Apollo seismometer might therefore thousand analysis are proposing remained therefore flat impacts per year and will that the faults of the Deep between two detected also discover if the Deep moonquakes sources, due quakes. Figure 7 shows Moonquakes need to be to the repeating activity of however that the number larger than a given magnitude these sources over the last of quake and impact is to occur. Very likely, a micro- hundred millions years, has increasing when the quakes seismic noise regime exists been abraded so much that have smaller amplitude. This on the Moon, generated by the surface of these fault is increase is roughly inversely the very numerous small extremely smooth and with proportional to the amplitude impacts and small deep very little asperity, which of the signals for impact moonquakes. It is estimated enable regularly the release and is even faster for the to be smaller than 1/100 of the relatively small tidal quakes from the strongest the Apollo resolution, which stress of the lunar solid tides. deep moonquake. Future open exiting perspective for instruments on the Moon very sensitive instruments, Despite the large number with better sensitivity will including for the detection of of detected quakes and therefore detect much more astrophysical signals to be their long duration, no signals, and instruments detailed in further section. micro-seismic noise was with performances better

Determining the Mechanisms of the Deep Moonquakes (DMQs)

The accurate determination attempts have been tide, i.e. tide-driven vs. tide- of the mechanisms of applied to determine the triggering, are quite difficult, the DMQs is the base for source mechanism, such considering the nature of simulating seismic wave as using polarization seismic signals records, and propagation inside the Moon, angle of S waves [38], P/S the sparse Apollo seismic deriving accurate structural amplitude ratio [59], [91], network. In the source model, and understanding and analysis of tidal stress mechanism inversion, long the physical and chemical [89]. However, further works period records are preferred state of the lunar interior. on determining whether the to avoid the high frequency It is a challenging problem source is a shear failure, and coda in Lunar seismic even for the earthquake full evaluation of the links records. Furthermore, large based studies. Several between the DMQs and the uncertainties in the source

太空|TAIKONG 19 locations and velocity Zhu and Helmberger [98] to equipped with broadband models require methods generate stable solutions. seismometers, will be the using whole waveform For future missions to Moon key to uncover the deep information with special or other planets, a well- quake sources underneath. treatment, as developed by distributed seismic network,

Lunar Structure

The internal structure of is consistent with the high down to 100 km depth is the Moon has been deeply porosity levels inferred from consistent with the GRACE inferred from seismological GRACE gravity data analysis data analysis suggesting data of Apollo Passive (about 12%). that the non zero porosity Seismic Experiment, but zone is extending below the also from active seismic The crustal structure, crust down into the mantle. experiments of Apollo in particular the crustal missions. These data have thickness estimate, varied Concerning the mantle been processed almost a lot with time. The average structure, the discontinuity continuously during the Moho depth was estimated at 500 km depth observed past 40 years, bringing at 60km by the early studies, in the early studies was new scientific results from and is now estimated demonstrated to be due to the seismic properties of between 45 km and 34 km the parametrisation of the the sub-surface through [6]. However, the Apollo model. Recent models favour the deep mantle and core data did not succeed in continuous evolution of the structure. a proper estimate of the seismic/density parameters, crustal thickness due to which is consistent with Figure 8 presents a the difficulty to retrieve the the geodynamical models compilation of global seismic body waves reflected and predicting that no strong models published by various converted at Moho within phase change is expected teams using mainly seismic the strong scattering signal into lunar mantle [37]. and geodetic information. coming from the crust. Other studies also estimated The deep mantle and The regolith and sub-surface the scattering level of the core structures are still structure obtained by the Moon, and demonstrated strongly debated. Some various teams presents very that the scattering is coming models predict a decrease low seismic velocities and from the first 100 km of the of seismic velocities (in densities in the first kilometer planet [27]. The extension of particular S wave velocities) of the planet [75] . This result the Moon scattering zone at the base of the mantle.

20 太空|TAIKONG 0 0 0

-10 -10 -10

-20 -20 -20

-30 -30 -30

-40 -40 -40 Depth (in km ) -50 -50 -50

Toksoz_RGSP_1974 Nakamura_JGR_1983-60 -60 -60 LognonneC_EPSL_2003 BeyneixC_PEPI_2006 Weber_Science_2011 Garcia_PEPI_2011-70 -70 -70 Khan_JGR_2014_mean Matsumoto_GRL_2015

-80 -80 -80 2468 024 2.5 3 3.5 Vp (in km/s) Vs (in km/s) Density (in g/cm 3)

0 0 0 Toksoz_RGSP_1974 Nakamura_JGR_1983 -200 -200 -200 LognonneC_EPSL_2003 BeyneixC_PEPI_2006 Weber_Science_2011 -400 -400 -400 Garcia_PEPI_2011 Khan_JGR_2014_mean Matsumoto_GRL_2015

-600 -600 -600

-800 -800 -800

-1000 -1000 -1000 Depth (in km )

-1200 -1200 -1200

-1400 -1400 -1400

-1600 -1600 -1600

2 46810 12 0246 468 Vp (in km/s) Vs (in km/s) Density (in g/cm 3)

Figure 8: Compilation of seismic and density models produced by various researchers since the first Apollo seismic data analysis. On top: zoom on the upper mantle and crust. On the bottom: whole Moon seismic structure.

太空|TAIKONG 21 The rationale is mainly that Concerning the lunar km and 380 km levels but in S waves are not observed at core, studies of Moon different seismic models of large epicentral distances, global rotational dynamics the Moon. Previous studies for which P waves can be demonstrated that a strong were also suggesting similar detected, and, consequently, dissipation of tides in the sizes based only on love either S waves are strongly Moon interior is needed. numbers and geodetic data attenuated or deflected by The best candidate for such [37]. However, the deep a low velocity layer at the a dissipation source is fluid Moon model, including base of the mantle. This movement in the core at core internal structure, will layer was interpreted as the core mantle boundary. only be known if broad presenting melt inclusions Under such assumptions, band sensors are deployed by some authors [90]. This the core size was estimated on the Moon surface in is consistent with analysis in the 300-380 km radius order to bypass the noise of love number values range. Two simultaneous generated by scattering at [37]. However, the S wave studies tried to detect body low frequencies. attenuation there can also waves reflected on the top of be interpreted by a different the core [90], [21]. The core material or temperature. radius was estimated at 330

Lunar Seismic Wave Propagation from Numerical Modeling

The Apollo lunar seismic wave propagation Wang et al. [87] performed seismograms are dominated in the Moon’s interior is not numerical modeling by intense scattering and well understood directly of seismic body wave reverberations of very long from limited lunar seismic propagation in the whole duration and slow decay waveform data. However, Moon based on recently of amplitude, as compared seismic wavefield modeling published whole-Moon with typical terrestrial provides a complementary model. They solved seismic seismic signals. The approach to observations wave equations in a 2-D commonly observed seismic for our understanding of cross-section of spherical body wave or surface wave seismic wave propagation Moon with a staggered grid phases for earthquake, can inside lateral heterogeneous pseudospectral and finite not be clearly identified in the and scattering Moon models difference hybrid method. lunar seismic waveforms. (e.g. [87], [32], [33], [34], The global P- and SV-wave Hence, the process of [10]). propagation in the whole

22 太空|TAIKONG Moon model generated the Moon’s surface. The frequency. Thus we expect from a 100 km deep shallow near surface low-velocity intense reverberations for moonquake and a 900 km layer in the global model higher frequency waveforms deep event are shown in plays significant roles in as observed on the Apollo Figure 9. Comparisons the development of long seismograms. However, between synthetic and reverberating wave modelling seismic wave seismograms obtained for trains following direct P propagation in the Moon with both deep and shallow and S arrivals. The multiple only the available 1-D model moonquakes and observed reflections and conversions does not seem to be enough Apollo seismograms showed which took place inside the to produce the slow decay that seismic body waves low-velocity layer formed of energy in observations can propagate efficiently constructive interferences, if other possible factors, inside the whole Moon. which propagate as such as scattering, are not Reflections and conversions trapped energy and appear considered in the model, of body wave phases which as reverberations in the which is fundamentally occurred at the internal synthetic waveforms. different from the seismic velocity discontinuities can Amplitude and duration of the wave propagation modeling be observed clearly on reverberations increase with on the Earth.

Figure 9: Wavefield snapshots showing the generation and propagation of lunar seismic phases in the whole-Moon model. (a) 100 km shallow moonquake and (b) 900 km deep event. Red and green colours show P and S waves, respectively. Solid circles are the free surface, crust and mantle interface, CMB and ICB. Two dashed circles represent the mid-mantle discontinuity and the core mantle transition zone. (Modified from[87] ).

太空|TAIKONG 23 Jiang et al. [32] modeled deep moonquakes at low- order to estimate the most global SH-wave propagation frequency range. realistic strength of velocity in the whole Moon with fluctuations. They proposed parallel modeling on a Jiang et al. [33], [34] that the standard deviation PC cluster. For shallow performed numerical of velocity fluctuations in the moonquakes, the direct modeling of scattering upper lunar crust should be wave and core–mantle effects caused by small- between 3% and 5%, and reflections can be identified scale heterogeneity in upper it's likely close to 3%. at a limited range of Moon crust in order to explain epicentral distance because the lunar seismic coda. Von- Analysis of lunar gravity data of interactions between them Karman auto correlation show drastic lateral variations and other phases. Waveform function is used to generate of crustal thickness around of each phase shows strong random velocity fluctuations, craters on the Moon [92]. reverberations caused which is superimposed Inside these craters, the by wave trapping effects on the background lunar crustal thickness has and multiple reflections velocity to simulate the very strong lateral variations, occurred in the surface heterogeneity of upper with the thinnest thickness low-velocity upper crust. Moon crust. Calculations nearly zero kilometer, For deep moonquakes, are performed for a 71 km which makes the mantle of the direct wave and core– shallow moonquake and an the Moon exposes to the mantle reflections can be 867 km deep moonquake. lunar surface directly, and clearly identified over wide Comparison of waveforms the thickest more than 60 range of epicentral distance. shows that scattering in kilometers. Chen and Wang Reverberations in waveform upper Moon crust can [10] performed numerical and surface multiples are effectively produce long- modeling of lunar seismic weaker than those for duration coda. Comparison wave propagation in a shallow event. Increasing with Apollo seismograms Moon model with laterally frequency enhances the shows that the duration heterogeneous crust. They strength and duration of and strength of coda calculated seismic wave waveform reverberation caused by scattering in propagation of recorded and reduces the possibility upper Moon crust can moonquakes along six of phase identification. This be well consistent with profiles Figure ( 11) of the suggests that SH body observations (Figure 10). laterally heterogeneous wave phases, such as core– Similarities between Moon crust and compared mantle reflections, can be synthetics and Apollo lunar with Apollo data. They found more clearly identified for seismograms are measured that the lateral variations via correlation coefficient in of crustal thickness cause

24 太空|TAIKONG Figure 10: Comparison between Apollo seismograms and synthetics for both shallow (SH1, 71km deep) and deep (A1, 867 km deep) moonquakes. Left and right figures are radial and vertical components. A0 is 1-D model and A1 is the model with scattering upper crust. (Modified from [33]).

太空|TAIKONG 25 Figure 11: Left: Crustal thickness of the Moon from GRAIL gravity and LRO topography [92] and 6 profiles along observed moonquake epicenters and Apollo stations used in numerical modeling[10] . Right: Comparing of modeling results between lateral heterogeneous and homogeneous crustal models [10]. multiple reflected and in above numerical heterogeneous Moon model converted waves, which modeling study. Numerical with topography for high interfere with each other, results support that strong frequency wavefield should thus contribute to the strong scattering, low attenuation be conducted on high and long duration wave and low velocity in the upper performance computing coda. Moon crust with drastic platform in the near future lateral variation may all to further enhance our The mechanism which contribute to the lunar understanding of seismic produces lunar seismic seismic coda. Numerical wave propagation in the coda is tentatively discussed modeling for 3-D lateral Moon.

26 太空|TAIKONG Post Apollo Science goals: Detection of Gravitational Waves on the Moon and Fundamental Physics

Gravitational waves domain were not known mode excitation, e.g., by (GWs) are propagating until the end of the 1960s. Earth and Moon quakes or perturbations of space- Freeman Dyson was the first atmosphere, is too frequent time produced by the most to calculate the response and loud to ever observe the powerful events in the of a homogeneous, elastic weak excitation from GWs. Universe such as the collision halfspace [16], and it took of two black holes or neutron much longer until the However, a recent series of stars. By the work of Pirani, formalism was developed papers has demonstrated Weber and others, it was to calculate the response that scientifically interesting known already by the end of a laterally homogeneous sensitivities can be achieved of the 1950s that GWs can spherical body by Ben- [12], [13], [14]. The new excite vibrations in elastic Menahem [4]. analyses were based on bodies [68], [88]. While sophisticated pipelines Weber started to develop In the meantime, Weber et al developed by the LIGO his famous bar detectors, he worked on the Lunar Surface and Virgo communities and and others also understood Gravimeter Experiment [26], modified to be applied to a that Earth itself responds to which was deployed on the network of seismometers GWs. A first analysis leading Moon in 1972 by the crew of or gravimeters monitoring to an upper limit for GW Apollo 17. Due to a design vibrations of the Earth [14], energy passing through the error, it could not accomplish [12] or Moon [13]. The Dyson Earth was obtained in 1961 its mission to detect GWs. half-space response was by Forward et al. [18], and With detailed studies in exploited in [14], [13], which a possible detection was the past two decades of is a valid model at higher claimed 10 years later by what GW amplitudes can frequencies were individual Tuman in 1971 [83]. be expected, and what normal modes cannot be the likely GW sources are resolved anymore, and the Calculations of cross- at the frequencies of the Ben-Menahem equations sections of elastic bodies lowest order Earth or Moon for normal-mode excitation towards GWs were quadrupolar normal modes, were used in [12]. In terms presented in some of the some scientists concluded of GW energy density, the earliest publications, but that GW detection with new constraints were better detailed calculations of the Earth or Moon is unlikely. It is by more than 10 orders coherent response in time typically claimed that intrinsic of magnitude than any

太空|TAIKONG 27 previous limits obtained for only about two orders the Moon today in terms example from high-precision of magnitude sensitivity of moonquakes and other laboratory experiments with improvement (in amplitude) seismic disturbances, it is in torsion bars. needs to be achieved to fact likely that GW excitations reveal known sources. can be found in the lowest To estimate the prospects order quadrupolar normal of future GW detection with 2) While moonquakes modes. Earth or Moon as response are known to exist, it can body, it is essential to realize be expected that the 3) Latest seismic that Moon is in fact one of the sensors as for example quietest, large bodies in developed for the Mars 1) The best searches the solar system lacking InSight Lander mission have so far reported in [12], [13], above all an atmosphere. sufficiently low instrumental [14] targeted a stochastic Until today, the lunar surface noise to make GW GW background. Better measurements essentially observations possible. sensitivity can in principle measured instrument noise be obtained by searching and occasional seismic For this reason, we strongly for known signals from transients. The typical, encourage further research known white-dwarf binaries ambient ground vibration on this topic, and to work exciting normal modes. With amplitudes might be so weak on a possible future mission respect to the less sensitive that GWs can be revealed. for deploying a new seismic stochastic GW searches, Given what is known about instrument on the Moon.

PLANETARY SEISMOLOGY

Mars

The NASA Viking mission related experiment, the stay on the lander deck, the was the first to land on seismometers onboard latter being in contact with Mars with seismometer in the two landers of Viking the Mars ground through feet 1976. Due to the priority couldn’t be deployed on the with shock absorbers. The given to the exobiology Martian surface and had to Viking lander 2 seismometer,

28 太空|TAIKONG the only one successfully large uncertainties remain: in March 2019. The payload unlocked, suffered during ±35 km in the mean crustal is a complete geophysical the day time from this poor thickness, ±450 km in the international observatory, seismic coupling and mostly core radius, leaving most, if with a seismometer (SEIS), detected the vibration of the not all, of the geodynamical a heat flux experiment (HP3, lander to the wind forces models of Mars evolution D), a geodesy experiment acting on the lander. At the weakly constrained. (RISE, US), a magnetometer night however, although the and the APSS (US) suite seismometer had a very low Seismology was therefore of atmospheric sensors noise, it did not detect clear put again in the payload of a measuring wind (TWINS, seismic signals. Only one Mars mission by the Russian Spain), atmospheric tentative event has been Space agency in the 1990s temperature, and pressure proposed as a marsquake, and great prospects were (US). SEIS is the primary but the lack of wind data put in the ambitious Mars96 instrument of the mission, at the time of the recording mission launched toward lead by the French Space leave several question Mars in November 1996 Agency and consists of marks on the true origin of with a very large orbiter, two a 3-axis very-broad-band this event. small autonomous stations (VBB, F) instrument and a equipped with short-period 3-axis short period (SP, UK) No Viking seismic data optimism seismometers, instrument mounted on a have therefore been used and two penetrators Levelling system (LVL, D) to constrain the interior with very short periods protected and connected by structure of the planet, accelerometers. That a Wind and Thermal Shield and all information as of mission failed, however, (WTS, US) and a Tether (US) today is related to geodesy, shortly after the launch, to the Instrument control gravity and mineralogical and fell down to the Pacific. and acquisition electronics investigations. This enables Seismology had again to (SEIS-AC, CH) located in planetologists to be rather wait more than 15 years. the lander. The two sensor confident in the fact that assemblies allow for highly Mars has a liquid core with The third attempt was Insight, sensitive measurements a radius of about half of the Discovery mission 13 over a very broad frequency the Martian radius and on selected by NASA in 2012, band, and will record most the increase of the crustal lead by the Jet Propulsion of the seismic spectrum of thickness beneath Martian Laboratory. It was launched the seismic signals. volcanoes and below the in May 2018 for a landing by South hemisphere, as the end of November 2018. The SEIS sensor heads, compared to North one. But Science operation will start composed by the combined

太空|TAIKONG 29 total of 6 axes between the calculation of the seismic might also be expected VBB and SP, will be installed moment release from during the full Mars year by a robotic arm 1-2 meters observed surface faults nominal mission duration, away from the lander and predict a level of activity enabling the detection of the will be protected against 100 greater than observed long period surface waves thermal variations and shallow moonquake activity. at periods larger than 50 direct wind effects on the This level would provide ~50 seconds as well as core sensor. Both the sensors’ quakes roughly equivalent to phase. Both the sensitivity, expected performances and terrestrial magnitude 4 per the very careful installation this installation will allow the (Earth) year. Several large and the expected activity are detection of ambient seismic quakes with magnitude 5 suggesting that several tens noise with amplitudes as low as 1.5 nm/ s2 (1.5 10-9 m/ s2) acceleration in one octave at 1Hz and 0.4 nm/ s2 (0.4 10-9 m/s2) at 10 seconds period. These sensitivities are up to three orders of magnitude better than those of Viking for body wave detection and Figure 12: InSight lander during the Earth test at the Lockheed Martin Facility, more than in Denver, Colorado. The lander with its robotic arm is in the back. The SEIS 50000 better for Sensor Assembly, with its golden color thermal protection is in the front, near surface waves. Earth reference seismometers used to assess the instrument noise after its integration in the spacecraft. Note that the SEIS instrument is tilted on one Theoretical side. As the Mars gravity is indeed 2.65 times smaller than the Earth one, it is estimates from necessary to tilt the instrument on Earth by almost 68°, in order to unsaturate thermoelastic one of the axis. On Mars, the 6 sensors located in the SEIS Sensor Assembly cooling and will be of course leveled. © JPL/LMA

30 太空|TAIKONG of quakes will be detected twice at the station, the the crust are much smaller every year by the mission second pass occurring after than those in the mantle. out of the Mars Global one turn around the planet. InSight is also expecting to seismic activity, with a focus As Mars has a radius twice detect impacts for which on the quakes and impacts smaller than the Earth's one, the location of the source occurring at distances the propagation distance will be known by comparing smaller than 3000 km. of these surface waves will remote sensing data taken be half of the Earth’s one, prior the InSight landing and As only one seismic station enabling better detection after the detection of the will be available, the seismic for the same magnitude. seismic signal, which will analysis of InSight will be As Mars circumference is provide not only the location different to the one used known, the two arrival times of the impact, but also on Earth, where seismic of these surface waves will the size of the crater and networks provide accurate provide the average velocity therefore an estimation of location of seismic sources. of surface waves around the the strength of the seismic For large quakes, InSight planet and a direct constrain source. Last but not least, will target the detection of on the crustal thickness, the SEIS instrument will surface waves train passing as the seismic waves in record, as the beginning

Figure 13: Picture taken by the CTX Camera on board the Mars Reconnaissance Orbiter of NASA, before and after an impact occurring on Mars. The impact appears clearly, through its removal of the surface dust and other surface modification. Such an impact will be a small quake, with magnitude up to 3, which might be detected by InSight at a rate of 3-5 events per year. The position of the impact being known by remote sensing, the differential arrival time of seismic waves (such as shear S waves with respect to compression P waves) will provide direct constraints on the crust interior.

太空|TAIKONG 31 of the mission, the seismic those related to the Mars atmospheric sources, such waves generated during interior, InSight will likely as the dust devils, a type of the penetration of the heat explore a new world in vortex already observed by flow experiment, which will seismology, by providing previous Mars missions, are penetrate by a succession an original way to monitor also expected to generate of strokes up to 5 meters the activity of the planetary such ground deformation, depth. These seismic waves atmospheric boundary layer. as well as infrasounds. will likely bounce on any It is indeed expected that Although a source of noise interface present in the near the atmospheric turbulences for the detection of remote subsurface, and the SEIS near the surface of the planet seismic activity, these recorded echo signals arrival will generate slight ground signals from the atmosphere times will provide the depth deformations and that the will likely be one additional of these interfaces. later will excite seismic discovery of this first seismic waves trapped in the near monitoring of Mars. If InSight major science surface regolith layers. Other goals remain of course

Giant Planets

Seismology applied to giant complicated by the fact making key contributions planets could drastically that their interior is thought to our understanding of change our understanding not to be homogeneous, Jupiter thanks to precise of their deep interiors, as so that spectroscopic measurements of its gravity it has happened with the determinations of and magnetic fields (e.g. Earth, the Sun, and many atmospheric abundances are [29]). Unfortunately, the main-sequence and evolved probably not representative presence of a denser core stars. The study of giant of the planet as a whole. of heavy elements only planets' composition is Instead, the determination weakly influences even the important for understanding of their composition and lowest order (quadrupole) both the mechanisms structure must rely on deviation in the gravity field enabling their formation indirect measurements and and the core mass will and the origins of planetary interior models. remain essentially model- systems, in particular dependent. On the other our own. Unfortunately, Since 2016, NASA's hand, seismology, which its determination is Juno mission has started consists of identifying

32 太空|TAIKONG global acoustic eigenmodes it is the biggest, closest, SDO [72]. As part of a 10- (p-modes), complements and brightest target. There day observing run in 2005, Juno science by offering have been several attempts the SYMPA instrument was a way to directly measure to detect Jovian oscillations able to produce a power the planet's sound speed using infrared photometry spectrum of Jupiter's profile, and thus its physical [15], Doppler spectrometry oscillations shown in properties from the outer [73], [56], [58], and careful Figure 14 [23], [25]. An envelope to the core. searches for excitation of excess of acoustic power is acoustic waves due to the observed in the frequency All of these questions can be impact of the Shoemaker- range predicted by theory, addressed with seismology. Levy 9 comet [86], [57]. In as well as the comb-like From an observational point most of these campaigns, structure of peaks that is of view, seismology of giant the signal-to-noise ratio also expected from interior planets is a natural extension was too low or instrumental models, thereby confirming of helioseismology. Their artifacts were present Jupiter's global pulsations. common fluid and convective that inhibited any positive Unfortunately, the level of nature is expected to lead to detection. The fast rotation noise in the data is too similar oscillations and to the of Jupiter also limits the high to identify individual possibility of using similar precision these instruments modes and decisively probe observational techniques. were able to obtain. Jupiter's interior. A second Theoretical works ([85], and independent detection [5]) predict that Jovian global Jovian seismology had to of these oscillations is highly oscillations should have a wait until 2011 to get a desired to confirm SYMPA’s frequency range of [0.8, convincing observational results. This is the purpose 3.5] mHz with 10 to 100 evidence of oscillations using of the JOVIAL/JIVE project, cm/s amplitude, values that the SYMPA instrument, led by Observatoire de are comparable to those an imaging spectrometer la Côte d’Azur and New of the Sun. However, more based on a Mach-Zehnder Mexico State University [28] , recent studies predict that interferometer at fixed optical which should start operating oscillation amplitude could path difference [74]. This in 2019. hardly be larger than 10 instrument was designed cm/s [50]. to overcome some of the Regarding Saturn, Marley & earlier limitations by imaging Porco [51] envisioned that Observationally, most of the full planetary disk, "the rings of Saturn may act the efforts dedicated to the similar to solar helioseismic as a seismograph, recording search of oscillations of giant instruments like GONG [30], gravitational perturbations planets regarded Jupiter, as MDI/SOHO [71], and HMI/ associated with acoustic

太空|TAIKONG 33 Figure 14: Detection of Jupiter’s oscillation from the SYMPA instrument. It shows the power spectrum of the mean velocity time series obtained in 2005. Excess oscillation power is detected between 800 and 3400 μHz, as well as a comb-like structure of regularly spaced peaks. The thick lines are smoothed data. From [24]. oscillations modes of the observations unexpectedly oscillations of giant planets, planet''. The basic idea is displayed a splitting of the they carry no information at that wave features in Saturn's oscillation frequencies for all about the amplitude of C rings could be created by several azimuthal orders. In the modes. resonant interactions with a first attempt to explain this, fundamental f-oscillation and to retrieve information In the coming years/ modes (i.e., radial order about the Physics of decades, seismology n=0), since these modes Saturn's interior, Fuller [19] applied to giant planets perturb the internal density proposed that a thermally needs a clear recording of profile and, therefore, the stable stratified layer might the oscillation spectrum external gravity field. Twenty surround the core. This of any giant planet, where years later, observations of would lead to the existence mode frequencies are stellar occultations of stars of a gravity g mode cavity, unambiguously determined. by the rings made with and these g-modes would Due to some peculiarities of NASA’s Cassini spacecraft interfere with Saturn's giant planets with respect to showed that density-wave f-modes and cause the the Sun or stars (resolved structures detected in the observed split frequencies, objects, wrapped by clouds, C-ring were compatible with somewhat like g-modes fast rotators), a specific resonances due to Saturn do with p-modes in red- instrumentation is required. f-modes [31], [19], [52]. giant stars [2]. Note that if Gaulme & Mosser [22] have These observations are the these Cassini observations explored the possibility of indirect evidence of these constitute the first using a space imager as wave forcings [52]. However, unambiguous evidence of the CNES CoRoT space-

34 太空|TAIKONG borne photometer for data are currently being Thales, NASA) to develop detecting the oscillations of processed and it is too early a prototype. The project giant planets in the reflected to reject the detection of named JOVIAL (in France) solar flux. They showed that planetary oscillations, even or JIVE (in the US) has answering to this question though they do not seem to now evolved into a three- depends strongly on the be detectable. instrument ground-based cloud vertical structure and network, with sites in France, microphysics, which are Overall, the best instrumental USA, and Japan, in the same mostly unknown. The Kepler approach to get good SNR way as helioseismic BiSON, K2 mission represented oscillations spectra of giant GONG, or asteroseismic a unique opportunity of planets are the same as for SONG. The objective testing this technique on the the Sun, i.e. Doppler spectro- is to observe Jupiter's two ice giants. It observed imaging. After the results of oscillations and identify its Neptune continuously for 49 the SYMPA instrument, a modes up to a degree l=10, days starting in December team led by François-Xavier to definitively answer the 2015 [76], and did the Schmider (Observatoire de question of the existence same with Uranus about la Côte d'Azur) proposed of a rocky core. Single-site a year later for 27 days. to place a payload inherited test campaigns were led in Neptune's observations from SYMPA onboard of 2015/2016/2017, and the have not brought any the JUICE ESA mission to network will be operational in detection of Neptune's the Jupiter's system. The 2019. First scientific results oscillations [70], but have payload was not selected in about Jupiter’s atmospheric allowed for detecting the the end of a 5-year process, dynamics are about to be Sun's oscillations reflected but we were granted enough published (Gonçalves et al., on Neptune [25]. K2 Uranus fundings (CNES, ESA, submitted).

Seismology on Small Bodies

Small bodies, compared under their own gravity, they much of the Earth’s interior with terrestrial planets, show highly irregular shapes structure can be captured pose additional questions – comet 67P/Churyumov- in one dimensional models and challenges for the Gerasimenko is probably the describing elastic properties application of seismic most prominent example, as function of depth, irregular experiments. Being too small although other bi-lobate bodies always require a to attain a spheroidal shape objects are known. While three dimensional treatment

太空|TAIKONG 35 e.g. of wave propagation, would result from seismic seismic experiments on a except perhaps at small experiments. comet in November 2014. scales. This can even extend to cartography, as the As part of the scientific The first touchdown on the surface of 67P/Churyumov- payload of Rosetta’s comet was recorded by Gerasimenko cannot be Philae lander, the CASSE all three accelerometers, uniquely described using Instrument (Comet resulting not only in a sound geographical latitude and Acoustic Surface Sounding file that became popular longitude. Experiment), consisting of in the internet, but also in three triaxial accelerometers an estimation of Young’s The Chelyabinsk event of in the feet of Philae, modulus (on the order of 10 February 15, 2013, reminded conducted the first active MPa) and the compressive us that small bodies pose strength (approx. 10 kPa) of a real threat – the Chelyabinsk meteorite was a 20 m sized rock, but deposited the equivalent of 550 kilotons of TNT in the atmosphere, and the shock wave alone resulted in significant destruction at the surface. Any system to redirect or destroy even larger objects before becoming a threat would profit from knowledge of the interior structure Figure 15: A cliff close to collapse on 67P/Churyumov-Gerasimenko. and mechanical Debris from a previous collapse is visible at the foot of the cliff, and a system properties of its of opening fractures is seen at its top in the middle and right of the middle target bodies, as of the image. Rosetta NAVCAM image NAC_2016-07-23T07.27.24.393Z_ ID30_1397549001_F22, image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

36 太空|TAIKONG the surface material at the Moreover, the recorded experiment by design, but Agilika landing site. signals show surface wave within the last decade or dispersion and corroborate so it has become clear After an unintended earlier models which that even small bodies excursion to a second site, predicted the formation provide surface processes named Abydos, CASSE of a sintered or cemented that can serve as natural listened to the hammering surface layer of 10 cm to sources. Mass wasting has of the MUPUS thermal 50 cm thickness on top of been observed on several probe, which attempted a less rigid material. For small bodies like asteroid to hammer itself into the Abydos, crucial information 4 Vesta, the Martian moon cometary material. A total of on surface topography Phobos, asteroid 25143 14 hammer strokes, and by underneath the lander is Itokawa, and comet 67P/ evaluation of relative travel not available with sufficient Churyumov-Gerasimenko. times a lower boundary for resolution (while it is for The Rosetta mission even shear wave velocity (vs≥79 Agilkia), hence the inversion returned before-and-after [ms]^(-1)) and, ultimately, resulted in lower limits only. images of collapsing cliffs, Young’s modulus (E≥7.2 as well as images of number MPa) could be derived for The SESAME experiment of fractures cliffs that are the Abydos site as well. was an active seismic

Figure 16: Outburst on the night side of 67P/Churyumov-Gerasimenko. The left image was taken by the OSIRIS Wide Angle Camera on 07:13 CET, 12 March 2015, the right image was taken two minutes later and shows an outburst on the night side not present two minutes earlier. Image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

太空|TAIKONG 37 probably close to collapsing of minutes. Also, some onto comet Tempel 1 to (Figure 15). outbursts began their study the effect of its impact activity during the local on the comet remotely. It is well known that comets night (Figure 16) implying The AIDA/DART mission sometimes break apart. that it took some time to concept of ESA and NASA Rosetta could document propagate the solar heat aimed to modify the orbit of the growth of a 500 m into a subsurface volatile a small body by an impactor long fracture in the “neck” reservoir, and that the and planned to observe the region of 67P/Churyumov- evaporated material was impact also using a lander Gerasimenko, which transported through some on the surface of the target extends over several of its pore or fracture system. body (the satellite of asteroid morphological regions and This transport might result 65803 Didymos, inofficially grew by several tens of in acoustic emissions similar named “Didymoon”). Thus meters in 2014 alone, and a to a boiling tea kettle, or several technologies with new crack of 150 to 300 m volcanic tremor. secondary uses as seismic length appeared until 2016. sources are available and The formation and growth At the time of writing, JAXA’s have a high technical of such cracks is most likely Hayabusa 2 mission is on readiness level. Together accompanied by seismic its way to asteroid 162173 with seismic sensors in events and could thus be Ryugu, while OSIRIS-Rex is small lightweight landers like used as source in a passive in cruise to asteroid 101955 Hayabusas’s MASCOT they experiment. Bennu. Both will arrive in could be used to investigate late 2018, and both carry the interior structure and Comets offer a second sampling mechanisms that mechanical properties of potential seismic source might serve as seismic small bodies- which are via their activity: Rosetta sources as well: Hayabusa 2 connected not only to the documented that outbursts fires projectiles onto Ryugu deflection of potentially emanating from the surface and collects the released hazardous asteroids but also sometimes show a very debris, while OSIRIS- to the formation of planets distinct on-off behaviour, Rex uses a fast-rotating at the beginning of the solar i.e. activity starts and stops scratching device. A big system. suddenly on a time scale projectile was already fired

38 太空|TAIKONG NEXT STEPS AND FUTURE PROJECTS

Projected ahead, it is quite geological provinces, the surface geomorphology possible that there will be advance of lunar seismology and atmospheric system a number of permanent or will have another meaning however, are drastically quasi-permanent research for human colonization of different from their facilities introduced to the Moon. That is, this array terrestrial counterparts. the Moon within 10 years of seismometers could The lack of plate tectonics according to the tempo serve as a forecast system but abundance of active of the Moon programs of for meteor storms. Up to volcanism, coupled with the different national agencies. now, impact events by presence of a thick envelope

This means the chance is interplanetary meteoroids of CO2 atmosphere, makes good that a global network of have been utilized as a source Venus a very strange world. seismometers would be set for seismic measurements. An in-depth study of the up on the lunar surface for When the sensitivities are interior structure of Venus Moonquake investigations. high enough such that the will be a fascinating topic An important outcome impact signals of meteoroids for comparative planetology of this Forum would be down to the size of cm or and unlocking the mystery pointing the direction and the smaller can be registered, surrounding the formation mechanism for international the measurements would of the terrestrial planets. cooperation. In Asia alone, be useful to reconstruct the The traditional approach the four Moon-faring particle size distributions to seismic measurements nations, namely, China, and orbital distributions of by employing ground- India, Japan and South individual meteor streams. level detectors a la InSight Korea, could potentially This information will be would encounter great be founding members of very useful in predicting difficulties at Venus because such a consortium. The the timing and magnitude of the forbidding surface key is to share the designs of bombardment hazards temperature of 450oC and and constructions of space of large meteor streams to the potential existence of qualified seismometers with artificial infrastructures and high wind and hence noises. high precision and hopefully astronauts on the Moon. In spite of the continuing better than those of Apollo. R/D effort in over-coming Besides a quantum jump Venus, the twin planet of these technical problems, in our understanding of the Earth, is likely to be the successful performance the lunar interior structures the next opportunity for of landing a seismometer at different depths and seismological study. Its experiment on Venus’

太空|TAIKONG 39 surface could be in distant born platforms or orbiting core would be obvious. future. For this reason, a vehicles can be applied But for Mercury, the large number of remote-sensing to Titan which also has surface temperature at methods by means of a very thick atmosphere daytime near the equatorial orbiting spacecraft or air- of nitrogen. Because of region presents a formidable born platform have been the experiences with the technical obstacle for investigated. scientific operations of the long-term ground-based Huygens probe, the ground- instruments. The large The atmospheric level environment is much diurnal temperature transmission and better known (and probably difference between day and propagation of acoustic far easier to cope in spite of night is equally challenging. waves and gravity waves it low surface temperature It is therefore possible that a generated from a quake of –180oC) at Titan than lander mission will choose a epicenter are more efficient at Venus, for surface polar or high-latitudinal site on Venus than on the Earth. seismometer experiments. for landing in order to mitigate This property can lead to It is therefore most tempting some of these issues. Since the possibility of monitoring to contemplate the inclusion it is unlikely that a multi- optical and infrared of a seismometer into the station seismometer system signatures of seismic events geophysical payload. Hence, as in the case of the Apollo by an orbiter [36] and the Titan might be second mission will be set up, the detection of infrasonic planetary body in addition seismic measurements by a pressure waves by high- to the Earth to witness the single station would require altitude balloons (or drones simultaneous seismological additional data inputs from for that matter). Another measurements by means other types of observations interesting method is to of surface detection, orbital such as the lunar impact examine the variability of the detection and air-born flash detections. total electron contents (TEC) platform. of the Venusian ionosphere Both the Grand Finale associated with seismic After the BepiColombo mission of the Cassini- activities by radio wave project of a dual spacecraft Huygens project to occultation observations mission to Mercury, the Saturn and Titan and the now being carried out on planning of a geophysical JUNO project to Jupiter Earth [48]. mission to probe the have provided first-hand subsurface and interior information on the interior Besides Venus, the remote- properties of this innermost structures of these two sensing techniques for planet known to possess giant planets. The scientific seismic activities on air- a proportionally large iron explorations of the internal

40 太空|TAIKONG structures of their respective plays an important role oceans, the tidal effects, icy satellites are expected in their planned scientific and finally the physical to soon follow. Indeed, the investigations [84]. We can properties of the inner Europa clipper mission of imagine that their findings cores. Only seismological NASA and ESA’s JUICE will lead to further questions measurements can mission to Ganymede can be on the structures and provide answers to these considered to be the first step depths of the icy crusts, the fundamental questions. even though astrobiology dimensions of the subsurface

A List of Proposed Seismometer Experiments

The critical nature of outer to be initiated by different would likely be few and planet exploration is about organizations, what matters infrequent. It is therefore its cost and time duration. is the promotion of multi- essential that research In the case of the Moon lateral cooperation to groups from the world-wide or Mars, there are already optimize and synergize scientific community can join mounting momentum in scientific returns. Any forces and be a part of these the coming decade with a new mission(s) to Europa, great chapters of human number of space projects Ganymede, or Enceladus endeavors.

Instrument Development

The research of planetary accelerometers, the space fundamental physics seismology requires space- technologies reviewed in this experiments. qualified seismometers to Forum are also suitable to measure the seismic events be used in planetary seismic In China, such technologies and force disturbances instrument development to have been studied at the of the planet on which promote the performance Huazhong University of they are mounted. As the and reliability. High precision Science and Technology measurement and controlling space inertial sensor/ (HUST) since around 2000. electronics of planetary electrostatic accelerometer Prototypes and engineering seismometers is similar is the key payload for satellite models have been developed with space electrostatic gravity measurement and and tested in laboratory.

太空|TAIKONG 41 High precision differential sensors or seismometers. In Europe, most of the capacitance sensors with A space flight model was recent development have 10-7pF/Hz1/2 resolution, low developed and launched been made in the frame of noise electrostatic feedback successfully in 2013. The the Insight Seismometer, controller electronics with technologies of space with the development of μV level resolution are inertial sensor are verified both a Short Period, and of developed. Progresses successfully in-orbit. A a Long Period seismometer are also achieved on the low power, high precision [45]. As shown on Figure 17, ground performance test charging/discharging an improvement of about and engineering studies differential capacitance 2500 at 1Hz and 200 000 of the accelerometers. sensor is also under study. at 0.1 Hz is expected in Pendulum test benches are The low frequency 1/f noise term of resolution with built to eliminate the seismic of the voltage read-out respect to Viking for the noise on ground and the system is reduced to be VBB, which is also better performance is under test, flat which allows evaluating by a factor of 5 than Apollo which can be used to test the low frequency noise of LPs in the range of the long the performance of high space inertial sensors. period body waves, of great precision space inertial importance for core phase

Figure 17: Root mean squared self-noise of the three main outputs of the SEIS instrument (VBB VEL, VBB POS and SP VEL), in acceleration for a 1/6 of decade bandwidth, as function of the central frequency of the bandwidth. This is compared to the Apollo and Viking resolution or LSB, as none of these instruments were able to record their self- noise due to limitation in the Acquisition System in the 70th (9 bits plus sign for Apollo, 7 bits plus sign for Viking). SEIS use acquisition on 23 bits plus sign.

42 太空|TAIKONG analysis. The InSIght SP is improved for Lunar version, by 20 with respect to both also better by a factor of 10 for which high vacuum will the Apollo LP and SPs than the Apollo SP. These reduce dramatically the can be achieved. At longer performances are limited sensors Brownian noise. term, instrument with even by the environmental noise In addition and if thermal better performances may associated to the interaction control can be achieved, as be expected, close to 10- of Mars atmosphere and for Apollo, the low frequency 11 m/s2/Hz1/2 in the seismic temperature variations feedback noise might bandwidth [35], [17]. with the SEIS assembly, furthermore be reduced and and might be easily further very likely, improvements

SUMMARY AND CONCLUSIONS

For the first time, 130 years after Earth and including those from China, comparative planetology, 50 years after the Moon, the thus providing a unique which was until today interiors of Mars, Jupiter and opportunity for the return to limited mostly to planetary Europa will likely be probed seismic study of the Moon. atmosphere and surface, in situ. It is indeed gratifying to might be soon possible for see that this Forum might the interior of telluric planets But Apollo was not enough have paved the way to the and giant planets. With its for understanding the Earth- initiative for deploying an successful launch on May Moon formation! A new step international lunar seismic 5, 2018, the InSight mission in the geophysical exploration network, enabling not only is finally on its way to Mars, of our satellite is required, the detailed imaging of and will restart the seismic this time made with much the lunar crust and deep exploration of planets, more more sensitive instruments, interior, but providing also than 20 years after the end detecting not only much the missing constraints for of the Apollo Lunar network subtler geophysical signals a complete understanding and landing of Viking. Other but looking even for the of the Moon formation projects are in development, detection of astrophysical scenario, with important either for monitoring Jupiter signals on the Moon. consequences on the long free oscillations from Earth or term habitability of the Earth. to explore the hidden ocean The next decade will witness of Europa with a lander the international plans to equipped with seismometer. deploy several landers,

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Participants

William Bruce Banerdt JPL, USA Michel Blanc International Space Science Institute-Beijing, China Jun Du Peking University, China Lijuan En International Space Science Institute - Beijing Wenzhe Fa Institute of Remote Sensing and GIS / Peking University, China Maurizio Falanga International Space Science Institute - Beijing Raphael Garcia ISAE, France Patrick Gaulme Department of Astronomy, New Mexico State University, USA Jan Harms Gran Sasso Science Institute, Italy Hiroaki Shiraishi Institute of Space and Astronautical Science (ISAS), Japan Heiner Igel Department of Earth and Environmental Sciences Geophysics, LMU, Germany Wing Huen Ip National Central University, Graduate Institute of Astronomy, Taiwan Xianghua Jiang China Earthquake Networks Centre, China Earthquake Administration, China Taichi Kawamura National Astronomical Observatory of Japan, Japan Amir Khan ETHZ, Switzerland Martin Knapmeyer DLR, Germany Brigitte Knapmeyer-Endrun MPS, Germany Patty Lin Taiwan Oceanographic Research Institute, Taiwan Philippe Lognonné University Paris Diderot/Institut de Physique du Globe de Paris, France Yosio Nakamura The University of Texas at Austin, Institute for Geophysics, USA Clive Neal University of Notre Dame, Notre Dame, USA Jieyuan Ning Peking University, China Shuoxian Ning Peking University, China Shaobo Qu Huazhong University of Science and Technology, China Sébastien de Raucourt IPGP, France Nick Schmerr University of Maryland, College Park, USA Daoyun Sun University of Science and Technology of China, China Chi Wang National Space Science Center, CAS, China Yanbin Wang Peking University, China Renee Weber MSFC, Huntsville, USA Mark Wieczorek LAGRANGE/Observatoire de la Cote d’Azur, France Ling Xin Science reporter, Bulletin of the CAS Anna Yang International Space Science Institute - Beijing Hsin-Ying Yang University of Science and Technology of China, China Peimin Zhu China University of Geosciences, China