太空|TAIKONG LUNAR AND PLANETARY SEISMOLOGY - International Space Science Institute
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国际空间科学研究所 - 北京
太空|TAIKONG
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
Phone: +86-10-62582811
experiments on Chinese lunar prospects of new science and future
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, This was the tenth science forum
Wuhan and its group. After these
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
Front Cover present activity of the Lunar and and was designed to provide a
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 11THE 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 13LUNAR 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|TAIKONGuncatalogued. 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 15Figure 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 17just 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|TAIKONGEarth’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 19locations 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|TAIKONG0 0 0
-10 -10 -10
-20 -20 -20
-30 -30 -30
Depth (in km)
-40 -40 -40
-50 -50 -50
Toksoz_RGSP_1974
Nakamura_JGR_1983
-60 -60 -60
LognonneC_EPSL_2003
BeyneixC_PEPI_2006
Weber_Science_2011
-70
Garcia_PEPI_2011 -70 -70
Khan_JGR_2014_mean
Matsumoto_GRL_2015
-80 -80 -80
2 4 6 8 0 2 4 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
Garcia_PEPI_2011
-400 -400 -400
Khan_JGR_2014_mean
Matsumoto_GRL_2015
-600 -600 -600
Depth (in km)
-800 -800 -800
-1000 -1000 -1000
-1200 -1200 -1200
-1400 -1400 -1400
-1600 -1600 -1600
2 4 6 8 10 12 0 2 4 6 4 6 8
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 21The 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 23Jiang 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|TAIKONGFigure 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|TAIKONGPost 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 27previous 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|TAIKONGthe 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
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