岩田 隆浩

The correlation between the spatial patterns of surface Thorium (Th) abundance measured by SELENE GRS data and the crustal thickness from the GRAIL gravity field and LRO LOLA data is investigated in the lunar highland area. Our analysis reveals that there are several areas of local minima for Th abundance exhibiting similar values but different crustal thicknesses. To explain the result, we propose a two-stage scenario for crustal formation. In the first stage, plural thin plateaus form on the surface of the lunar magma ocean (LMO), which corresponds to the observed surface Th distribution. In the second stage, a global crust with dichotomy forms by solidification of the LMO under the plateaus.

<p>We are planning to explore the caverns through the skylight holes on the Moon and Mars. The holes and their associated subsurface caverns are among the most important future exploration targets. The importance of the lunar and Martian holes and their associated caverns is categorized from two aspects: (1) fresh materials are easily observed and sampled there, and (2) the subsurface caverns provide a safe, quiet environment. The expectation of lunar and Martian hole and cavern exploration is increasing in Japan. We name the project as UZUME (Unprecedented Zipangu (Japan) Underworld of the Moon Exploration) whose name is after a Japanese mythology. The ultimate purpose of the UZUME project is to investigate how to expand human activity and survival in space and on extraterrestrial bodies. </p>

The solidification of the Lunar Magma Ocean (LMO) and formation of impact basins are important events that took place on the early Moon. The relative timing of these events, however, is poorly constrained. The aim of this study is to constrain the formation ages of old impact basins based on inferences of their thermal state. Most proposed basins formed before Pre-Nectarian (PN) 5 stage do not exhibit clear concentric features in either topography or gravity, suggesting substantial viscous lateral flow in the crust. Recent geodetic measurements reveal that the lunar crust is thinner than previously estimated, indicating that an extremely high crustal temperature is required for lateral flow to occur. In this study, we calculate lunar thermal evolution and viscoelastic deformation of basins and investigate the thermal state at the time of basin formation using recent crustal thickness models. We find that a Moho temperature 1300-1400. K at the time of basin formation is required for substantial viscous relaxation of topography to occur; the implied elastic thickness at the time of loading is <30. km. Such a high temperature can be maintained only for a short time (i.e., <50. Myr for most conditions) after solidification of the LMO or after mantle overturn if it took place; relaxed impact basins forming ≥150 Myr later than LMO solidification are unlikely. This result is in conflict with an intensive Late Heavy Bombardment (LHB) model, which assumes that most impact basins were formed at ~3.9. Ga, since it requires LMO solidification time much later than previous theoretical estimates. Either the LHB was moderate, or the majority of proposed early PN basins were not in fact formed by impacts.

The South Pole-Aitken (SPA) basin is the largest clearly recognized basin on the lunar surface. Determining the composition and structure of the SPA basin interior provides critical constraints on the deep crustal and/or mantle composition of the Moon and improves our understanding of large-basin-forming impact processes. Here we present a new mineralogical map of the SPA basin interior, based on high-spatial-resolution reflectance spectra using the SELENE (Kaguya) multiband imager, which is combined with topographic data in order to interpret the geologic context. The derived mineralogical map suggests extensive distribution of ejected low-Ca pyroxene-dominant mantle material with the presence of purest anorthosite crustal materials surrounding a possible melt pool of 0.26 to 0.33 of the basin diameter near the basin center, which is significantly smaller than that suggested by the crater-scaling law. The absence of clear evidence of lower crustal material is consistent with recent impact simulation results. Key Points Stratigraphy of the lunar mantle was revealed within the Moon's largest basin A smaller melt pool was suggested than that derived by the crater-scaling law Absence of clear evidence of lower crustal material is suggested by mineralogy ©2014. American Geophysical Union. All Rights Reserved.

Diverse geological characteristics found for the three major lunar provinces (i.e., the Feldspathic Highlands Terrane (FHT), the South Pole-Aitken Terrane (SPAT), and the Procerallum KREEP Terrane (PKT)) strongly suggest their distinctly different thermal histories. Quantitative differences among these provinces in their early thermal histories and crustal radioactive element concentrations, however, are highly unknown. One of the few observables that retain a record of the ancient lunar thermal structure is the viscoelastic state of impact basins. This study investigates the long-term evolution of basin structures using global lunar gravity field data obtained by Kaguya tracking and derives constraints for (1) the paleo-thermal state of impact basins and for (2) crustal column-averaged radioactive element concentrations for each province. Our calculation results indicate that impact basins in the central anorthositic region of the FHT (i.e., the FHT-An) require a very cold interior (dT dr ≤ 20 K km - 1 on the surface). This result strongly suggests that the deep portion of the thick farside highlands crust is highly depleted in radioactive elements (Th ≤ 0.5 ppm), indicating that the Th-rich SPA basin floor crust is clearly different from the lower crust underneath the FHT-An and cannot be accounted for by simple exposure of the lower crust. Our analysis also indicates that the observed basin structure allows as high as ∼ 6 ppm of column-averaged Th concentration in the crust inside the PKT. These results indicate that radioactive element concentrations deep in the crust probably vary greatly region by region, similarly to those observed on the surface. Key Points We investigate viscoelastic deformation of major lunar impact basins Constraints on the early thermal state vary greatly among geological provinces The thick lunar farside crust needs to be depleted in radioactive elements ©2013. American Geophysical Union. All Rights Reserved.

DESTINY which stands for "Demonstration and Experiment of Space Technology for INterplanetary voYage" is a mission candidate for the third mission of ISAS small science satellite series. DESTINY is launched by an Epsilon launch vehicle, JAXA's next-generation solid fuel rocket, and is firstly placed into a low elliptical orbit. It raises its altitude by the use of ion engine and reaches the Moon. Then, it is injected into transfer orbit for L 2 Halo orbit of the Sun-Earth system by using lunar gravity assist. On the way to L2 Halo orbit, DESTINY conducts demonstration and experiment on key advanced technology for future deep space missions. This paper presents the overview of DESTINY mission as well as its importance and significance in Japanese space science program.

DESTINY which stands for "Demonstration and Experiment of Space Technology for INterplanetary voYage" is a mission candidate for the third mission of ISAS small science satellite series. DESTINY is launched by an Epsilon launch vehicle, JAXA's next-generation solid fuel rocket, and is firstly placed into a low elliptical orbit. It raises its altitude by the use of ion engine and reaches the Moon. Then, it is injected into transfer orbit for L-2 Halo orbit of the Sun-Earth system by using lunar gravity assist. On the way to L-2 Halo orbit, DESTINY conducts demonstration and experiment on key advanced technology for future deep space missions. This paper presents the overview of DESTINY mission as well as its importance and significance in Japanese space science program.

Measurements of lunar gravity and rotation are important because they provide information of the physical state of the lunar interior. Previously only passive LLR (Lunar Laser Ranging) using CCR (corner cube reflectors) has been applied for the detailed study of lunar librations, rotation variability. As for candidate instruments for SELENE-2 (forthcoming lunar landing mission by JAXA) and future lunar missions, we propose VLBI (inverse-VLBI and differential VLBI) for gravity measurement to constrain tidal Love number, LLR and ILOM (In-situ Lunar Orientation Measurement) for libration measurements.

Using 4-way Doppler tracking with relay satellite OKINA, KAGUYA obtained the first precise gravity field of the lunar farside. Multi-frequency differential Very Long Baseline Interferometry (VLBI) observation using OKINA and OUNA improved the accuracy of gravity, through precise determination of OKINA's orbit. The current gravity model is SGM100i involving VLBI data. Laser altimeter (LALT) on board KAGUYA obtained the first precise global topography of the Moon with range accuracy of 5m. The correlation of spherical harmonics coefficients between gravity and topography become higher than that of the previous model. Gravity signatures of far-side impact basins are mostly explained by topography except for the central high. Combined with topography data, we estimate Bouguer anomaly and the crustal thickness variation of the Moon.

Variations of Mars' rotation provide us information concerning both the interior structure and the surface mass redistribution of Mars. Precession and nutation of Mars mainly reveal the core-mantle subsystem, besides length-of-day variation and polar motion of Mars are generally referred to the atmosphere-cryosphere sub-system. As one of the missions of MELOS (Mars Exploration with Lander-Orbiter Synergy), we are proposing areodetic observations using space geodetic techniques including four-way Doppler measurement (FWDP) and inverse VLBI (IVLBI). FWDP is ranging rate measurement of target spacecraft via a relay spacecraft. Utilizing the heritage of four-way Doppler measurements by SELENE, we plan to track the MELOS Landers relayed by the MELOS Orbiter. We also introduce the new technology of IVLBI. In the framework of this technology, one ground radio telescope observes the phase-shift between mutually coherent signals on the orbiter and the landers. The estimated accuracy for the rotation obtained by IVLBI is better than 1 mas (milli-arc second) including the systematic phase noise. The measurements will provide the core radius estimation with the errors of less than 200 km by 50 weeks observations with two landers.

The Radio Science experiment (RS) in the Akatsuki mission of JAXA aims to determine the vertical structure of the Venus atmosphere, thereby complementing the imaging observations by onboard instruments. The physical quantities to be retrieved are the vertical distributions of the atmospheric temperature, the electron density, the H2SO4 vapor density, and small-scale density fluctuations. The uniqueness of Akatsuki RS as compared to the previous radio occultation experiments at Venus is that low latitudes can be probed many times thanks to the near-equatorial orbit. Systematic sampling in the equatorial region provides an opportunity to observe the propagation of planetary-scale waves that might contribute to the maintenance of the super-rotation via eddy momentum transport. Covering the subsolar region is essential to the understanding of cloud dynamics. Frequent sampling in the subsolar electron density also helps the understanding of ionosphere dynamics. Another unique feature of Akatsuki RS is quasi-simultaneous observations with multi-band cameras dedicated to meteorological study; the locations probed by RS are observed by the cameras a short time before or after the occultations. An ultra-stable oscillator provides a stable reference frequency which is needed to generate the X-band downlink signal used for RS.

The effect of the phase patterns of antennas onboard flying spin satellites on the Doppler measurements is reported. Phase patterns mean that there are deviations in wave fronts from perfect sphericity, expressed as a function of the angular position around the antenna. We analyzed what effect the phase patterns of dipole and patch antennas onboard two flying spin satellites, Rstar and Vstar, used in the Japanese lunar mission, SELENE (KAGUYA), had on 2-way and 4-way Doppler measurements, and detected higher harmonics in the spin frequency up to an order of 26 in the Doppler frequency. We developed a low-pass filter (LPF) using a Kaiser window, with the optimal parameters empirically determined, to remove the influence of phase patterns and to precisely conserve information on the lunar gravity field. We processed the 2-way and 4-way Doppler data of SELENE by using LPF. After using LPF, a high degree of accuracy of about 0.001 Hz was achieved for the 2-way Doppler measurements, and signals that reflected the gravity field on the far side of the Moon were first detected from the 4-way Doppler data. We also suggested a method for estimating the phase response of satellite antennas using the Doppler frequency variations. In order to estimate the Doppler frequency variation, a filtering technique was adopted to extract the harmonics of interest in the residual signal, from which the antenna phase pattern was derived.

Measurements of physical and free librations of lunar rotation are important because they provide information of the physical state of the lunar interior. For example, we can discuss if the lunar core is molten from the amplitudes of libration terms. Previously only passive LLR (Lunar Laser Ranging) using CCR (corner cube reflectors) has been applied for the detailed study of lunar librations. As for candidate instruments for SELENE-II (forthcoming lunar landing mission by JAXA), we propose detailed measurements of lunar rotation by ILOM (In-situ Lunar Orientation Measurement) and IVLBI (Inverse-VLBI) in addition to LLR.

On the basis of the gravity model of the Moon developed by SELENE (Kaguya), we propose new classification and compensation mechanism of lunar impact basins. Impact basins on lunar far side and limb are classified into Type I and II basins depending on the magnitude of central gravity high in free-air and Bouguer gravity anomalies. Among previously known gravity anomalies, most typical mascons are referred as primary mascon basins. Topographic depression and rim of both Type I and II basins show good correlation between topography and free-air gravity anomaly suggesting elastic support of lunar lithosphere. Central gravity high of Type I basin is inferred to be a result of mantle uplift at the time of basin formation, and is elastically supported, too. On the other hand, free-air anomalies at the center of Type II basins are lower than Bouguer anomalies indicating brittle deformation of the basins. Topographic depression and rim of primary mascon basins on near side of the Moon show little to no free-air gravity anomalies. This indicates a result of elastic relaxation that occurred probably after eruption of mare basalts. Plateau-like signature of gravity anomalies of primary mascon basins implies viscous relaxation at crust-mantle boundary beneath the basins and significant heat (or volatile) transport by basaltic magma.

The electron density profiles above the lunar surface are being observed by the radio occupation technique during the mission using the Vstar and Rstar sub-satellites. In addition to a traditional technique which uses one orbiter, we are conducting another method which uses two orbiters with the second one being used to measure the terrestrial ionosphere contribution. Previous radio occultation measurements have indicated the existence of an ionosphere with densities of up to 1000 cm-3 above the dayside lunar surface. These densities are difficult to explain theoretically when the removal of plasma by the solar wind is considered, and thus the generation mechanism of the lunar ionosphere is a major issue, with even the validity of previous observations still under debate. The SELENE radio science experiment will establish the morphology of the lunar ionosphere and will reveal its relationship with various conditions to provide possible clues to the mechanism.

Same beam very long baseline interferometry (VLBI) observations of the two subsatellites of SELENE (KAGUYA) are demonstrated for purpose of the precise gravimetry of the Moon. Same beam VLBI contributes a great deal to cancel out the tropospheric and ionospheric delays and to determine the absolute value of the cycle ambiguity by using the multifrequency VLBI method. As a result, the differential phase delay of the X-band signal is estimated within an error of below 1 ps. This accuracy is more than 1 order of magnitude smaller than former VLBI results. The preliminary results for the orbit determination of the subsatellites show a decrease of the orbit error from a few hundreds of meters to around 10 m when the differential phase delay data are added to the conventional range and Doppler data. These results reveal the possibility of precise gravimetry.

The farside gravity field of the Moon is improved from the tracking data of the Selenological and Engineering Explorer (SELENE) via a relay subsatellite. The new gravity field model reveals that the farside has negative anomaly rings unlike positive anomalies on the nearside. Several basins have large central gravity highs, likely due to super-isostatic, dynamic uplift of the mantle. Other basins with highs are associated with mare fill, implying basalt eruption facilitated by developed faults. Basin topography and mantle uplift on the farside are supported by a rigid lithosphere, whereas basins on the nearside deformed substantially with eruption. Variable styles of compensation on the near- and farsides suggest that reheating and weakening of the lithosphere on the nearside was more extensive than previously considered.

SELENE (Kaguya), which was successfully launched on Sep. 14, 2007, consists of the main orbiter, and two small free-flying sub-satellites, called Rstar (OKINA) and Vstar (OUNA). We use multi-frequency VLBI to measure the angular distance between the two sub-satellite radio sources Okina and Ouna in order to improve the accuracy of the low degree gravitational harmonics and the gravity field near the limb. The observations are made at three frequencies in S-band, (2212, 2218 and 2287 MHz), and one in X-band, (8456MHz) with carrier waves. The Japanese domestic VLBI network, VERA, will conduct VLBI observations for the whole mission period of one year. In addition, we will conduct two periods of international observations, each one month in duration, which will also include the international stations, Shanghai, Urumqi, Hobart, and Wettzell. We have succeeded in making VLBI observations of Okina/Ouna with VERA and the international network, and have also succeeded in correlating of signals from Okina/Ouna. We obtained phase delays with an accuracy of several pico-seconds in S-band.

SELENE Main Orbiter (KAGUYA) has separated two small sub-satellite (1) the Relay Satellite "Rstar (OKINA)", and (2) the VLBI Radio Satellite "Vstar (OUNA)". These sub-satellites started to perform 4-way Doppler measurements using Relay Satellite Transponder (RSAT) and multi frequency differential VLBI using VLBI Radio Sources (VRAD) for selenodesy. Initial check out was executed and properties of satellite bus equipments, onboard mission instruments, and observation systems including ground stations were evaluated. Electric power and thermal control subsystems have shown that they conduct as designed and inspected in the ground tests. The release mechanisms have given the spin which can maintain the stability of the satellite attitudes. Communication functions of mission instruments conform to the link budgets. These results suggest that Rstar and Vstar have enough performances to produce efficient selenodetic data by RSAT/VRAD observations.

In the very long baseline interferometry (VLBI) radio sources mission of selenological and engineering explorer, the differential phase delay between the Rstar and Vstar sub-satellites is obtained by using the multifrequency VLBI method during the switching VLBI observation period. The cycle ambiguity is successfully determined and the differential phase delay is estimated within an error of 7 picoseconds. The RMS error is somewhat larger than that for the case of same-beam VLBI because fluctuations of propagation delays whose periods are shorter than the switching interval cannot be canceled out between Rstar and Vstar. However, the differential phase delay during the switching VLBI period is sufficiently accurate and, together with Doppler and range measurements, can be a useful means for precisely determining satellite orbits and precisely estimating the lunar gravity field.

The electron density profiles above the lunar surface will be observed by the radio occultation technique during the SELENE mission using the Vstar sub-satellite. Previous radio occultation observations have indicated the existence of an ionosphere with densities of up to 1000 cm(-3) above the dayside lunar surface. The measured densities are difficult to explain theoretically when the removal of plasma by the solar wind is considered, and thus the generation mechanism of the lunar ionosphere is a major issue, with even the validity of previous observations still under debate. The SELENE radio science experiment will establish the morphology of the lunar ionosphere and will reveal its relationship with various physical conditions to provide possible clues to the mechanism.

In the JAXA's long-term vision, "JAXA 2025" at last March, the importance of exploration and utilization of the Moon is clearly identified as one of the major goals for the next two decades. The assigned long-term goal of the Moon exploration is the sound technology development for the lunar base after 2025. In this presentation, we will introduce the summary of this JAXA vision related to the Moon exploration, describe the interim result of the voluntary 60 days study team for the concrete plan of this JAXA's Moon exploration vision, and present quick report of some technological examination. © 2006 Elsevier Ltd. All rights reserved.

In the JAXA's long term vision in March 2004, the exploration and utilization of the Moon for human lunar base is clearly identified as one of the JAXA's main goals for the next two decades. For this Moon goal, the ELR (Eternal Light Region) is going to be recognized as the major target for Japanese first Moon landing. In this paper, we will introduce the ELR, our expectations, topics of ELR exploration, and several PSR exploration concepts as the neighborhood of ELR.

In the JAXA's long term vision, "JAXA 2025" at last March, the importance of exploration and utilization of the Moon is clearly identified as one of the major goals for the next two decades. The assigned long term goal of the moon exploration is the sound technology development for the Lunar Base after 2025. In this presentation, we will introduce the summary of this JAXA vision related to the moon exploration, describe the interim result of the voluntary 60 days study team for the concrete plan of this JAXA's moon exploration vision, and present quick report of some technological examination.

The solar radiation pressure model for the sub-satellites RSAT and VSAT in the SELENE project is improved to correct the mean acceleration due to an evolving tip-off of the spin during the life time of satellites. The shape of the satellites is assumed to be a regular octagonal pillar. Solar radiation pressure force components acting on each surface element of the satellite are calculated independently and summed vectorially during a total period of Euler's free nutation of the satellite to obtain the mean acceleration of the satellite center of mass. The Doppler tracking data reduction process for the RSAT is simulated after incorporating the modified model into the orbit analysis software. Comparing with two other types of solar radiation pressure models, the standard cannonball model and a non tip-off model, it is found that when the tip-off becomes larger than 5°/s, the orbit determination result of using the modified model is better than the results of using other ones.

In the SELenological and ENgineering Explorer (SELENE) project which is the Japanese lunar program to be launched in 2003 by the Institute of Space and Astronautical Science (ISAS) and National Space Development Agency of Japan (NASDA), we measure angular distance between a radio transmitter on a relay satellite, that on the Moon and quasars by differential VLBI and determine amplitudes of the physical Librations, gravitational harmonic coefficients of the Moon and lunar ephemeris with an accuracy one or two orders higher than before in cooperation with 4-way Doppler measurements and two-way Doppler and ranging m(e)asurements using the lunar orbiter and the relay satellite. We are proposing another selenodetic mission, In situ Lunar Orientation Measurement (ILOM) to study lunar rotational dynamics by direct observations of the lunar physical libration from the lunar surface with an accuracy of 1 milliarcsecond in a post-SELENE project which will be launched about three years after SELENE. Year-long trajectories of the stars provide information on various components of the physical librations and we will also try to detect the lunar free librations in order to investigate the lunar mantle and the liquid core.