Icy planetesimals are thought to contribute to the volatile inventory of terrestrial planets and serve as building blocks of icy bodies in the outer Solar System. Samples from the C-type asteroid Ryugu, collected by the Hayabusa-2 spacecraft, indicate a low-temperature history with aqueous alteration and organic materials. In contrast, iron meteorites with isotopic ratios similar to those of carbonaceous chondrites suggest exposure to higher temperatures. These findings imply that the thermal evolution of icy planetesimals is highly diverse. Since direct exploration provides only localized data, understanding this diversity requires comparing observational results with model calculations that incorporate key evolutionary processes. We develop a model, including radial growth, impact heating, water phase changes, aqueous alteration, and structural differentiation, to re-evaluate the thermal evolution of icy planetesimals during the first 100 Myr after the formation of calcium–aluminum-rich inclusions (CAIs). The model considers final radius (10–1000 km), timing of growth onset (1.0 or 2.0 Myr after CAI), growth duration (0.4 or 4.0 Myr), and growth mode (linear or runaway). Our results show that larger planetesimals generally reach higher temperatures, but growth timing and mode significantly affect thermal evolution. Early accretion leads to higher temperatures, with some bodies reaching the Fe–FeS eutectic (1250 K), while delayed or prolonged growth reduces heating. Our results show that the constituent materials of Ryugu, which kept below 40 ^∘ C, likely formed near the surface of a hydrated mineral layer. This is possible even in planetesimals several hundred kilometers in size due to efficient heat transport via convection. If accretion begins 2.0 Myr after CAI and completes in 0.4 Myr, a wide region in such a body could yield Ryugu’s material. Evolution into a 250 km body with a 170–200 km hydrous core and overlying liquid water layer may resemble Saturnian icy moon Enceladus. For a later onset and longer duration of growth, even aqueous alteration could be prevented. In contrast, metal melting in the deep regions of rapidly formed icy planetesimals larger than 200 km could originate iron meteorites with isotopic signatures similar to those of carbonaceous chondrites.

In the outer solar system beyond Jupiter, water ice is a dominant component of planetary bodies, and most solid objects in this region are classified as icy bodies. Icy bodies display a remarkable diversity of geological, geophysical, and atmospheric processes, which differ fundamentally from those of the rocky terrestrial planets. Evidence from past and ongoing spacecraft missions has revealed subsurface oceans, cryovolcanic activity, and tenuous but persistent atmospheres, showing that icy bodies are active and evolving worlds. At the same time, major questions remain unresolved, including the chemical properties of icy materials, the geological histories of their surfaces, and the coupling between internal evolution and orbital dynamics. Current knowledge of the surfaces, interiors, and atmospheres of the principal icy bodies is built on spacecraft measurements, telescopic observations, laboratory experiments, and theoretical modeling. Recent contributions from Juno, JWST, and stellar occultation studies have added valuable constraints on atmospheric composition, interior structure, and surface activity. Looking ahead, missions such as JUICE, Europa Clipper, Dragonfly, and the Uranus Orbiter and Probe are expected to deliver substantial progress in the study of icy bodies. Their findings, combined with continued Earth- and space-based observations and laboratory studies, will be critical for assessing the potential habitability of these environments and for placing them within a broader framework of planetary system formation and evolution.

We present visible wavelength observations of Saturn’s rings conducted using 1.6 m ground-based telescope. The goal is to assess long-term compositional changes in the rings by comparing spectral characteristics with data acquired by the Cassini spacecraft. Reflectance spectra from 450 to 900 nm were obtained during four nights in 2021 and 2022. Spectral slopes, sensitive to water ice purity, were derived for two wavelength ranges: 450–550 and 550–900 nm. Our measured slopes agree well with values previously reported from Cassini’s VIMS data, suggesting minimal compositional change over the past decade. In addition, no clear correlation is found between spectral slope and the orbital position of Enceladus. These results support the reliability of visible-wavelength ground-based monitoring as a complementary tool to past spacecraft observations.

Europa’s tenuous atmosphere mainly consists of molecular oxygen, with traces of sodium, potassium, and other elements, potentially reflecting the composition of its surface and interior. While sodium- and magnesium-bearing hydrated salts have been suspected to exist on the surface, other materials like calcium-bearing salts dissolved in the subsurface ocean could also be present. Atmospheric emissions could originate from sputtering or sublimation processes, and potential water plume activity, possibly influenced by tidal interactions, could significantly contribute to the atmosphere. Additionally, the composition of the atmosphere and surface could be affected by neighboring moons, particularly Io. We searched for calcium emissions using the Multi-Spectral Imager of a 1.6 m Pirka telescope at Hokkaido University’s Astronomical Observatory. Observations from 2021 September to 2023 October revealed suggestive emission lines attributed to calcium atoms. These suggestive emission lines originated from the trailing hemisphere, suggesting that sputtering is likely the dominant atmospheric source.

Icy planetesimals are likely to supply volatiles to terrestrial planets and serve as building blocks of icy bodies in the outer Solar System. Samples from the C-type asteroid Ryugu, collected by the Hayabusa-2 spacecraft, indicate a low-temperature history with aqueous alteration and organic materials. In contrast, iron meteorites with isotopic ratios similar to carbonaceous chondrites suggest exposure to higher temperatures. These findings imply that the thermal evolution of icy planetesimals is highly diverse. Since direct exploration provides only localized data, understanding this diversity requires comparing observational results with model calculations incorporating key evolutionary processes. We develop a model including radial growth, impact heating, water phase changes, aqueous alteration, and structural differentiation, to re-evaluate the thermal evolution of icy planetesimals during the first 100 Myr after CAI formation. The model considers final radius (10-1000 km), growth onset (1.0 or 2.0 Myr after CAI), growth duration (0.4 or 4.0 Myr), and growth mode (linear or runaway). Our results show that larger planetesimals generally reach higher temperatures, but growth timing and mode significantly affect thermal evolution. Early accretion leads to higher temperatures, with some bodies reaching the Fe-FeS eutectic (1250 K), while delayed or prolonged growth reduces heating. Our results show that the constituent materials of Ryugu, which kept below 40 degC, likely formed near the surface of a hydrated mineral layer. This is possible even in planetesimals several hundred kilometers in size due to efficient heat transport via convection. If accretion begins 2.0 Myr after CAI and completes in 0.4 Myr, a wide region in such a body could yield Ryugu's material.

The Ganymede Laser Altimeter (GALA) on the Jupiter Icy Moons Explorer (JUICE) mission, is in charge of a comprehensive geodetic mapping of Europa, Ganymede, and Callisto on the basis of Laser range measurements. While multiple topographic profiles will be obtained for Europa and Callisto during flybys, GALA will provide a high-resolution global shape model of Ganymede while in orbit around this moon based on at least 600 million range measurements from altitudes of 500 km and 200 km above the surface. By measuring the diurnal tidal deformation of Ganymede, which crucially depends on the decoupling of the outer ice shell from the deeper interior by a liquid water ocean, GALA will obtain evidence for (or against) a subsurface ocean on Ganymede and will provide constraints on the ice shell thickness above the ocean. In combination with other instruments, it will characterize the morphology of surface units on Ganymede, Europa, and Callisto providing not only topography but also measurements of surface roughness on the scale of the laser footprint, i.e. at a scale of about 50 m from 500 km altitude, and albedo values at the laser wavelength of 1064 nm. GALA is a single-beam laser altimeter, operating at a nominal frequency of 30 Hz, with a capability of reaching up to 48 Hz. It uses a Nd:YAG laser to generate pulses with pulse lengths of 5.5 ± 2.5 ns. The return pulse is detected by an Avalanche Photo Diode (APD) with 100 MHz bandwidth and the signal is digitized at a sampling rate of 200 MHz providing range measurements with a sub-sample resolution of 0.1 m. Research institutes and industrial partners from Germany, Japan, Switzerland and Spain collaborated to build the instrument. JUICE, conducted under responsibility of the European Space Agency (ESA), was successfully launched in April 2023 and is scheduled for arrival at the Jupiter system in July 2031. The nominal science mission including multiple close flybys at Europa, Ganymede, and Callisto, as well as the final Ganymede orbit phase will last from 2031 to 2035. In May 2023 GALA has completed its Near-Earth Commissioning, showing full functionality of all units. Here we summarize the scientific objectives, instrument design and implementation, performance, and operational aspects of GALA.

The JUpiter ICy moons Explorer (JUICE) of ESA was launched on 14 April 2023 and will arrive at Jupiter and its moons in July 2031. In this review article, we describe how JUICE will investigate the interior of the three icy Galilean moons, Ganymede, Callisto and Europa, during its Jupiter orbital tour and the final orbital phase around Ganymede. Detailed geophysical observations about the interior of the moons can only be performed from close distances to the moons, and best estimates of signatures of the interior, such as an induced magnetic field, tides and rotation variations, and radar reflections, will be obtained during flybys of the moons with altitudes of about 1000 km or less and during the Ganymede orbital phase at an average altitude of 490 km. The 9-month long orbital phase around Ganymede, the first of its kind around another moon than our Moon, will allow an unprecedented and detailed insight into the moon's interior, from the central regions where a magnetic field is generated to the internal ocean and outer ice shell. Multiple flybys of Callisto will clarify the differences in evolution compared to Ganymede and will provide key constraints on the origin and evolution of the Jupiter system. JUICE will visit Europa only during two close flybys and the geophysical investigations will focus on selected areas of the ice shell. A prime goal of JUICE is the characterisation of the ice shell and ocean of the Galilean moons, and we here specifically emphasise the synergistic aspects of the different geophysical investigations, showing how different instruments will work together to probe the hydrosphere. We also describe how synergies between JUICE instruments will contribute to the assessment of the deep interior of the moons, their internal differentiation, dynamics and evolution. In situ measurements and remote sensing observations will support the geophysical instruments to achieve these goals, but will also, together with subsurface radar sounding, provide information about tectonics, potential plumes, and the composition of the surface, which will help understanding the composition of the interior, the structure of the ice shell, and exchange processes between ocean, ice and surface. Accurate tracking of the JUICE spacecraft all along the mission will strongly improve our knowledge of the changing orbital motions of the moons and will provide additional insight into the dissipative processes in the Jupiter system. Finally, we present an overview of how the geophysical investigations will be performed and describe the operational synergies and challenges.

Europa's interior is expected to be divided into the metallic core, rocky mantle and hydrosphere based on the moment of inertia factor estimated from gravity field measurements. Specifically, the thickness of the outermost water layer is 120-170 km, and the radius of the metallic core is 0.12-0.43 times the surface radius. No systematic study of Europa's internal evolution has been conducted to estimate the current state of the subsurface ocean and to explain the absence of a core dynamo field within such uncertainty for internal structure and material properties. Herein, I performed a numerical simulation of the long-term thermal evolution of Europa's interior and investigated the temporal changes in the ocean thickness as well as the temperature and heat flow of the metallic core. If the ice reference viscosity is greater than 5$\times$10$^{14}$ Pa s, the ocean can persist even in the absence of tidal heating. In the case of a tidal heating of 10 and 20 mW/m$^{2}$, the ice shell thickness is $\le$90 km if the ice viscosity is $\ge$1$\times$10$^{15}$ and 1$\times$10$^{14}$ Pa s, respectively. Regardless of the ice viscosity, if the tidal heating is $\ge$50 mW/m$^{2}$, the shell thickness will be $\le$40 km. The thermal history of the metallic core is determined by the hydrosphere thickness and the metallic core density, and is unaffected by variations in the ice shell (ocean) thickness. Preferred conditions for the absence of the core dynamo include CI chondritic abundance for the long-lived radioactive isotopes, lower initial core-mantle boundary (CMB) temperature and thicker hydrosphere. The core may be molten without convection if the composition is near the eutectic in a Fe-FeS alloy, or not molten (without convection) if the composition is near the Fe or FeS endmember.

Ultraviolet (UV) spectroscopy is one of the most powerful tools used in a wide range of scientific fields from planetary science to astronomy. We propose a future UV space telescope, LAPYUTA (Life-environmentology, Astronomy, and PlanetarY Ultraviolet Telescope Assembly), selected as a candidate for JAXA's 6th M-class mission in 2023. Launch is planned for the early 2030s. LAPYUTA will accomplish the following four objectives related to two scientific goals: understanding (1) the habitable environment and (2) the origin of structure and matter in the universe. Objective 1 focuses on the subsurface ocean environments of Jupiter's icy moons and the atmospheric evolution of terrestrial planets. Objective 2 characterizes the atmosphere of the exoplanets around the habitable zone and estimates their surface environment by detecting their exospheric atmosphere. In cosmology and astronomy, Objective 3 tests whether the structures of presentday galaxies contain ubiquitous Ly-alpha halos and reveals the physical origins of Ly-alpha halos. Objective 4 elucidates the synthesis process of heavy elements based on observations of ultraviolet radiation from hot gas immediately after neutronstar mergers. LAPYUTA will perform spectroscopic and imaging observations in the far-UV range of 110-190 nm with an effective area of >300 cm(2) and a high spatial resolution of 0.1 arcsec. The apogee is 2,000 km, and the perigee is 1,000 km to avoid the influence of the geocorona when observing oxygen and hydrogen atoms and the Earth's radiation belt.

The Ganymede Laser Altimeter (GALA) on board the Jupiter Icy Moons Explorer (JUICE) is currently on its way to its targets, the Galilean Moons. Following the launch of the mission in April 14th 2023 the instruments on board have been checked for functionality and performance. While these regular checkouts were performed in cruise, the upcoming flyby at Earth’s Moon will give a unique opportunity to receive ground returns, assess ranging performance, and calibrate the instrument.GALA is an active instrument emitting short laser pulses (about 5 ns) of infrared radiation (at 1064 nm) to its target. Nominally, GALA emits 30 shots per second with a pulse energy of 17 mJ and a pulse divergence of 100 µrad (full cone). The receiver collects a small fraction of the reflected laser light and the round-trip travel time of the pulse is measured by the instrument electronics. In contrast to previous planetary laser altimeters, GALA features a high-frequency (200 MHz) temporal sampling of the return pulse. This enhances significantly the precision of range measurements and allows a reliable estimate of the surface roughness and albedo at the footprint scale. Performance estimates based on dark-noise measurements in cruise checkouts and models of surface properties suggest a maximal ranging distance of 1400 km for Ganymede, 1600 km for Europa, and 1100 km for Callisto. At these distances the signal-to-noise ratio for a large fraction of possible return pulse widths is larger than 1, which comprises the detection limit for GALA (see Figure 1).Current performance estimates of GALA will come to a powerful test at the lunar flyby, where measurement conditions are challenging: (1) the range to the lunar surface is above 800 km; (2) observation geometries are at an oblique angle due to the fixed inertial pointing of JUICE and (3) the albedo of the lunar surface, in particular the mare areas, is lower than on the icy satellites. Despite these challenges, current modelling suggests that GALA will be able to obtain a topographic profile of the lunar surface, which will be used to calibrate, in particular, GALA’s albedo measurement and the orientation of the transmitter boresight vector. The latter will be also determined by a cross-calibration to the JANUS camera on JUICE using data from the night side. For that purpose, JANUS will take long-exposure images during GALA operation. With that procedure it is expected to precisely locate GALA’s footprint in the detector of JANUS and thus to precisely constrain the relative orientation of the boresight vectors of the two instruments.Figure 1: Signal-to-noise ratio (SNR) for GALA at Europa (top), Ganymede (middle) and Callisto (bottom). The black dashed line shows SNR values of 10 and the dashed white line an SNR of 1.

We present near-infrared high-dispersion spectroscopic observations of Europa using the Infrared Camera and Spectrograph (IRCS) onboard the Subaru Telescope, seeking direct evidence of water plumes on Europa and exploring spatial variations in plume activity. Using the high spectral/spatial resolution and sensitivity of Subaru/IRCS, our observations have enabled a spatially resolved search for water plumes on Europa. Within our detection limits and time of observation, we found no evidence for the presence of water emission. For a rotation temperature of 50 K, we derived an upper limit on the H2O abundance of 9.46 x 10(19)-5.92 x 10(20) m(-2) in each divided slit area and 4.61 x 10(19) m(-2) in the entire area covered by the slit. This upper limit lies below the inferred water abundance from previous UV observations by the Hubble Space Telescope, while being less sensitive by a factor of three compared to the Keck telescope and by one order of magnitude or more than the James Webb Space Telescope observations. Our results align with previous studies and demonstrate that using Subaru/IRCS is an effective strategy for searching for water plumes on Europa with high spatial resolution. Continued observations across different surface areas and orbital phases are essential to fully characterize Europa's plume activity and complement upcoming space missions.

The Wispy Terrain is the region of chasmata characterized by quasi-parallel fault systems, formed by extensional and shear stresses of the icy crust of Dione, a moon of Saturn. Besides the basic, satellite-scale geological mapping and very general definition of the phenomenon, only a few studies focus on the Wispy Terrain and its chasmata from the angle of detailed tectonic reconstruction, with others mainly targeting, e.g., the timing of its formation. This study provides a detailed geological and cryotectonic analysis in the surroundings of the Eurotas and Palatine Chasmata and proposes additional, until now, unidentified tectonic processes and a formation model. The relationship between fragmentary impact craters and tectonic features indicates other newly suspected tectonic movements, namely thrust, and splay and décollement fault systems. In contrast to the commonly expected and identified dilatational processes, such fault types show compression and are characteristic of subduction in a terrestrial environment. Theoretically, the appearance of such tectonic processes means that the already-known rift and the newly discovered subsumption (subduction-like) processes may appear together in the Wispy Terrain. The appearance of both features may suggest the presence of some of the components (phases) of a Wilson cycle analog cryotectonic cycle (or possibly cycles) in icy planetary bodies like Dione.

The first images of Jupiter's moon Europa from the Voyager missions sparked the curiosity about the lineament system appearing at the surface. Curiosity quickly turned into profound interest, following the discovery of its subsurface ocean, which may harbour life below the thick ice crust. This study revisits Europa and reinvestigates its surface using high-resolution Galileo data and mapping one of its most characteristic surface patterns: the complex network of lineaments. The analysis of the morphological type of over two hundred lineaments and their crosscutting relationship based on relative age indicate the influence of cyclical tidal force, orbital forcing, and the nonsynchronous rotation triggered periodic stress in the formation of three characteristic lineaments-generations, along with additional and unaccounted forces that may contribute to surface renewal. Despite ongoing debate, which suggests that Earth-like tectonism (e.g., subduction) is less likely plausible on Europa, the result of this study calls for some process acting along with tidal forces, causing shortening, horizontal movement and increasing extension in the ice plate located in the region of a suspected subduction (or low-relief subsumption) zone and may be part of the “subducting” crust. This study raises new questions and encourages additional scientific discussions, which, along with the Europa Clipper and JUICE missions, will help to understand the nature of Europa's ocean and ice-tectonic processes.

<p>The Ganymede laser-altimeter (GALA) is one of 10 instruments on ESA’s Jupiter Icy Moons Explorer (JUICE) mission. The scientific goals cover a wide range  from geology, geophysics to geodesy of the icy moons Ganymede, Europa and Callisto. JUICE will explore Jupiter, its magnetosphere and satellites first in orbit around Jupiter before going finally into polar orbit around Ganymede.  GALA is developed under responsibility of the DLR Institute of Planetary Research in collaboration with industry and institutes from Germany, Japan, Switzerland and Spain. GALA has two main objectives: (1) providing Ganymede’s topography from global to local scales (2) determination of Ganymede's tidal variations of surface elevations. GALA is a single-beam laseraltimeter: a laser pulse (1064 nm) is emitted by using a Nd:YAG laser firing at 30 Hz (nominal). After about 3 msec (500 km altitude) the reflection of the pulse from the surface of Ganymede is received by a telescope and transferred to the detector (Avalanche Photo Diode). The signal is digitized and transferred to the range finder module, which determines (a) time of flight (b) pulse shape, and (c) energy of the received pulse. Including information on the spacecraft position and attitude the height of the terrain above a reference surface is determined for each shot from time-of-flight measurements. The GALA flight model was delivered to ESA in August 2021. After several tests on instrument level the integration on the JUICE spacecraft started in September 2021 and first tests were performed successfully in October 2021. With the launch scheduled for 2023, GALA will go through several tests, among them an end-to-end test including laser-receiver measurements. Here we present the instrument's current status with respect hardware integration and regarding the verification of its performance.</p>

The Jupiter Icy Moons Explorer (JUICE) is a science mission led by the European Space Agency, being developed for launch in 2023. The Ganymede Laser Altimeter (GALA) is an instrument onboard JUICE, whose main scientific goals are to understand ice tectonics based on topographic data, the subsurface structure by measuring tidal response, and small-scale roughness and albedo of the surface. In addition, from the perspective of astrobiology, it is imperative to study the subsurface ocean scientifically. The development of GALA has proceeded through an international collaboration between Germany (the lead), Japan, Switzerland, and Spain. Within this framework, the Japanese team (GALA-J) is responsible for developing three receiver modules: the Backend Optics (BEO), the Focal Plane Assembly (FPA), and the Analog Electronics Module (AEM). Like the German team, GALA-J also developed software to simulate the performance of the entire GALA system (performance model). In July 2020, the Proto-Flight Models of BEO, FPA, and AEM were delivered from Japan to Germany. This paper presents an overview of JUICE/GALA and its scientific objectives and describes the instrumentation, mainly focusing on Japan's contribution.

The difference between the inactive surface of Mimas and the active surface of Enceladus is puzzling. We investigate the conditions under which both have a thick subsurface ocean and the thermal lithosphere of Mimas is thicker than that of Enceladus by using a one-dimensional simulation of thermal evolution. We adopt the initial core temperature, initial methane concentration, and tidal heating rate as free parameters in the calculation. The initial methane concentration and tidal heating rate greatly affect the current ocean thickness, although the initial core temperature does not affect the thickness. Methane hydrate forms at the base of the icy shell if the initial methane concentration is not 0. The methane hydrate layer plays an insulative role in an icy shell. When the initial methane concentration is 1000 , ∼2 GW is needed to achieve more than 50 km of the subsurface ocean on Mimas and ∼7.5 GW is needed to achieve more than 25 km of the subsurface ocean on Enceladus. These values are smaller than those needed when the initial methane concentration is 0 . The existence of the methane hydrate layer promotes the survival of the subsurface ocean because it insulates internal heat. In addition, it is found that the surface heat flux is depressed if the methane hydrate layer exists, which is consistent with the unrelaxed craters in Mimas. Methane hydrate may explain the thick oceans in Mimas and Enceladus and the inactive shell of Mimas.

The difference between the inactive surface of Mimas and the active surface of Enceladus is puzzling. We investigate the conditions under which the both have a thick subsurface ocean and the thermal lithosphere of Mimas is thicker than that of Enceladus by using a one-dimensional simulation of thermal evolution. We adopt the initial core temperature, initial methane concentration, and tidal heating rate as free parameters in the calculation. The initial methane concentration and tidal heating rate greatly affect the current ocean thickness, although the initial core temperature does not affect the thickness. Methane hydrate forms in a subsurface ocean if the initial methane concentration is not 0. The methane hydrate layer plays an insulative role in an icy shell. When the initial methane concentration is 1000 \(\text{m}\text{o}\text{l}\hspace{0.17em}{\text{m}}^{-3}\), ∼3 GW is needed to achieve more than 50 km of the subsurface ocean on Mimas and ∼10 GW is needed to achieve more than 25 km of the subsurface ocean on Enceladus. These values are smaller than those needed for when the initial methane concentration is 0 \(\text{m}\text{o}\text{l}\hspace{0.17em}{\text{m}}^{-3}\). The existence of the methane hydrate layer promotes the survival of the subsurface ocean because it insulates internal heat. In addition, it is found that the surface heat flux is depressed if the methane hydrate layer exists, which is consistent with the unrelaxed craters in Mimas. Methane hydrate may explain the thick oceans in Mimas and Enceladus and the inactive shell of Mimas.

A global classification map of lunar absorption spectra is developed under unified classification criteria by applying the K-means and ISODATA unsupervised classification methods. The spectra were obtained by the Spectral Profiler (SP) onboard the Kaguya (SELENE) lunar orbiter spacecraft. The spectra are mostly categorized into seven spectral groups by the K-means method: K1, K2, K3, K4, K5, K6, and K7. Then, each K-means group is divided into 66 total classes by the ISODATA method. The K1 and K3 groups are classified into 21 and 14 classes, respectively, for which the average spectra are high-calcium pyroxene (HCP)-abundant compared to the other groups. Both groups are mainly located on maria and pyroclastic deposits. However, while the K1 group can be found primarily near the center of large maria, the K3 group can be found mainly at small maria and the margins of large maria. The K2 group contains eight classes and shows average spectra, such as low-calcium pyroxene (LCP), in abundance, which are found mainly on the South Pole-Aitken (SPA) basin and on the coast of Mare Frigoris. The K5 (eight classes), K6 (two classes) and K7 (three classes) groups indicate average spectra with shallower absorption (between 0.9 mu m and 1.2 mu m wavelength) than those of the K1, K2, and K3 groups. These groups are located in highland regions, and the K5 group is located in high-albedo regions, such as fresh craters. The K6 group is found on nearside highland areas and the K7 group is found on farside highland areas. The K4 group with nine classes shows average spectra, such as mixtures of K1/K2/K3 and K5/K6/K7, and these classes are located around boundaries between mare/SPA and highland. In addition, one class is independently defined without using K-means and ISODATA because its spectra are unsuitable for automatic unsupervised classification. However, this class is still geologically meaningful. The average spectra of the ISODATA classes show not only the characteristics of the major host material belonging to a spectral group, but they also show the small influence of materials from other groups in nearby regions on that host material. This influence is considered to be caused by horizontal material mixing of materials in that location with the ejecta from nearby regions. However, occasionally the ISODATA classes of the K4, K5, and K6 groups influenced by the Kl/K3 groups are clustered geographically in regions far from the K1/K3 regions. Some locations of the clusters correspond to locations of the well-known cryptomaria and small pyroclastic deposits. On the other hand, some locations of them also contain candidates of hidden subterranean basaltic materials, such as undiscovered cryptomare, dike, and sill estimated based on recent lunar gravitational anomalies. This agreement suggests to be appeared hidden basaltic materials by the material mixing occurring vertically. The total area of the clusters except for known cryptomaria is approximately two times larger than that of the known cryptomaria, and this area reaches approximately 20% of the total area of exposed maria. The total volume of hidden basaltic materials, except for the known cryptomaria, is estimated to be 10(4) to 10(6) km(3), depending on assumptions of the relative abundance between cryptomare and dike/sill. This volume is comparable to that of the known cryptomaria and approximately 10% of that of the exposed mare.