表紙は，地球誕生後からはじめの約6億年間における層状構造の進化のようすを示した模式図である．4.55 Gaにドライな地球が誕生したあと，4.53 Gaまでにマントルは固化し層状構造を形成した．この時はスタグナントリッドテクトニクスが機能していた．4.37-4.20 GaにABEL爆撃が起き，ドライな地球に大気・海洋成分が二次的に付加された．巨大隕石の衝突によって，スタグナントリッドは破壊され，プレートテクトニクスへと移行した．そして，構造侵食によって原初大陸はマントル深部へと運ばれ，密度のもっとも高いKREEP I玄武岩は4.0 Ga頃までにほぼすべてがコア-マントル境界へ崩落した．コア直上に蓄積したKREEP I玄武岩は放射性元素を多量に含むために自己発熱し，プルームを形成した（Maruyama et al., 2018）．

（丸山茂徳）

 “Tandem” planet formation, a new theory of planet formation, is described. A steady-state, 1-D model of the accretion disk of a protostar is obtained taking into account magneto-rotational instability (MRI) and porous aggregation of solid particles. The disk is found to be divided into an outer turbulent region, a MRI suppressed region, and an inner turbulent region. The outer turbulent region is fully turbulent because of MRI. However, MRI is suppressed around the midplane of the gas disk, and a quiet area without turbulence appears at r_out ( 8-60 AU) from the central star, because the degree of ionization of gas becomes sufficiently low. The disk becomes fully turbulent again at r_in ( 0.2-1 AU), because the midplane temperature becomes sufficiently high (> 1000 K). Planetesimals are formed through gravitational instability at both outer and inner MRI fronts. At the outer MRI front, icy particles grow through low-velocity collisions into porous aggregates with low densities (down to ≃ 10⁵ g cm⁻³). They eventually undergo gravitational instability to form icy planetesimals. Rocky particles, on the other hand, accumulate at the inner MRI front due to the local maximum in gas pressure. They undergo gravitational instability in a sub-disk of pebbles to form rocky planetesimals. They are likely to be volatile-free because of the high temperature (> 1000 K) at this formation site. This is consistent with a model in which the Earth was initially formed as a completely volatile-free planet. Water and other volatile elements came later through the accretion of icy particles with occasional scatterings in the outer regions after solidification of the planet surface. Our new proposed tandem planet formation regime shows that planetesimals are formed at two distinct sites. The former is likely to be the source of outer gas giants and the latter of inner rocky planets. The tandem regime also explains the gap in the distribution of solid components (2-4 AU), and therefore a relatively small Mars and a very small mass in the main asteroid belt. This tandem regime is found not to take place when the vertical magnetic field of the disk is five times weaker than that assumed, because the outer MRI front shifts outward beyond 100 AU. Such a “dispersed planetary formation” regime may explain high eccentricity planets, which are detected in exosolar planetary systems. On the other hand, when the ionization rate due to galactic cosmic-rays is 100 times larger than that of the present value, the outer MRI front shifts down to the inside of the water sublimation zone. Such a “single star formation” regime might explain super Earths or hot Jupiters, because almost all of the rock components in the disk transported to the inner MRI front contribute to planetary formation.

 Habitable Trinity is a new concept for a habitable environment proposed by Dohm and Maruyama (2015). This concept indicates that the coexistence of an atmosphere, an ocean, and a landmass, accompanied by a continuous circulation of material among these three components driven by the Sun, is one of the minimum requirements for life to emerge and evolve. Because a life body consists of carbon (mainly from the atmosphere), oxygen (mainly from an ocean), hydrogen (mainly from an ocean), nitrogen (mainly from the atmosphere), and various nutrients (supplied from a landmass), the presence of water alone is not a sufficient condition. The Habitable Trinity concept can also be applied to other planets such as Mars, Europa, and Titan, and even exoplanets, as a useful index in the quest for life-containing planetary bodies.

 Seismic tomography reveals significant lateral heterogeneities in the lunar interior. A correlation is found between S-wave velocity tomography and distribution of thorium. The area with a high distribution of thorium exhibits a distinct low S-velocity, which extends to a depth of ∼ 300 km below the Procellarum KREEP Terrane (PKT), perhaps reflecting a thermal and compositional anomaly beneath the PKT. The distribution of deep moonquakes shows a correlation with tomography in the deep lunar mantle, which is similar to earthquakes affected by structural heterogeneities in the terrestrial crust and upper mantle. The occurrence of deep moonquakes and seismic-velocity heterogeneities implies that the lunar interior may contain fluids; therefore, it is still thermally and dynamically active at present. Because there are no plate tectonics in the Moon, the lunar surface and interior structure formed at an early stage of the Moon's history have been preserved until today. Consequently, the results of lunar tomography provide useful information for our understanding of the Hadean Earth.

 The Earth's core is thought to composed of Fe–Ni alloys with large amounts of light elements. The composition of the present Earth's outer core reflects various processes, such as core formation, inner core growth, and core-mantle chemical interactions. Although oxygen, silicon, carbon, nitrogen, sulfur, and hydrogen have been proposed as candidates for light components (Stevenson, 1981), little is known yet about amounts and species. This is partly because experimental determination of the physical properties of liquid states is still not practical at the outer core pressure and temperature due to technical limitations. However, the ab initio density functional computation method is quite powerful for investigating liquid properties under such extreme conditions. The thermodynamic properties of liquid iron alloys may provide fundamental data for thermochemical modeling of the Earth's core. Presented are comprehensive discussions covering density jump at the inner-outer core boundary, phase relations of iron alloys, geochemical constraints on the bulk composition of the Earth, heterogeneities of P-wave velocity in the outer core, and partitioning of elements during core formation processes, along with density and P-wave velocity of pure Fe and Fe-light elements alloy liquids by means of an ab initio molecular dynamics simulation at whole outer core P, T conditions.

 In the recently proposed ABEL model, the Earth was born as a dry planet without atmosphere and ocean components at 4.56 Ga. From 4.37 to 4.20 Ga, bio-elements, such as carbon (C), hydrogen (H), oxygen (O), and nitrogen (N), were secondarily accreted on the Earth. This two-step formation model of the Earth is referred to as the advent of the bio-elements model (ABEL Model) and the event of the advent of bio-elements (water component) is referred to as ABEL Bombardment. It is evident that the solid Earth originated from enstatite chondrite-like dry material because of its oxygen isotopic composition and similar isotopes. On the other hand, the Earth's water originated primarily from carbonaceous chondrite material based on the hydrogen isotopic ratio. The ABEL model successfully explains this enigma between solid Earth and water. In addition, the importance of the secondary accretion of oxidizing bio-elements should be emphasized because this event became a precursor to initiating the metabolism for life to emerge on a highly reductive planet. If ABEL Bombardment had not occurred, life would never have emerged on the Earth. Therefore, ABEL Bombardment was one of the most important events for this planet to evolve into a habitable planet. Moreover, ABEL Bombardment is thought to be the trigger to switch from stagnant lid tectonics to plate tectonics on this planet because of the injection of volatiles into the initial dry Earth.

 The Earth is a unique planet, on which plate tectonics has operated to form highly evolved and diversified surface environments throughout geologic time. The occurrence of 4.03 Ga orthogneiss in the Acasta Gneiss Complex suggests that plate tectonics goes back to the Eoarchean, but it is still debated when plate tectonics began because of a lack of geologic evidence on the early Earth. Supracrustal rocks with Eoarchean ages provide key evidence to solve the debate on tectonics in the early Earth, but few supracrustal rocks have been discovered. Therefore, geological and geochronological studies were conducted on supracrustal rocks in the Saglek Block, northern Labrador to find key evidence related to the operation of plate tectonics. The Saglek Block is underlain by orthogneiss and supracrustal rocks, which are subdivided into two groups based on the geologic relationship with the Mesoarchean Saglek dyke. The pre-Saglek dyke suite comprises Uivak Gneiss and Nulliak supracrustals, whereas the post-Saglek suite is composed of Lister Gneiss and Upernavik supracrustals, respectively. Our geological mapping of Nulliak supracrustal belts in the Saglek Block shows that the belts contain ultramafic rock, mafic volcanic rock, chemical sedimentary rocks of banded iron formation, chert and carbonate rock, and clastic sedimentary rocks of pelitic rock and conglomerate. In particular, the belt in the St. John's Harbour South area comprises piles of fault-bounded subunits of ultramafic rock, mafic rock, and sedimentary rock in ascending order, namely the ophiolite sequence. In addition, small-scale duplex structures are found over the area. Their similarity to lithostratigraphy and geological structures of modern accretionary complexes suggests that the Nulliak supracrustal belt is one of the Eoarchean accretionary complexes. Detailed sketch maps are also made of outcrops in order to observe cross-cutting relationships among supracrustal rocks and orthogneisses. One of them comprises six generations of mafic and felsic magmatic components; thus U–Pb dating of zircons was conducted from the third and sixth generations and ages of 3.95 and 3.87 Ga were obtained, respectively. The former provides the minimum age of the supracrustal rocks. So far, the oldest supracrustal rock is an 3.83 Ga Akilia association in southern West Greenland; thus, the Nulliak supracrustal rocks are the oldest supracrustal rocks in the world. The presence of accretionary complex, ophiolite, and granitoid provides the oldest evidence for plate tectonics on the early Earth.

 The extensive occurrence of a felsic continental crust is one of the unique features of the Earth. The growth history of the continental crust has been a key issue in understanding the origin and evolution of the Earth. In particular, recent geological studies indicate that subduction of the continental crust into the mantle has been greater than previously imagined. The current understanding of the growth of continents and the differentiation of the crust and the mantle of the Earth is reviewed based on a detrital zircon geochronology. One of the most important achievements arises from the analysis of the age structure of individual continents and secular changes over time. The new detrital zircon geochronology suggests that the sizes of the continents have changed over time, which has been an important factor in the growth of the continents. Large continents, such as the modern examples, can preserve older crusts in their interiors, which are separated from active continental margins. Conversely, in the early Earth, continents were probably formed by the amalgamation of small fragments of crust, such as oceanic island arcs. It is speculated that the smallness of the continents was the most significant cause of the poor preservation of Hadean and Archean crusts, despite putative expected active crustal production. Consequently, the recycling of the continental crust occurred in great magnitudes during the early Earth's history. The large-scale subduction of felsic crust represents one of the most important aspects in studies of the early Earth.

 Zircon is the only candidate of Hadean terrestrial material that can be collected because it can survive physically and chemically, as well as maintain its U–Pb age system during omnigenous geological events. However, Hadean zircons are rare, so many age analyses of zircons are required to isolate Hadean zircon grains. There are two ways to improve the analyses; decrease the time required for pre-analyzing processes and decrease the time required for age analysis. New equipment, high-voltage pulse power fragmentation device and automatic zircon pick-up system are effective for crushing rock sample and separating zircon grains, respectively, in a short time. Another potential age analysis, the ²⁰⁷Pb/²⁰⁶Pb age screening analysis, shows good performance in selecting old zircons quickly. Integrative operation of equipment and methods will result in successfully finding numerous Hadean zircons.