The earliest history of the Moon is poorly preserved in the surface geologic record due to the high flux of impactors, but aspects of that history may be preserved in subsurface structures. Application of gravity gradiometry to observations by the Gravity Recovery and Interior Laboratory (GRAIL) mission results in the identification of a population of linear gravity anomalies with lengths of hundreds of kilometers. Inversion of the gravity anomalies indicates elongated positive-density anomalies that are interpreted to be ancient vertical tabular intrusions or dikes formed by magmatism in combination with extension of the lithosphere. Crosscutting relationships support a pre-Nectarian to Nectarian age, preceding the end of the heavy bombardment of the Moon. The distribution, orientation, and dimensions of the intrusions indicate a globally isotropic extensional stress state arising from an increase in the Moon's radius by 0.6 to 4.9 kilometers early in lunar history, consistent with predictions of thermal models.

Spacecraft-to-spacecraft tracking observations from the Gravity Recovery and Interior Laboratory (GRAIL) have been used to construct a gravitational field of the Moon to spherical harmonic degree and order 420. The GRAIL field reveals features not previously resolved, including tectonic structures, volcanic landforms, basin rings, crater central peaks, and numerous simple craters. From degrees 80 through 300, over 98% of the gravitational signature is associated with topography, a result that reflects the preservation of crater relief in highly fractured crust. The remaining 2% represents fine details of subsurface structure not previously resolved. GRAIL elucidates the role of impact bombardment in homogenizing the distribution of shallow density anomalies on terrestrial planetary bodies.

The pressure-induced phase transformations of a form of amorphous silicon (a-Si) with well characterized impurity levels and structure are examined at pressures up to 40 GPa using in situ synchrotron X-ray radiation. At similar to 12 GPa crystallization commences, but it is not completed until similar to 16 GPa. At higher pressures, not all the crystalline phases observed for crystalline silicon (c-Si) appear. On pressure release, none of the metastable crystalline phases observed for c-Si nucleate. Instead an amorphous phase is re-formed. This is in contrast to all previous diamond-anvil studies on a-Si. If full pressure-induced crystallization occurred, the material remained crystalline on unloading. The formation of a-Si upon unloading was only observed when a high-density amorphous phase was reported on loading. The fully characterized nature of the a-Si used in this current study allows for the interpretation of this significant diversity in terms of impurity content of the a-Si used. Namely, this suggests that 'ideal' (pure, voidless, structurally relaxed) a-Si will follow the same transition pathway as observed for c-Si, while crystallization of a-Si forms with a high impurity content is retarded or even inhibited. The a-Si used here straddles both regimes and thus, although full crystallization occurs, the more complex crystalline structures fail to nucleate.

The nucleation of metastable germanium polymorphs on decompression is studied using in situ synchrotron x-ray diffraction. We show that the transition pathway is critically dependent on the hydrostaticity. Quasihydrostatic conditions result in the nucleation of the rhombohedral r8 phase, followed by the cubic bc8 and hexagonal diamond phases. In contrast, the presence of shear yields the tetragonal st12 phase. Thus, targeted nucleation of a metastable polymorph is now possible. This observation has implications for the technological exploitation of Ge, but also for other tetrahedral systems.

The Gravity Recovery and Interior Laboratory (GRAIL) mission has sampled lunar gravity with unprecedented accuracy and resolution. The lunar GM, the product of the gravitational constant G and the mass M, is very well determined. However, uncertainties in the mass and mean density, 3345.56 +/- 0.40 kg/m(3), are limited by the accuracy of G. Values of the spherical harmonic degree-2 gravity coefficients J(2) and C-22, as well as the Love number k(2) describing lunar degree-2 elastic response to tidal forces, come from two independent analyses of the 3 month GRAIL Primary Mission data at the Jet Propulsion Laboratory and the Goddard Space Flight Center. The two k(2) determinations, with uncertainties of similar to 1%, differ by 1%; the average value is 0.02416 +/- 0.00022 at a 1 month period with reference radius R = 1738 km. Lunar laser ranging (LLR) data analysis determines (C - A)/B and (B - A)/C, where A < B < C are the principal moments of inertia; the flattening of the fluid outer core; the dissipation at its solid boundaries; and the monthly tidal dissipation Q = 37.5 +/- 4. The moment of inertia computation combines the GRAIL-determined J(2) and C-22 with LLR-derived (C - A)/B and (B - A)/C. The normalized mean moment of inertia of the solid Moon is I-s/MR2 = 0.392728 +/- 0.000012. Matching the density, moment, and Love number, calculated models have a fluid outer core with radius of 200-380 km, a solid inner core with radius of 0-280 km and mass fraction of 0-1%, and a deep mantle zone of low seismic shear velocity. The mass fraction of the combined inner and outer core is <= 1.5%.

Observations from the Gravity Recovery and Interior Laboratory (GRAIL) mission indicate a marked change in the gravitational signature of lunar impact structures at the morphological transition, with increasing diameter, from complex craters to peak-ring basins. At crater diameters larger than similar to 200 km, a central positive Bouguer anomaly is seen within the innermost peak ring, and an annular negative Bouguer anomaly extends outward from this ring to the outer topographic rim crest. These observations demonstrate that basin-forming impacts remove crustal materials from within the peak ring and thicken the crust between the peak ring and the outer rim crest. A correlation between the diameter of the central Bouguer gravity high and the outer topographic ring diameter for well-preserved basins enables the identification and characterization of basins for which topographic signatures have been obscured by superposed cratering and volcanism. The GRAIL inventory of lunar basins improves upon earlier lists that differed in their totals by more than a factor of 2. The size-frequency distributions of basins on the nearside and farside hemispheres of the Moon differ substantially; the nearside hosts more basins larger than 350 km in diameter, whereas the farside has more smaller basins. Hemispherical differences in target properties, including temperature and porosity, are likely to have contributed to these different distributions. Better understanding of the factors that control basin size will help to constrain models of the original impactor population.

The kinetics of two metastable polymorphs of silicon under thermal annealing was investigated. These phases with body-centered cubic bc8 and rhombohedral r8 structures can be formed upon pressure release from metallic silicon. In this study, these metastable polymorphs were formed by two different methods, via point loading and in a diamond anvil cell (DAC). Upon thermal annealing different transition pathways were detected. In the point loading case, the previously reported Si-XIII formed and was confirmed as a new phase with an as-yet-unidentified structure. In the DAC case, bc8-Si transformed to the hexagonal-diamond structure at elevated pressure, consistent with previous studies at ambient pressure. In contrast, r8-Si transformed directly to diamond-cubic Si at a temperature of

The Orientale basin is the youngest and best-preserved major impact structure on the Moon. We used the Gravity Recovery and Interior Laboratory (GRAIL) spacecraft to investigate the gravitational field of Orientale at 3- to 5-kilometer (km) horizontal resolution. A volume of at least (3.4 +/- 0.2) x 10(6) km(3) of crustal material was removed and redistributed during basin formation. There is no preserved evidence of the transient crater that would reveal the basin's maximum volume, but its diameter may now be inferred to be between 320 and 460 km. The gravity field resolves distinctive structures of Orientale's three rings and suggests the presence of faults associated with the outer two that penetrate to the mantle. The crustal structure of Orientale provides constraints on the formation of multiring basins.

Context. Imaged in the gap of a transition disk and found at a separation of about 195 mas (similar to 22 au) from its host star at a position angle of about 155 degrees, PDS 70 b is the most robustly detected young planet to date. This system is therefore a unique laboratory for characterizing the properties of young planetary systems at the stage of their formation.

We present L'-band Keck/NJRC2 imaging and H-band Subaru/A0188 HiCIA0 polarimetric observations of the CQ Tau disk with a new spiral arm. Apart from the spiral feature, our observations could not detect any companion candidates. We traced the spiral feature from the r2-scaled High-Contrast Coronographic Imager for Adaptive Optics (HiCIA0) polarimetric intensity image and the fitted result is used for forward modeling to reproduce the ADI-reduced NJRC2 image. We estimated the original surface brightness after throughput correction in the L' band to be-126 mJy arcsec-2 at most. We suggest that the grain temperature of the spiral may be heated up to-200 K in order to explain both of the H- and L'-band results. The H-band emission at the location of the spiral originates from the scattering from the disk surface while both scattering and thermal emission may contribute to the L'-band emission. If the central star is only the light source of scattered light, the spiral emission at the L' band should be thermal emission. If an inner disk also acts as the light source, the scattered light and the thermal emission may equally contribute to the L'-band spiral structure.