We present chemical abundance measurements for seven stars with metallicities ranging from Fe/H] = -3.3 to [Fe/H] = -2.4 in the Tucana II ultra-faint dwarf galaxy (UFD), based on high-resolution spectra obtained with the MIKE spectrograph on the 6.5 m Magellan-Clay Telescope. For three stars, we present detailed chemical abundances for the first time. Of those, two stars are newly discovered members of Tucana II and were selected as probable members from deep narrowband photometry of the Tucana II UFD taken with the SkyMapper telescope. This result demonstrates the potential for photometrically identifying members of dwarf galaxy systems based on chemical composition. One new star was selected from the membership catalog of Walker et al. The other four stars in our sample have been reanalyzed, following additional observations. Overall, six stars have chemical abundances that are characteristic of the UFD stellar population. The seventh star shows chemical abundances that are discrepant from the other Tucana II members and an atypical, higher strontium abundance than what is expected for typical UFD stars. While unlikely, its strontium abundance raises the possibility that it may be a foreground metal-poor halo star with the same systemic velocity as Tucana II. If we were to exclude this star, Tucana II would satisfy the criteria to be a surviving first galaxy. Otherwise, this star implies that Tucana II has likely experienced somewhat extended chemical evolution.

Hydrous minerals in subducted crust can transport large amounts of water into Earth's deep mantle. Our laboratory experiments revealed the surprising pressure-induced chemistry that, when water meets iron at the core-mantle boundary, they react to form an interlayer with an extremely oxygen-rich form of iron, iron dioxide, together with iron hydride. Hydrogen in the layer will escape upon further heating and rise to the crust, sustaining the water cycle. With water supplied by the subducting slabs meeting the nearly inexhaustible iron source in the core, an oxygen-rich layer would cumulate and thicken, leading to major global consequences in our planet. The seismic signature of the D" layer may echo the chemical complexity of this layer. Over the course of geological time, the enormous oxygen reservoir accumulating between the mantle and core may have eventually reached a critical eruption point. Very large-scale oxygen eruptions could possibly cause major activities in the mantle convection and leave evidence such as the rifting of supercontinents and the Great Oxidation Event.

Lithium titanium oxide (Li4Ti5O12, LTO), a 'zero-strain' anode material for lithium-ion batteries, exhibits excellent cycling performance. However, its poor conductivity highly limits its applications. Here, the structural stability and conductivity of LTO were studied using in situ high-pressure measurements and first-principles calculations. LTO underwent a pressure-induced amorphization (PIA) at 26.9 GPa. The impedance spectroscopy revealed that the conductivity of LTO improved significantly after amorphization and that the conductivity of decompressed amorphous LTO increased by an order of magnitude compared with its starting phase. Furthermore, our calculations demonstrated that the different compressibility of the LiO6 and TiO6 octahedra in the structure was crucial for the PIA. The amorphous phase promotes Li+ diffusion and enhances its ionic conductivity by providing defects for ion migration. Our results not only provide an insight into the pressure depended structural properties of a spinel-like material, but also facilitate exploration of the interplay between PIA and conductivity.

The response of forsterite, Mg2SiO4, under dynamic compression is of fundamental importance for understanding its phase transformations and high-pressure behavior. Here, we have carried out an in situ X-ray diffraction study of laser-shocked polycrystalline and single-crystal forsterite (a-, b-, and c-orientations) from 19 to 122 GPa using the Matter in Extreme Conditions end-station of the Linac Coherent Light Source. Under laser-based shock loading, forsterite does not transform to the high-pressure equilibrium assemblage of MgSiO3 bridgmanite and MgO periclase, as has been suggested previously. Instead, we observe forsterite and forsterite III, a metastable polymorph of Mg2SiO4, coexisting in a mixed-phase region from 33 to 75 GPa for both polycrystalline and single-crystal samples. Densities inferred from X-ray diffraction data are consistent with earlier gas-gun shock data. At higher stress, the response is sample-dependent. Polycrystalline samples undergo amorphization above 79 GPa. For [010]- and [001]-oriented crystals, a mixture of crystalline and amorphous material is observed to 108 GPa, whereas the [100]-oriented forsterite adopts an unknown phase at 122 GPa. The first two sharp diffraction peaks of amorphous Mg2SiO4 show a similar trend with compression as those observed for MgSiO3 in both recent static- and laser-driven shock experiments. Upon release to ambient pressure, all samples retain or revert to forsterite with evidence for amorphous material also present in some cases. This study demonstrates the utility of femtosecond free-electron laser X-ray sources for probing the temporal evolution of high-pressure silicate structures through the nanosecond-scale events of shock compression and release.

We report the early discovery and multicolor (BVI) high-cadence light-curve analyses of the rapidly declining sub-Chandrasekhar Type Ia supernova KSP-OT-201509b (= AT 2015cx) from the KMTNet Supernova Program. The Phillips and color stretch parameters of KSP-OT-201509b are Delta M-B,M-15 1.62 mag and s(BV) 0.54, respectively, at an inferred redshift of 0.072. These, together with other measured parameters (such as the strength of the secondary I-band peak, colors, and luminosity), identify the source to be a rapidly declining Type Ia of a transitional nature that is closer to Branch-normal than 91bg-like. Its early light-curve evolution and bolometric luminosity are consistent with those of homologously expanding ejecta powered by radioactive decay and a Type Ia SN explosion with 0.32 0.01 M of synthesized Ni-56 mass, 0.84 0.12 M of ejecta mass, and (0.61 0.14) x 10(51) erg of ejecta kinetic energy. While its B - V and V - I colors evolve largely synchronously with the changes in the I-band light curve, as found in other supernovae, we also find the presence of an early redward evolution in V - I prior to -10 days since peak. The bolometric light curve of the source is compatible with a stratified Ni-56 distribution extended to shallow layers of the exploding progenitor. Comparisons between the observed light curves and those predicted from ejecta-companion interactions clearly disfavor Roche lobe-filling companion stars at large separation distances, thus supporting a double-degenerate scenario for its origin. The lack of any apparent host galaxy in our deep stack images reaching a sensitivity limit of similar to 28 mag arcsec(-2) makes KSP-OT-201509b a hostless Type Ia supernova and offers new insights into supernova host galaxy environments.

There has been considerable recent interest in the high-pressure behavior of silicon carbide, a potential major constituent of carbon-rich exoplanets. In this work, the atomic-level structure of SiC was determined through in situ X-ray diffraction under laser-driven ramp compression up to 1.5 TPa; stresses more than seven times greater than previous static and shock data. Here we show that the B1-type structure persists over this stress range and we have constrained its equation of state (EOS). Using this data we have determined the first experimentally based mass-radius curves for a hypothetical pure SiC planet. Interior structure models are constructed for planets consisting of a SiC-rich mantle and iron-rich core. Carbide planets are found to be similar to 10% less dense than corresponding terrestrial planets.

The Earth's fluid budget is dominated by species in the system C-O-H-N-S together with halogens such as F and Cl. H2O is by far the most abundant. Such fluids are one of the two main mass transport agents (fluid and magma) in the Earth. Among those, in particular aqueous fluids are efficient solvents of geochemically important components at high temperature and pressure. The solution capacity of aqueous fluids can be enhanced further by dissolved halogens and sulfur. CO2 or nitrogen species has the opposite effect.

The complexity of microbial communities suggests prevalent interactions involving more than just pairs of species. These so-called higher-order interactions may reveal new molecules that enable bacteria to deal with complex environments. This forum article discusses how higher-order interactions can be detected and why molecular biologists might care.

An outstanding goal in planetary science is to understand how terrestrial cores evolved to have the compositions, thermal properties, and magnetic fields observed today. To achieve that aim requires the integration of datasets from space missions with laboratory experiments conducted at high pressures and temperatures. Over the past decade, technological advances have enhanced the capability to conduct in situ measurements of physical properties on samples that are analogs to planetary cores. These challenging experiments utilize large-volume presses that optimize control of pressure and temperature, and diamond-anvil cells to reach the highest pressures. In particular, the current experimental datasets of density, compressional velocity, viscosity, and thermal conductivity of iron alloys are most relevant to the core conditions of small terrestrial planets and moons. Here we review the physical properties of iron alloys measured in the laboratory at conditions relevant to the cores of Mars, the Moon, and Mercury. We discuss how these properties inform models of core composition, as well as thermal and magnetic evolution of their cores. Experimental geochemistry (in particular, metal-silicate partitioning experiments) provides additional insights into the nature and abundance of light elements within cores, as well as crystallization processes. Emphasis is placed on the Martian core to discuss the effect of chemistry on core evolution.