The extraterrestrial materials returned from asteroid (162173) Ryugu consist predominantly of low-temperature aqueously formed secondary minerals and are chemically and mineralogically similar to CI (Ivuna-type) carbonaceous chondrites. Here, we show that high-temperature anhydrous primary minerals in Ryugu and CI chondrites exhibit a bimodal distribution of oxygen isotopic compositions: 16O-rich (associated with refractory inclusions) and 16O-poor (associated with chondrules). Both the 16O-rich and 16O-poor minerals probably formed in the inner solar protoplanetary disk and were subsequently transported outward. The abundance ratios of the 16O-rich to 16O-poor minerals in Ryugu and CI chondrites are higher than in other carbonaceous chondrite groups but are similar to that of comet 81P/Wild2, suggesting that Ryugu and CI chondrites accreted in the outer Solar System closer to the accretion region of comets.

The worldwide proliferation of harmful algal blooms (HABs) both in freshwater and marine ecosystems make understanding and predicting their occurrence urgent. Trait-based approaches, where the focus is on functional traits, have been successful in explaining community structure and dynamics in diverse ecosystems but have not been applied extensively to HABs. The existing trait compilations suggest that HAB taxa differ from non HAB taxa in key traits that determine their responses to major environmental drivers. Multi-trait comparisons between HAB-forming and other phytoplankton taxa, as well as within the HAB groups to characterize interspecific and intraspecific differences will help better define ecological niches of different HAB taxa, develop trait-based mechanistic models, and better identify environmental conditions that would likely lead to HABs. Building databases of HAB traits and using them in diverse statistical and mechanistic models will increase our ability to predict the HAB occurrence, composition, and severity under changing conditions, including the anthropogenic global change.

We report the discovery of Specter, a disrupted ultrafaint dwarf galaxy revealed by the H3 Spectroscopic Survey. We detected this structure via a pair of comoving metal-poor stars at a distance of 12.5 kpc, and further characterized it with Gaia astrometry and follow-up spectroscopy. Specter is a 25 degrees x 1 degrees stream of stars that is entirely invisible until strict kinematic cuts are applied to remove the Galactic foreground. The spectroscopic members suggest a stellar age tau greater than or similar to 12 Gyr and a mean metallicity <[Fe/H]> = -1.84(-0.18)(+0.16), with a significant intrinsic metallicity dispersion sigma([Fe/H]) = 0.37(-0.13)(+0.21). We therefore argue that Specter is the disrupted remnant of an ancient dwarf galaxy. With an integrated luminosity M-v approximate to -2.6, Specter is by far the least-luminous dwarf galaxy stream known. We estimate that dozens of similar streams are lurking below the detection threshold of current search techniques, and conclude that spectroscopic surveys offer a novel means to identify extremely low surface brightness structures.

Mercury has a compositionally diverse surface that was produced by different periods of igneous activity suggesting heterogeneous mantle sources. Understanding the structure of Mercury's mantle formed during the planet's magma ocean stage could help in developing a petrologic model for Mercury, and thus, understanding its dynamic history in the context of crustal petrogenesis. We present results of falling sphere viscometry experiments on late-stage Mercurian magma ocean analogue compositions conducted at the Advanced Photon Source, beamline 16-BM-B, Argonne National Laboratory. Owing to the presence of sulfur on the surface of Mercury, two compositions were tested, one with sulfur and one without. The liquids have viscosities of 0.6-3.9 (sulfur-bearing; 2.6-6.2 GPa) and 0.6-10.9 Pa center dot s (sulfur-free; 3.2-4.5 GPa) at temperatures of 1600-2000 degrees C. We present new viscosity models that enable extrapolation beyond the experimental conditions and evaluate grain growth and the potential for crystal entrainment in a cooling, convecting magma ocean. We consider scenarios with and without a graphite flotation crust, suggesting endmember outcomes for Mercury's mantle structure. With a graphite flotation crust, crystallization of the mantle would be fractional with negatively buoyant minerals sinking to form a stratified cumulate pile according to the crystallization sequence. Without a flotation crust, crystals may remain entrained in the convecting liquid during solidification, producing a homogeneous mantle. In the context of these endmember models, the surface could result from dynamical stirring or mixing of a mantle that was initially heterogeneous, or potentially from different extents of melting of a homogeneous mantle.

The nature of dark matter is one of the most important unsolved questions in science. Some darkf matter candidates do not have sufficient nongravitational interactions to be probed in laboratory or accelerator experiments. It is thus important to develop astrophysical probes which can constrain or lead to a discovery of such candidates. We illustrate this using state-of-the-art measurements of strong gravitationally lensed quasars to constrain four of the most popular sterile neutrino models, and also report the constraints for other independent methods that are comparable in procedure. First, we derive effective relations to describe the correspondence between the mass of a thermal relic warm dark matter particle and the mass of sterile neutrinos produced via Higgs decay and grand unified theory (GUT)-scale scenarios, in terms of large-scale structure and galaxy formation astrophysical effects. Second, we show that sterile neutrinos produced through the Higgs decay mechanism are allowed only for mass >26 keV, and GUT-scale scenario >5.3 keV. Third, we show that the single sterile neutrino model produced through active neutrino oscillations is allowed for mass >92 keV, and the three sterile neutrino minimal standard model (nu MSM) for mass >16 keV. These are the most stringent experimental limits on these models.