Mercury's graphite crust redraws the chemistry of its giant core
High-pressure experiments indicate that Mercury's reducing chemistry kept carbon out of its giant core and in a graphite crust, leaving silicon and sulfur as leading explanations for the core's low density. Around AU Microscopii, a separate simulation finds that an unseen planet on a tilted orbit could reproduce fast-moving dust clumps. The results build pictures of planetary structure from indirect traces while preserving the models' uncertainty.
Science··Evening
Carbon stayed in Mercury's crust instead of its core
High-pressure and high-temperature experiments in Nature Communications narrow a long-standing puzzle in Mercury's formation chemistry. Carbon binds to iron and enters the core under relatively oxidizing conditions, but remains in silicate melt and crystallises as graphite under the planet's extremely reducing conditions. In the experimental range that best matches the crustal thickness inferred from MESSENGER data, the core contains less than 5000 micrograms of carbon per gram. Silicon and sulfur therefore become the leading candidates for the low density of a core that makes up roughly 70 per cent of the planet's mass.[1]
AU Microscopii's escaping dust points to an unseen planet
A simulation in The Astrophysical Journal offers a planetary explanation for dust clumps racing away from AU Microscopii's debris disk. N-body calculations fitted to the observed clump motions show that a planet of about 2 Jupiter masses on an orbit tilted by 30 degrees could kick dust above the disk plane. Radiation and wind from the star would then drive those particles outward. The planet has not been observed directly: the favoured arrangement is a model that reproduces the structure, and the fit does not exclude other orbital configurations.[2]
The dust-clump rhythm maps the possible orbit
The favoured fit for AU Microscopii describes a planet of 2 Jupiter masses with a semimajor axis of 3.52 astronomical units, an eccentricity of 0.37 and an orbital tilt of 30 degrees. In that arrangement, the debris disk begins between 5.36 and 5.86 astronomical units from the star, and dust ejections recur about every 8.5 years. A radiation-and-wind factor close to 1.76 best reproduces the clumps' outward motion. The planet has not been observed directly; these values describe one model configuration consistent with the measured motions, and the analysis leaves other orbital arrangements open.[2]
