The chemical case for Mercury mantle stripping

Mercury, the Solar System’s innermost planet, has an unusually massive core prompting speculation that the planet lost silicate after it formed. Using the unusually high sulfur and low iron composition of its surface and space geodetic constraints on its core composition, we show Mercury’s chemistry to be compatible with formation in a larger planet at minimum 1.4–2.5 times Mercury’s present mass and possibly 2–4 times its mass by similarity with other rocky Solar System bodies. To do this, we apply an experimentally determined metal-silicate partitioning model for sulfur to Mercury’s silicate. The model is validated by applying it to Vesta, which, when evaluated at the conditions of Vestan self-differentiation, yields sulfur contents in its silicate in the range of HED meteorites. Mercury could have lost a substantial fraction of its rocky material through impacts or by being itself a remnant impactor. Independent of any stripping, because a significant amount of silicon resides in Mercury’s core, silicate meteoritic debris from Mercury would likely be characterized by 30Si isotopic enrichment >+ 0.10‰ relative to parent sources that could aid identification of a new meteorite class.

K2-141 b

We report on the discovery of K2-141 b (EPIC 246393474 b), an ultra-short-period super-Earth on a 6.7 h orbit transiting an active K7 V star based on data from K2 campaign 12. We confirmed the planet’s existence and measured its mass with a series of follow-up observations: seeing-limited MuSCAT imaging, NESSI high-resolution speckle observations, and FIES and HARPS high-precision radial-velocity monitoring. K2-141 b has a mass of 5.31 ± 0.46 M⊕ and radius of 1.54−0.09+0.10 R⊕, yielding a mean density of 8.00−1.45+1.83 g cm−3 and suggesting a rocky-iron composition. Models indicate that iron cannot exceed ~70% of the total mass. With an orbital period of only 6.7 h, K2-141 b is the shortest-period planet known to date with a precisely determined mass.