Robert E. Grimm | Searcharxiv

arXiv2026

Chondrite Parent Bodies as Escaped Satellites of Proto-Planetary Embryos

Chondrites are composed of formerly partially molten material, known as chondrules, surrounded by fine-grained matrix. They date from the earliest times in Solar System history. However, their role in the formation of the planets is uncertain because, in part, it is not clear how they were produced. Here, we show a robust pathway for forming meteorite-producing asteroids that contain chondrules through embryo-embryo collisions during the late stages of terrestrial planet formation. Melted material from these impacts cool into chondrules and mix with unmelted material in embryo-centric disks that formed from the ejecta. This material accretes into numerous asteroid-sized satellites. These objects are later ejected onto heliocentric orbits because of gravitational encounters with other embryos, thereby becoming the parent bodies of chondrites. This mechanism provides a pathway to form chondrites in Solar System history at times commensurate with measured chondrule ages, while explaining many of their physical properties.

astro-ph.EP↗

arXiv2023

Lunar Mantle Structure and Composition Inferred From Apollo 12 -- Explorer 35 Electromagnetic Sounding

Constraints on the interior of the Moon have been derived from its inductive response, principally as measured by the magnetic transfer function (TF) between the distantly orbiting Explorer 35 satellite and the Apollo 12 surface station. The most successful prior studies used a dataset spanning 0.01-1 mHz, so the lunar response could be modeled as a simple dipole. However, earlier efforts also produced transfer functions up to 40 mHz. The smaller electromagnetic skin depth at higher frequency would better resolve the uppermost mantle-where key information about primitive lunar evolution may still be preserved-but requires a multipole treatment. I compute new profiles of electrical conductivity vs depth using both low- and high-frequency ranges of published Apollo-Explorer TFs. Using the low-frequency data, I derive temperature profiles at depths >400 km (<1 mHz) consistent with conductive heat loss and expectations of the iron (and possibly water) content of the mantle. The near-constant iron fraction (Mg# 81 +/- 10) could imply efficient mixing due to now-defunct convection. Alternatively, incomplete overturn of gravitationally unstable magma-ocean cumulates could have left a heterogeneous distribution of minerals at hundred-km scales that are not resolved by electromagnetic sounding. A third explanation is that the electromagnetically probed region may be the initial equilibrium crystallization in a mantle that did not buoyantly overturn. In contrast, the high-frequency data produced higher conductivities than expected, requiring unrealistically low Mg# or high water content. Either the published transfer functions >> 1 mHz are incorrect, or the TF multipole method at the Moon is unreliable. Future electromagnetic sounding using the magnetotelluric method can operate up to 100s Hz and is largely insensitive to multipole effects, resolving structure to 100 km or less.

astro-ph.EP↗