¹Brynna G. Downey, ¹Robin M. Canup
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2026.117208]
¹Solar System Science and Exploration Division, Southwest Research Institute, 1301 Walnut Street, Boulder, 80302, CO, USA
Copyright Elsevier

In the giant impact theory for the origin of the Moon, a protoplanet collided with the Earth, producing a disk of melt-vapor debris from which the Moon accreted. Simulations of canonical impacts in which the impactor is  Mars-sized produce disks with mass 1 to  (lunar masses). However, most prior models of lunar accretion require that the initial disk have mass  because of the assumed disk state, modelled processes, and initial conditions. This inconsistency has been a challenge for the canonical impact model. To bridge this gap, we (i) update a model of the disk interior to the Roche limit to treat melt and vapor as separate, vertically stratified layers, and (ii) adopt as initial conditions for the accretion model more realistic radial mass distributions for the disk based on impact simulations. We find that treating the inner melt and vapor as vertically stratified layers lowers the final Moon mass by 20% on average compared to prior work that assumed they remained well-mixed, a relatively small effect. In contrast, the initial radial mass distribution has a substantial effect. We show that for outer disk mass , which is often 60% of the total disk mass, an  Moon accretes in as little as  days and at most  months. For almost all successful cases, only 5% of the final Moon is from the inner melt and vapor layers that might have isotopically equilibrated with the Earth’s vapor atmosphere. The Moon’s rapid accretion from material originally emplaced in an outer canonical disk requires that the isotopic similarities between the Earth and Moon be inherited from similarities between Earth and the impactor Theia, rather than through disk-planet equilibration.

¹Dylan M. Seal, ¹Melody Z. Chen, ²Robert W. Nicklas, ¹Ethan F. Baxter, ³James M.D. Day, ⁴Ben G. Rider-Stokes, ⁵Anthony B. Love, ⁴James Malley
Geochimica et Cosmochimica Acta (in Press) Link to Article [10.1016/j.gca.2026.06.035]
¹Department of Environmental Sciences, Boston College, Chestnut Hill, MA 02467, USA
²Lunar and Planetary Institute, USRA, Houston, TX 77058, USA
³Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0244, USA
⁴School of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK
⁵Department of Geological and Environmental Sciences, Appalachian State University, Boone, NC 28608, USA
Copyright Elsevier

Here we report on the petrology, mineralogy, geochemistry, and O and Sm-Nd isotope compositions for Northwest Africa (NWA) 13441, an olivine-phyric shergottite rich in melt glass that was recovered from Algeria in 2019. The meteorite is a basalt containing abundant olivine megacrysts set in a groundmass of pigeonite, olivine, and maskelynite with accessory chromite and merrillite. The Fe/Mn ratios of olivine (56.1 ± 7.2; 2σ, n = 15) and pyroxene (30.1 ± 2.1; 2σ, n = 10), and bulk rock O isotope ratios of Δ17O = 0.270 ± 0.014 ‰, confirm its martian origin. Pyroxene Ti/Al barometry indicates that crystallization began near the crust-mantle boundary of Mars. The meteorite contains ∼7 vol% pyroxene-dominated melt glass that together with undulatory extinction and mosaicism in olivine, mechanical twinning in pyroxene, and amorphized plagioclase, suggests a relatively high level of shock metamorphism at estimated peak conditions of ∼28–34 GPa and ∼200–250 °C. The bulk rock rare earth element pattern ((La/Yb)CI = 0.64) suggests an affinity to intermediate shergottites. Hand-picked mineral separates and leachates define a 147Sm-143Nd errorchron corresponding to an age of 1273 ± 21 Ma (MSWD = 19; n = 9) and a chondritic εNdi composition of + 0.93 ± 1.04 that is distinct from other shergottites, which are typically younger (<600 Ma). Comparison with the 147Sm-143Nd evolution of different martian sources indicates that the chondritic composition of NWA 13441 could represent a previously unsampled undifferentiated reservoir or mixing between known enriched and depleted shergottite sources. Regardless, NWA 13441 expands the temporal and isotopic range of shergottite magmatism and demonstrates that the martian meteorite record incompletely samples the diversity of compositions produced from the melting of Mars’ mantle.