¹Mizuho Koike,^2,3Yuji Sano,^1,2Naoto Takahat,⁴ Tsuyoshi Iizuka,^4,5Haruka Ono,^4,5Takashi Mikouchi
Earth and Planetary Science Letters 549, 116497 Link to Article [https://doi.org/10.1016/j.epsl.2020.116497]
¹Earth and Planetary Systems Science Program, Graduate School of Advanced Science and Engineering, Hiroshima University, 1-3-1 Kagamiyama, Higashihiroshima-shi, Hiroshima, 739-8526, Japan
²Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba, 277-8564, Japan
³Institute of Surface-Earth System Science, Tianjin University, Nankai District, Tianjin, 300072, PR China
⁴Department of Earth and Planetary Science, The University of Tokyo, Bunkyo-ku, Tokyo, 113-0033, Japan
⁵The University Museum, The University of Tokyo, Bunkyo-ku, Tokyo, 113-0033, Japan
Copyright Elsevier

The late heavy bombardment (LHB) hypothesis, wherein the terrestrial planets are thought to have suffered intense collisions ca. 3.9 billion years ago, is under debate. Coupled with new dynamical calculations, re-examination of geochronological data seem to support an earlier solar system instability and a smooth monotonic decline in impacts, as opposed to a “cataclysm.” To better understand this collisional history, records from the asteroidal meteorites are required. Here, we report a uranium–lead (U–Pb) chronological dataset for eucrite meteorites thought to originate from the asteroid 4 Vesta; this dataset indicates to a continuous history of collisions prior to 4.15 Ga. Our 207Pb⁎/206Pb⁎ model ages of apatite [Ca5(PO4)3(F,Cl,OH)] and merrillite [Ca9NaMg(PO4)7] from three brecciated basaltic eucrites—Juvinas (4150.3 ± 11.6 million years ago (Ma); merrillite only), Camel Donga (disturbed around 4570–4370 Ma), and Stannern (4143.0 ± 12.5 Ma)—record multiple thermal metamorphic events during the period of ∼4.4–4.15 Ga. We interpret this to mean that Vesta or the vestoid cluster underwent multiple impacts and moderate high-temperature reheating during this time. The ages of ∼4.4–4.15 Ga are distinctly younger than the initial magmatic process on Vesta (>4.5 Ga) but are significantly older than a later “impact peak” based on some interpretations of 40Ar–39Ar chronologies (∼3.9–3.5 Ga). The intense collisions prior to 4.15 Ga on Vesta are at odds with the conventional LHB hypothesis but not inconsistent with the much earlier bombardment and monotonic decline scenario. Different radiometric chronologies of the asteroid likely represent the different stages of a continual collisional process. Conversely, the model 207Pb⁎/206Pb⁎ ages of apatite in the unbrecciated basaltic eucrite, Agoult, returned an age of 4524.8 ± 9.6 Ma. This may represent slow cooling from an earlier global reheating of the crust on Vesta at 4.55 Ga, as documented by other radiometric chronologies. The apatite in Juvinas recorded a coincident timing of 4516.9 ± 10.4 Ma, which could be due to either slow crustal cooling or impact.

¹Giampaolo P. Sighinolfi,^1,2Federico Lugli,¹Federica Piccione,³Vincenzo DE Michele,^1,4Anna Cipriani
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13550]
¹Department of Chemical and Geological Sciences, University of Modena and Reggio Emilia, Via Campi 103, Modena, 41225 Italy
²Department of Cultural Heritage, University of Bologna, Via degli Ariani 1, Ravenna, 48121 Italy
³Museo di Storia Naturale, Milan, 20121 Italy
⁴Lamont‐Doherty Earth Observatory, Columbia University, Palisades, New York, 10964 USA
Published by arrangement with john Wiley & Sons

Strontium isotopes and selected trace elements (Rb, Sr, REE, Zr, Hf, Th, and U) were measured on samples of Libyan Desert Glass (LDG) and a series of terrestrial materials (rocks, LDG‐bearing soils, eolic sand) collected over a large area of southwestern Egypt to identify the LDG terrestrial parent material and the site where impact melting occurred. Samples include Upper Cretaceous hypersilicic sandstones outcropping at or near the LDG strewn field and Lower Cretaceous to Silurian sandstones from the Gilf Kebir Plateau highlands. Strontium isotopes and partially Zr, Hf, Th, and U, possibly reflecting the composition of detrital zircon grains, are effective indicators of the geochemical affinity between terrestrial materials and LDG, unlike Rb, Sr, and REE abundances. The best geochemical affinity with LDG was found in LDG‐bearing soils collected at the base of intradunal corridors in the Great Sand Sea. Remarkably, abundances of the Zr group elements of the LDG Zr‐bearing phase are distinct from all terrestrial detrital zircons from the area. We suggest a mixture of weathering products from sandstones of different ages, including Devonian and Silurian rocks from the Gilf Kebir highlands, as the most likely source for LDG. A loose sedimentary formation exposed 29 Ma ago at the Earth’s surface, superimposed over hard bedrock, might have been the true terrestrial target of the impact, but because of its incoherent nature, it was rapidly destroyed, explaining the complete absence of any evidence of an impact structure.

^1,2James S. New,³Bahar Kazemi,²Mark C. Price,²Mike J. Cole,²Vassi Spathis,^1,3Richard A. Mathies,²Anna L. Butterworth
Meteoritics & Planetary Science (in Press) Link to Articie [https://doi.org/10.1111/maps.13554]
¹Space Sciences Laboratory, University of California, Berkeley, California, 94720 USA
²School of Physical Sciences, University of Kent, Canterbury, Kent, CT2 7NH UK
³Department of Chemistry, University of California, Berkeley, California, 94720 USA
Published by arrangement with John Wiley & Sons

Enceladus is a compelling destination for astrobiological analyses due to the presence of simple and complex organic constituents in cryovolcanic plumes that jet from its subsurface ocean. Enceladus plume capture during a flyby or orbiter mission is an appealing method for obtaining pristine ocean samples for scientific studies of this organic content because of the high science return, reduced planetary protection challenges, and lower risk and expense compared to a landed mission. However, this mission profile requires sufficient amounts of plume material for sensitive analysis. To explore the feasibility and optimization of the required capture systems, light gas gun experiments were carried out to study organic ice particle impacts on indium surfaces. An organic fluorescent tracer dye, Pacific Blue™, was dissolved in borate buffer and frozen into saline ice projectiles. During acceleration, the ice projectile breaks up in flight into micron‐sized particles that impact the target. Quantitative fluorescence microscopic analysis of the targets demonstrated that under certain impact conditions, 10–50% of the entrained organic molecules were captured in over 25% of the particle impacts. Optimal organic capture was observed for small particles (d ~ 5–15 µm) with velocities ranging from 1 to 2 km s−1. Our results reveal how organic capture efficiency depends on impact velocity and particle size; capture increases as particles get smaller and as velocity is reduced. These results demonstrate the feasibility of collecting unmodified organic molecules from the Enceladus ice plume for sensitive analysis with modern in situ instrumentation such as microfluidic capillary electrophoresis (CE) analysis with ppb organic sensitivity.