¹Sohei Wada,¹Ken-ichi Bajo,^1,2Tomoya Obase,¹Hisayoshi Yurimoto
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14346]
¹Department of Natural History Sciences, Hokkaido University, Sapporo, Hokkaido, Japan
²Department of Earth and Planetary Sciences, Institute of Science Tokyo, Meguro, Tokyo, Japan
Published by arrangement with John Wiley & Sons

Solar wind (SW) is preserved in meteorites as abundant solar noble gases. We performed in situ 4He isotope imaging of mineral grains in the CR2 chondrite matrix of Northwest Africa 801 using time-of-flight secondary neutral mass spectrometry with strong-field post ionization. 4He+ signals were detected along the surfaces of individual grains of Fe-Ni metal, ferrihydrite, olivine, pyroxene, and troilite. The high 4He concentrations along the surfaces indicate implantation of SW into the mineral grains. We determined the SW-4He fluence of eight mineral grains from the line profiles across the grain boundaries. SW-4He fluence ranged from 2.7 × 1016 to 58 × 1016 atoms cm−2. These fluences were then used to calculate the SW irradiation durations. Assuming irradiation occurred at 4 astronomical units, the durations ranged from 3.8 to 82 kyr. These durations correspond to the residence time of individual mineral grains on the surface of the parent body. The variation in residence time for the mineral grains suggests variable durations for local mixing and burial processes on the parent body. The SW exposure ages provide insights into the gardening rate driven by small-scale impact mixing processes on the parent body.

^1,2Peter Jenniskens et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14337]
¹SETI Institute, Mountain View, California, USA
²NASA Ames Research Center, Moffett Field, California, USA
Published by arrangement with John Wiley & Sons

The Aguas Zarcas (Costa Rica) CM2 carbonaceous chondrite fell during nighttime in April 2019. Security and dashboard camera videos of the meteor were analyzed to provide a trajectory, light curve, and orbit of the meteoroid. The trajectory was near vertical, 81° steep, arriving from an ~109° (WNW) direction with an apparent entry speed of 14.6 ± 0.6 km s−1. The meteoroid penetrated to ~25 km altitude (5 MPa dynamic pressure), where the surviving mass shattered, producing a flare that was detected by the Geostationary Lightning Mappers on GOES-16 and GOES-17. The cosmogenic radionuclides were analyzed in three recovered meteorites by either gamma-ray spectroscopy or accelerator mass spectrometry (AMS), while noble gas concentrations and isotopic compositions were measured in the same fragment that was analyzed by AMS. From this, the pre-atmospheric size of the meteoroid and its cosmic ray exposure age were determined. The studied samples came from a few cm up to 30 cm deep in an object with an original diameter of ~60 cm that was ejected from its parent body 2.0 ± 0.2 Ma ago. The ejected material had an argon retention age of 2.9 Ga. The object was delivered most likely by the 3:1 or 5:2 mean motion resonances and, without subsequent fragmentation, approached the Earth from a low i < 2.8° inclined orbit with a perihelion distance q = 0.98 AU close to the Earth’s orbit. The steep entry trajectory and high strength resulted in deep penetration in the atmosphere and a relatively large fraction of surviving mass.