Collisional history of asteroid Itokawa

Abstract

12In situ extraterrestrial samples returned for study (e.g., from the Moon) are crucial in 13 understanding the origin and evolution of the Solar System as, contrary to meteorites, they 14 provide a known geological context for the samples and their analyses. Asteroid 25143 Itokawa 15 is a rubble pile asteroid consisting of reaccumulated fragments from a catastrophically disrupted 16 monolithic parent asteroid, and from which regolith dust particles have been recovered by the 17 Hayabusa space probe. We analyzed two dust particles using Electron Backscatter Diffraction 18 (EBSD) and 40 Ar/ 39 Ar dating techniques. One of the grains showing signs of 15-25 GPa impact 19 shock pressure, yielded a 40 Ar/ 39 Ar plateau age of 2.3 ± 0.1 Ga. We develop a novel temperature-20 pressure-porosity model, coupled with diffusion models to show that the relatively low pressure 21 and high temperature involved in the impact process can be reconciled only if the asteroid was 22

Thermal and impact histories of 25143 Itokawa recorded in Hayabusa particles

“…These outcomes are thought to be vestiges of a thermal metamorphism age and a shock age recorded in Itokawa particles, respectively. The younger age of 1.51 ± 0.85 Ga is consistent with Ar-Ar ages of (1.3–1.4) ± 0.3 Ga for three Itokawa particles 16 but different from the Ar-Ar age of 2.3 ± 0.1 Ga for the single grain 17 . It should be noted that the U-Pb system is highly resistant to secondary events in contrast to the Ar-Ar system, which is sensitive to shock.…”

Impact History and Regolith Evolution on the Moon: Geochemistry and Ages of Glasses from the Apollo 16 Site

“…Larger impactors such as those inferred for terrestrial Archean spherule layers (10 km; Johnson & Melosh, ) would produce an abundance of young craters ~100 km diameter on the Moon that is not observed. The predominantly local provenance implied by the compositions of lunar regolith glasses is also consistent with their formation by relatively small craters, possibly enhanced by the elevated temperatures at relatively modest pressures that can be produced in porous targets such as the lunar regolith (Jourdan et al, ; Wünnemann et al, ). As such, the age distribution of the regolith glasses cannot be compared directly with the crystalline lunar melt rocks, which reflect much larger (basin‐scale?)…”

Ultraprecise age and formation temperature of the Australasian tektites constrained by40Ar/39Ar analyses

“…The three‐ to fourfold improvement in precision of individual plateau ages is due in part to improved electronic stability of the Faraday cups, but mostly due to the ability to precisely measure the small 36 Ar mass on a CDD (ultralow‐noise ion counter), which can yield ultraprecise measurements even for extremely small ion beam signals (e.g., Jourdan et al. ). Precise measurement of 36 Ar is important to correct for trapped (nonradiogenic) 40 Ar contribution in a sample, especially for young samples where the amount of 40 Ar* produced is still low, and thus, the large mass of sample needed for a precise analysis results in the addition of a substantial amount of naturally trapped argon released during step‐heating extraction (Annex 2).…”

“…(1) We use the ArArDIFF algorithm (Jourdan and Eroglu ; Jourdan et al. ), which allows the modeling of 40 Ar transport in a sphere by diffusion and thus, calculating the effect of given T – t histories on the shape of a synthetic age spectrum and (2) we use a forward modeling Monte Carlo approach applied to a simplified sets of diffusion equations from McDougall and Harrison () for a sphere. This approach can accommodate a large number of adjustable parameters and partially take into account the uncertainties of D 0 and E a measured in this study.…”

“…Modeled age spectra for a 5‐mm radius sphere with a pre‐ 40 Ar* loss age of 1.5 Ma calculated using the ArAr DIFF algorithm (Jourdan and Eroglu ; Jourdan et al. ). We simulate various peak temperature values followed by a cooling phase down to 1100 °C (~solidus temperature) of either 10 s or 30 s, and subsequent cooling phases from 1100 °C to 700 °C for the following 20 s and from 700 °C to 25 °C for the following 10 min.…”