Asteroid Itokawa … but when did form exactly?
F. Jourdan1, N.E. Timms1, T. Nakamura2, W.D.A. Rickard1, C. Mayers1
1
Curtin University, Perth, GPO Box U1987, WA, Australia
2
Tohoku University, Aoba, Sendai, Miyagi 980-8578, Japan
Rubble piles asteroids consist of reassembled fragments from shattered monolithic asteroids and are much moreabundant than previously thought in the solar system. In earlier studies (1, 2), we analysed five regolith dust
particles recovered by the Hayabusa space probe from the rubble pile asteroid 25143 Itokawa, using a workflowof microstructural analysis by electron backscatter diffraction (EBSD), element mapping by time-of-flightsecondary ion mass spectrometry (ToF-SIMS ± atom probe) and 40Ar/39Ar geochronology. We showed that
Itokawa must have formed before or at ca. ≥ 4. 2 Ga and concluded that rubble pile asteroids can survive
ambient solar system bombardment processes for extremely long periods. Here, we continue our quest tounderstand the bombardment history of Itokawa with the so far seemingly elusive goal of precisely pinpointing
the age and nature of the catastrophic breakup of Itokawa’s parent body. Only through a larger number of40
Ar/39Ar ages on single particles and U/Pb ages on phosphate grains (3) coupled with careful particle
characterizations, will we be able to achieve this goal. With that in mind, we analysed three new particles.
RA-QD02-0300 (dia. = 190 µm) is composed of olivine, plagioclase, with minor troilite. EBSD and ToF-SIMS
analyses show that the plagioclase in this particle is distributed in vein-like structures and has a finepolycrystalline texture of <10 µm grains with abundant inclusions and K-rich exsolutions. This suggests itrepresents recrystallised melt domains. Olivine crystals show no sign of shock deformation. We obtained aplateau age of 4130 ± 33 Ma (P = 0.55). Noticeably, this age is very similar to previous results from otherparticles at 4219 ± 35 and 4149 ± 41 Ma (1), indicating an age concentration around that time. Could it be thatthe parent body of Itokawa broke up at ca. 4.2 Ga while some of the impact heated material slowly cooled down
inside the newly formed insulating rubble pile asteroid over ca. 100 million years? This scenario is tantalizing as
it would be similar to the scenario proposed by (4) for the breakup of an initially hot chondritic parent body at
4.5 Ga and with subsequent debris cooling at rates of ca. 1 to 10 °C/Ma inside the rubble pile asteroids.
RA-QD02-0306 (160 µm) is composed of olivine and plagioclase with minor troilite and chromite. This particle
is the only 40Ar/39Ar-dated particle which contains measurable solar wind, with a 38Ar/36Ar ratio of 0.188 ±0.049 (2σ) that is indistinguishable from modern solar wind value. Along with the presence of abundant microcraters, melt drop and blisters observed on the surface of the particle observed by (5), this indicates that thisparticle spent some time directly at the surface of the asteroid. This particle shows a structured 40Ar/39Ar age
spectrum, with two different portions attributed to the gas released by the plagioclase and olivine phases.
Plagioclase yielded a mini-plateau (59 % 39Ar released) age of 4557 ± 61 Ma (P = 0.71) similar to the U-Pbphosphate age of 4640 ± 360 Ma obtained by (3) on another particle, suggesting that it recorded the initialcooling deep inside Itokawa’s parent asteroid and was not affected by heat during the breakup process. This
demonstrates that, during the breakup of Itokawa’s parent body, part of the asteroid experienced minimum tonegligible shock pressure and temperature increase. This is similar to the conclusions reached via impact
modelling calculated for the rubble pile asteroid Ryugu (6). This is also in excellent agreement with breakup
models of structurally weakened monolith asteroids that do not require a particularly large impactor to shatter(7).
RA-QD02-0311 (108 µm) is composed of olivine, Ca-rich pyroxene, plagioclase and troilite. Elemental maps
show that this particle contains very little plagioclase compared to the other particles and most of the K is
contained in a rim-like structure exposed at the edge of the particle. Olivine crystals show no sign of shock
deformation, yet 40Ar/39Ar dating reveals a complex age spectrum with maximum weighted-mean age of 705 ±53 Ma (P=0.39) including 47% of 39Ar released and indicating an impact ≤ 0.7 Ga. This maximum age, coupledwith an available impact age of 2291 ± 139 Ma (2) and maximum age estimates (1, 8), suggests that impact
events happened many times during the history of Itokawa and that the high porosity of the rubble pile material
was able to convert low-shock events into high temperature spikes, sufficient to reset and/or partially reset the40
Ar/39Ar chronometer (2) in some material.
In summary, we propose that Itokawa’s parent body was disrupted ca. 4.2 billion years ago in a process
involving heterogenous temperature distribution during the impact. Some of the heated material, mixed withcold material, cooled down inside a larger version of Itokawa, possibly over a period of ca. 100 Ma. Itokawa
kept being bombarded and eroded by small impactors since its formation with some of these events recorded by
the 40Ar/39Ar system.
References[1] Jourdan et al. 2023. Proceedings of the National Academy of Sciences 120, e2214353120. [2] Jourdan et al.,2017. Geology 45, 819-822. [3] Terada et al., 2018. Scientific Reports 8, 11806. [4] Lucas et al., 2020. Geochim
Cosmochim Ac 290, 366-390. [5] Matsumoto et al., 2018. Icarus 303, 22-33. [6] Nakamura et al., 2022. Science379, eabn8671. [7] Michel et al. 2004. Icarus 168, 420-432. [8] Park et al., 2015. Meteoritics and PlanetaryScience 50, 2087-2098.