Shocked regolith in asteroid 25143 Itokawa surface
Josep M. Trigo-Rodríguez1, S. Tanbakouei1 and Mark Burchell2
Institute of Space Sciences (CSIC-.IEEC), Campus UAB, C/ Can Magrans s/n, 08193 Cerdanyola del Vallés (Barcelona),
Catalonia, Spain,
2
Centre for Astrophysics and Planetary Science, University of Kent, Canterbury Kent CT2 7NR, Kent, United Kingdom,

1

A major milestone in space exploration was to achieve the first sample return from an asteroid by the Japanese Space Agency(JAXA) [1]. The detailed images of asteroid 25143 Itokawa transformed a faint point of light seen by telescope into an
intrincated rubble-pile asteroid, with a complexity that exemplifies the type of challenging bodies we need to confront assource of hazard to humans. These bodies surrounding our planet exhibit challenging properties because they have been
exposed over the eons to space weathering processes and numerous impacts. As a consequence, their crumbly surfaces arecovered by small particles, pebbles, and boulders [1]. The Japanese JAXA/ISAS Hayabusa mission collected micron-sizedparticles from the regolith of asteroid 25143 Itokawa [2]. We have studied some regolith grains using different analyticaltechniques in our Meteorite and Sample Return Clean Lab at the CSIC Institute of Space Sciences in Barcelona in order tounveil some of the processes in which they were formed [3].
There is an important debate about the dominant physical processes at work in the surface of these bodies. Certainly NearEarth Asteroids (NEAs) are exposed to short-perihelion approaches that produce significant thermal stress changes in the rockforming minerals present in their surfaces. Nowadays, it is usually considered that most small regolith particles are producedby thermal fatigue [4], but obviously impacts should play an important role over longer time scales. Fine-grained regolith
could be useful to apply future In Situ Resource Utilization (ISRU) tests in NEAs, so there is specific interest in deciphering
the most dominant mechanisms. Then, which of these two mechanisms dominates the production of regolith over a body like
Itokawa?. This can be tested in a non-destructive way, just by studying the silicate phases using Raman spectroscopy. Itokawa
asteroid is a nice example of a rubble pile and its surface consists of heterogeneously distributed boulders, large rocks andpebbles: In addition, some regions contain fine-grained regolith made of pounded stones and small grits (Saito et al, 2006). A
particularly useful mineral to infer the existence of shock is olivine, as pointed out by Harriss and Burchell [5].
We have studied 3 Itokawa particles provided by JAXA with numbers S14, S23 and S47 were investigated (Table 1). The
polished upper sides of the three particles are shown in Figure 1. To study their specific mineralogy the samples were analyzed
by SEM/EDX (Quanta 650 FEG equipped with EDX Inca 250 SSD XMax20 detector). To identify possible shocked minerals
several regions of interest (ROIs) were defined. Then we have performed Micro-Raman spectra using a spot size of about 1 µm
and a laser power of 0.6 mW cm-2. We studied carrefully the chemical and mineralogical structure to get some clues on theirshock history. Raman spectra were taken at room temperature using the 5145 Å line of an Argon-ion laser with a Jobin-Yvon
T-64000 Raman spectrometer attached to an Olympus microscope, and using a CCD detector cooled by liquid nitrogen. We
concentrated in the silicate phases, so we decided to acquire the spectra in a working window between 100 and 1400 cm-1.
From the EDX images and the peaks found in the Raman spectra we are able to distinguish the major rock-forming minerals(Fig. 1).

Table 1.

Catalog numbers, maximum size and composition of the studied regolith samples.

Sample#Size (µm)

Main mineral phases

RA-QD02-0014

131.2 ± 0.1

Olivine, low-Ca pyroxene, plagioclase

RA-QD02-0023149.4 ± 0.1

Olivine, troilite

RA-QD02-0047

108.0 ± 0.1

Olivine, low-Ca and high-Ca pyroxeneAs the main results of the study of the selected ROIs, our Raman spectra of olivine, found for two out of three grains, show two drifted peaks
P1=820 cm-1 and P2=850 cm-1 which are considered characteristics of a shocked phase [6]. A Raman spectrum for S14 particle is shown inFig. 1. The precise peak location depends on the forsterite (Fo) and fayalite (Fa) content of the olivine [7,8]. A Raman spectrum for S14particle is shown in Fig. 1. The precise peak location depends on the forsterite (Fo) and fayalite (Fa) content of the olivine [8-9]. In any case,Harriss and Burchell suggested that shocked olivines above 65 GPa exhibit permanent shifts in the 820 and 850 cm-1 peaks in their Raman
spectra [5].

Figure 1.

Raman spectrum obtained for olivine in sampleRA-QD02-0014

In conclusion, the shock experienced by Itokawa’s regolith grains can be inferred from the deformation found in the lattice of the olivine
grains. Our Raman study of three particles seems to indicate that a significant fraction of these grains could have experienced collisionalprocessing. Obviously the number of studied grains so far is not statistically significant, but this finding could encourage other groups tocomplete a more comprehensive study. If these preliminary results are correct, a significant fraction of the regolith particles collected in theMuses Sea were shocked, and fragmented by impact excavation more than by thermal fatigue. In fact, such scenario is likely because nanoindentation studies we performed on these grains [1] seem to point that the minerals exhibit similar mechanical properties than ordinarychondrites of similar composition [3].
Acknowledgements
JMTR and ST acknowledge financial support from research project PGC2018-097374-B-I00, funded by FEDER/Ministerio de Ciencia eInnovación – Agencia Estatal de Investigación (PI: JMTR).

References[1] Nakamura T., et al., 2012. Science 333, 1113. [2] Saito, J., et al. 2006. Science, 312, 1341. [3] Tanbakouei, S., et al., 2019.
Astronomy & Astrophysics, 629, p.A119. [4] Delbo M. et al. 2014. Nature, Volume 508, 233. [5] Harriss, K.H., and Burchell,M.J., 2016. Meteoritics & Planetary Science 51, 1289. [6] Mohanan, K., et al. 1993. American Mineralogist, 78,.42. [7]Chopelas, A., 1991. American Mineralogist, 76, 1101. [8] Kuebler K. E., et al. 2006, Geochim. Cosmochim. Acta, 70, 6201.[9]Foster N.F., et al. 2013. Cosmochimica et Geochimica Acta 121, 1.