EDP Sciences
Free Access
Issue
A&A
Volume 599, March 2017
Article Number L1
Number of page(s) 4
Section Letters
DOI https://doi.org/10.1051/0004-6361/201630346
Published online 20 February 2017

© ESO, 2017

1. Introduction

After the success of the Hayabusa mission, which returned to Earth in 2010 with samples of the S-type near-Earth asteroid (NEA) (25143) Itokawa (Fujiwara et al. 2006; Nakamura et al. 2011), the Japan Aerospace Exploration Agency (JAXA) launched the Hayabusa2 mission in December 2014, with arrival at the C-type NEA (162173) Ryugu in July 2018, and return to the Earth of the asteroid samples in December 2020. The Hayabusa2 mission also implements the Small Carry-on Impactor experiment designed to remove the asteroid surface regolith locally and create an artificial crater (Saiki et al. 2016), as well as the German/French (DLR/CNES) compact landing package MASCOT (Mobile Asteroid surface SCOuT), designed to perform in-situ investigations of the surface (Ho et al. 2016).

As a primitive asteroid (with a most likely dynamical origin in the inner main belt; Campins et al. 2013), a sample return mission from Ryugu will provide a better understanding of the origin and evolution of the solar system. In particular, the mission will allow for the investigation of key topics, such as the original organics and water present in the early solar system, as well as the delivery of such materials to the primitive Earth (e.g., Barucci et al. 2012). Unfortunately, the nature of Ryugu still presents some obscure points that could affect the mission operations and limit its scientific return.

First of all, there are uncertainties about the homogeneity of the surface composition of Ryugu. A worldwide observational campaign of this body was carried out during the observational opportunity in 2012. Moskovitz et al. (2013) noticed that there is no detectable rotational variation in the spectra at 0.50.9 μm to an accuracy of ~4%. On the contrary, Lazzaro et al. (2013) suggested that there could be differences in the UV wavelength region, showing a spectral drop-off at ~0.4 μm by 1030%. Hiroi et al. (1996) noticed that, for primitive asteroids, there is a strong correlation between the presence of the UV drop-off and the hydrated silicate absorption around 3 μm. Thus, the spectral variation in Lazzaro et al. (2013) can be explained by a heterogeneity in the surface distribution of hydrated minerals. Noteworthy, this UV/blue absorption (at approximately <0.55 μm), typical of Cg asteroids, had already been detected in the spectrum of Ryugu by Binzel et al. (2001). A further absorption feature related to hydrated minerals at ~0.7 μm was reported by Vilas (2008) but has not been confirmed in any subsequent observation, including those presented in this work. Such a feature is attributed to Fe2 + → Fe3 + charge-transfer transitions in oxidized iron in phyllosilicates. All of these features are due to aqueous alteration (e.g., Rivkin et al. 2002), that is, a chemical alteration of a material by the interaction of that material with water. Aqueous alteration in asteroids may be connected to early aluminium-26 heating, a now decayed short-lived isotope, or to electrical induction heating due to a strong early (T Tauri phase) solar wind. For the sake of proper Hayabusa2 mission planning, it is important to confirm the presence of aqueously-altered material on particular regions of Ryugu. Indeed, their presence would put constraints on thed by the historymA;am:97e fulch ma(at apprRlgu. Thyzcludinals tolre therntroducti to thets that coullready meta rations and ct

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