In-Situ Investigation of the Aseroid (162173) Ryugu by the Hayabusa2 MASCOT Lander

Abstract

JAXA’s Hayabusa2 mission was launched in December 2014 to explore the pristine C-type asteroid (162173) Ryugu, whose bulk material can be considered representative for the volatile rich building blocks that formed the terrestrial planets. C-types are the most common variety of asteroids, and 75% of all known asteroids are of this type. They mainly occur at the outer edge of the asteroid belt, where 80% of asteroids are C-type, but chondritic meteorites are rare in our sample collections, which makes (162173) Ryugu a special target. Hayabusa2 reached Ryugu after a 3.5 years cruise on June 27th, 2018, and after initial characterization and landing site selection, the Hayabusa2 spacecraft delivered the DLR/CNES built MASCOT lander to the asteroid’s surface on October3rd, 2018, where MASCOT successfully performed in situ investigations. The MASCOT payload consisted of a wide angle camera (MasCAM), an imaging IR spectrometer (MicrOmega), a multichannel radiometer (MARA), and a magnetometer (MasMAG), gathering a wealth of data during the 17h and 7min surface operations. The asteroid’s surface was characterized down to the millimeter scale, and MasCAM images revealed a surface dominated by rocks and boulders without showing fine-grained material. Most boulders appear dark with cauliflower-like crumbly surfaces, and inclusions observed in high-resolution images have strong similarities with carbonaceous chondrites. Diurnal temperature variations of a ~60 cm diameter rock were found to be consistent with low thermal inertia around 300 J m-2 K-1 s-1/2, much lower than anticipated when compared to samples in our meteorite collections. This indicates that rocks on Ryugu are highly porous and likely very friable. The remnant magnetization of Ryugu's surface material on scales larger than 1 m has an upper limit of 3·10-6 Am2/kg, and a preliminary analysis of the induction effect in Ryugu caused by the variation in the solar wind magnetic field reveals high electrical conductivity of the order of 1 S m-1. In summary, MASCOT measurements indicate that C-type asteroids appear similar to carbonaceous chondrites, and that magnetization did not play a major role during the accretion of the terrestrial planets.