The Albedo of Ryugu: Evidence for a High Organic Abundance, as Inferred from the Hayabusa2 Touchdown Maneuver

Abstract

The Hayabusa2 mission successfully collected samples from the asteroid Ryugu last year and will return these to Earth in December 2020. It is anticipated that the samples will enable the analysis of terrestrially uncontaminated organic matter and minerals. Such analyses are in turn expected to elucidate the evolution of organic matter through Solar System history, including the origination and processing of biogenically important molecules, which could have been utilized by the first organisms on Earth. In anticipation, studies have made predictions concerning the properties of Ryugu, including its composition. The spectral characteristics of Ryugu, such as albedo, have been employed to relate the asteroid to members of the carbonaceous chondrite group that have been identified on Earth. However, the recent Hayabusa2 touchdown highlights a disparity between the color of surfaces of displaced platy fragments, indicating a brightening trend for the surface exposed to space compared to that facing into the body. Here we present a mass balance calculation with reference to data from the literature, which indicates that Ryugu may contain a significantly higher abundance of organic matter (likely >50%) than the currently most accepted meteorite analogues. A high organic content may result in high levels of extractable organic matter for the second touchdown site, where the spacecraft sampled freshly exposed material. However, high abundances of insoluble aromatic/graphitic rich organic matter may be present in the first touchdown site, which sampled the surface of Ryugu that had been exposed to space. Moreover, we suggest that the potentially high organic abundance and the rubble-pile nature of Ryugu may originate from the capture of rocky debris by a comet nucleus and subsequent water-organic-mineral interactions and sublimation of water ice.

The asteroid 162173 Ryugu: a cometary origin

“…If the cometary nucleus is mainly composed of water ice that contains 1 wt.% refractory organic matters, and captures 1 wt.% rocky debris as assumed in Figures 2 and 3, the masses of organic matters and rocky debris left behind become comparable. The mass ratio is consistent with the estimate of the organic content of Ryugu's surface layer from its albedo [2]. The organic-rich surface of comet 67P/Churyumov-Gerasimenko [21] may also be explained by the sublimation-induced concentration of organic matter.…”

“…The third feature is the extremely high organic matter content compared to those in carbonaceous chondrites. Mass balance calculations based on the difference in albedo between the surface and the underground materials recognized after the touchdown of Hayabusa2 spacecraft inferred that the surface layer of Ryugu would contain about 60% organic matter by area, if the coexisting silicate components have optical properties similar to those of the CM chondrites [2]. This estimate is much larger than the typical organic content in carbonaceous chondrites (likely < 10 wt.% [11]).…”

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The Japanese Hayabusa2 mission has revealed in detail the physical characteristics of the C-type asteroid 162173 Ryugu, in particular, its spinning top-shaped rubble pile structure [1] and the potentially extremely high organic content [2,3]. A widelyaccepted formation scenario for Ryugu is catastrophic collision between larger asteroids and the subsequent slow gravitational accumulation of collisional debris [4,5].

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“…However, the collisional reaccumulation scenario does not explain the origin of the abundant organic matter. An alternative scenario is that Ryugu is an extinct comet, which lost its icy components [2,6,3]. Here, the sublimation of water ice from a uniform porous cometary nucleus was numerically simulated until the refractory components, such as silicate rocks and organic matter were left behind as evapora-…”

“…An alternative scenario, which can satisfy the three different major features of the asteroid Ryugu simultaneously, is a cometary origin [2,6,3].…”