隕石中の炭酸塩から推測される外惑星領域からのD型小惑星の移動

Recent dynamical models of Solar System evolution and isotope studies of rock-forming elements in meteorites have suggested that volatile-rich asteroids formed in the outer Solar System beyond Jupiter’s orbit, despite being currently located in the main asteroid belt1,2,3,4. The ambient temperature under which asteroids formed is a crucial diagnostic to pinpoint the original location of asteroids and is potentially determined by the abundance of volatiles they contain. In particular, abundances and ¹³C/¹²C ratios of carbonates in meteorites record the abundances of carbon-bearing volatile species in their parent asteroids. However, the sources of carbon for these carbonates remain poorly understood5,6,7,8. Here we show that the Tagish Lake meteorite contains abundant carbonates with consistently high ¹³C/¹²C ratios. The high abundance of ¹³C-rich carbonates in Tagish Lake excludes organic matter as their main carbon source5,9. Therefore, the Tagish Lake parent body, presumably a D-type asteroid10, must have accreted a large amount of ¹³C-rich CO₂ice. The estimated ¹³C/¹²C and CO₂/H₂O ratios of ice in Tagish Lake are similar to those of cometary ice11,12. Thus, we infer that at least some D-type asteroids formed in the cold outer Solar System and were subsequently transported into the inner Solar System owing to an orbital instability of the giant planets1,3.