地球近傍小惑星「ベンヌ」の初期観測結果

Unexpected surface features of the near-Earth asteroid (101955) Bennu are reported in seven papers published in Nature, Nature Astronomy, Nature Geoscience and Nature Communications this week. Although the findings confirm some aspects of radar and lightcurve observations from Earth, the newly reported features provide clues to the origin of Bennu and suggest it may be older than expected.

On 3 December 2018, NASA’s OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) spacecraft arrived at Bennu to characterize the asteroid and bring back a sample. Such an asteroid may have once introduced hydrated, carbon-rich material to Earth. Asteroids and comets are the leftover debris from the formation of the Solar System, thus the information encoded in the surface, shape and dynamical properties of Bennu, developed over its lifetime, may provide insights into different stages of Solar System evolution.

The early observations from instruments onboard OSIRIS-REx confirm the presence of widespread and abundant hydrated minerals. The observations also identified the unexpected presence of numerous, large boulders. Several features, such as the lack of small craters and the heterogeneous appearance of the surface, suggest that the surface includes different regions from different eras - remnants of Bennu’s parent body and signs of recent activity, for example. The authors suggest that Bennu has an estimated age of between 100 million and one billion years old - which is older than expected - and probably originated in the main Asteroid Belt.