Assertion: comet's origin | /r/HasProperty | mystery
Facets:
value: to astronomers
type: TRANSITIVE-OBJECT
Source sentences:
text: During the Voyager missions to the planet, planetary scientists identified 13 crater chains on Callisto and three on Ganymede, the origin of which was initially a mystery.
text: The origin of Centaurs has been a mystery to astronomers since the first member of the group, called Chiron, was recognised in 1977.