The Limited Role of the Streaming Instability During Moon and Exomoon Formation
arxiv(2024)
摘要
It is generally accepted that the Moon accreted from the disk formed by an
impact between the proto-Earth and impactor, but its details are highly
debated. Some models suggest that a Mars-sized impactor formed a silicate
melt-rich (vapor-poor) disk around Earth, whereas other models suggest that a
highly energetic impact produced a silicate vapor-rich disk. Such a vapor-rich
disk, however, may not be suitable for the Moon formation, because moonlets,
building blocks of the Moon, of 100 m-100 km may experience strong gas drag and
fall onto Earth on a short timescale, failing to grow further. This problem may
be avoided if large moonlets (≫ 100 km) form very quickly by streaming
instability, which is a process to concentrate particles enough to cause
gravitational collapse and rapid formation of planetesimals or moonlets. Here,
we investigate the effect of the streaming instability in the Moon-forming disk
for the first time and find that this instability can quickly form ∼ 100
km-sized moonlets. However, these moonlets are not large enough to avoid strong
drag and they still fall onto Earth quickly. This suggests that the vapor-rich
disks may not form the large Moon, and therefore the models that produce
vapor-poor disks are supported. This result is applicable to general
impact-induced moon-forming disks, supporting the previous suggestion that
small planets (<1.6 R_⊕) are good candidates to host large moons because
their impact-induced disks would be likely vapor-poor. We find a limited role
of streaming instability in a satellite formation in an impact-induced disk,
whereas it plays a key role during planet formation.
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