Its notable flat plains are dotted with irregular dark areas surrounded bright halos of material.  
Its notable flat plains are dotted with irregular dark areas surrounded bright halos of material.  Originally discovered by William Lassell, its thin (*) atmosphere of nitrogen and methane was discovered during a close passage by Voyager II, and its unique landscape is thought to be caused by "icy volcanism." For 10 points— identify this icy moon, possibly the coldest object in the solar system and the largest known moon of Neptune.
Its notable flat plains are dotted with irregular dark areas surrounded bright halos of material.  
Triton