 evidence for a thin atmospheric layer on Europa , Ganymede , and Callisto known as a ' surface @-@ bound exosphere ' ; 
 evidence for a thin atmospheric layer on Europa , Ganymede , and Callisto <a href='uninflect' title='1'><mark>known</mark></a> as a ' surface @-@ bound exosphere ' ; 
Evidence for a thin atmospheric layer on Europa , Ganymede , and Callisto knew as a ' surface @-@ bound exosphere ' ;