(S (S (S (NP (BASENP (DT Some)) (PP (IN of) (BASENP (DT the) (NN ring) (NNS features)))) (VP (VBP are) (VP (VBN known) (S (VP (TO to) (VP (VB be) (VP (VBN caused) (PP (IN by) (NP (BASENP (NNP Saturn) (POS 's)) (NNS moons)))))))))) (CONJP (COMMA ,) (CC but)) (S (BASENP (NNS others)) (VP (VBP are) (ADJP (JJR harder) (S (VP (TO to) (VP (VB explain)))))))) (PERIOD .))