( Saturn ( ( ( is mostly ) ( ( gas , ) ( ( most ( of which ) ) ( is ( ( hydrogen and ) helium ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Saturn)) (VP (VBZ is) (ADVP (RB mostly)) (NP (NP (NN gas)) (, ,) (SBAR (WHNP (JJS most) (WHPP (IN of) (WHNP (WDT which)))) (S (VP (VBZ is) (NP (NP (NN hydrogen)) (CC and) (NP (NNP helium)))))))) (. .)))
Saturn is mostly gas , most of which is hydrogen and helium .
(ROOT (S (NP (NNP Saturn)) (VP (VBZ is) (VP (VBN made) (ADVP (RB mostly)) (PP (IN of) (NP (NN helium) (CC and) (NN hydrogen))))) (. .)))