( Mercury ( ( rotates ( on ( its ( axis ( ( once every ) ( fifty-nine ( earth days ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Mercury)) (VP (VBZ rotates) (PP (IN on) (NP (PRP$ its) (JJ axis) (ADJP (RB once) (DT every)) (JJ fifty-nine) (NN earth) (NNS days)))) (. .)))
Mercury rotates on its axis once every fifty-nine earth days.
(ROOT (S (NP (NNP Earth)) (VP (VBZ rotates) (PP (PP (IN on) (NP (PRP$ its) (NNS axis)) (ADVP (RB once))) (CC times) (PP (IN in) (NP (CD one) (NN day))))) (. .)))