( ( While ( ( ( ( Mercury , ) ( ( the ( closest planet ) ) ( to ( the Sun ) ) ) ) , ) ( is ( relatively ( close ( to Earth ) ) ) ) ) ) ( , ( it ( ( remains ( one ( of ( the ( most ( mysterious planets ) ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN While) (S (NP (NP (NNP Mercury)) (, ,) (NP (NP (DT the) (JJS closest) (NN planet)) (PP (TO to) (NP (DT the) (NNP Sun)))) (, ,)) (VP (VBZ is) (ADJP (RB relatively) (JJ close) (PP (TO to) (NP (NNP Earth))))))) (, ,) (NP (PRP it)) (VP (VBZ remains) (NP (NP (CD one)) (PP (IN of) (NP (DT the) (JJS most) (JJ mysterious) (NNS planets))))) (. .)))
While Mercury, the closest planet to the Sun, is relatively close to Earth, it remains one of the most mysterious planets.
(ROOT (S (NP (NNP Mercury)) (VP (VBZ is) (ADJP (ADJP (JJS closest)) (PP (TO to) (NP (PRP$ our) (NN sun))))) (. .)))