( Mars ( ( ( looks red ) ( because ( ( iron ( in ( its ( surface soil ) ) ) ) ( ( long ago ) ( ( ( reacted ( with ( ( the ( tiny amount ) ) ( of ( oxygen ( left ( on Mars ) ) ) ) ) ) ) , ) ( making rust ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Mars)) (VP (VBZ looks) (ADJP (JJ red)) (SBAR (IN because) (S (NP (NP (NN iron)) (PP (IN in) (NP (PRP$ its) (NN surface) (NN soil)))) (ADVP (RB long) (RB ago)) (VP (VBD reacted) (PP (IN with) (NP (NP (DT the) (JJ tiny) (NN amount)) (PP (IN of) (NP (NP (NN oxygen)) (VP (VBN left) (PP (IN on) (NP (NNP Mars)))))))) (, ,) (S (VP (VBG making) (NP (NN rust)))))))) (. .)))
Mars looks red because iron in its surface soil long ago reacted with the tiny amount of oxygen left on Mars, making rust.
(ROOT (S (NP (NP (NNP Iron)) (PP (IN in) (NP (DT the) (NN soil)))) (VP (VBZ causes) (S (NP (NNS mars)) (VP (TO to) (VP (VB look) (ADJP (JJ red)))))) (. .)))