( ( ( ( ( ( Right now ) ( there ( are ( ( ice caps ) ( on ( the surface ) ) ) ) ) ) , ) but ) ( ( at ( one time ) ) ( there ( may ( have ( ( been ( water ( in ( liquid form ) ) ) ) ( under ( ( the surface ) ( of ( the planet ) ) ) ) ) ) ) ) ) ) . )
(ROOT (S (S (ADVP (RB Right) (RB now)) (NP (EX there)) (VP (VBP are) (NP (NP (NN ice) (NNS caps)) (PP (IN on) (NP (DT the) (NN surface)))))) (, ,) (CC but) (S (PP (IN at) (NP (CD one) (NN time))) (NP (EX there)) (VP (MD may) (VP (VB have) (VP (VBN been) (NP (NP (NN water)) (PP (IN in) (NP (JJ liquid) (NN form)))) (PP (IN under) (NP (NP (DT the) (NN surface)) (PP (IN of) (NP (DT the) (NN planet))))))))) (. .)))
Right now there are ice caps on the surface, but at one time there may have been water in liquid form under the surface of the planet.
(ROOT (S (NP (NNP Water)) (VP (VBZ takes) (NP (NP (DT the) (NN form)) (PP (IN of) (NP (NN ice)))) (PP (IN on) (NP (NN planet) (NNS mars)))) (. .)))