The orbiter ' s first mission is to find landing sites for the phoenix mars lander , set to arrive on mars in may 2008 to dig for subsurface water ice , and for the 2009 arrival of the mars science laboratory , a larger version of the twin robotic geologists spirit and opportunity , which have been traversing the planet ' s surface since 2004 Mro的第一個任務將是為鳳凰號火星登陸車和火星科學尋找合適的降落點。前者將于2008年抵達火星,搜集地下固態(tài)水,后者則是勇氣號和機遇號火星車的增強版,