For centuries,explorers have risked their lives venturing into the unknown for reasons that were to varying degrees economic and nationalistic,Columbus went west to look for better trade routes to the Orient and to promote the greater glory of Spain.Lewis and Clark journeyed into the American wilderness to find out what the U.S.had acquired when it purchased Louisiana,and the Apollo astronauts rocketed to the moon in a dramatic show of technological muscle during the cold war.

　　Although their missions blended commercial and political-military imperatives,the explorers involved all accomplished some science simply by going where no scientists had gone before.

　　Today Mars looms as humanity's next great terra incognita(未探明之地).And with doubtful prospects for a short-term financial return,with the cold war a rapidly fading memory and amid a growing emphasis on international cooperation in large space ventures,it is clear that imperatives other than profits or nationalism will have to compel human beings to leave their tracks on the planet's reddish surface.Could it be that science,which has long played a minor role in exploration,is at last destined to take a leading role?The question naturally in vites a couple of others:Are there experiments that only human could do on Mars?Couldthose experiments provide insights profound enough to justify the expense of sending people across interplanetary space?

　　With Mars the scientific stakes are arguably higher than they have ever been.The issue of whether life ever existed on the planet,and whether it persists to this day,has been highlighted by mounting evidence thatthe Red Planet once had abundant stable,liquid water and by the continuing controversy over suggestions the bacterial fossils rode to Earth on a meteorite(隕石)from Mars.A mor conclusive answer about life on Mars,past or present,would give researchers invaluable data about the range of conditions under which a planet can generate the complex chemistry that leads to life.If it could be established that life arose independently on Mars and Earth,the finding would provide the first concrete clues in one of the deepest mysteries in all of science:the prevalence of life in the universe.

　　1.For What purposes did the explorers go the unknown places in the past?

　　2.In the exploration of Mars,_______________________still remains uncertain.

　　3.What has long been regarded as unimportant in the past explorations?

　　4.What has been found on a meteorite from Mars?

　　5.The conditions under which life originates would be revealed with the proof of_________________on Mars.