For decades, scientists have been fascinated by the ____1____ that Earth’s next-door neighbor once harbored life. And although no hard ____2____ has yet been found, scientists are still hopeful that signs of life---past and ____3____ present--- will still turn up. Because Mars missions have shown that in many ways Mars isn’t so very different from our __4__. At its poles, the so-called Red Planet is very much like the Earth. For example, the Phoenix Lander found that, just like on Earth, Mars once had large salty lakes and rushing rivers. And where there was once water, there may very well have been life. But even though Martian water is mostly long gone from the ___5___, Mars still resembles Earth in many ways. Similar to Antarctica, Mars’ north polar ___6___ contains ice just below the surface. And at least some of that ice is mixed with soil containing some ____7____ that could support life. And every few million years, the Martian ice caps melt. This could mean winter snowstorms and even summer wetlands similar to tundra ( 苔原 ) regions on Earth. So does this mean that Mars once was full of living things? _______8_______. But some organisms can live even in the most extremely cold and dry environments on Earth. So life may not only have existed on Mars in the ___9___ past. It could still be there today, hiding in the icy subsurface and waiting to grow when the ___10___ is right.