About 4 billion years ago, Mars was similar to now Earth, warm and wet. Water flowed in its surface, passed rivers and finally emptied to shallow sea. Thick atmosphere covered Mars and kept the warm climate. There were microbes, and even a few scientists believe that there was a life just like in Earth.
But why Mars become a cold and dry planet? No more rivers or seas. And its atmosphere is very thin. No microbes over there. Or maybe they're hiding under dusts.
Then what happen? Why Mars is dry and frozen now? This question makes scientist confuse about this. Maybe scientists will know it soon because NASA has launched a satellite to Mars called MAVEN (short for Mars Atmosphere and Volatile Evolution).
Below is an image of Mars' sea long time ago, but now it's dry:
