Earth is the only planet known to have liquid water, and water ice has been present over parts of the Earth for much of its history. Scientists have only recent... » 

Earth is the only planet known to have liquid water, and water ice has been present over parts of the Earth for much of its history. Scientists have only recently come to understand how widespread the presence of ice is in our solar system. Deposits of water ice may exist in unexpected places, such as in the polar craters of Mercury, the closest planet to the Sun. Other ices, such as methane ice and nitrogen ice, abound in our solar system. «