Corrected entry: The aliens are attacking Earth because it's the only planet with water in a liquid state. In other words, the aliens are capable of interstellar travel but aren't smart enough to melt a block of ice from somewhere else.
Nauticalisimo
25th Jun 2011
Correction: They use the water as fuel. Melting the ice would require fuel. See the dilemma? Besides, perhaps they ARE mining ice elsewhere. But they learn of Earth, and it's ready supply of liquid water, and decide the opportunity is too good to pass up.
Phixius ★
Not to mention, much of the "ice" available on other planets is not water ice.
Nauticalisimo