Water is ubiquitous on Earth – about 70% of Earth’s surface is covered by the stuff. Water is in the air, on the surface and inside rocks. Geologic evidence suggests water has been stable on Earth since about 4.3 billion years ago.
The history of water on early Mars is less certain. Determining when water first appeared, where and for how long, are all burning questions that drive Mars exploration. If Mars was once habitable, some amount of water was required.
