What happened to Mars's water? New study suggests most was trapped in underground rocks Posted TueTuesday 16 updated WedWednesday 17 MarMarch 2021 at 2:35am The early Martian environment (right), believed to contain liquid water and a thicker atmosphere, is a world away from the cold, dry environment of Mars today (left). ( Share Print text only Cancel Some 4 billion years ago, Earth and Mars were young, warm and wet. If they weren't Solar System twins, they were certainly siblings. Key points: Much of Mars's water has disappeared since the planet formed Planetary scientists suspect most was split into oxygen and hydrogen in the atmosphere, and the hydrogen lost to space