When we look at Mars today, we see a planet without the deep blue oceans, lakes, and rivers that our own planet has in abundance.
Many scientists believe, however, that Mars once had much more water than is visible today.
Billions of years ago, Earth and Mars might have been very similar places. The answer to that question is one of the greatest mysteries we hope to solve in our exploration of Mars. If there is any liquid water flowing onto the surface of Mars today from below ground, it doesn't last long. Following the water really means looking for scientific evidence that water was present in the past or is present today, either below the surface or possibly in rare locations near small, hydrothermal vents like those we might find at Yellowstone. Our Mars missions have already sent back views of the Martian surface that seem to show evidence of dry riverbeds, flood plains, rare gullies on Martian cliffs and crater walls, and sedimentary deposits that suggest the presence of water at some point in the history of Mars.