Science Education Standards: Earth in the Solar System:

• The Earth is the third planet from the sun in a system that includes the Moon, the sun, seven other planets and their moons, and smaller objects, such as asteroids and comets.

Structure of the Earth System:

• Water, which covers the majority of the Earth's surface, circulates through the crust, oceans, and atmosphere in what is known as the "water cycle." Water evaporates from the Earth's surface, rises and cools as it moves to higher elevations, condenses as rain or snow, and falls to the surface where it collects in lakes, oceans, soil, and in rocks underground.

• Water is a solvent. As it passes through the water cycle it dissolves minerals and gases and carries them to the oceans.

Short Description: Students investigate several physical properties of liquid water to better understand why we think water is important for life. An extension activity investigates the connection between liquid water and plant productivity on Earth.

Mars: A Dry Planet Compared to Earth

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. If Earth is the "big blue marble," then Mars is the red, dusty one. 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.

But if Mars once had lots of water, where did it go? The answer to that question is one of the greatest mysteries we hope to solve in our exploration of Mars.

Follow the Water

If there is any liquid water flowing onto the surface of Mars today from below ground, it doesn't last long. Temperature and pressure conditions on the surface would either freeze or "evaporate" it right away.

Without a clear presence of surface water, it may seem strange that the science strategy for Mars exploration is "to follow the water." How can we do that, when it doesn't seem like there is any to be found?

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.

The Story of Water on Mars: Follow the Evidence

Take a look at our images from Mars, and be your own detective in looking for clues that indicate the past or present presence of water.

In the meantime, follow "The Case of the Missing Mars Water," brought to you by NASA Science News.