Mars

Our Exploration Strategy: Follow the Water!
To discover the possibilities for life on Mars--past, present or our own in the future--the Mars Program has developed an exploration strategy known as "Follow the Water."
Following the water begins with an understanding of the current environment on Mars. We want to explore observed features like dry riverbeds, ice in the polar caps and rock types that only form when water is present. We want to look for hot springs, hydrothermal vents or subsurface water reserves. We want to understand if ancient Mars once held a vast ocean in the northern hemisphere as some scientists believe and how Mars may have transitioned from a more watery environment to the dry and dusty climate it has today. Searching for these answers means delving into the planet's geologic and climate history to find out how, when and why Mars underwent dramatic changes to become the forbidding, yet promising, planet we observe today.

http://mpfwww.jpl.nasa.gov/programmissions/overview/