The Mars Reconnaissance Orbiter

While the sudden appearance of depository streaks thrilled many scientists, others began to question the flash-flood theory. Further analysis of at least one of the gullies suggested that its shape didn't match up with the way fast-moving water flows. The deposits' finger-like shape suggested something granular and dry rushed through the valley -- like fine-grained sand [source: National Geographic].

However, this doesn't completely discount the possibility that there was liquid water involved. Even if the gullies were carved by sand, wet conditions could've initiated the landslide or a small amount of water could've mixed with dirt and sand to form slurry [source: National Geographic].

The images that got scientists speculating came from the High Resolution Imaging Science Experiment (HiRISE), an instrument that can capture minute landscape details and geologic structures. It's one of six instruments that make up the Mars Reconnaissance Orbiter. The orbiter was launched in 2005 to determine if there was ever life on Mars. While scientists hope the mission will also clarify the climate and geology of the planet, the orbiter's scientific payload focuses on the detection of water.

The orbiter's cameras and spectrometers search for mineral deposits left by water. A sounder uses radar to find underground liquid reserves. Other cameras monitor clouds and dust storms. All of the information from the Mars Reconnaissance Orbiter is relayed back to Earth through X-band and Ka-band radio waves that are picked up by the Deep Space Network antenna in Canberra, Australia.

Why water? We tend to take water's properties for granted, but it's an unusual compound. Most planetary scientists agree: Liquid water is necessary for life. But why? The powerful solvent is able to dissolve molecules easily allowing nutrients and metabolites to circulate.

But while the orbiter's primary mission is to see if life ever existed on Mars -- a goal the presence of liquid water would strongly support -- it has a secondary agenda as well. The orbiter's high-resolution cameras create a record of Mars' rocky terrain. This information could help future manned missions know which spots are safe for landing, paving the way to human exploration of the red planet.

To learn more about Mars, the Mars Odyssey and water, explore the links on the next page.