Is there really water on Mars?

When scientists compared high-resolution 3-D images of Mars taken in 2005 to those taken in 1999, they noticed something unusual. A series of bright, depositary streaks had formed in gullies during the intervening years. Since flash floods can carve away soil and leave behind new sediments on Earth, some observers believed they finally had strong evidence that liquid water, and therefore the potential for life, existed on Mars.

Image courtesy NASA/JPL/ University of Arizona The Mars Reconnaissance Orbiter's High Resolution Imaging Science Experiment (HiRISE) camera took captured images of gully channels on Mars.

Since li­fe as we know it -- even the most bizarre or minute varieties of life -- depends on liquid water, scientists assume that it would also be a necessity for extraterrestrial organisms. Mars is replete with water, but most (if not all) of it is in frozen or vapor form. Ice caps the planet's poles and might also circle areas near the equator [source: The New York Times]. Patches of ice lie over dunes in craters. Mars also shows signs of having had liquid water in the past. Studies of craters like the Holden crater show erosion patterns suggestive of ancient Martian rain.
But until recently, most people assumed that if Mars had liquid water, it no longer did and hadn't for quite some time. Just a decade ago, scientists speculated that the planet hadn't had liquid water for several billion years. The atmosphere and temperature of Mars makes the idea of liquid water -- or at least water as we know it -- seem nearly impossible. The planet is extremely dry and its distance from the sun keeps the temperature between 22 and -124 degrees Fahrenheit (5.5 and -86.6 degrees Celsius).
However, liquid water on Mars wouldn't necessarily be the same as liquid water on Earth. If the water was highly acidic, for example, it would have a lower freezing point and could maintain its liquid state in the chilly climate [source: National Geographic].
But where would liquid Martian water come from? What else could have caused the depository streaks? Go to the next section to find out.