As interesting as the martian atmosphere is, there's one compound that's presence is probably more important than all the others combined: water. Whilst some early astronomers convinced themselves they saw vast canal systems on the martian surface, it wasn't till Mariner 9 reached the red planet in 1971 that direct evidence of the presence of water was observed - in the form of erosion patterns, weather and vast canyons and floodplains later photographed in more detail by the Viking missions. The importance of confirming the presence of water on Mars cannot be understated, simply because we believe water to be a pre-requisite for all known life on Earth - so it's presence on a foreign world within our solar system goes a long way towards answering one of mankind's biggest and most enduring questions: are we alone in the cosmos? The image above shows some convincing evidence for the presence of large amounts of liquid water on the red planet's surface at some point in its history, and until we sent landers and rovers to Mars - this was all the evidence we had. However, since the Viking landers made their way to Mars' surface in 1970s, we now know there IS water on Mars, but it's usually in the form of ice or clouds rather than liquid. This is in part due to the frigid temperatures at the surface and also due to the low pressures, meaning that rather than melting water on Mars can transform straight from a solid into a gas - a process known as sublimation (similar to what dry ice, frozen carbon dioxide, does here on Earth).
Like Earth, mars has two polar ice caps that shrink and grow depending on the seasons. This is where the majority of ice on mars is found, but also there are small amounts in frosts, glaciers and snow storms, all of which have been witnessed on the martian surface. Stream beds, eroded craters and minerals directly connected to the existence of liquid water have also been observed that strongly suggest the existence of liquid water on mars, yet the question remains: is there liquid water on the surface there now? NASA's next mars mission, the Curiosity Mars Rover, that is due to land in August 2012 is set to answer this question as well as to detect the chemical signatures of microbial life on mars.