The Search for Life on Mars (A Photo Timeline)

Since humans first began to wonder if there was life beyond Earth, one of the first places we hoped to find it was Mars. While our expectations have changed from the idea of cities and walking, talking Martians, to the thought that microbial organisms may lie deep beneath Mars' surface, this hope hasn't died. Here's a timeline of humanity's attempts to look for life on Mars through the years

As outrageous as it may sound to us now, a prominent 19th century astronomer named Percival Lowell was convinced not just that there was life on Mars, but that this life was intelligent and had built cities and a complex network of canals across the face of the planet. The theory went that Mars was once lush, but now was drying out, so the Martians had engineered the canals to route water down from the planet's polar ice caps to feed their civilization. Though Lowell popularized the idea, Italian astronomer Giovanni Schiaparelli was the first to propose Martian canals in the 1870s. It wasn't until the 20th century that astronomers had observational techniques improved enough to disprove the canal theory. Eventually, Martian canals were revealed to be merely optical illusions.

The first photos of Mars from orbit came from the NASA probe Mariner 4, which flew by the Red Planet in 1965. The probe's 21 photos of a dry, dusty surface disappointed those hoping for rivers, oceans or any signs of life. Its findings largely killed the hope that Mars had human-like intelligent life, and shifted the focus to searching for more basic organisms.

It was more than a decade later that the first Mars landers were able to send back photos from the surface of the planet. NASA's Viking program sent two probes to orbit the Red Planet, which each ejected landers that touched down on its surface. The first man-made vehicle to come in contact with the surface of Mars was Viking 1, which landed July 20, 1976. It was followed on September 3 by Viking 2, which landed in a different spot. The two landers beamed back photos dispelling the notion completely that intelligent life was walking around Mars, or that liquid water was flowing on its surface. Viking was equipped with experiments specifically designed to detect life in Martian soil, if it existed. One experiment did appear to find evidence for metabolic reactions. However, because other Viking experiments failed to detect organic molecules, the results were largely dismissed as being caused by non-living agents in the soil. Later studies, though, suggested that the organic building blocks of life may have been present in the Martian dirt tested by Viking, but that other chemicals there would have destroyed these organics when heated for the experiment. Ultimately, some researchers still debate what Viking's measurements mean.

A Martian meteorite discovered in Antarctica in 1984 called ALH84001 was found to have a mineral called magnetite that on Earth is associated with the presence of microorganisms. The meteorite is thought to have formed on Mars at least 16 million years ago, and landed on Earth roughly 13,000 years ago. Carbonate materials in the meteorite also indicate that liquid water was present when they formed. Finally, some have claimed that small structures inside the meteorite appear to be fossilized nanobacteria, though this idea has been controversial. Similar signatures, and debates over what they mean, have been found in other meteorites originating on Mars in the years since.

The first rover sent to Mars was NASA's Sojourner, part of the Mars Pathfinder mission, which also included a stationary lander called Pathfinder. The rover accomplished the most sophisticated studies of Martian geology and climate to date, though it found no direct evidence of water or life.

NASA's twin Mars Exploration Rovers Spirit and Opportunity landed on the Red Planet in January 2004. Within a few months, they'd found evidence that liquid water once existed on the Martian surface. Opportunity had found particular rock patterns inside a crater that indicated the presence of flowing water sometime in the past, and nearby chemicals suggested the site was once the shore of a salt-water sea.

The Mars Reconnaissance Orbiter arrived to circle the Red Planet in March 2006, taking high-resolution photos of many areas on the world's surface. The orbiter has found further evidence that Mars was once wetter than it is now, with observations of surface patterns that could only have been made by flowing liquid, most likely carbon dioxide or water, in its recent geological past.

On May 25, 2008 NASA landed the Phoenix spacecraft on Mars' arctic plains to study whether liquid water may have existed there, creating an environment habitable to life. After about a month of operations, NASA announced that Phoenix had found the first proof of current water ice under the surface of Mars. The lander had dug a small trench and observed chunks of a bright substance there that vaporized after a few days. Eventually, Phoenix's mass spectrometer was able to detect water vapor from a sample of Mars dirt. Significantly, this water was found near Mars' north pole, but not in its polar icecaps, indicating that water extends farther than just those areas of trapped water ice, dust and carbon dioxide.

In November 2011, NASA launched its most expensive, ambitious probe to Mars yet. The Mars Science Laboratory rover, called Curiosity, is due to land on Mars Aug. 5, 2012, to begin a two-year journey around the surface hunting for signs that Mars is, or ever was, habitable to life.

On March 12, 2013, NASA announced ancient Mars was capable of supporting primitive, microbial life. The discovery came from the Mars rover Curiosity, based on the rover's first rock sample drilled from 'John Klein' a flat bedrock target, in Gale Crater. [Full Story]