Nasa has announced that its Mars Rover Curiosity has failed to find a key chemical associated with life.

The $2.5 billion robot is 14 months into a planned two-year mission to search for signs of organic life on the dusty planet.

It had been hoped that the rover might find traces of methane in an area of the planet known as the Gale Crater.

Previous studies involving data from satellites and telescopes had indicated that small but potentially crucial amounts of the gas might be present on the surface.

If it had been found it might hinted that organic life had been present at one time - on Earth the vast majority of methane in the atmosphere in caused by organic life.

But after fresh analysis and '"exhaustive tests" to search for the gas it now appears that Curiosity has been unable to find traces of methane. At best there is 1.3 parts per billion of methane in the atmosphere - one-sixth the earlier estimates.

Nasa said the results were "a surprise to researchers" and that it reduced the possibility that life had ever survived on the strange world.

"This important result will help direct our efforts to examine the possibility of life on Mars," said Michael Meyer, Nasa's lead scientist for Mars exploration.

"It reduces the probability of current methane-producing Martian microbes, but this addresses only one type of microbial metabolism. As we know, there are many types of terrestrial microbes that don't generate methane."

Scientists also played down the chance that methane previously observed might have been removed through some unknown mechanism.

"Methane is persistent," said Sushil Atreya of the University of Michigan.

"It would last for hundreds of years in the Martian atmosphere. Without a way to take it out of the atmosphere quicker, our measurements indicate there cannot be much methane being put into the atmosphere by any mechanism, whether biology, geology, or by ultraviolet degradation of organics delivered by the fall of meteorites or interplanetary dust particles."

But the Rover itself - or rather, its managers - tweeted later that there is still a chance life once existed on the planet.

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  • Curiosity at Work on Mars

    This artist's concept depicts the rover Curiosity, of NASA's Mars Science Laboratory mission, as it uses its Chemistry and Camera (ChemCam) instrument to investigate the composition of a rock surface. ChemCam fires laser pulses at a target and views the resulting spark with a telescope and spectrometers to identify chemical elements. The laser is actually in an invisible infrared wavelength, but is shown here as visible red light for purposes of illustration.

  • Daybreak At Gale Crater

    This computer-generated view depicts part of Mars at the boundary between darkness and daylight, with an area including Gale Crater beginning to catch morning light.

  • Curiosity Launch Vehicle

    The Atlas V 541 vehicle was selected for the Mars Science Laboratory mission because it has the right liftoff capability for the heavy weight requirements of the rover and its spacecraft.

  • Mars Science Laboratory Spacecraft During Cruise

    This is an artist's concept of NASA's Mars Science Laboratory spacecraft during its cruise phase between launch and final approach to Mars. The spacecraft includes a disc-shaped cruise stage (on the left) attached to the aeroshell. The spacecraft's rover (Curiosity) and descent stage are tucked inside the aeroshell.

  • Curiosity Approaching Mars

    The Curiosity rover is safely tucked inside the spacecraft's aeroshell. The mission's approach phase begins 45 minutes before the spacecraft enters the Martian atmosphere. It lasts until the spacecraft enters the atmosphere.

  • Curiosity Inside Aeroshell

    The Curiosity rover and the spacecraft's descent stage are safely tucked inside the aeroshell at this point. The aeroshell includes a heat shield (on the right, facing in the direction of travel through the atmosphere) and backshell. The diameter of the aeroshell is 14.8 feet (4.5 meters), the largest ever used for a mission to Mars.

  • Mars Science Laboratory Guided Entry At Mars

    The mission's entry, descent, and landing (EDL) phase begins when the spacecraft reaches the top of Martian atmosphere, about 81 miles (131 kilometers) above the surface of the Gale crater landing area, and ends with the rover safe and sound on the surface of Mars. During the approximately seven minutes of EDL, the spacecraft decelerates from a velocity of about 13,200 miles per hour (5,900 meters per second) at the top of the atmosphere, to stationary on the surface.

  • Deceleration of Mars Science Laboratory in Martian Atmosphere

    This artist's concept depicts the interaction of NASA's Mars Science Laboratory spacecraft with the upper atmosphere of Mars during the entry, descent and landing of the Curiosity rover onto the Martian surface.

  • Mars Science Laboratory Parachute

    This is an artist's concept of the Mars Science Laboratory Curiosity rover parachute system.

  • Curiosity While On Parachute

    This is an artist's concept of NASA's Curiosity rover tucked inside the Mars Science Laboratory spacecraft's backshell while the spacecraft is descending on a parachute toward Mars. The parachute is attached to the top of the backshell. In the scene depicted here, the spacecraft's heat shield has already been jettisoned.

  • Curiosity And Descent Stage

    This is an artist's concept of the rover and descent stage for NASA's Mars Science Laboratory spacecraft during the final minute before the rover, Curiosity, touches down on the surface of Mars.

  • Curiosity's Sky Crane Maneuver

    The entry, descent, and landing (EDL) phase of the Mars Science Laboratory mission begins when the spacecraft reaches the Martian atmosphere, about 81 miles (131 kilometers) above the surface of the Gale crater landing area, and ends with the rover Curiosity safe and sound on the surface of Mars.

  • Curiosity Touching Down

    This artist's concept depicts the moment that NASA's Curiosity rover touches down onto the Martian surface.

  • A Moment After Curiosity's Touchdown

    This artist's concept depicts the moment immediately after NASA's Curiosity rover touches down onto the Martian surface.

  • Curiosity Mars Rover

    This artist concept features NASA's Mars Science Laboratory Curiosity rover, a mobile robot for investigating Mars' past or present ability to sustain microbial life.

  • Curiosity's Close-Up

    In this picture, the mast, or rover's "head," rises to about 2.1 meters (6.9 feet) above ground level, about as tall as a basketball player. This mast supports two remote-sensing instruments: the Mast Camera, or "eyes," for stereo color viewing of surrounding terrain and material collected by the arm; and, the ChemCam instrument, which is a laser that vaporizes material from rocks up to about 9 meters (30 feet) away and determines what elements the rocks are made of.

  • Mars Rover Curiosity

    This artist concept features NASA's Mars Science Laboratory Curiosity rover, a mobile robot for investigating Mars' past or present ability to sustain microbial life.