Nasa scientists have admitted that the Martian soil is too lumpy for the Phoenix probe to analyse as intended.

Mission controllers attempted over the weekend to deliver a soil sample to Phoenix's Thermal and Evolved-Gas Analyser (TEGA).

The instrument comprises an array of eight ovens in which samples are heated so that the gas they emit can be analysed.

But the soil clump is still sitting on top of the sieve-like screen covering the instrument.

Nasa scientists suggested that the soil particles may be bonded by salts, or even moisture created by Phoenix when it landed.

But the boffins are not defeated yet. TEGA is fitted with a vibrator which can be used to shake soil particles free from the clump so that they fall into the ovens.

Should this technique fail, the controllers believe that they can use the scoop on the end of the robotic arm to feed samples directly into the analyser.