Images clearly show the soil sample covering the entrance to the oven
Scientists working on Nasa's Phoenix lander are trying to work out why a soil sample dropped on to an instrument bay was not registered.
Images sent back from Mars clearly show the sample lying across a screen protecting the opening to a tiny oven.
But it seems the soil may have been too lumpy to pass through into the Thermal and Evolved-Gas Analyzer.
The oven would have baked the soil to gain some insights into its possible water content and mineral make-up.
The sample was the first delivered to Phoenix's onboard laboratory by the 2.4m-long robot arm.
Until this time, the arm had merely been practising its digging technique.
The screen which covers the entrance to the analyzer is designed to let through only particles that are smaller than one millimetre, to stop a funnel leading to the tiny oven becoming clogged with larger fragments.
Polygon patterns are similar to those in permafrost regions on Earth
But an infrared beam crossing the pathway failed to register any material falling through the funnel, suggesting none of the sample material made it into the analyzer.
Scientists and engineers on the Phoenix mission team are now working through ideas on what to do next.
"I think it's the cloddiness of the soil and not having enough fine granular material," said Ray Arvidson, of Washington University in St Louis.
"In the future, we may prepare the soil by pushing down on the surface with the arm before scooping up the material to break it up, then sprinkle a smaller amount over the door," he explained.
Phoenix landed on Mars' northern plains on 25 May. It has sent back pictures of a flat, rust-coloured landscape which is marked by polygonal shapes in the ground.
Scientists think these polygonal features are the result of large quantities of water-ice expanding and contracting just below the dusty surface.
Phoenix will spend the next three months investigating its landing location for signs that life could ever have found the conditions there habitable.
The Phoenix lander carries seven science instruments