LOS ANGELES - The Phoenix lander's first chemical sniff of Martian soil did not turn up any trace of the building blocks of life. Its next whiff could be its last.

Engineers said a short circuit that occurred last month in one of its test ovens designed to shake and bake minuscule soil samples could happen again when the instrument is turned on.

"Since there is no way to assess the probability of another short circuit occurring, we are taking the most conservative approach and treating the next sample ... as possibly our last," the NASA mission's chief scientist, Peter Smith of the University of Arizona in Tucson, said in a statement Wednesday.

Phoenix, which landed near the Martian north pole May 25, has eight single-use ovens that heat and analyze Martian soil and ice for signs of organic, or carbon-based, compounds that are essential for life.

The lander delivered its first soil sample scooped up from the surface to one of its ovens last month. The experiment did not yield any ice or organics. Initially, the clumpy dirt could not fit through the oven's opening, so scientists vibrated the instrument several days to break it up. Engineers think the short circuit occurred as a result of the repeated shakes.

Scientists want to bake another soil sample mixed with icy bits in another oven next week. Since this could be the last time researchers conduct this experiment, they planned extensive testing on Earth to make sure they can quickly get the icy soil into the oven before the ice evaporates.

Meanwhile, Phoenix's robotic arm was set to sprinkle soil particles taken from a trench dubbed Snow White onto its microscope today for analysis. If there are leftovers, the rest will be dumped into its wet chemistry lab.