LOS ANGELES — Will the Mars lander's next baking test of soil and ice be its last?

Scientists worry that it could be, thanks to an electrical glitch that threatens the $420 million quest to find the chemical ingredients for life near the Martian north pole.

The Phoenix Mars lander suffered a short circuit several weeks ago to one of its eight tiny test ovens. Scientists fear another outage could render the crucial equipment useless.

So they've speeded up their mission, skipping plans for a slow, deliberate set of heating experiments and moving ahead for the dramatic conclusion.

The change in game plan is the biggest challenge yet for a mission that got off to an impressive start.

Phoenix survived a 10-month journey through space and nailed a perfect touchdown on May 25 on the polar plains. It has wowed scientists by touching ice and finding Earth-like soil.

But it's a complicated, odd contraption. The lander's equipment includes the eight miniature ovens, two microscopes and a chemistry lab to conduct experiments. It also has a long arm for digging trenches. Plans called for it to take several scoops of Martian dirt and ice at different depths over a period of weeks. Each sample was to be baked in one of the ovens, with tests run on the vapors produced to check for the carbon compounds essential to life.

Scientists wanted to understand how the soil chemistry changed according to depth, an analysis that would help them when they test the Martian ice.

"We really feel we need a slow deliberate process to make sure that when we go for the paydirt — that icy soil down at the bottom of the trench — that we're fully prepared to do it properly," chief scientist Peter Smith of the University of Arizona in Tucson said last month after confirming the presence of ice at the landing site.

Last week, Smith said in a statement: "We are taking the most conservative approach and treating the next sample to (the oven) as possibly our last."

After the outage that zapped an oven, the science team decided to skip several steps. In recent days, they've been working toward their big ice dig.

This effort too is encountering snags.

Earlier this week, Phoenix used the blade at the end of its robotic arm scoop to chip at the hard ice. None of the ice bits made it into the scoop, forcing scientists to break out a power tool to drill into the ice. The next oven test could happen as early as next week.

Researchers who have no role in the three-month project said they would be saddened if the oven became disabled since it's the only instrument that can detect carbon.

Planetary scientist David Paige of the University of California, Los Angeles, said it makes sense for mission scientists to go after the ice. But he worried that studying frozen water without a full knowledge of the soil could make it difficult to interpret the results.

"It's a tough predicament," said Paige, who is not part of the mission. "The fact that they managed to land in such a promising locale makes the potential loss all that more difficult."

NASA's checklist for "full mission success" requires Phoenix to analyze at least three oven samples. So far, the lander has completed only one — enough to achieve "minimum mission success" last weekend, said project manager Barry Goldstein of NASA's Jet Propulsion Laboratory in Pasadena.

Results from the first heating of soil detected water vapor and carbon dioxide but no signs of carbon.

It was that first oven test that led to the problematic electrical short. The scoop dumped so much soil that it clogged a mesh screen filter over the oven. To break up the dirt, technicians shook the instrument for several days.

Engineers think the shaking caused the short circuit, and an independent engineering group reported that the problem could happen again if an oven is turned on.

Goldstein of JPL said he doesn't expect future problems, but the team did not want to chance it.

"It's not that we expect one to occur," said Goldstein of another possible short. "It's just us being very cautious."