This red-filter image taken by the lander's Surface Stereo Imager shows the sun rising on the morning of Day 90, Aug. 25, the last day of the Phoenix nominal mission. It was taken at 51 minutes past midnight local solar time during the slow sunrise that followed a 75-minute "night."

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Though the sun has begun to set on the UA-led Phoenix Mars lander, the spacecraft's mission is far from over.

After digging through the red planet's arctic soil for three months — delivering scientific results that have expanded our knowledge of Mars — Phoenix and its mission are moving into the home stretch.

The Martian summer, which has bathed Phoenix in round-the-clock sunlight, is yielding to a winter that eventually could encase the spacecraft in dry ice.

Facing the very real prospect of losing the spacecraft permanently, mission planners are working to gather as much information as possible about Mars' soil composition while trying to tie up loose ends and confirm previous test results.

"We are pushing," said Peter Smith, the University of Arizona's lead scientist for the mission. "We're trying to get that last bit of data."

Earlier this week, the lander completed its 90th day on Mars, marking what originally was planned to be the mission's final day by sending back a picture of a Martian sunrise.

Leaders initially planned for the mission to last three months after Phoenix landed in May, but in July NASA extended it through September.

Rather than shutting down, the lander dug into a deep trench and prepared to deliver another soil sample to one of its on-board experiments.

But as the mission continues, sunlight — and the solar energy used to power Phoenix — grows increasingly scarce.

Last week, the sun dipped halfway below the horizon for about a half-hour, and mission planners predict the sun will fully set and rise on Phoenix in the coming days.

Though Phoenix will continue to draw in solar energy for weeks to come and has enough energy stored in its batteries to conduct the rest of the mission, the spacecraft is expected eventually to run low on power as winter sets in.

And even if Phoenix continues to have enough power, mission planners predict the dry ice that covers the Martian arctic each winter could render the spacecraft inoperable.

Given the tight timeline, mission leaders are trying to determine how to use the remaining experiments to bring some closure to the mission, said Chris Lewicki, a Phoenix mission manager with the Jet Propulsion Laboratory in California.

But scientists know they probably won't be able to answer every lingering question, Lewicki said.

"We realize that we're going to end this mission with unfinished business," he said. "We're really trying to figure out in the next month what questions we need to answer to help the final outcome of the science."

For example, questions remain about the presence of perchlorate in the soil, Smith said.

Experiments found conflicting results on the presence of the mineral, which is an oxidizing agent that can be used as an energy source for organisms and also is the main ingredient in rocket fuel.

Scientists also will try to see whether the soil holds any organic materials, Smith said.

But even after researchers with Phoenix gather the last data from Mars, it could take a while before scientists begin to fully comprehend the results.

The data will have to be analyzed, and experiments will need to be duplicated in laboratories.

Following that, other scientists will have the opportunity to comb through the research and scrutinize findings made by the Phoenix team.

It's a process that takes time and requires patience, Smith said. "This is the harvest period," he said. "We haven't made the wine yet. We still have to stomp the grapes."