The spacecraft arrived in the northern polar region of Mars in May. Since then, the lander has been digging trenches and conducting science experiments to study whether the environment on Mars could support primitive life.
Project manager Barry Goldstein said Monday the team has not heard from Phoenix since Nov. 2.
Goldstein said the space agency will continue to use overhead orbiters to try to communicate with Phoenix, but the chances are slim that the spacecraft will wake up.
The mission was scheduled to last three months, but Phoenix has operated for more than five months. During its three-month prime mission, the sun stayed above the horizon, allowing Phoenix to dig trenches in the soil and collect ice bits for its various instruments to analyze.
NASA extended the mission to the end of September in August after soil tests revealed similarities with earth's soil and the lander discovered water on the surface of the planet. The soil and water tests bolstered scientists hope that Mars may be able to sustain life.
"There's nothing about it that would preclude life. In fact, it seems very friendly," mission scientist Samuel Kounaves of Tufts University said of the soil after the tests in June. "There's nothing about it that's toxic."
In its final days, overnight temperatures plunged to minus 141 degrees, and daytime temperatures reached only minus 50, the lowest temperatures during the mission. The lander also battled a dust storm, in late October, which drained its energy.
Click Here to see the CNET photo gallery of the Phoenix Mars Lander dig.