The National Aeronautics and Space Administration said the seasonal decline in sunshine at the robot's arctic landing site is not providing enough sunlight to charge batteries that operate the lander's instruments.
"The project team will be listening carefully during the next few weeks to hear if Phoenix revives and phones home," NASA said in a statement. "However, engineers now believe that is unlikely because of the worsening weather conditions on Mars."
While the spacecraft's work has ended, the analysis of data from the instruments is only in its earliest stages.
"Phoenix has given us some surprises, and I'm confident we will be pulling more gems from this trove of data for years to come," said Phoenix Principal Investigator Peter Smith of the University of Arizona in Tucson.
Launched Aug. 4, 2007, Phoenix landed May 25, 2008.
"Phoenix not only met the tremendous challenge of landing safely, it accomplished scientific investigations on 149 of its 152 Martian days as a result of dedicated work by a talented team," said Phoenix Project Manager Barry Goldstein at NASA's Jet Propulsion Laboratory in Pasadena, Calif.