Hello Rebecca
The Phoenix Mars lander which landed near Mars' north polar region uses sunlight for energy off its solar panels. Due to Mars' orbit and tilt, the northern hemisphere, especially the polar region will go into virtually a (night-time) pattern shortly (November). Therefore, Phoenix will loose power from the Sun, and it will eventually cease operations. Its not known when the summer months return if it will re-emerge from its "sleep" since it will get very COLD, on the average of more than 135 Deg below zero. To learn more about the mission visit their webpage at;
http://www.nasa.gov/mission_pages/phoenix/main/index.html
PS There's also a really neat animation in recent images showing the low sunlight angle as the Sun skims low to the horizon!