You can't beat the seasons. If you're solar powered, and in the Martian Arctic, your days are numbered.

the PHX landing site will be in 24/7 darkness within a few months. When it landed, it was 24/7 daylight. We're now in that transition phase in the drop off from about 3kwhr/sol to zero. At some point on that slope, the arrays no longer generate enough energy to keep the avionics and battery warm, and then it's game over.

To double the problem - as the solar power decreases with the arrival of winter, the heating requirements increase. On landing the daily cycle went from about -30 to -80 deg c. Now it's -40 to -95 - and it takes a lot of energy to keep an extra 15deg C of heat within a lump of aluminium, circuitry and batterys.

Even MSL, with it's RTG power source, would struggle to survive the sort of cold that will kill off Phoenix in the next month or so.

The Martian arctic is a nasty nasty place, and a brutal engineering challenge.