Even during the 24/7 sunshine - PHX was experiencing cycles from -30 to -80 every day. But the circuitry inside is being kept nice and warm, steady, and avionic-friendly with heaters.

As it gets to winter, with zero power, it'll probably see -150 enough to freeze the batteries, and potentially trash circuitry (i.e. break cables, solder joints etc). It's one almighty, lengthy, and deep thermal cycle.

MER is an interesting comparison. Because PHX is the '01 lander, reborn - it's really not a great design for the polar region. The spacecraft bus was actually headed to Meridiani Planum before being cancelled. It's not like the MER warm electronics box. That, with little heating, can keep the electronics inside above -50 (just) in the depths of equatorial winter. Indeed, on just 250whrs/sol, the MER design can survive the winter. PHX needs probably 4x that just for survival heaters alone, and each whr is harder to come by up there.

I put a Tamagochi I was given free once, in our freezer. -10 maybe, for 24 hrs. It never worked again. I miss that guy. ;)

Doug