Strange Planet by Nathan W. Pyle

Then you don't know how to calibrate thermometers.

You can't use boiling or freezing water. The boiling and freezing point of water changes based on pressure.

Any mixture of liquid water, frozen water, and ammonium hydroxide is 0°F at any pressure (assuming it itself isn't boiling or freezing), which you can then calibrate to STP with a bit of math.