What is the maximum height could a weather balloon achieve on Mars without rupturing?
Assume that the balloon is adapted for Mars' atmosphere and gravity from standard high altitude balloons used on Earth.
Most weather balloons can go up to about 40 km. At that altitude, the pressure is about 2.9 millibars. The Martian atmosphere is about 6 millibars at surface level. At the summit of Olympus Mons (21.25 km) the pressure is about 0.3 millibar. That would set 2.9 millibar at about 9.8 km above the Martian surface.
The record for highest high-altitude balloon is 53 km, where the pressure is only 0.55 millibar. If released on Mars that balloon would pass the top of Olympus Mons going up to 28 km.