system
Given the following post, respond as if you're commenting on Reddit:
human
How is altitude measured on other planets if they don't have seas and hence they don't have a mean sea level?
gpt
A zero-altitude reference surface is defined.  Depending on the body and the usage, this surface might be a sphere, spheroid, or ellipsoid whose dimensions are taken from average measurements of the planet's dimensions.

The reference surface for large solid bodies with slow rotation (e.g. Mercury, Venus) are often simply spheres with the average radius of the planet.  

For faster rotaters (Earth, Mars) an oblate spheroid is more appropriate.  This shape approximates the equatorial bulge caused by planet's rotational momentum.

For a *tidally locked* fast rotater (Io, Europa) an ellipsoid usually provides the best fit reference surface.

For the gas giants, the convention has been to define zero altitude to be the atmospheric depth at which the pressure equals 1 bar (100 kPa).