system
Given the following post, respond as if you're commenting on Reddit:
human
How do we establish elevation on another planet that has no ocean, ex. Mars?

Do we pick the lowest point on the planet and call that zero? Or what do we use for "zero" elevation, when there is no ocean?
gpt
On Mars we use atmospheric pressure to determine at what elevation water molecules would no longer be stable if they WERE present (6.105 mbar) and use that as a reference point that’s similar to our sea level. It’s called the Mars areoid.