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NikolaiFyodorov  ·  3634 days ago  ·  link  ·    ·  parent  ·  post: Astronomers discover remnants of 1,500 km long river on Mars.  ·  

I realise it's 473 days since we had this conversation, but I've just finished reading this novel and I was overjoyed to discover an answer to the question. Elevation on Mars is based on atmospheric pressure. Zero altitude is defined as 610.5 Pascals.

So now we know.