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How height on Martian surface is determined if there are no seas ?

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"It's actually a very tricky question," says Dr Simon O'Toole from the Australian Astronomical Observatory. "A lot of people think it's just taking the highest and lowest points on the planet and finding the average, but it's not that simple. Because there's no sea level on Mars any more, zero altitude is defined as a specific atmospheric pressure of 610.5 Pascals, about six millibars. This value was chosen because it's the triple point of water on Mars, where it can exist as gas, liquid or solid."

Courtesy of ABC News. I'm not sure whether it's correct or not, but it sounds plausible and I'd personally believe it unless corrected by an expert.

Edited by Jaelommiss
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Yeah, it's arbitrary. They just picked a level and called that the datum. I hadn't realized they picked it because it was the triple point of water -- that's interesting info. But they could have picked any other point too. The key is that whatever they pick, everybody agrees to use the same one.

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Basically, much like Earth, the mean surface is defined by an equipotential gravitational surface.

Two methods are/were employed :

- Surface pressure : Assuming the temperature plays only a small role, the only reason for surface pressure to exist is due to gravitational forces. Hence, if two points were to have the same surface pressure, it must have been affected by the same gravitational force - it is on the same equipotential surface.

- Gravitational acceleration : Obviously this is a lot harder to measure directly, but in places where the value are the same, regardless of direction, it must be located on the same equipotential surface.

The values attached to them are purely from a unifying and convenience perspective.

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