On earth, the alitude of mountains is measured from the sea level.
But what is the reference point for the altitude of mountains (like the famous Olympus Mons) on other planets?
“The deepest basin on Mars is Hellas, which measures 2200 km by 1800 km and is about 3 km deep (reckoned from the level on Mars where the atmospheric pressure is 6.1 millibars).”
Taken from the Guiness Book of Astronomy.
Not a very satisfying answer.
this is a little better…
The surface of Mars exhibits two main types of terrain. The northern hemisphere contains an array of geologic features that include large volcanoes, a great rift valley, and extensive plains. Generally the landmasses in the northern hemisphere lie below the Martian “reference level,” the altitude at which we call zero. On Earth, we use sea level for this purpose, but Mars has no sea level. Thus the reference level of Mars is where the atmospheric pressure is 6.1 millibars. The southern hemisphere is dominated by many impact craters and generally has a higher elevation than the northern hemisphere. The southern highlands are considered to be the oldest terrain on Mars.
Err…Thanks, but is there any particular reason to involve the triple point of water ???
Besides, what is the result? Is most of Mars below or above this reference altitude? Is this reference actually used when people mention casually the altitude of some point on Mars or do people/medias actually refer to something more in phase with common sense (like the alitude above the lowest point on Mars or somesuch)?
Finally, how does it works for other planets/satellites. Let’s take the moon, for instance. Lacking an athmosphere, you could hardly use the same reference point. Or at the contrary, what about Venus with its high athmospheric pressure?
I agree. As some places will have nowhere harboring conditions at the triple-point of water, that benchmark seems highly arbitrary. Why not just call 0 the mean elevation, as well as can be determined?
The conditions for the triple-point of water can be created artificially on Mars, and used to calibrate temperature and pressure measuring instruments. Having a benchmark based on a physical constant, however arbitrary, is useful in that instruments can be calibrated locally, instead of being assumed to be accurate after sitting in storage for months in conditions of varying gravity, temperature, and pressure (i.e., in some bin in the back of the vehicle carrying it to Mars from Earth).
Why would a mean elevation be a good place to set “0”? Why not the elevation of the lowest exposed point on the surface? Why not the elevation of the highest exposed point? Any reference point is necessarily going to be arbitrary.
Well, any of those would do, as they are applicable to any body. I guess I figured the mean would be convenient because it’s easier to get a good idea of elevations on a global scale, average that, and be reasonably accurate, than finding with precision the highest or lowest point on a remote body. It’s pretty obvious that Olympus Mons is the highest point on Mars, but what about some of the moons of Jupiter? Can we say more accurately what the highest or lowest point on Ganymede is, or what the mean elevation is, given what we know?
Lowest point doesn’t sound like a very useful reference - it’ll probably be the bottom of some crevasse or canyon. Mean isn’t very convenient either, since much of the planet would be at negative altitude. I think it’s just something you have to re-define for each planet and moon.
FWIW, the “surface” of the sun is the point where it becomes optically thick (opaque) in visible light. It’s a very sharp and well-defined transition. I think the same definition works for gas giants.
What’s wrong with negative altitudes? It’s just numbers, after all.
If I read correctly, the datum is set at the mean radius of the body, except for Earth and Mars.