Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 22/08/2012 10:30:02

Title: What is ground level on another planet?
Post by: thedoc on 22/08/2012 10:30:02

Steven Wasmer  asked the Naked Scientists:
   
Good morning Chris,

Like the rest of the world I have been spellbound by the curiosity rover over the last few weeks.

I was wondering, when they refer to elevations of different planetary features, what are they using as a reference datumn? Obviously here we use the ocean to calculate elevation in terms of MSL, but without an ocean what do scientists use?

Thanks for an awesome show (http://www.thenakedscientists.com/HTML/podcasts/)!

Steve
USA

What do you think?

Title: Re: What is ground level on another planet?
Post by: Phractality on 22/08/2012 17:19:21

Since the choice is arbitrary, they could select the highest peak or the lowest valley. Per Wikipedia (http://en.wikipedia.org/wiki/Geography_of_Mars#Zero_elevation): Instead, at least for the Mariner 9 mission, they chose the height at which the atmospheric pressure is equal to the tripple point of water. Since they do have storms, the pressure varies significantly, so they must have taken the average pressure. Below that elevation it is theoretically possible for liquid water to exist. That datum is useful if you are exploring the surface in search of water.
If you are making a map for satellites in low Mars orbit, you might choose the top of Mount Olympus for the zero elevation datum.