The triple point pressure of water was used during the Mariner 9 mission to Mars as a reference point to define "sea level".
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After the spacecraft Mariner 9 provided extensive imagery of Mars in 1972, a small crater (later called Airy-0), located in the Sinus Meridiani ('Middle Bay' or 'Meridian Bay') along the line of Beer and Mädler, was chosen by Merton Davies of the RAND Corporation to provide a more precise definition of 0.0° longitude when he established a planetographic control point network.
He was a postdoctoral associate and research scientist at the Jet Propulsion Laboratory working on the Mariner 9 project.