mars datum question

From: (Ken Edgett)
Subject: mars datum question
Date: Sat, 25 Jan 1997 12:43:44 -0700 (MST)

Dear Mars Educators,

This is in reply to a question by "Barb in NJ" regarding the martian
datum ("sea level").  I thought it would be useful to share with the
whole group.

The following is from The Planetary Society's MARSLINK Kit #2,
(which is out of print and not available, so bug TPS if you want
them to reprint it, not me...)

The text below on Mars topography is (c) 1994 by The Planetary Society,
so be aware of this when using it.  I am quoting this text because
I wrote this part for them...

- Ken Edgett
  Arizona Mars K-12 Education Program


Topographic mapping of Mars is very similar to methods used to map Earth.
The exception is that Mars has no seas or oceans, hence it has no
actual sea level.  How do cartographers determine martian "sea level"?
Mars scientists refer to the sea level as the *topographic datum*.
This datum is the elevation designated as zero.  For Mars, the zero
elevation is defined by the mean martian radius, 3382.9 kilometers,
and the [average] atmospheric pressure [which] is 6.1 millibars
(6.1 thousandths of the Earth's atmosphere).  If you were standing on
the martian surface and the center of the planet were 3382.9 kilometers
beneath your feet, then you would be standing at 0 kilometers elevation.
Likewise, if the surface depth to the center of the planet were 3407.9
kilometers, you would be at an elevation of 25 kilometers. You'd
probably be standing on [the summit of] Olympus Mons.

To determine the elevations... scientists combined 1972 Mariner 9
mission data with Earth-based radar.  The ultraviolet spectrometer
instrument on Mariner 9 measured the intensity of ultraviolet light
scattered between the orbiting spacecraft and the surface.  By
analyzing the amount of atmosphere under the Mariner, scientists
measured elevations to an accuracy of 0.6 kilometers.  The higher
the atmospheric pressure, the lower the elevation (when compensated
for temperature).  The lowest elevation is in the huge impact crater,
Hellas Planitia.  This is also the location of the highest
atmospheric pressure on Mars.