Re: zero elevation and mean Mars radius

[Dave Smith <dsmith@tharsis....nasa.gov>]

You are absolutely correct.

We define the potential of the mean equatorial radius to be zero elevation.  We extend this equipotential surface across the whole planet and the MOLA topography is measured with respect to this equipotential surface so that we can  determine which way is "downhill".

Hope this helps.  We will add the word "equatorial" to our statement on our web site.

Dave Smith
PI, MOLA



> Dear Dr. Smith,
>
> in
> Linkname: Mars Geodetic parameters from MOLA
> URL: http://ltpwww.gsfc.nasa.gov/tharsis/geodesy.html
>
>
> the definition 
> # Zero elevation is defined as the gravitational equipotential surface
> whose average value at the equator is equal to the mean EQUATORIAL radius as
> determined by MOLA.
>
> seems to me a bit confusing -- should it not read 
> ... to the mean equatorial radius 
> ?
>
> Otherwise, the mean height of equator features would be some 6.7 km, as is
> the difference 
> mean equatorial radius - mean radius
>
> but from the map at
> Linkname: Quarter-degree global topographic map of Mars
> URL: http://ltpwww.gsfc.nasa.gov/tharsis/quart.map.html
> it is apparent it is not the case, mean equatorial altitude seems to
> be close to zero. 
>
> I want to point my Czech colleagues to your excellent page (in order to
> clear the confusion with ``the highest mountain on Mars''), but it should
> be really clear, what is meant by ``height'' (of course, the difference
> between Olympus and Ascraeus montes altitudes would change very little by
> chosing another equipotential surface).
>
> yours sincerely
> Jenik 
>
> --------------------------------------------------------------------------
> Jan Hollan
> N. Copernicus Observatory and Planetarium in Brno
> KravÌ hora 2, CZ - 616 00 Brno                        +420 (5) 41 32 12 87
>
> home:
> Lipov· 19, 602 00 Brno                                         43 23 90 96
>
> member of the Society for Sustainable Living
> and volunteer of the Ecological counselling Veronica
> Pansk· 9, 602 00 Brno, Czechia                  fax:  +420 (5) 42 21 05 61
>
> e-mail: hollan@ped....cz             http://astro.sci.muni.cz/pub/hollan
> --------------------------------------------------------------------------