When I did my short OU course 'exploring Mars' planitia was the definition of a low-lying plain and planum was a high plain.
Does anyone know where the line is drawn, for example is a planum above the Mars datum and of course is Meridiani planum a planitiae?

Roy F

According to the USGS Astrogeology Gazetteer of Planetary Nomenclature, a planitia is a low plain and a planum is a high plain, as you said. But they say a planum is a "plateau or high plain," so perhaps the line between them is defined according to a local relative datum rather than an absolute one. I'm guessing if the plain is higher than most of its suroundings it is a planum, and it is a planitia if it is lower.

http://planetarynames.wr.usgs.gov/append5.html

Cosmicrocker is correct. The definitions are not tied to absolute elevation but to the average elevation of the surrounding area. But Meridiani is relatively high in elevation. It would be a planum either way.

Phil

Wasn't it originally called Sinus Meridiani - the Meridian Bay?

Sinus Meridiani is the name of the large two-lobed albedo marking in the same area.

Phil

Thank you Cosmicrocker,
height relative to local surounding sounds good to me.
I've bookmarked that url

Roy F

Phil,
The highest point is the Olympus Mons : 22 km above of what??? and the lowest point is the bottom of crater Argyre is 8 km below of what??? I would like to know what is the height reference for Mars as the ones of Earth with sea level reference.

From what reference, the flat land is refered as Planum and also as Plantia....I know that the southern of Mars has the high level land and the northern of Mars is of low level land.

Rodolfo

Hi Rodolfo
I believe the martian equivalent of sea level is called the datum and is based on atmospheric pressure, exactly what pressure is used I'm not sure but it's about 6 millibars but of course this can change if more gases are liberated into the atmosphere.

Roy F

On Earth the Datum is sea level. On Mars it is the elevation where atmospheric pressure might allow transient liquid water to exist briefly (If I recall correctly - at least that was the original idea, but I'm away from my office and can't look it up).

Phil

Many thanks to Reckless and Phil.

The corrected references for Olympus Mons altitude which was typed as 22 km. Indeed it has 21,270 meters of altitude. The deepest Mars's basin has 8.180 meters. It is not the southern crater Argyre but also of another southern crater which is the biggest Mar's crater: Hellas.

I have found the explanation about the meaning of datum from the

http://quest.arc.nasa.gov/mars/ask/terrain...vel_on_Mars.txt

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.

Rodolfo

That's interesting, and quite logical that the Mean Martian Radius would define the datum. It would be interesting to find a map showing the areas that were below the datum. I'm sure they exist, but I am too tired to look for them tonight.