Exploratour - The Surface of Mars
Altimeter Shows the Difference
Mars Global Surveyor carries an instrument which measures the altitudes of things. The instrument is called an altimeter, or "altitude-meter". An altitude-meter is the same sort of instrument which is used to measure the height of the Earth's oceans.
The picture to the left shows the measurement of the size of the bluff which separates the southern and northern hemispheres of Mars. The picture shows the surface of Mars from the north pole to the south pole. The red line traces the height of the lower, northern plains to the left, then the abrupt cliff which separates the northern plains from the highlands of the south, then the highlands at the right. The height of the bluff at any place along the red line can be read in both kilometers and in miles at either side of the picture.
It can be seen from the picture that the southern hemisphere as a whole is much more elevated than the northern hemisphere. The southern hemisphere is also much older than the northern hemisphere. The different sizes of the two hemispheres is suggestive that Mars has a giant southern continent.