Thursday, May 27, 1999 Published at 20:52 GMT 21:52 UK
Mars in sharper focus
A global view of Mars
By BBC News Online Science Editor Dr David Whitehouse
The Mars Global Surveyor (MGS) has produced the first detailed map of the planet's surface.
"We now have a definitive picture of the shape of the whole planet," says David Smith of Nasa's Goddard Space Flight Center.
The new map was made by bouncing laser light off the Martian surface to determine the height of features to an accuracy of just 13 metres.
The southern hemisphere is old and heavily cratered. The north, however, is much younger, having been resurfaced by magma from volcanoes early in Martian history.
Overall, the northern hemisphere is five kilometres lower than the south. Geologists say that on Mars "downhill is north".
The laser altimeter shows that Mars is not round. A slice through the planet through the poles shows just how much it deviates from a perfect circle.
Clearly visible in this planetary profile, on the bottom right of the image, is the gigantic Hellas impact basin.
This is a structure that dominates the entire southern hemisphere of Mars. Billions of years ago, an asteroid-sized chunk of rock struck Mars leaving scars that are still visible today.
MGS observations show that the vertical distance between the high and low points of this impact basin are nine kilometres. The diameter of the crater caused by the impact explosion is 2,300km.
The data also allows an estimate of the amount of ice on the surface. Concentrated at the north and south polar caps, there is about four million cubic kilometres of ice.
Details of the map are reported in the journal Science.