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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

View the new map and how it was made
We now have a better view of Mars than we do of some parts of the Earth thanks to a spacecraft orbiting the Red Planet.

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.

Nasa'a Jim Garvin: "The map is extraordinarliy precise"
The polar caps, the vast plains, the extinct volcanoes, the canyons and all the ancient impact structures have all been brought into sharper focus.

Martian history

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.

[ image: Red is high; Blue is low]
Red is high; Blue is low
The main thing that emerges from this global view is the profound difference between the northern and southern hemispheres.

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".

Oddball planet

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.

[ image: Mars is not round]
Mars is not round
There is a 20 kilometre difference between the polar and equatorial radius of Mars.

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.

Ice coverage

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.

[ image: Giant impact: The Hellas crater]
Giant impact: The Hellas crater
Geologists point out that this structure has many similarities to the south pole-Aitken basin on the Moon, which was also caused by a titanic impact.

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.

Take a virtual flyover of Mars
That is enough to cover the entire planet to a depth of 30 metres.

Details of the map are reported in the journal Science.

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