Wednesday, March 24, 1999 Published at 18:24 GMT
Geologists call them grabens
By BBC News Online Science Editor Dr David Whitehouse
Now firmly settled in its final mapping orbit, the Mars Global Surveyor (MSG) spacecraft has started sending back to Earth a series of stunning, detailed images of the Martian surface.
Pavonis Mons is one of three large volcanoes in the Tharsis Montes region of Mars' western hemisphere. It stands about 7 kilometres (4 miles) above the surrounding plains.
The spacecraft recently spied a chain of elliptical pits on the lower east flank of Pavonis Mons.
The pits are aligned down the centre of a 485-metre-(530 yards)-wide, shallow trough. The straight trough and the pits were both formed when the ground was pulled apart and collapsed - troughs of this type are known to geologists as grabens.
These include the floor of Alexey Tolstoy Crater, which shows a dark surface that is extremely rough and rocky.
The small crater in the picture is inside the larger Tolstoy crater. It is only about 850 metres (930 yards) wide.
Alexey Tolstoy Crater was named by the International Astronomical Union in 1982 to honour the Soviet writer who died in 1945. It is one of only a few craters on Mars designated by both the first and last names of the honoured person.
Last year, MGS passed over a relatively small impact crater located on the Vastitas Borealis plain.
The picture shows bright patches of snow or frost left over from the Martian winter. These snowfields are so small that you could walk across one of them in a matter of minutes.
In winter, the entire scene would be covered by frost.