Wednesday, October 6, 1999 Published at 11:05 GMT 12:05 UK
Evening clouds on a Martian volcano
Looking down through the clouds at Arsia Mons
By BBC News Online Science Editor Dr David Whitehouse
As if to show that the loss of the Mars Climate Orbiter spacecraft last week was just a relatively minor setback in the exploration of the Red Planet, the Mars Global Surveyor (MGS) has just celebrated two years in orbit by sending back some stunning new pictures.
MGS has imaged one of the largest volcanoes not just on Mars but anywhere in the Solar System, Arsia Mons.
This so-called shield volcano is actually part of a string of three volcanoes known as the Tharsis Montes. They were called Montes or mountains before it was realised that they were extinct volcanoes.
The three volcanoes, Ascraeus Mons, Pavonis Mons and Arsia Mons, form one of the most striking series of surface features on the planet.
These huge, broad volcanoes rise out of the Tharsis plain, spaced in a line about 700 km (435 miles) apart. The Tharsis plain stretches for thousands of kilometres north-west from the chaotic upland region where the mighty Valley of the Mariners can be found.
Billions of years ago, this region of Mars must have been a magnificent and foreboding sight. Torrential floods from the uplands created the Valley of the Mariners and the floodwater gushed onto the plains separating into a myriad of tributaries from which the water evaporated.
On the horizon would have been the three mighty volcanoes of Tharsis bellowing volcanic gas into the atmosphere and drowning the region with vast lava floes.
In the new image, bright water ice clouds (the whitish-bluish wisps) can be seen hanging above the volcano. Such clouds are a common sight nearly every Martian afternoon in this region.
Mars Global Surveyor, launched in November 1996, has been in orbit around Mars since September 1997, and is returning unprecedented detail about planet's surface features, atmosphere, and magnetic properties.
With the loss of the orbiter last week, it has been decided that MGS will have to act as a part-time relay for the Mars Polar Lander, due to land on 3 December. The lander is also capable of sending data directly to Earth.