Thursday, October 21, 1999 Published at 13:05 GMT 14:05 UK
Moon not quite dead
Clementine image of the Aristarchus region
By BBC News Online Science Editor Dr David Whitehouse
The Moon may not be the dead and unchanging world that many scientists believe. From time to time, parts of it may stir in a pale reminder of the volcanic and meteoric violence of its youth.
Some astronomers, who claim to have seen what they term as transient lunar phenomena (TLP) on its surface, have long suspected that there is some activity on the Moon.
For more than 450 years, there have been reports of TLP's - bright flashes to reddish or bluish glows hovering over certain craters and valleys. There are several explanations for them - weak volcanic activity, perhaps, or the escape of gasses trapped beneath the Moon's surface, or even impact by meteors.
Many lunar geologists have doubted their existence. The Moon, they have argued, is a dead world. There might perhaps be a rock fall here and there, or a small impact by a meteor, but the possibility that volcanic outgassing might be going on has not been regarded as a plausible theory.
But now seemingly definitive evidence has been obtained that strange things do happen sometimes on the Moon. During its two-month mapping mission in 1996, the Clementine lunar orbiting satellite returned over two million images. For astronomers, they provided an unprecedented opportunity to study the question of TLP's.
As Clementine photographed the Moon from orbit, a team of amateur astronomers were mobilised to provide almost continuous observations of the satellite.
Now a team of astronomers led by Bonnie Buratti at Nasa's Jet Propulsion Laboratory have seen TLP's and, in at least four cases, Clementine images were obtained before and after reports of TLP's by ground-based observers.
One exciting case concerned an incident near the so-called Cobrahead feature on a region called the Aristarchus plateau. On 23 April, 1994, amateur astronomers noticed a "possible obscuration over the region". Clementine images taken on 3 March and again on 27 April do indeed show a change. Part of the region is a slightly different colour.
It is a significant discovery as the Aristarchus plateau is one of the youngest regions of the Moon and astronomers believe that if there is any residual volcanic activity left on the world then it is most likely in such a place.
The Cobrahead feature is a collapsed lava tube that came from a volcano that had its heyday billions of years ago. It is also a region where TLP's have been seen in the past. Perhaps pockets of gas seep up through the ground and, when caught by the bright rays of the lunar dawn, glow in reds and blues. Or perhaps heating effects cause sub-surface explosions.
Either way, as a world, the Moon is dead. But it seems that occasionally, in specific places, something does stir.