By Dr David Whitehouse
BBC News Online science editor
The highlands of Venus are covered by a heavy metal "frost", say planetary scientists from Washington University.
The highlands may be covered in metal
Because it is hot enough to melt lead at the surface, metals vaporise and condense at cooler, higher elevations.
This may explain why radar observations made by orbiting spacecraft show that the highlands are highly reflective.
Detailed calculations, to be published in the journal Icarus, suggest that lead and bismuth are to blame for giving Venus its bright, metallic skin.
Frequently seen as a brilliant point of light in the evening or morning sky, Venus has been identified with beauty by many cultures. But the truth is somewhat different.
Although it is about the same size as the Earth, its closer proximity to the Sun means that it is a very different planet.
Its thick atmosphere - composed chiefly of carbon dioxide - gives it an intense greenhouse effect, whereby trapped solar radiation heats the surface of the planet to an average of temperature of 467 Celsius.
Also, its pressure is 90 times greater than that at the Earth's surface.
The only way to glimpse what lies beneath its opaque clouds is by radar, and several missions have carried our radar surveys from orbit, principally the Magellan probe which operated from 1990 to 1994.
Magellan's images astounded astronomers who were able to see the surface of Venus in detail for the first time. They showed the planet was covered in volcanic features, such as vast lava plains, fields of small lava domes, and large shield volcanoes.
But the images were puzzling as well. It appeared that parts of the highlands were abnormally bright, reflecting radar beams much better than lower elevations.
Several explanations were put forward ranging from the presence of a loose soil to a coating of metal - specifically, tellurium.
Lined with lead
The theory suggests at Venus's hot lower layers any metal would be vaporised and exist as a metallic mist. Only at higher elevations, where it is a little cooler, would that metal condense to form a thin, highly reflective layer on the ground.
Using detailed chemical calculations involving 660 metal compounds, Laura Schaefer and Bruce Fegley, of the Washington University in St Louis, conclude that tellurium is not responsible, but that common lead probably is.
The researchers estimate that the timescale for the coating of the Venusian highlands by metallic frost is somewhere between a few thousand and a few million years, demonstrating that it is an active process.
They point out that at the highest elevations on Venus there is evidence that the metallic frost is absent - possible evidence of weathering, they say.
If it were possible to examine these lead deposits, from a Venus lander craft, the respective abundances of certain atom types, or isotopes, could give astronomers an estimate for the age of Venus.