NAMED after the Roman goddess of love, Venus could have been a sister planet to Earth. The two orbs are almost identical in shape, volume and mass. Both have solid surfaces, an atmosphere and a weather system. Even their distances from the sun are not so different: Venus is about 108m kilometres (67m miles) from the centre of the solar system while the Earth is about 150m kilometres out.
Yet Venus is far from lovely. Probes sent to the planet by America and the Soviet Union from the 1960s to the 1980s revealed that Venus would be deadly to humans. The atmosphere is mostly noxious carbon dioxide, the pressure at the planet's surface is equivalent to being almost a kilometre under water and the temperature is a roasting 465 °C.
On November 9th, the European Space Agency launched its bid to discover why the two planets are so different. The spacecraft Venus Express aims to arrive at the target world in April 2006, and is expected to orbit the planet for 500 Earth days, equivalent to just two Venusian days, while carrying enough fuel to double the duration of the mission.
Planetary scientists want to know whether Venus is geologically active. On Earth, the continents lie on tectonic plates that move relative to each other, creating mountains and deep sea shelves. Volcanoes erupt at pressure points at their edges. Venus, however, seems to be covered by a single plate. Paradoxically, its surface appears to be shaped by the deformation of the crust and by volcanic activity. Lava flows extend for hundreds of kilometres across its surface.
Both planets are pockmarked by craters caused by meteorites. However, the oldest craters on Venus appear to have been made less than 500m years ago. Scientists suspect that the planet might periodically undergo some global eruption that completely resurfaces the planet. If this were so, it is possible that, instead of Venus experiencing regular volcanic eruptions that release the pressure within the planet, it has no geological activity between its catastrophic outbreaks.
Seven instruments aboard Venus Express will attempt to solve the mystery. They will analyse the planet's lower atmosphere, looking for telltale gases associated with volcanic activity, and for related temperature changes. The spacecraft is also equipped to detect the Venusian equivalent of earthquakes.
Venus has various other odd aspects that will also be examined in more detail. High in its atmosphere, the winds race at hurricane pace, creating enormous vortices at the poles. The planet has no internal magnetic field, which might be due to its core failing to circulate in the way the Earth's does. As a result, it is bombarded by the solar wind and this might have resulted in the loss of water and oxygen. Venus's weather system involves sulphur, carbon monoxide and water vapour. The uppermost parts of its thick cloud layer mysteriously absorb half of the ultraviolet light received by the planet. Visible flashes in its atmosphere have been detected on Earth. Could these be lightning? Venus Express will take a closer look.