Earth and Venus: Twins that grew apart
28 November 2007
Scientists have revealed the results of the first year of observing Venus, our nearest planetary neighbour, using ESA’s Venus Express spacecraft.
The international team report their research in nine papers published this week in a special edition of Nature. The picture they paint is of a planet once very similar to our own but where a carbon dioxide-driven greenhouse effect, the erosion of the atmosphere by solar particles, and a slower rotation rate, have resulted in a world stripped of water and ravaged by hurricane-force winds and frequent lightning strikes.
The results give a sobering insight into how Earth might evolve in the future if manmade and natural processes were to lead to catastrophic changes in our climate.
Professor Fred Taylor from Oxford University’s Department of Physics is an Interdisciplinary Scientist for the Venus Express mission, making him a member of all the project teams and responsible for interpreting their combined results on the climate of Venus. He was also co-investigator of the Venus Express VIRTIS infrared imaging spectrometer project and was involved in the overall mission design and planning – he is an author on three of the Nature papers.
‘It is now becoming clear why the climate on Venus is so different from the Earth, when the planets themselves are otherwise quite similar,’ said Professor Taylor. ‘These differences are not just down to Venus being closer to the sun, we now know that the lack of a protective magnetic field and the differing planetary rotation rates also play a role in ensuring that many of the atmospheric processes we observe on Earth occur at a much faster rate on Venus. Our new data make it possible to construct a scenario in which Venus started out like the Earth – possibly including a habitable environment, billions of years ago – and then evolved to the state we see now.’
Scientists used the VIRTIS infrared imaging spectrometer on board the Venus Express spacecraft to take three-dimensional ‘soundings’ of the planet’s atmosphere. ‘The first year’s worth of data from VIRTIS has enabled us to resolve the confused picture of the general circulation of the atmosphere and show that it can be divided into four major components,’ said Professor Taylor. ‘First there is a turbulent region near the equator. Then, at higher latitudes, the flow is much ‘smoother’, travelling in parallel layers with no disruption between. Above and below that the poles are marked by swirling vortices contained inside a cold ‘polar collar’. Overlaying all of this is a flow in the upper atmosphere that travels from the part of Venus that is closest to the sun to an opposite point on the planet’s dark side – this replaces the equator-to-pole flow that we see lower down in the atmosphere.’
Out of these four components the pole is of particular interest with its unusual ‘double eye’ cloud formation that sits within a vast polar vortex measuring several thousand kilometres across. ‘This ‘double eye’ rotates approximately every two and a half Earth days,’ said Professor Taylor. ‘Through simultaneous observation across the ultraviolet and infrared spectrum we can see that while the double vortices are only visible in the cloud top region, the vortex-like structure extends down to where clouds begin to form 50-55km above the surface of the planet. We now have a much better idea of how the behaviour of the polar vortex gives rise to the ‘double eye’.’
Commenting on other findings from the first year of Venus Express, Professor Taylor said: ‘Elsewhere, thanks to the work of other research groups, we have begun to understand the processes by which water has been removed from Venus. We knew that chemical species in the atmosphere of Venus are ‘ripped apart’ (dissociated) by ultraviolet solar radiation, and then blown into space. However, it has been established for the first time that oxygen, which is a relatively heavy atom, is lost in large amounts – along with lighter atoms such as helium and hydrogen – in fact oxygen is lost at about half the rate for hydrogen, consistent with the source being water. This happens at a faster rate on Venus than on the Earth not just because Venus is closer to the Sun but also because it lacks the magnetic field that protects the Earth from the solar wind and rotates more slowly than the Earth (once every 243 Earth days). These loss processes must have stripped away large amounts of water from Venus during the first billion years or so after the formation of the Solar System.’
Scientists have also found the first reliable evidence for lightning on Venus. Data from a magnetometer aboard the spacecraft suggests that lightning is more common on Venus than the Earth, a phenomenon that is likely to have important implications for the chemistry of the planet’s atmosphere.
For more information you can listen in on the ESA press conference taking place on 28 November between 14:00 GMT and 16:30 GMT, where the full findings will be presented, by dialling +33 158 995742
Alternatively contact the University of Oxford Press Office on +44 (0)1865 283877 or by email at press.office@admin.ox.ac.uk [Please note: Professor Fred Taylor will be presenting at the ESA press conference in Paris and so will be unavailable for interview on Wednesday 28 November]