Venus is Earth's near twin in mass and radius, and our nearest planetary neighbour, yet conditions there are very different in many respects. Its atmosphere, mostly composed of carbon dioxide, has a surface temperature and pressure far higher than those of Earth. Only traces of water are found, although it is likely that there was much more present in the past, possibly forming Earth-like oceans. Here we discuss how the first year of observations by Venus Express brings into focus the evolutionary paths by which the climates of two similar planets diverged from common beginnings to such extremes. These include a CO₂-driven greenhouse effect, erosion of the atmosphere by solar particles and radiation, surface–atmosphere interactions, and atmospheric circulation regimes defined by differing planetary rotation rates.

1. ESA/ESTEC, PB 299, 2200AG Noordwijk, The Netherlands
2. Max Planck Institute for Solar System Research, Max-Planck-Strasse 2, 37191 Katlenburg-Lindau, Germany
3. Space Research Institute (IKI), Profsojuznaja ul. 84/32, 117997 Moscow, Russia
4. Department of Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford OX1 3PU, UK

Correspondence to: Håkan Svedhem¹ Correspondence should be addressed to H.S. (Email: h.svedhem@esa.int).

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