Sunday, 2 December 2007

Venus as a more Earth-like planet

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.
The overall sense of the results from the first year of operation of Venus Express is that the differences, particularly in climate, between Venus and Earth are much less mysterious than previously thought after the early phase of spacecraft exploration. They are consistent with theoretical ideas and interpretations suggesting that the two planets had similar surface environments in the past and that they evolved differently, with Earth's oceans converting most of its atmospheric CO2 to carbonate rocks, and Venus losing most of its water to space. Both processes can now be seen to be still going on. The high zonal winds and near-equatorial turbulence on Venus, as well of course as the high surface temperatures, result from the depth of the atmosphere and huge inventory of greenhouse gas retained by Venus. The slow rotation of Venus, as well as possibly being responsible for the lack of magnetic field that makes erosion of the atmosphere by the solar wind so effective, permits the Earth-like Hadley cell component of the atmospheric circulation to extend closer to the poles, where it breaks down in spectacular fashion to form mid-latitude jets and polar vortices that are larger and more energetic than Earth's but are in many respects quite similar (Read more).

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