Backspin inevitable

VENUS IS strange. It rotates from east to west. Other planets in the Solar System turns west to east. It has been assumed that this is because a freak event up-ended Venus on its axis at some point in the past. Now astronomers suggest in Nature that there are other ways our Venus could have gone into backspin.

Researchers in France, calculate that Venus has four states available to it: two that spin the normal way, and two retrograde. Under most conditions, retrograde motion is the most likely final state, they conclude. .

They have previously shown that, the tilted rotation axes of all the inner planets Mercury, Venus, Earth and Mars can wobble chaotically. This makes their behaviour sensitive to tiny effects and highly unpredictable. Tides sloshing Venus's thick atmosphere could thus have caused its rotation axis to flip. But only if the initial tilt was large.

But Venus need not have been in this special initial state to acquire retrograde rotation. The team have calculated how Venus moves around its orbit, allowing for tidal effects and for rubbing between the planet's rocky mantle and its molten core.

The four rotation states that they settle upon are likely to apply to other planets with dense atmospheres, such as the Earth. But for Venus, the two prograde states are much less stable than the two retrograde states for a wide range of initial conditions. .

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