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From: Larry Klaes (
Date: Mon Jun 18 2001 - 07:22:09 PDT


>From Andrew Yee <>

Thursday, 14 June 2001

Celestial backspin inevitable

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

Alexandre Correla and Jacques Laskar of Astronomie et Systèmes Dynamiques in
Paris, France, calculate that Venus has only 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 [1].

Laskar has previously shown that, thanks to myriad influences, the tilted
rotation axes of all the inner planets -- Mercury, Venus, Earth and Mars --
can wobble chaotically. This makes their behaviour highly 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. Corella and Laskar 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.

They find no unique answer. Because of the chaotic nature of the motion, the
researchers had to run lots of computer simulations for different initial
conditions, and look for the most common outcomes.

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. So its unusual rotation may be less a
matter of chance, and more an inevitability.

[1] Correla, A. C. M. & Laskar, J. The four final rotation states of Venus.
    Nature 411, 767-770 (2001).

© Macmillan Magazines Ltd 2001 - NATURE NEWS SERVICE

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