-> Mercury spins on its axis three times for every two of its orbits around 
-> the Sun. How the tiny, baked world got locked in such a spin-orbit 
-> resonance has remained somewhat mysterious; accepted models gave the 
-> planet a seven percent chance of orbiting the Sun the way it does. Now a 
-> paper by Alexandre C. M. Correia (University of Aveiro, Portugal) and 
-> Jacques Laskar (Paris Observatory) published in the June 24th issue of 
-> NATURE makes the resonance seem more plausible. 
 
-> Their research focused on long-term changes in the shape of Mercury's 
-> orbit. New models demonstrate that over millions of years the planet's 
-> eccentricity ranges chaotically between nearly zero (a circular orbit) and 
-> 0.45 (very distinctly elliptical). When the eccentricity of Mercury's 
-> orbit increases, the probability of landing in a 3:2 spin-orbit resonance 
-> increases. Assuming that the planet's eccentricity has changed this way 
-> during the last 4 billion years, their models suggest that Mercury has 
-> better than even odds -- 55 percent -- of locking into a 3:2 state. 
  
I don't believe this. Sure, the 3:2 resonance is stable when the orbit 
is substantially elliptical, but it would *not* be stable when the 
orbit is close to circular. At those times, tidal drag would tend to 
slow down Mercury's rotation, making it approach a 1:1 resonance - 
which would also be stable when the orbit is elliptical. 
  
No. To my mind, the fact that the 3:2 resonance exists demonstrates 
that Mercury must *always* have been in a considerably elliptical 
orbit. If someone has written a simulation that suggests otherwise, 
then the simulation is wrong. 
  
I have a deep mistrust of these simulations that are claimed to track 
planetary motions through millions or even billions of orbital 
revolutions. Digital computers always introduce errors, and in such 
prolonged calculations the errors can become large enough to be 
seriously misleading. 
  
                    dow 
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