  Mercury's eccentricity is chaotic and can increase so much that collisions
with Venus or the Sun become possible (Laskar, 1989, 1990, 1994, 2008, Batygin
& Laughlin, 2008, Laskar & Gastineau, 2009). This chaotic behavior results from
an intricate network of secular resonances, but in this paper, we show that a
simple integrable model with only one degree of freedom is actually able to
reproduce the large variations in Mercury's eccentricity, with the correct
amplitude and timescale. We show that this behavior occurs in the vicinity of
the separatrices of the resonance g1-g5 between the precession frequencies of
Mercury and Jupiter. However, the main contribution does not come from the
direct interaction between these two planets. It is due to the excitation of
Venus' orbit at Jupiter's precession frequency g5. We use a multipolar model
that is not expanded with respect to Mercury's eccentricity, but because of the
proximity of Mercury and Venus, the Hamiltonian is expanded up to order 20 and
more in the ratio of semimajor axis. When the effects of Venus' inclination are
added, the system becomes nonintegrable and a chaotic zone appears in the
vicinity of the separatrices. In that case, Mercury's eccentricity can
chaotically switch between two regimes characterized by either low-amplitude
circulations or high-amplitude librations.
