  Mass-spring model simulations are used to investigate past spin states of a
viscoelastic Phobos and Deimos. From an initially tidally locked state, we find
crossing of a spin-orbit resonance with Mars or a mean motion resonance with
each other does not excite tumbling in Phobos or Deimos. However, once tumbling
our simulations show that these moons can remain so for an extended period and
during this time their orbital eccentricity can be substantially reduced. We
attribute the tendency for simulations of an initially tumbling viscoelastic
body to drop into spin-synchronous state at very low eccentricity to the
insensitivity of the tumbling chaotic zone volume to eccentricity. After a
tumbling body enters the spin synchronous resonance, it can exhibit long lived
non-principal axis rotation and this too can prolong the period of time with
enhanced tidally generated energy dissipation. The low orbital eccentricities
of Phobos and Deimos could in part be due to spin excitation by nearly
catastrophic impacts rather than tidal evolution following orbital resonance
excitation.
