  The largest Lyapunov exponent of a system composed by a heavy impurity
embedded in a chain of anharmonic nearest-neighbor Fermi-Pasta-Ulam oscillators
is numerically computed for various values of the impurity mass $M$. A
crossover between weak and strong chaos is obtained at the same value
$\epsilon_{_T}$ of the energy density $\epsilon$ (energy per degree of freedom)
for all the considered values of the impurity mass $M$. The threshold $\epsi
lon_{_T}$ coincides with the value of the energy density $\epsilon$ at which a
change of scaling of the relaxation time of the momentum autocorrelation
function of the impurity ocurrs and that was obtained in a previous work ~[M.
Romero-Bastida and E. Braun, Phys. Rev. E {\bf65}, 036228 (2002)]. The complete
Lyapunov spectrum does not depend significantly on the impurity mass $M$. These
results suggest that the impurity does not contribute significantly to the
dynamical instability (chaos) of the chain and can be considered as a probe for
the dynamics of the system to which the impurity is coupled. Finally, it is
shown that the Kolmogorov-Sinai entropy of the chain has a crossover from weak
to strong chaos at the same value of the energy density that the crossover
value $\epsilon_{_T}$ of largest Lyapunov exponent. Implications of this result
are discussed.
