  We study the long-term (t >> 10 s) evolution of the accretion disk after a
neutron star(NS)-NS or NS-black hole merger, taking into account the
radioactive heating by r-process nuclei formed in the first few seconds. We
find that the cumulative heating eventually exceeds the disk's binding energy
at t ~ 10^2 s (\alpha/0.1)^{-1.8} (M/2.6 Msun)^{1.8} after the merger, where
\alpha is the Shakura-Sunyaev viscosity parameter and M is the mass of the
remnant object. This causes the disk to evaporate rapidly and the jet power to
shut off. We propose that this is the cause of the steep flux decline at the
end of the extended emission (EE) or X-ray plateau seen in many short gamma-ray
bursts (GRBs). The shallow flux evolution before the steep decline is
consistent with a plausible scenario where the jet power scales linearly with
the disk mass. We suggest that the jets from NS mergers have two components --
a short-duration narrow one corresponding to the prompt gamma-ray emission and
a long-lasting wide component producing the EE. This leads to a prediction that
"orphan EE" (without the prompt gamma-rays) may be a promising electromagnetic
counterpart for NS mergers observable by future wide-field X-ray surveys. The
long-lived disk produces a slow ejecta component that can efficiently
thermalize the energy carried by beta-decay electrons up to t ~ 100 d and
contributes 10% of the kilonova's bolometric luminosity at these late epochs.
We predict that future ground-based and JWST near-IR spectroscopy of nearby (<
100 Mpc) NS mergers will detect narrow (~0.01c) line features a few weeks after
the merger, which provides a powerful probe of the atomic species formed in
these events.
