  A fraction of the thermal protons in the outer envelope of an X-ray cluster
have velocities that exceed the local escape speed from the cluster
gravitational potential. The Coulomb mean-free-path of these protons is larger
than the virial radius of the cluster at temperatures >2.5 keV. The resulting
leakage of suprathermal particles generates a collisionless shock in
neighboring voids and fills them with heat and magnetic fields. The momentum
flux of suprathermal particles cannot be confined by magnetic tension at the
typical field strength in the periphery of cluster halos (<<1 micro-gauss).
Over a Hubble time, thermal evaporation could drain up to a tenth of the
cluster gas at its virial temperature. The evaporated fraction could increase
dramatically if additional heat is deposited into the gas by cluster mergers,
active galactic nuclei or supernovae. Thermal evaporation is not included in
existing cosmological simulations since they are based on the fluid
approximation. Measurements of the baryon mass fraction in the outer envelopes
of hot clusters (through their Sunyaev-Zel'dovich effect or X-ray emission) can
be used to empirically constrain their evaporation rate.
