  A long-term sublimation model to explain how Phaethon could provide the
Geminid stream is proposed. We find that it would take $\sim6$ Myr or more for
Phaethon to lose all of its internal ice (if ever there was) in its present
orbit. Thus, if the asteroid moved from the region of a 5:2 or 8:3 mean motion
resonance with Jupiter to its present orbit less than $1$ Myr ago, it may have
retained much of its primordial ice. The dust mantle on the sublimating body
should have a thickness of at least $15$ m but the mantle could have been less
than $1$ m thick $1000$ years ago. We find that the total gas production rate
could have been as large as $10^{27}\rm~s^{-1}$ then, and the gas flow could
have been capable of lifting dust particles of up to a few centimeters in size.
Therefore, gas production during the past millennium could have been sufficient
to blow away enough dust particles to explain the entire Geminid stream. For
present-day Phaethon, the gas production is comparatively weak. But strong
transient gas release with a rate of $\sim4.5\times10^{19}\rm~m^{-2}s^{-1}$ is
expected for its south polar region when Phaethon moves from $0^\circ$ to
$2^\circ$ mean anomaly near perihelion. Consequently, dust particles with radii
of $<\sim260~\mu m$ can be blown away to form a dust tail. In addition, we find
that the large surface temperature variation of $>600$ K near perihelion can
generate sufficiently large thermal stress to cause fracture of rocks or
boulders and provide an efficient mechanism to produce dust particles on the
surface. The time scale for this process should be several times longer than
the seasonal thermal cycle, thereby dominating the cycle of appearance of the
dust tail.
