  The Centaurs are recent escapees from the Kuiper belt that are destined
either to meet fiery oblivion in the hot inner regions of the Solar system or
to be ejected to the interstellar medium by gravitational scattering from the
giant planets. Dynamically evolved Centaurs, when captured by Jupiter and close
enough to the Sun for near-surface water ice to sublimate, are conventionally
labeled as "short-period" (specifically, Jupiter-family) comets. Remarkably,
some Centaurs show comet-like activity even when far beyond the orbit of
Jupiter, suggesting mass-loss driven by a process other than the sublimation of
water ice. We observed a sample of 23 Centaurs and found nine to be active,
with mass-loss rates measured from several kg/s to several tonnes/s. Considered
as a group, we find that the "active Centaurs" in our sample have perihelia
smaller than the inactive Centaurs (median 5.9 AU vs. 8.7 AU), and smaller than
the median perihelion distance computed for all known Centaurs (12.4 AU). This
suggests that their activity is thermally driven. There are several
possibilities for the origin of the mass-loss from the active Centaurs. We
consider the possibility that activity in the Centaurs is triggered by the
conversion of amorphous ice into the crystalline form accompanied by the
release of trapped gases, including carbon monoxide. By imposing the condition
that crystallization should occur when the crystallization time is shorter than
the orbital period we find a qualitative match to the perihelion distribution
of the active Centaurs and conclude that the data are consistent with the
hypothesis that the Centaurs contain amorphous ice.
