6.2 Activity of Centaurs
As noted earlier, Centaurs are thought to be objects in transition between the
scattered disc and the Jupiter family of comets. Approximately 10% of the Centaurs
discovered show some evidence of activity (Mazzotta Epifani et al. 2017; Peixinho
et al. 2020) through having a diffuse coma and/or tail in optical images. Several of
these objects have received comet designations including 174P/Echeclus and 29P/
Schwassmann-Wachmann 1 (e.g. Schambeau et al. 2019). The mass losses from
these objects can be substantial. Wong et al. (2019) measured the loss from P/2011
S1 and C/2012 Q1 to be 140 Æ 20 and 250 Æ 40 kg s
À1 , respectively which is
comparable to the loss from 67P (e.g. Fig. 3.48). One should be aware that the
methods to derive the losses from Centaurs are based upon broad-band photometry
of the dust comae and are therefore markedly different to the methods used for 67P.
With the perihelia of Centaurs being outside the orbit of Jupiter, simple water ice
sublimation models cannot explain this activity. However, the exothermic crystallization of amorphous water ice (Sect. 2.9.3.4) remains a plausible model although
Jewitt et al. (2017) have found activity in an object, C/2017 K2 (PANSTARRS), for
which this explanation is probably inadequate because of its extreme distance to the
Sun (23.7 AU) when seen to be active. 174P/Echeclus exhibits outbursts with mass
loss rates of the order of 86 kg s
À1 occurring at 12.9 AU from the Sun (Rousselot
2008). The sporadic and variable activity seen in some Centaurs is probably not
consistent with homogeneous objects undergoing crystallization leading Wong et al.
(2019) to suggest that outbursts are an indication of nucleus inhomogeneity. On the
other hand, super-volatiles such as CO may be significant. Wierzchos et al. (2017)
detected CO J(2-1) emission towards 174P/Echeclus with a production rate of
around 36 kg s
À1 at 6.1 AU from the Sun which they noted was 40 times less than
seen at 29P/Schwassmann–Wachmann 1 at a similar heliocentric distance
(Gunnarsson et al. 2008).
The driving process(es) behind the activity of Centaurs remains unknown at this
point and the differences between Centaurs in terms of, for example, CO loss per unit
surface area, are significant (see e.g. Wierzchos et al. 2017). On the other hand,
outbursts are, in themselves, complex phenomena that may lead to many processes
(exposure of the sub-surface, lifting of ice-rich material volatiles into the coma, etc.)
that may have stoichastic components that will be difficult to disentangle. But if
these objects really are JFC pre-cursors, then understanding of their current activity
is clearly of major importance.
6.3 Interstellar Visitors
The discovery of the first object with an inbound trajectory placing its origin outside
our Solar System (1I/(2017 U1) ’Oumuamua) was a major milestone (Meech et al.
2017). It was not observed to become active during its perihelion passage. However,
430
6 Comet-Like Activity in Related Objects
As noted earlier, Centaurs are thought to be objects in transition between the
scattered disc and the Jupiter family of comets. Approximately 10% of the Centaurs
discovered show some evidence of activity (Mazzotta Epifani et al. 2017; Peixinho
et al. 2020) through having a diffuse coma and/or tail in optical images. Several of
these objects have received comet designations including 174P/Echeclus and 29P/
Schwassmann-Wachmann 1 (e.g. Schambeau et al. 2019). The mass losses from
these objects can be substantial. Wong et al. (2019) measured the loss from P/2011
S1 and C/2012 Q1 to be 140 Æ 20 and 250 Æ 40 kg s
À1 , respectively which is
comparable to the loss from 67P (e.g. Fig. 3.48). One should be aware that the
methods to derive the losses from Centaurs are based upon broad-band photometry
of the dust comae and are therefore markedly different to the methods used for 67P.
With the perihelia of Centaurs being outside the orbit of Jupiter, simple water ice
sublimation models cannot explain this activity. However, the exothermic crystallization of amorphous water ice (Sect. 2.9.3.4) remains a plausible model although
Jewitt et al. (2017) have found activity in an object, C/2017 K2 (PANSTARRS), for
which this explanation is probably inadequate because of its extreme distance to the
Sun (23.7 AU) when seen to be active. 174P/Echeclus exhibits outbursts with mass
loss rates of the order of 86 kg s
À1 occurring at 12.9 AU from the Sun (Rousselot
2008). The sporadic and variable activity seen in some Centaurs is probably not
consistent with homogeneous objects undergoing crystallization leading Wong et al.
(2019) to suggest that outbursts are an indication of nucleus inhomogeneity. On the
other hand, super-volatiles such as CO may be significant. Wierzchos et al. (2017)
detected CO J(2-1) emission towards 174P/Echeclus with a production rate of
around 36 kg s
À1 at 6.1 AU from the Sun which they noted was 40 times less than
seen at 29P/Schwassmann–Wachmann 1 at a similar heliocentric distance
(Gunnarsson et al. 2008).
The driving process(es) behind the activity of Centaurs remains unknown at this
point and the differences between Centaurs in terms of, for example, CO loss per unit
surface area, are significant (see e.g. Wierzchos et al. 2017). On the other hand,
outbursts are, in themselves, complex phenomena that may lead to many processes
(exposure of the sub-surface, lifting of ice-rich material volatiles into the coma, etc.)
that may have stoichastic components that will be difficult to disentangle. But if
these objects really are JFC pre-cursors, then understanding of their current activity
is clearly of major importance.
6.3 Interstellar Visitors
The discovery of the first object with an inbound trajectory placing its origin outside
our Solar System (1I/(2017 U1) ’Oumuamua) was a major milestone (Meech et al.
2017). It was not observed to become active during its perihelion passage. However,
430
6 Comet-Like Activity in Related Objects