Experiments have indicated that there are two main forms of amorphous ice but
the formation and structure is highly dependent on deposition conditions. The high
density phase (called I a h) has a density of about 1.1 g cm
À3 and exists at temperatures <70 K. The low density phase (~0.94 g cm
À3
) is called I a l and exists between
70 and 120 K. An amorphous phase (I a r) can exist above 120 K coexisting with a
cubic crystalline phase.
In a series of experiments, Jenniskens and co-workers (Jenniskens and Blake
1994; Jenniskens et al. 1995) have demonstrated amorphous ice formation by gas
phase deposition onto cold substrates with the amorphous phase dependent upon the
deposition rate. The importance for comets is that cold dust particles in the protosolar nebula should form a substrate for low temperature deposition of water
molecules and these particles may then be incorporated into the cometary nucleus
in this form. The low temperatures of nuclei in the Oort cloud and the Kuiper Belt
would then be sufficient to maintain the ice in this state to the present day. The
presence of amorphous ice on interstellar grains has been established since
Jenniskens et al. (1995) and it is now assumed that amorphous ice is the dominant
form of water in the universe, even though it does not occur naturally on Earth
(Loerting et al. 2015).
The possible presence of amorphous ice in the nucleus has provoked considerable
discussion because the transition from the amorphous phase to the crystalline phase
is exothermic, producing 9 Â 10
4 J kg
À1 , and is therefore an internal heat source.
The amorphous-crystalline transition has been invoked as an explanation for
observed activity of comets at high heliocentric distance (see Tancredi et al. 1994;
Gronkowski 2007; Hosek et al. 2013).
Theoretical work (e.g. Kouchi et al. 1994) shows that the crystallization rate
(from the amorphous to crystalline phase) is strongly dependent upon temperature.
At temperatures below about 70 K, the low density form of amorphous ice (I a l) can
be preserved over timescales comparable to the age of the Solar System. However,
the timescale for transition drops by roughly an order of magnitude every 5 K (see
Mastrapa et al. (2013) for a detailed plot). Consequently, if amorphous ice, interior to
the nucleus, is warmed to about 90–100 K, theory suggests the transition must occur
producing additional heat.
The presence of amorphous ice can have significant implications for heat transport within the nucleus. Although there is uncertainty in this value at very low
temperatures, Haruyama et al. (1993), following Kouchi et al. (1992), used
κ ¼ 7:1 10
À8 T
ð2:123Þ
for the thermal conductivity of amorphous water ice (in units of [W m
À1 K
À1 ])
which, when compared with Eq. (2.111), can be seen to be appreciably lower than
that of crystalline ice.
In principle, the different forms of water ice can be determined through infrared
spectroscopy (Kokaly et al. 2017). The absorption coefficient for water ice at 190 K
2.9 Surface Processes
105
the formation and structure is highly dependent on deposition conditions. The high
density phase (called I a h) has a density of about 1.1 g cm
À3 and exists at temperatures <70 K. The low density phase (~0.94 g cm
À3
) is called I a l and exists between
70 and 120 K. An amorphous phase (I a r) can exist above 120 K coexisting with a
cubic crystalline phase.
In a series of experiments, Jenniskens and co-workers (Jenniskens and Blake
1994; Jenniskens et al. 1995) have demonstrated amorphous ice formation by gas
phase deposition onto cold substrates with the amorphous phase dependent upon the
deposition rate. The importance for comets is that cold dust particles in the protosolar nebula should form a substrate for low temperature deposition of water
molecules and these particles may then be incorporated into the cometary nucleus
in this form. The low temperatures of nuclei in the Oort cloud and the Kuiper Belt
would then be sufficient to maintain the ice in this state to the present day. The
presence of amorphous ice on interstellar grains has been established since
Jenniskens et al. (1995) and it is now assumed that amorphous ice is the dominant
form of water in the universe, even though it does not occur naturally on Earth
(Loerting et al. 2015).
The possible presence of amorphous ice in the nucleus has provoked considerable
discussion because the transition from the amorphous phase to the crystalline phase
is exothermic, producing 9 Â 10
4 J kg
À1 , and is therefore an internal heat source.
The amorphous-crystalline transition has been invoked as an explanation for
observed activity of comets at high heliocentric distance (see Tancredi et al. 1994;
Gronkowski 2007; Hosek et al. 2013).
Theoretical work (e.g. Kouchi et al. 1994) shows that the crystallization rate
(from the amorphous to crystalline phase) is strongly dependent upon temperature.
At temperatures below about 70 K, the low density form of amorphous ice (I a l) can
be preserved over timescales comparable to the age of the Solar System. However,
the timescale for transition drops by roughly an order of magnitude every 5 K (see
Mastrapa et al. (2013) for a detailed plot). Consequently, if amorphous ice, interior to
the nucleus, is warmed to about 90–100 K, theory suggests the transition must occur
producing additional heat.
The presence of amorphous ice can have significant implications for heat transport within the nucleus. Although there is uncertainty in this value at very low
temperatures, Haruyama et al. (1993), following Kouchi et al. (1992), used
κ ¼ 7:1 10
À8 T
ð2:123Þ
for the thermal conductivity of amorphous water ice (in units of [W m
À1 K
À1 ])
which, when compared with Eq. (2.111), can be seen to be appreciably lower than
that of crystalline ice.
In principle, the different forms of water ice can be determined through infrared
spectroscopy (Kokaly et al. 2017). The absorption coefficient for water ice at 190 K
2.9 Surface Processes
105