Observations of Saturn’s moon Enceladus by NASA’s Cassini
spacecraft have revealed the presence of jets of water vapour and ice particles
emanating from warm tectonic ridges at the south pole of the moon. The heat
required to power these jets is much larger than what may be produced by the
decay of radioactive elements in the moon’s rocky core. Instead, the heating
power is most likely produced by tidal heating resulting from the damping of
Enceladus’ orbital eccentricity.
Convective processes occurring within Enceladus’ ice shell
is believed to be causing the activity currently observed at the south pole. A
study done by M. Behounkova et al. (2013) investigates the effect of tidal
heating on the onset of convection in Enceladus’ ice shell. Convection in Enceladus’
ice shell can only occur if there is sufficient tidal heating and if the ice
grains are smaller than a critical size. In the study, the amount of tidal
heating depends on the orbital eccentricity of Enceladus and on the width of
the internal liquid water reservoir at the boundary between Enceladus’ ice
shell and rocky core. For this study, the internal liquid water reservoir is
assumed to be centred under the south pole and reservoir widths of 120°, 180°
and 360° are considered. Also, a variety of ice grain sizes are also
considered.
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The study indicates that for low tidal heating rates,
convection only occurs for ice grain sizes smaller than 0.5 mm. However, it is
unlikely that ice grain sizes smaller than 0.5 mm are present within Enceladus’
ice shell. Ice grains of somewhat larger sizes are more realistic. Since the
minimum ice grain size needed to drive convection increases with the tidal
heating rate, larger ice grain sizes will require a higher amount of tidal
heating to drive convection.
In the presence of tidal heating, the width of the internal
liquid water reservoir has a strong effect on the onset of convection. A
considerable larger orbital eccentricity is required to generate sufficient
tidal heating to drive convection if Enceladus has a small internal liquid
water reservoir. For example, if the ice grain size is 1 mm, the eccentricity
needed to trigger convection is 4 times current eccentricity for a reservoir
width of < 120°, while it is only twice current eccentricity for a reservoir
width of > 180°.
The present-day value of Enceladus’ orbital eccentricity is
too small to generate enough tidal heating to account for the heat loss by
thermal convection. This has led to the suggestion that Enceladus may have experienced
a recent period of enhanced orbital eccentricity followed by a rapid damping of
orbital eccentricity through tidal heating. Hence, Enceladus is expected to
switch between conductive (low activity) and convective (high activity) states
depending on its orbital configuration. The present-day Enceladus may be near
the end of its convective state.
During periods of low activity, the orbital eccentricity of
Enceladus is expected to gradually increase over timescales on the order of 100
million years. When the orbital eccentricity of Enceladus reaches 2 to 5 times
the present-day value, tidal heating then becomes large enough to drive convection
even for a maximum ice grain size of 1.5 mm. This period of high activity is
expected to last for less than 10 million years as the orbital eccentricity of
Enceladus damps out. As a result, high activity periods associated with
convection and internal melting should be brief (~ 1 to 10 million years),
followed by relatively long periods of low activity (~ 100 million years)
during which convection is likely to cease.
Reference:
M. Behounkova et al., “Impact of tidal heating on the onset
of convection in Enceladus’s ice shell”, Icarus 226 (2013) 898-904