Wednesday, August 28, 2013

Convection in Enceladus’ Ice Shell

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.

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.

M. Behounkova et al., “Impact of tidal heating on the onset of convection in Enceladus’s ice shell”, Icarus 226 (2013) 898-904