Dr Carly Howett

Abstract:

NASA’s Cassini mission revealed endogenic activity at the south pole of Saturn’s moon Enceladus. The activity is concentrated along four fractures in Enceladus’ ice shell, which are much warmer than their surroundings and the source of Enceladus’ plumes. This work provides a review of the current state of knowledge of the energy and mass lost by Enceladus through this activity. Specifically, we discuss the composition of the plumes, along with their spatial and temporal variation. The mass flux loss predicted for the three plume constituents (gas, dust and charged particles) is reviewed and a total mass flux of ejected material that subsequently escapes Enceladus is estimated to be 2.1×1011 kg over a Saturn year. Given that Enceladus’ ocean is predicted to be 1019 kg this loss is sustainable in the very long term (∼1.5 billion Earth years). However, unless a resupply mechanism (such as serpentinization) exists molecular hydrogen is expected to be depleted within ∼1 million Earth years. The difficulty in determining Enceladus’ current heat flow is outlined, along with the advantages and disadvantages of the various techniques used to derive it. We find a robust lower limit for Enceladus’ exogenic production is 7.3 GW. Tidal heating models show endogenic emission of this level is sustainable, and Enceladus may have long-term near-surface heating (a result supported by studies of Enceladus’ geology). Finally, we offer suggestions for future observations, instrumentation, and missions. Enceladus remains a high-priority target for NASA, and as such it is highly likely that we will return to study this enigmatic world. Hopefully these missions will answer some of the questions that remain

Abstract:

The New Horizons encounter with the Pluto system revealed Pluto to have an extremely spatially variable surface with expansive dark, bright, and intermediate terrains, refractory and volatile ices, and ongoing/recent endogenous and exogenous processes. Albedo is useful for understanding volatile transport because it quantifies absorbed solar energy; albedo may also provide insights into surface processes. Four filters of the New Horizons LORRI and MVIC imagers are used to approximate the bolometric (flux-weighted, wavelength-integrated) albedo. The bolometric hemispherical albedo (local energy balance albedo) as a function of the incidence angle of the solar illumination is measured for both Cthulhu and Sputnik Planitia, which are extensive, extreme dark and extreme bright terrains on Pluto. For both terrains, the bolometric hemispherical albedo increases by >30% from 0° to 90° incidence. The incidence-angle-average bolometric hemispherical albedo of Cthulhu is 0.12 ± 0.01, and that of Sputnik Planitia is 0.80 ± 0.06, where uncertainties are estimates based on scatter from different photometric functional approximations. The bolometric Bond albedo (global energy balance albedo) of Cthulhu is 0.12 ± 0.01, and that of Sputnik Planitia is 0.80 ± 0.07. A map of Pluto’s incidence-angle-average bolometric hemispherical albedo is produced. The incidence-angle-average bolometric hemispherical albedo, spatially averaged over areas north of ≈30° S, is ≈0.54. Pluto has three general albedo categories: (1) very low albedo southern equatorial terrains, including Cthulhu; (2) high-albedo terrains, which constitute most of Pluto’s surface; and (3) very high albedo terrains, including Sputnik Planitia. Pluto’s extraordinary albedo variability with location is also spatially sharp at some places

Abstract:

On 2010 May 18 Cassini’s Composite Infrared Spectrometer (CIRS) observed Dione’s leading hemisphere as its surface went into solar eclipse. Surface temperatures derived from each of CIRS’ focal plane 3 (FP3, 600−1100 cm−1) show a rapid decrease in Dione’s surface temperature upon eclipse ingress. This change was compared to the model surface emission to constrain bolometric Bond albedo and thermal inertia. Seven FP3 detectors were able to constrain the observed surface’s thermophysical properties. The bolometric Bond albedo derived from these detectors are consistent with one another (0.54 ± 0.05 to 0.62 ± 0.03) and that of diurnal studies (e.g., 0.49 ± 0.11, Howett et al. 2014). This indicates that Dione’s albedo is uniform to within the uncertainties across the observed region of its leading hemisphere. The derived thermal inertias are consistent across detectors, 9 ± 4 J m−2 K−1 s−1/2 (MKS) to 16 ± 8 MKS, and with previous diurnal studies (e.g., 8 to 12 MKS, Howett et al. 2014). The skin depth probed by the eclipse thermal wave is ∼0.6–1 mm, which is much shallower than that probed by diurnal cycles (∼50 mm). Thus, the agreement in thermal inertia between the eclipse and diurnal studies indicates that Dione’s subsurface structure is uniform from submillimeter to subcentimeter depths. This is different from the Jovian system, where eclipse-derived thermal inertias are much lower than those derived from diurnal studies. The cause of this difference is not known, but one possibility is that the E-ring grains that bombard Dione’s leading hemisphere overturn it, causing uniformity to centimeter depths.