Constraining Enceladus’ energy emission outside the South Polar Terrain

We revisit the 12 years of Cassini Composite InfraRed Spectrometer (CIRS) observations of Enceladus.  Spectra from the CIRS FP1 and FP3 detectors (spanning 10 - 600 cm^-1 and 600– 1100 cm^-1) are used to evaluate whether endogenic “hot spots” might exist outside of Enceladus’ South Polar Terrain (SPT).  These detectors afford contrasting spatial resolution and temperature sensitivity for detecting the temperature of very cold scenes.  Significant endogenic heat is observed in the SPT principally from to the four long fissures or ‘sulci’, from which extensive cryovolcanic plumes of water vapour, volatiles, solid ice and organic species emanate (e.g. Porco et al. 2006; Spencer et al. 2006).  This is considered to be the primary mechanism by which Enceladus dissipates energy from tidal heating (Spencer et al., 2018), with possible further conductive heat loss through the ice shell.

The evolution of Enceladus, particularly as it relates to the tidal heating and how long a global ocean might be sustained, depends on a fine balance of heating and dissipation processes. 

We have conducted an extensive survey of the CIRS Enceladus observations from which we derive surface temperature and compared the observed results to a passive thermal model used to derive expected passive emission temperatures.  We have improved the estimate of temperature uncertainty associated with the CIRS observations for the coldest scenes by extensive statistical modelling.  For FP1, which is sensitive to colder temperatures than FP3, we find that temperature errors for the coldest scenes with higher signal to noise ratio (25 - 40 K) are still well characterised and appropriate for the expected sensitivity of the detector.  Minimum bounds of temperature uncertainty are more challenging to establish below 30 K but upper error limits are considered robustly quantified.  For FP3, temperature errors are well characterized down to 75 K, below which co-addition of spectra should be considered for confidence in temperature and error evaluation. 

We attempt to quantify thermal model error by using perturbations that represent the uncertainty in thermal inertia and albedo, which themselves were derived from CIRS observations.  This model uncertainty has multivariate dependencies that impact insolation, including local time, time of year, latitude and the occurrence of eclipses.  Even with well quantified model and observation uncertainty, the unambiguous identification of anomalous CIRS observations is still challenged by the spatial paucity and limited temporal sampling.  The results of this study and implications will be discussed. 

CIRS observations have shown that some of the coldest scenes in the Solar System exist on the icy moons of Saturn.  As such, it is possible – in the case of Enceladus – to use these observations to infer constraints on posited conductive heat flow through Enceladus’ icy shell.  Several Saturn moon evolution models that allow for a variable Q factor as Saturn evolves predict conductive heat loss of 25-40 GW for Enceladus (Hemmingway and Mittal 2019, Lainey et al., 2012, 2017, Fuller et al., 2016, Nimmo et al., 2018), but potentially much more.  The coldest CIRS temperatures of circa 30 K are found in the north polar region, where the ice shell is relatively thin.  This indicates that conductive heat loss is unlikely to be more than 25 GW.  This does not discount the possibility of highly spatially varying conductive heat flow, or temporal variation on timescales beyond Cassini’s observations. As such future observations (particularly of the north polar region) would be welcomed. 

References 

Fuller, J., Luan, J., Quataert, E., 2016. Resonance locking as the source of rapid tidal migration in the Jupiter and Saturn moon systems. Mon. Not. R. Astron. Soc. 458, 3867–3879. http://arxiv.org/abs/1601.05804 https://doi.org/10.1093/mnras/ stw609 arXiv:1601.05804.

Hemmingway, D.J. and T. Mittal, Enceladus's ice shell structure as a window on internal heat production Icarus, 332, 2019, https://doi.org/10.1016/j.icarus.2019.03.011

Lainey, V., Karatekin, Ö., Desmars, J., Charnoz, S., Arlot, J.-E., Emelyanov, N., Le Poncin-

Lafitte, C., Mathis, S., Remus, F., Tobie, G., Zahn, J.-P., 2012. Strong tidal dissipation in Saturn and constraints on Enceladus' thermal state from astrometry. Astrophys. J. https://doi.org/10.1088/0004-637X/752/1/14. http://stacks.iop.org/0004-637X/ 752/i=1/a=14?key=crossref.d68a903d62213eb4f281cc2f4669349f.

Lainey, V., Jacobson, R.A., Tajeddine, R., Cooper, N.J., Murray, C., Robert, V., Tobie, G., Guillot, T., Mathis, S., Remus, F., Desmars, J., Arlot, J.-E., De Cuyper, J.-P., Dehant, V., Pascu, D., Thuillot, W., Poncin-Lafitte, C.L., Zahn, J.-P., 2017. New constraints on Saturn's interior from Cassini astrometric data. Icarus 281, 286–296. http:// linkinghub.elsevier.com/retrieve/pii/S0019103516304183 https://doi.org/10. 1016/j.icarus.2016.07.014.

Nimmo, F., Barr, A. C., Běhounková, M., & Mckinnon, W. B. (2018). The thermal and orbital evolution of Enceladus: observational constraints and models. In P. M. Schenk, R. N. Clark, C. J. A. Howett, & A. J. Verbiscer (Eds.), Enceladus and the Icy Moons of Saturn (pp. 79–94). Tucson, Arizona: University of Arizona Press. https://websites. pmc.ucsc.edu/ fnimmo/website/Enceladus_in_press.pdf

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