Constraining Enceladus&#8217; energy emission outside the South Polar Terrain
Authors: Miles Georgina , Howett Carly , Spencer John 
Published: September 2022

&lt;p&gt;We revisit the 12 years of Cassini Composite InfraRed Spectrometer (CIRS) observations of Enceladus.&amp;#160; Spectra from the CIRS FP1 and FP3 detectors (spanning 10 - 600 cm&lt;sup&gt;-1&lt;/sup&gt; and 600&amp;#8211; 1100 cm&lt;sup&gt;-1&lt;/sup&gt;) are used to evaluate whether endogenic &amp;#8220;hot spots&amp;#8221; might exist outside of Enceladus&amp;#8217; South Polar Terrain (SPT).&amp;#160; These detectors afford contrasting spatial resolution and temperature sensitivity for detecting the temperature of very cold scenes.&amp;#160; Significant endogenic heat is observed in the SPT principally from to the four long fissures or &amp;#8216;sulci&amp;#8217;, 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).&amp;#160; 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.&lt;/p&gt;
&lt;p&gt;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.&amp;#160;&lt;/p&gt;
&lt;p&gt;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.&amp;#160; We have improved the estimate of temperature uncertainty associated with the CIRS observations for the coldest scenes by extensive statistical modelling.&amp;#160; 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.&amp;#160; Minimum bounds of temperature uncertainty are more challenging to establish below 30 K but upper error limits are considered robustly quantified.&amp;#160; 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.&amp;#160;&lt;/p&gt;
&lt;p&gt;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.&amp;#160; This model uncertainty has multivariate dependencies that impact insolation, including local time, time of year, latitude and the occurrence of eclipses.&amp;#160; 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.&amp;#160; The results of this study and implications will be discussed.&amp;#160;&lt;/p&gt;
&lt;p&gt;CIRS observations have shown that some of the coldest scenes in the Solar System exist on the icy moons of Saturn.&amp;#160; As such, it is possible &amp;#8211; in the case of Enceladus &amp;#8211; to use these observations to infer constraints on posited conductive heat flow through Enceladus&amp;#8217; icy shell.&amp;#160; 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.&amp;#160; The coldest CIRS temperatures of circa 30 K are found in the north polar region, where the ice shell is relatively thin.&amp;#160; This indicates that conductive heat loss is unlikely to be more than 25 GW.&amp;#160; This does not discount the possibility of highly spatially varying conductive heat flow, or temporal variation on timescales beyond Cassini&amp;#8217;s observations. As such future observations (particularly of the north polar region) would be welcomed.&amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;em&gt;References&amp;#160;&lt;/em&gt;&lt;/p&gt;
&lt;p&gt;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&amp;#8211;3879. http://arxiv.org/abs/1601.05804 https://doi.org/10.1093/mnras/ stw609 arXiv:1601.05804.&lt;/p&gt;
&lt;p&gt;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&lt;/p&gt;
&lt;p&gt;Lainey, V., Karatekin, O&amp;#776;., Desmars, J., Charnoz, S., Arlot, J.-E., Emelyanov, N., Le Poncin-&lt;/p&gt;
&lt;p&gt;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.&lt;/p&gt;
&lt;p&gt;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&amp;#8211;296. http:// linkinghub.elsevier.com/retrieve/pii/S0019103516304183 https://doi.org/10. 1016/j.icarus.2016.07.014.&lt;/p&gt;
&lt;p&gt;Nimmo, F., Barr, A. C., Be&amp;#780;hounkova&amp;#769;, M., &amp; 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, &amp; A. J. Verbiscer (Eds.), Enceladus and the Icy Moons of Saturn (pp. 79&amp;#8211;94). Tucson, Arizona: University of Arizona Press. https://websites. pmc.ucsc.edu/ fnimmo/website/Enceladus_in_press.pdf&lt;/p&gt;
&lt;p&gt;Porco, C.C., Helfenstein, P., Thomas, P.C., Ingersoll, A.P., Wisdom, J., West, R., Neukum, G., Denk, T., Wagner, R., Roatsch, T. and Kieffer, S., 2006. Cassini observes the active south pole of Enceladus.&amp;#160;&lt;em&gt;science&lt;/em&gt;.&lt;/p&gt;
&lt;p&gt;Spencer, J.R., Pearl, J.C., Segura, M., Flasar, F.M., Mamoutkine, A., Romani, P., Buratti, B.J., Hendrix, A.R., Spilker, L.J. and Lopes, R.M.C., 2006. Cassini encounters Enceladus: Background and the discovery of a south polar hot spot.&amp;#160;&lt;em&gt;science&lt;/em&gt;&lt;/p&gt;