Uranian Mid-Sized Icy Moons: Unique Targets of Exploration in the Outer Solar System
Authors: Schenk Paul , Cartwright Richard , Beddingfield Chloé , Castillo-Rogez Julie 
Published: September 2022

&lt;p&gt;The five major Uranian satellites provide important constraints on compositional and dynamical evolution in the Uranian system.&amp;#160; These bodies experienced endogenic activity in the past, morphologically distinct from those on Saturnian icy moons.&amp;#160; Uranian moons also exhibit compositional differences with their Saturnian counterparts.&amp;#160; Because our understanding of these bodies is based on the single 1986 Voyager flyby and telescope observations, much remains to be discovered and understood about how these moons formed and evolved.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;ul&gt;
&lt;li&gt;&lt;strong&gt;1&lt;/strong&gt;&lt;strong&gt;. Introduction&lt;/strong&gt;&lt;/li&gt;
&lt;/ul&gt;
&lt;p&gt;The five major Uranian moons (diameters 930 to 1575 km and best density estimates of ~1.2 to 1.7 kg/m&lt;sup&gt;3&lt;/sup&gt;)&lt;sup&gt; &lt;/sup&gt;form a regular family of ice-rich planetary bodies, and record accretional conditions in the Uranian system.&amp;#160; Voyager 2 mapping of &lt;40% of their surfaces provides our only resolved record of these worlds [1,2]. Earth- and space-based telescope observations provide additional constraints. Here we review these moons in context with the Saturnian system, with a view towards the now likely return to the Uranian system as a major NASA Flagship.&lt;/p&gt;
&lt;p&gt;&lt;img src=&quot;&quot; alt=&quot;&quot; width=&quot;1039&quot; height=&quot;339&quot; /&gt;&lt;/p&gt;
&lt;p&gt;Figure 1: Voyager views of the 5 major Uranian icy moons.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;ul&gt;
&lt;li&gt;&lt;strong&gt;2&lt;/strong&gt;&lt;strong&gt;. Composition and Interiors&lt;/strong&gt;&lt;/li&gt;
&lt;/ul&gt;
&lt;p&gt;Uranian satellites are markers of the composition of the region they formed in. Telescope observations indicate surface compositional differences from Saturnian moons. Saturnian moons have bright surfaces with strong H&lt;sub&gt;2&lt;/sub&gt;O ice absorption bands (except Iapetus&amp;#8217; dark), partly due to E-ring deposition [e.g., 3]. Uranian moons have darker surfaces of H&lt;sub&gt;2&lt;/sub&gt;O-ice mixed with spectrally neutral components possibly rich in organics and/or hydrated silicates. CO&lt;sub&gt;2&lt;/sub&gt; on Saturnian moons is complexed with other species (except CO&lt;sub&gt;2&lt;/sub&gt;-ice on Enceladus) [4,5]; CO&lt;sub&gt;2&lt;/sub&gt; is in crystalline form on Uranian moons [4,5].&amp;#160;&amp;#160;&lt;/p&gt;
&lt;p&gt;Limited topography indicates Uranian moons are more rugged than inner Saturnian moons [7], provisionally attributed to global thermal resetting in saturnian moons.&amp;#160; Enceladus is the only confirmed ocean world among these moons (deep oceans have been suggested at Dione [8] and Mimas [9]). &amp;#160;Saturnian moons are involved in mean motion resonances that drive obliquity or eccentricity, whereas Uranian moons are not but likely shared such resonances in the past [10,11].&amp;#160;&amp;#160;&lt;/p&gt;
&lt;p&gt;Radioactive heating estimates range from ~0.1 mW/m&lt;sup&gt;2&lt;/sup&gt; for Miranda (similar to Mimas) to ~0.7 mW/m&lt;sup&gt;2&lt;/sup&gt; for Ariel/Umbriel (similar to Dione/Rhea, but three orders magnitude less than measured at Enceladus) to&amp;#160;~1.1 mW/m&lt;sup&gt;2&lt;/sup&gt; for Titania/Oberon [12]. &amp;#160;If Uranian moons preserve liquid, it is likely as residual oceans &lt; 30 km thick, or brine-filled porous cores [12]. Miranda is unlikely to preserve liquid presently unless recently tidally heated.&amp;#160;&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;ul&gt;
&lt;li&gt;&lt;strong&gt;3&lt;/strong&gt;&lt;strong&gt;. Tectonics, Volcanism, Past Heat Flow&lt;/strong&gt;&lt;/li&gt;
&lt;/ul&gt;
&lt;p&gt;The five Uranian moons betray various levels of tectonic activity, from deep walled troughs of Ariel (Fig. 2) to cryptic deep features on Umbriel&amp;#8217;s limb [2,7].&amp;#160; Some are ancient but global extent and associated stresses are uncertain due to mapping limits.&amp;#160; (Cryo)volcanism of higher relief than in the Saturnian system is widespread on Miranda and Ariel (and unresolved on the others) but composition and eruption state are poorly known [7].&lt;/p&gt;
&lt;p&gt;&lt;img src=&quot;&quot; alt=&quot;&quot; /&gt;&lt;/p&gt;
&lt;p&gt;Figure 2.&amp;#160; Voyager stereo-derived DEM showing tectonic and resurfacing features on Ariel. Cylindrical map projection.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;p&gt;Ariel&amp;#8217;s estimated past heat flux (6&amp;#8211;92 mW/m&lt;sup&gt;2&lt;/sup&gt;) [13,14] overlaps Tethys, Dione, Rhea, Triton, Pluto, and Charon (Fig. 3), and are consistent with tidal heating from past resonances [13,14]. Miranda&amp;#8217;s maximum estimated heat flux (31&amp;#8211;140 mW/m&lt;sup&gt;2&lt;/sup&gt;) [14,15] is higher than Ariel, Tethys, Dione, and Rhea, and overlaps estimates for Enceladus [16,17]. High heat fluxes for Miranda suggest increases to eccentricity from past resonances [10,11].&amp;#160;&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;img src=&quot;&quot; alt=&quot;&quot; width=&quot;1036&quot; height=&quot;747&quot; /&gt;&lt;/p&gt;
&lt;p&gt;Figure 3. Comparison of heat flux estimates on icy bodies.&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;&amp;#160;&lt;/strong&gt;&lt;/p&gt;
&lt;ul&gt;
&lt;li&gt;&lt;strong&gt;4&lt;/strong&gt;&lt;strong&gt;. Comparison and Exploration&lt;/strong&gt;&lt;/li&gt;
&lt;/ul&gt;
&lt;p&gt;Like Saturn&amp;#8217;s icy moons, the five major Uranian satellites are diverse, from little geologic activity to near global resurfacing as little as ~0.1 Gyr ago [17]. &amp;#160;The two satellite systems are not identical, representing formation in distant compositionally distinct part of the Solar System, and under possibly much different formation conditions.&amp;#160; Cassini showed the value of a dedicated orbiter with instrumentation across the spectrum. Aside from resurfacing of Dione, Tethys, and esp. Enceladus [18], Cassini revealed magnetospheric and ring particle surface alterations [18, 19], satellite ring deposition [18,20] and other processes. Whether these occur(red) in the Uranian system must await a dedicated Uranian orbiter. &amp;#160;Such observations and interior property constraints from close encounters, will allow better understanding of formation processes of these moons and the Uranian system.&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;&amp;#160;&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Reference&lt;/strong&gt;&lt;strong&gt;s&lt;/strong&gt;&lt;/p&gt;
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