A Wunda-full world? Carbon dioxide ice deposits on Umbriel and other large moons of Uranus Michael M. Sori, Shane Byrne, Jonathan Bapst, Ali M. Bramson, and Margaret E. Landis, Lunar and Planetary Laboratory, University of Arizona ([email protected]) Umbriel and Uranus Thermal Modeling I thermal inertia R gas constant Wundafill c heat capacity T Temperature Umbriel is one of five large (r = 584.7 km) We use a 1D semi-implicit thermal E escape velocity r Umbriel radius Uranian moons. It is dark (average bond conduction model to calculate p vapor pressure µ molecular mass albedo ~0.1), but has a noticeable bright surface temperatures. We use I = i sublimation rate k Boltzmann constant (~0.5) annulus inside the ~130 km 15 J m-2 K-1 s-0.5 for regolith [8] and g surface gravity v velocity -2 -1 -0.5 Above: Table of parameters used. diameter equatorial crater Wunda [1–3]. I = 940 J m K s for pure CO2 We hypothesize that the bright annulus, ice. Based on temperature results 450 which we call “Wundafill,” represents a we estimate sublimation rates, South 400 deposit of carbon dioxide ice. Pole which are given by: --- Regolith 350 --- Equator --- Thick CO2 ice --- North pole ) 2 300 Recent detections of CO2 on the trailing µ --- South pole hemispheres of Uranian satellites [4,5] i = p 250 support this hypothesis. The CO2 may be 2π RT 200 produced radiolytically [6]. We test our Above: Temperatures for regolith at the equator, Above: Temperatures for regolith and thick CO2 ice 150 north pole, south pole of Umbriel. at Wunda’s location. hypothesis using thermal modeling and Sublimation (kg/ m We have performed thermal model 100 ballistic transport models. simulations at varying thermal 50 -7 The sublimation on a surface with thermal properties of thick CO2 ice at Wunda is < 3×10 inertias and find our results are 2 The extreme obliquity of the Uranian 0 kg/m per Uranian year (~84 Earth years). robust to uncertainty in this −80 −60 −40 −20 0 20 40 60 80 system (~98°) makes our study Latitude Above: Uranus as seen from the Hubble Space Telescope parameter. fundamentally different from similar work and Umbriel as seen from Voyager 2 (not to scale or during Above: Total annual sublimation of CO2 ice A thick (10s of m) CO2 ice deposit on Umbriel is stable over the age of the solar system. on other planets [e.g., 7]. the same season). mixed with regolith on Umbriel. Ballistic Transport Carbon Dioxide Budget Topographic Effects Other Uranian Moons When a molecule sublimates, it is launched on a Using the temperatures and sublimation rates from our thermal model Miranda Ariel Umbriel Why does Wundafill appear in a crater, and not a Why have we not observed similar 0°0° 0° 1581U2-001 ballistic trajectory in a random direction, and with a as an input for our ballistic transport model, as has been done for latitudinal band? And why is Wundafill in the shape of an bright spots on the other four 1667U2-001 1705U2 -001 1699U2 velocity according to the Maxwell-Boltzmann probability other planets [e.g., 7] we calculate how an initial distribution of -001 1665U2 annulus? We propose that Wunda is a complex crater, as large moons of Uranus? Miranda 1717U2-001 1663U2-001 1334U2-001 -001 1714U2-001 Wunda distribution: surface CO2 evolves over time. expected from its size [9]. The crater topography would 270° 1708U2 90° is too small to retain CO2 (E = -001 1661U2-001 1546U2 3 1577U2 -001 −µv2 1702U2-001 -001 2 have 3 thermal effects: (1) sloped terrain would 193 m/s) and detections of CO2 1550U2-001 ⎛ µ ⎞ 2 (a) (b) f = 4π ⎜ ⎟ v e 2kT experience different insolation, (2) walls and central peak on Oberon are uncertain. Voyager ⎝ 2πkT ⎠ would provide shadowing to the floor during parts of the 20 3 2 was only able to image ~1/3 of year, and (3) crater topography obstructs portions of the 1523U2-001 If a molecule’s velocity exceeds Umbriel’s escape theDidosurfaces of Ariel, Umbriel, and 1111U2-001 ) 2 2 0 velocity of E = (2gr)0.5 = 517 m/s, it is lost from the sky from reradiation of heat from surrounding topography. Titania, so it is not surprising that 270° 90° 1 1137U2-001 system via Jeans’ escape. -20 we have only observed a bright 0 Left: Cassini image of Dione. The crater deposit on 1 of the 3 moons. We Flux (kg/m Flux -40 180° 180° 2 S -1 Dido has a central peak. Relevant 0.01 N S N Dido predict that complete coverage Titania Oberon parameters are very similar between -60 -2 would reveal a similar bright 0.009 Dione/Dido and Umbriel/Wunda, so we Above: Voyager 2 image coverage of the five 0.008 Left: Velocity -3 deposit of CO2 ice on all three of Annual CO expect Wunda to have a central peak. 2020 K distributions of -80 large Uranian satellites. 0.007 5555 K -44 these moons. 9090 K sublimated CO2 0.006 -100 131 km molecules at < -5 0.005 (a) temperatures 20 80 km 5 0.004 ) 2 The case for Wundafill as CO Ice relevant to 0 2 0.003 0 Umbriel. Dashed 4.0 km 6.7 km 1. CO has been detected on the trailing hemisphere of Umbriel, and Wunda Probability Distribution Probability −20 2 0.002 line represents the -5 20 km lies very near the center of the trailing hemisphere. 0.001 −40 Flux (kg/m Flux escape velocity. 2 -10 10 2. CO2 ice migrates to low latitudes over short timescales, and Wunda lies 0 −60 0 100 200 300 400 500 600 (b) -15 N nearly at the equator. Velocity (m/s) −80 3. Umbriel occupies a sweet spot for CO : it is warm enough for CO to be -20 9 2 2 ) Annual CO −100 2 -2 mobile, cold enough so a thick CO2 ice deposit is resistant to sublimation −120 −80 −60 −40 −20 0 20 40 60 80 −80 −60 −40 −20 0 20 40 60 80 -4 over long timescales, and large enough to retain CO2. Latitude Latitude 8 4. The albedo of Wundafill is consistent with CO2 ice. Flux (kg/m -6 2 5. The annular shape of Wundafill is explained by Wunda as a complex crater. Above: Flux of CO2 molecules at a given location on Umbriel’s surface over one -8 7 Uranian year, starting from an initial condition of CO2 everywhere on the surface Right: Fraction of -10 (left column) or everywhere except depleted from regions poleward of 50° latitude Annual CO sublimated CO (right column). Top shows results displayed over one hemisphere, bottom shows 6 Why do we care? 2 -12 molecules that flux as a function of latitude. 1. We make a prediction about the Uranian satellite system that can are lost to Jeans -14 easily be tested with a Uranus orbiter or fly-by. log10(Escape Fraction) Once CO starts migrating, the the latitudinal band experiencing net < 5 escape versus 2 2. If correct, bright ice deposits will have implications for other -16 CO influx narrows as the system evolves. These results assume the temperature. 20 30 40 50 60 70 80 90 2 Above: (a) Our estimate of Wunda’s topography, extrapolated from thermal properties of regolith; thick CO ice is instead effectively processes on these bodies, e.g. radiolysis or cryovolcanism. Temperature (K) 2 Saturnian moons [10]. (b) The CO2 budget for the walls, floor, and immobile. central peak of Wunda crater taking into account the 3 thermal 3. 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