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Ice Sublimation, Outgassing, and Melting on Ceres: Models and Observations

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Ice Sublimation, Outgassing, and Melting on Ceres: Models and Observations 47th Lunar and Planetary Science Conference (2016) 2736.pdf ICE SUBLIMATION, OUTGASSING, AND MELTING ON CERES: MODELS AND OBSERVATIONS. P. O. Hayne1 and O. Aharonson2, 1NASA – Jet Propulsion Laboratory, California Institute of Technology ([email protected]), 2Weizmann Institute of Science, Israel Introduction: Ceres is the largest member of the 190 K. In the polar perennial shadows, water ice is Main Belt asteroids and orbits near the Solar System’s stable indefinitely. Within ~2° of both poles, it is stable “frost line” where water readily condenses. With a on illuminated surfaces on billion-year time scales as bulk density of ~2 g cm-3, Ceres may have a significant long as it maintains high albedo [8]. Subsurface ice is water component, either in the form of an icy mantle stable for > 1 Gyr within ~1 m depth above ~45° lati- [1], hydrated silicates [2], or both. Because its growth tude. We also model ice sublimation over a range of was apparently arrested at the protoplanetary stage, ice grain sizes from microns to meter-size blocks or observations of Ceres could test models for planetary bedrock, which have much longer lifetimes. formation, incorporation of water, and interior evolu- Ice exposure and removal: By analogy with obser- tion [3]. One of the key observations to be made by the vations of fresh, icy craters on Mars [9], we model the Dawn mission is the presence (or absence) and distri- exposure and gradual disappearance of icy ejecta mate- bution of ice on the surface and near-subsurface. rials. The thinner outer portions of the ejecta blanket Motivation: On billion-year timescales water ice disappear first, producing the highest outgassing rates, is unstable against sublimation at nearly all latitudes on which then decay as the surface area decreases. Out- Ceres’ surface, but may exist in the near subsurface at gassing rates and vapor densities are estimated using a high latitudes [4][5]. Therefore, if Ceres has an ice-rich simple plume model, where vapor escape is deter- mantle, processes such as mass wasting on slopes and mined by the mean thermal velocity, v , at the sur- impacts may expose ice, which would then sublimate th and outgas to space. Water vapor and OH observed face. In this case, the gas density is approximated by around Ceres may be evidence of such outgassing [6] E! ρ ~ = E! m/(k T) , (2) [7]. Thus, depending on the excavation rate, exposures gas v B th of ice could be common at high latitudes. Conversely, where m is the molecular mass, and k is Boltz- a lack of exposed ice could indicate a crust heavily B processed by meteorite impacts or an intrinsically low mann’s constant. An instrument measuring the line-of- ice fraction. Here we compare models to reported ob- sight column density would see M ~ ρ D , where D servations by the Dawn mission, to place limits on the gas ice content of Ceres’ uppermost crust, and investigate is the diameter of the source region. the role of water in surface modification and outgas- Melting of buried ice: Melting is not expected un- sing. der normal conditions on Ceres, because solar and geo- Models: We simulate surface and subsurface tem- thermal heat fluxes are too low and there is no appre- peratures on Ceres with a numerical heat and vapor ciable atmosphere. However, high heating rates during diffusion model, using realistic topography and illumi- impacts may produce transient liquid H2O in the sub- nation conditions [8]. Sublimation at the surface is controlled by radiative heating, balanced against cool- ing by thermal emission and latent heat removal: 4 εσT = F − LE! (1) solar where ε is the bolometric infrared emissivity, σ is the Stefan-Boltzmann constant, T is temperature, F is solar the absorbed insolation flux, L is the latent heat of ! sublimation, and E is the sublimation rate. At typical noontime equatorial temperatures of ~240 K, granular H O frost would sublimate extremely quickly, with a 2 timescale of days. However, water ice will never reach Figure 1: Lifetime of H2O ice blocks/bedrock on Ceres’ these temperatures, due to the efficiency of latent heat surface (defined as time to remove 1 m), based on numerical removal. Instead, the peak temperature of ice at the modeling. (Adapted from [8]). Shown are lifetime contours spaced logarithmically, and the red ‘X’ indicates the albedo equator is given by solving equation (1), giving T ≈ max and latitude of the bright spots in Occator. 47th Lunar and Planetary Science Conference (2016) 2736.pdf 1010 108 106 R = 10 km 104 Lifetime (yr) 102 R = 1 km R = 100 m 100 0 10 20 30 40 50 60 70 80 90 Latitude (degrees) Figure 2: The “bright spots” in Occator crater near local Figure 3: Lifetimes of ice excavated by impacts on Ceres, noon, where we find ice to be highly unstable on time- for three different crater radii, R. These lifetimes are cal- scales of ~5 – 100 yr. culated using the numerical model described in the text, along with an assumed ~1/r3 ejecta thickness distribution. surface. We investigate possible melting by modeling impact heating and conduction into the subsurface, deposits with short lifetimes. Therefore, the ice must accounting for latent heat removal and vapor diffusion be resupplied on short time scales if sublimation out- through the regolith [10]. gassing is observed. Given the moderately long life- Results: Lifetimes of exposed ice vary widely de- times of impact-excavated water ice at high latitudes, it pending on latitude, surface slope, albedo, and thermal would be surprising if no such exposures were ob- inertia [8] [11]. Figure 1 summarizes some of our re- served. sults. At the equator, lifetimes of >104 yr are possible Models of heat transfer following hypervelocity for cohesive ice blocks with high albedo (>0.55). impacts suggest both sustained vaporization and melt- However, such features would have outgassing rates ing of H2O are possible. This process could remove below detection limits. Large outgassing rates can be large quantities of subsurface ice, especially at high achieved in the low- to mid-latitudes for darkened or latitudes. In this case, instability and structural failure granular ice deposits, which will disappear rapidly. For are possible, resulting in observable signatures such as example, bright ice deposits (albedo ~ 0.25) at the slumping, landslides or collapsed crater walls. We will ~20° latitude of Occator [12] (Fig. 2) would have life- present several possible models that will be tested with times of ~5 – 100 yr, corresponding to sublimation existing and future Dawn observations. rates of 10-7 to 10-5 kg m-2 s-1. Given their surface area of ~100 km2, only ~1 – 20% of the Occator bright References: [1] Castillo, J. C. and T. B. McCord spots would need to have been active to explain the the (2010), Icarus 205, 443-459. [2] Zolotov, M. Y. vapor production rate of ~6 kg s-1 inferred from obser- (2009), Icarus 204, 183-193. [3] McCord, T. B. and C. vations by [7]. This could be the case, for example, if a Sotin (2005), JGR 110, E5. [4] Fanale, F. P. and J. R. large mass-wasting event temporarily exposed water Salvail (1989), Icarus 82, 97-110. [5] Schorghofer, N. ice, which was subsequently removed by sublimation. (2008), ApJ 682, 697. [6] A’Hearn, M. F. and P. D. Impacts may produce sustained H2O vapor produc- Feldman (1992), Icarus 98, 54-60. [7] Küppers, M., et tion at Ceres by exposure of icy ejecta materials. We al. (2014), Nature 505, 525-527. [8] Hayne, P. O. and calculate lifetimes (Fig. 3) and outgassing rates for O. Aharonson (2015), JGR 120, 1567-1584. [9] Dun- such events. Although long lifetimes are possible, the das, C. M. and S. Byrne (2010), Icarus 206, 716-728. development of a darkened sublimation lag deposit [10] Hayne, P. O., et al. (2010), Science 330, 477-479. would obscure the ice and affect its sublimation. None- [11] Titus, T. N. (2015), GRL 42, 2130-2136. [12] theless, these results show that impact-excavated ice Nathues, A., et al. (2015), Nature 528, 237-240. could persist on Ceres’ surface for >104 yr at latitudes >65° even for small craters < 1 km in radius. Acknowledgement: Part of this work was per- Conclusions: Lifetimes of exposed water ice lay- formed at the Jet Propulsion Laboratory, California ers on Ceres vary widely, depending on latitude as well Institute of Technology, under contract with the Na- as surface slope, albedo and thermal inertia. Outgas- tional Aeronautics and Space Administration. sing from exposed ice can be rapid, but only for those .
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