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Ceres, an Unexpectedly Active Dwarf Planet: Findings from the Dawn Mission

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Ceres, an Unexpectedly Active Dwarf Planet: Findings from the Dawn Mission Ceres, an unexpectedly active dwarf planet: Findings from the Dawn mission Bethany Ehlmann California Institute of Technology (with thanks to the Dawn Science team) February 21, 2018 NASA/JPL-Caltech/UCLA/MPS/DLR/IDA Ceres 2.7 AU Largest asteroid ~30% of the mass of the main belt Dwarf Planets Asteroids record the formation and dynamical history of the solar system Asteroids record the formation and dynamical history of the solar systemrocky hydrous icy DeMeo & Carry, 2014, Nature We have pieces from some of the asteroids in meteorites Carbonaceous Ordinary Chondrites Chondrites Increasing water content Increasing “processing” (heating and differentiation) Irons Achondrites Stony Irons http://www.astro.washington.edu/users/smith/Astro150/Labs/Meteorites/ Asteroids come in many types with characteristic spectra Metal Rocky Hydrous (Ice + Phyllosilicates) Psyche (artist’s concept) Vesta Itokawa Ceres Indicate • originally accreted materials • effects of early heating and differentiation driven by short-lived radiogenic isotopes Asteroids come in many classes with characteristic spectra linkable to lab data and meteorite spectra Telescopic spectra of asteroids (reflected light) Lab spectra Mg-OH Mg-OH Fe(II) serpentine Mg2Si2O5(OH)4 howarditemeteorite Fe (II) Fe (II) high-Ca pyroxene Ca(Fe,Mg)Si2O6 Fe (II) Cold Bokkeveld Pieters & McFadden, 1994 meteorite • Rocky meteorites show mafic minerals olivine and pyroxene • Hydrous (inferred from density) asteroids are dark More information on Hydrous, C- and D-Class Asteroid (Spectral Properties in longer λ SWIR metal-OH Fe(II) phyllosilicates H2O-ice +organics? (the asteroid; not “Ceres-class” NH4? Jovian moon) Water ice? Mg(OH)2? ? Takir & Emery, 2012, Icarus Ceres is weird (size, spectral properties)… not like most other C- and D-class dark asteroids Ice melted; rxns with rockà hydrous minerals (metal-OH) Ice didn’t melt; water ice leftover Weird…needs further study Based on its position in the solar system and accretion time, Ceres should have heated enough to melt all water, differentiate, host hydrothermal systems Takir & Emery, 2012; after McSween et al., after Grimm & McSween, 1993 Ceres, Pre-Dawn Mission Dark, flat spectrum similar to carbonaceous chondrites? ρ = 2.2 g/cm3 (Larson et al. 1979) D ~ 963km x 891km Debated composition based on telescopic IR spectra: Rotation+shape à ice, phyllosilicates, carbonates, magnesite, brucite… differentiated, icy (Lebofsky et al. 1981, King et al. 1992, Rivkin et al. 2006, mantle & rocky core Milliken & Rivkin 2009) (Thomas et al. 2005) No clearly associated meteorite family. Fanale & Salvail 1989 100 Thermal models suggested icy outer 50% porosity; Specific shell, possible rpore=10 µm predictions of interior ocean. (McCord & Sotin 10 near-surface ice 10% porosity; stability 2005, Castillo- rpore=1 µm & Rogez & McCord Possible polar 2010) frosts 1 11 Dawn Mission Dawn launched in September 2007 to visit Vesta and Ceres to answer – What chemical variations in the nebular disk? – How did time of formation affect asteroids? – How have materials been processed post-formation? FC GRaND VIR Framing Camera Gamma Ray & Neutron Visible and Infrared (multispectral VNIR) Detector (bulk chemistry) Mapping Spectrometer Dawn Mission: The Journey to Vesta and Ceres Rocky, pyroxene-bearing Hydrous silicates? Rock + Ices? NASA/JPL-Caltech/UCLA/MPS/DLR/IDA Typical Dark Ceres: Dawn VIR, Ceres requires NH4 phyllosilicates, Mg-OH phyllosilicates like serpentines • Ceres is very dark, fairly homogeneously (except for bright spots) • VIR detects Mg-phyllosilicate (like serpentine), NH4-phyllosilicate, Mg/Ca- carbonate, and dark component Mg-OH CO3 CO3+CH NH4 DeSanctis et al., 2015, Nature Ceres’ Dark Surface is Globally Homogeneous Ammannito et al., 2016, Science • Band center maps at 2.7 and 3.1 µm show ubiquitous presence of Mg phyllosilicates and ammoniated materials • Strikingly uniform. In dark materials, no variation in band center; only small variation in intensity, even around large craters • May indicate globally homogeneous aqueous alteration of silicates Typical Dark Ceres: Dawn VIR, Ceres requires NH4 phyllosilicates, Mg-OH phyllosilicates like serpentines • Ceres is very dark, fairly homogeneously (except for bright spots) • VIR detects Mg-phyllosilicate (like serpentine), NH4-phyllosilicate, Mg/Ca- carbonate, and dark component Mg-OH CO3 CO3+CH NH4 DeSanctis et al., 2015, Nature Ceres is weird (size, spectra)… not exactly like carbonaceous chondrites De Sanctis et al., 2015, Nature Carbonaceous chondrite matrix: carbonate, serpentine, smectites (Blinova et al., 2014) Ceres is weird (size, spectra)… not exactly like carbonaceous chondrites Mg-OH NH4 CO3+CH? CO3 • Ceres has – Strong, characteristic NH4 absorption – distinctively sharp 2.72μm Mg-OH absorption relative to many carbonaceous meteroties – More carbonate than typical meteorite – Less organics than some? Or different type? 3.06-um absorption indicates ammoniated phyllosilicates M-OH, • Predicted by King et al., NH4 H2O 1994 based on telescopic data M-OH, NH4 • H2O Mg-saponite best fit but several classes fit NH4 inserts here Maintenance of volatiles over 1 Ga volatile lost volatile retained Ceres Brown, 2012, Ann. Rev. Where did Ceres come from? Ceres may have formed in the trans- Neptunian disk, before it was implanted into the main belt (e.g., Dynamically difficult and Ceres does not resemble TNOs/KBOs McKinnon, 2012) Ceres formed closer to its present position by accreting material that drifted inward from greater distances (e.g. NH3 would be lost when crossing Jupiter’s gap (Turner et al., 2012) Mousis & Alibert, 2005) Ceres formed between the orbits of the giant planets and was scattered due to planet growth and migration Emerging paradigm consistent with cosmochemical models (Raymond +Izodoro, 2017) favored De Sanctis et al., Nature,2015, 10.1038/nature16172 Typical Dark Ceres: Dawn VIR, Ceres requires NH4 phyllosilicates, Mg-OH phyllosilicates like serpentines • Ceres is very dark, fairly homogeneously (except for bright spots) • VIR detects Mg-phyllosilicate (like serpentine), NH4-phyllosilicate, Mg/Ca- carbonate, and dark component Mg-OH CO3 CO3+CH NH4 DeSanctis et al., 2015, Nature Distinctly Mg-rich phyllosilicate suggests high degree of alteration Mg-OH Fe-OH Takir et al., 2015, MAPS Group 2 (less altered) Ehlmann et al., in revision, Group 3 MAPS (more altered) GRaND Hydrogen Maps of Ceres Prettyman et al., 2017, Science • Ice-free regolith in equatorial region is similar to CI/CM composition but more water rich that all but most hydrous CIs (Ceres: ~16 wt. %) • Water ice table near surface at poles, receding deeper at the equator Prettyman et al., Science, 2016 GRaND Hydrogen Maps of Ceres Prettyman et al., 2017, Science @ poles • Ice-free regolith in equatorial region is similar to CI/CM 1 cm dark material composition but more water rich (~16 wt. %) dark + 10% • Water ice table near surface at poles, receding deeper at the ice equator Prettyman et al., Science, 2016 Iron content of Ceres from GRaND Prettyman et al., 2017, Science • Iron abundance is lower than the average value for CI/CM chondrites, even when these are diluted by additional water Prettyman et al., Science, 2016 A single region near Ernutet crater more organics-rich: exogenic vs. endogenic origin debated De Sanctis et al., 2017, Science; Pieters et al., 2018, MAPS Framing Camera enhanced color mosaic (MPS) Overall, elevated C from GRaND data globally Prettyman et al., 2018, LPSC • C in organics or carbonates? Typical Dark Ceres: Dawn VIR, Ceres requires NH4 phyllosilicates, Mg-OH phyllosilicates like serpentines • Ceres is very dark, fairly homogeneously (except for bright spots) • VIR detects Mg-phyllosilicate (like serpentine), NH4-phyllosilicate, Mg/Ca- carbonate, and dark component CO3 Mg-OH Candidate darkening CO +CH 3 agents: NH4 Magnetite (Fe3O4)? Iron sulfides? Carbon? DeSanctis et al., 2015, Nature Quantifying the composition of typical Ceres is challenging for models – some of our current research • Unusually dark properties of carbonaceous chondrite-type materials • Many very fine particles approaching wavelength of light Kurokawa et al., LPSC, 2018; from DeSanctis et al., 2015 Good fit. Unrealistically high dark carbon- or Fe- bearing phases Quantifying the composition of typical Ceres is challenging for models – some of our current research • Unusually dark properties of carbonaceous chondrite-type materials • Many very fine particles approaching wavelength of light New approach: examine differences from Kurokawa et al., LPSC, 2018; DeSanctis et al., 2015 carbonaceous chondrites Serp. More NH4 materials than meteorites: A captured body from the outer solar system? There are planums and planitae, and rougher and Ceres Topography smoother areas. Crater morphology is only weakly modulated by latitude (temperature) non-hydrostatic degree two topography of about+/- 2 km Vendimia Planitia Hanami Planum Park et al., 2016, Nature Spatial resolution ~100 m km Ceres Interior Evidence for physical differentiation: Was there a subsurface ocean? ü Gravity and shape data indicate partial differentiation (Park et al., 2016, Fu et al. 2017) consistent with models that produce an ocean (McCord+Sotin, 2005; Zolotov, 2009; Castillo-Rogez and McCord, 2010) ü Ice-rich regolith, altered chondritic elemental composition and low Fe measured by GRaND (Prettyman et al., 2016) indicate extensive water-rock fractionation and volatile mobility ü Presence of pervasive phyllosilicates (De Sanctis et al., 2015; Ammannito
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