Exploring Mercury: Scientific Results from the MESSENGER Mission Larry R. Nittler Carnegie Institution of Washington ASU SESE Colloquium April 16, 2014 Acknowledgements • Richard D. Starr, Shoshana Z. Weider, Patrick N. Peplowski, Ellen Crapster-Pregont, Larry G. Evans, Timothy J. McCoy, Dave Blewett, William V. Boynton, Paul Byrne, Nancy Chabot, Brett Denevi, Denton S. Ebel, Carolyn M. Ernst, Jeffrey J. Gillis-Davis, John O. Goldsten, Tim Goudge, David K. Hamara, Steven A. Hauck, II, Jim Head, Christian Klimzcak, David J. Lawrence, Ralph L. McNutt, Jr., Scott Murchie, Louise Prockter, Edgar A. Rhodes, Mark Robinson, Charles E. Schlemm II, Sean C. Solomon, Ann L. Sprague, Karen R. Stockstill-Cahill • MESSENGER Science Team, Engineers, Mission Operations (APL) Mercury Mercury Venus Jupiter Mars May 12, 2011, from NZ (M. White, Flickr) • Naked-eye planet, but very difficult to observe due to proximity to Sun Mercury Is IsDifficult Study Mercury Difficult toto Study …by telescope … …or spacecraft. Only prior visit was by Mariner 10, 1974-1975 Mercury Exploration Contributions of Mariner 10 flybys: • Imaged 45% of Mercury’s surface • Discovered Mercury’s magnetic field and dynamic magnetosphere • Detected H, He, O in Mercury’s “exosphere” Mariner 10 mosaic of the Caloris basin Mercury Exploration Contributions of Earthbased astronomy • Discovery of Mercury’s 3:2 spin-orbit resonance (1965) • 3 days in 2 years • Discovery of sodium (1985), potassium (1986), and calcium (2000) in Mercury’s exosphere Na emission Potter et al. 2002 Mercury Exploration Contributions of Earthbased astronomy • Discovery of Mercury’s polar deposits (1992) • Discovery of Mercury’s molten outer core (2007) Harmon et al. 1999 Mercury: planet of extremes • Smallest, densest planet • Closest to Sun • Highest diurnal variation in temperature – −170 ºC to +430 ºC • Very high Fe:silicate ratio – Core ~70% of mass, 80% radius • Magnetic field: dynamic magnetosphere • Low FeO in surface silicates “end-member of planet formation” Extrasolar Planetary Context Mercury Extrasolar Planetary Context Mercury Mercurylike? • First spacecraft to orbit Mercury • 7th NASA Discovery mission – PI: Sean C. Solomon Challenge: Surviving at Mercury Ceramic cloth sunshade Low-mass composite structure Ceramic cloth sunshade Solar panels are 2/3 mirrors Solar panels 2/3 mirrors Phased-array high-gain antenna Trajectory allows orbit insertion Scientific Payload Vibration test, December 2003 Launch, August 2004 Getting to Mercury Earth (August 2005) Venus (October 2006) Mercury Flybys (2008-2009) • >90% of surface imaged M1 (Jan 2008) M2 (Oct 2008) • Flyby results: • Extensive volcanism, color/albedo variations • Dynamic magnetosphere • Na, Mg, Ca variability in exosphere M3 (Sep 2009) Mercury Orbit Insertion (March 18, 2011) Mercury Orbit Insertion (March 18, 2011) 2988 orbits completed as of April 16, 2014 After 3 flybys +2 years orbit, MESSENGER Near-Global Mosaic increased imaging Image coverage to 100%! Carolyn Ernst, [BeckerFeb et 2013 al., morning poster] Mercury in “true” color – RGB: 630, 560, 480 nm Mercury in “enhanced” color – RGB: PC2, PC1, 430/1000 Caloris basin: 1550 km diameter Mercury radius: 2440 km RGB: PC2, PC1, 430/1000 Formation of Mercury • Terrestrial planets shared common formation process: accretion Dust -> Rocks -> Planetesimals -> Planets • Why does Mercury have such a large Fe core? (pre-MESSENGER) Mercury Formation Models • Accretion at high-T? (Lewis 1973) • Evaporation by hot Sun? (Cameron 1985) • Giant impact stripping? (Wetherill, Benz 1988) Aerodynamic sorting (Weidenschilling 1978) Composition of Mercury Gamma-ray/ Neutron Spectrometer (GRS/NS) measures surface composition by detecting cosmic-ray- and radioactivity-induced grays and neutrons XRS measures surface composition by detecting solar-induced X-ray fluorescence Major elements on Mercury 0.8 Moon rocks Earth rocks Highlands Mercury Peridotites 1.0 (Nittler et al. 2011) Komatiites 0.5 Mare basalts Cont. crust 0.0 0.0 0.2 Oceanic basalts Lunar Highlands 0.4 0.6 Al/Si (wt. ratio) 0.8 1.0 Ca/Si (wt. ratio) Mg/Si (wt. ratio) 1.5 0.6 Mare basalts 0.4 Oceanic basalts 0.2 Komatiites 0.0 Peridotites 0.0 0.2 Cont. crust 0.4 0.6 Al/Si (wt. ratio) 0.8 • XRS flare data indicate Mercury Mg-rich, AlCa-poor, relative to Earth and Moon surface 1.0 Major elements on Mercury 0.8 Moon rocks Earth rocks Highlands Mercury Peridotites 1.0 (Nittler et al. 2011) Komatiites No lunar-like flotation crust 0.5 Mare basalts Cont. crust 0.0 0.0 0.2 Oceanic basalts Lunar Highlands 0.4 0.6 Al/Si (wt. ratio) 0.8 1.0 Ca/Si (wt. ratio) Mg/Si (wt. ratio) 1.5 0.6 Mare basalts 0.4 Oceanic basalts 0.2 Komatiites 0.0 Peridotites 0.0 0.2 Cont. crust 0.4 0.6 Al/Si (wt. ratio) 0.8 • XRS flare data indicate Mercury Mg-rich, AlCa-poor, relative to Earth and Moon surface 1.0 Major elements on Mercury 0.8 Moon rocks Earth rocks Highlands Mercury Peridotites 1.0 Komatiites (Nittler et al. 2011) +Weider et al. 2012 +unpub No lunar-like flotation crust 0.5 Mare basalts Cont. crust 0.0 0.0 0.2 Oceanic basalts Lunar Highlands 0.4 0.6 Al/Si (wt. ratio) 0.8 1.0 Ca/Si (wt. ratio) Mg/Si (wt. ratio) 1.5 0.6 Mare basalts 0.4 Oceanic basalts 0.2 Komatiites 0.0 Peridotites 0.0 0.2 Cont. crust 0.4 0.6 Al/Si (wt. ratio) 0.8 • XRS flare data indicate Mercury Mg-rich, AlCa-poor, relative to Earth and Moon surface 1.0 Major elements on Mercury 0.8 Moon rocks Earth rocks Highlands Mercury Peridotites 1.0 Komatiites (Nittler et al. 2011) +Weider et al. 2012 +unpub No lunar-like flotation crust 0.5 Mare basalts Cont. crust 0.0 0.0 0.2 Oceanic basalts Lunar Highlands 0.4 0.6 Al/Si (wt. ratio) 0.8 1.0 Ca/Si (wt. ratio) Mg/Si (wt. ratio) 1.5 0.6 Mare basalts 0.4 Oceanic basalts 0.2 Komatiites 0.0 Peridotites 0.0 0.2 GRS results et al 2012) Cont. crust(Evans 0.4 0.6 Al/Si (wt. ratio) 0.8 • XRS flare data indicate Mercury Mg-rich, AlCa-poor, relative to Earth and Moon surface • GRS data consistent 1.0 High Sulfur, Low Fe XRS GRS 0.3 0.5 0.2 6 0.1 Number Ca/Si 0.0 Fe (wt%) 1.0 1.5 2.0 0.0 0.00 0.05 0.10 S/Si 0.15 • S ~1-4 wt% • S strongly correlated with Ca – Presence of CaS? 0.20 2.5 3.0 0.10 0.12 GRS XRS 4 2 0 0.00 0.02 0.04 0.06 Fe/Si 0.08 • Surface Fe very low (<2 wt%) despite large Fe core K, Th, Na, Cl on Mercury • K~1000 ppm • Th ~ 100 ppb • K/Th~8000±3000 Mercury similar to Mars, Earth – No sign of volatile depletion as seen in Moon • Na ~3 wt% on average (Evans et al 2012, Peplowski et al 2013) • Cl ~0.14 wt% on average (Evans et al. 2014, LPSC 2014) (Peplowski et al., 2011,2012) (pre-MESSENGER) Mercury Formation Models • Accretion at high-T? (Lewis 1973) • Evaporation by hot Sun? (Cameron 1985) • Giant impact stripping? (Wetherill, Benz 1988) (pre-MESSENGER) Mercury Formation Models • Accretion at high-T? (Lewis 1973) very high Predict low K, S, Na • Evaporation by hot Sun?Al,(Cameron 1985) ? • Giant impact stripping? (Wetherill, Benz 1988) • High S, low Fe -> less O in starting materials compared to other planets • Role of ice? Ebel & Alexander, PSS 2011 Aerodynamic sorting (Weidenschilling 1978) Photophoresis? Wurm et al. [ 2013] Mercury’s Geology Widespread Volcanism Northern Plains Murchie et al. [2008] Head et al. [ 2011] Volatiles: “Hollows” • Bright deposits within impact craters show fresh-appearing, rimless depressions, commonly with halos. Raditladi • Formation from recent volatile loss? 5 km [Blewett et al., 2011] Tectonics • Mercury covered with “lobate scarps” (cliffs) • Due to contraction of planet as it cooled 50 km • Detailed analysis of MESSENGER data indicates much more contraction than previous work (Byrne et al. 2014) Major-Element Heterogeneity Nittler et al. (2013) Weider et al. (2012-14); smooth plains outlines from Denevi et al (2013) Major-Element Heterogeneity Evidence that smooth plains “more basaltic” (higher Al, lower Mg) than older intercrater plains/ heavily cratered terrain Nittler et al. (2013) Weider et al. (2012-14); smooth plains outlines from Denevi et al (2013) Major-Element Heterogeneity Nittler et al. (2013) Weider et al. (2012-14); smooth plains outlines from Denevi et al (2013) Major-Element Heterogeneity Coherent region with highest Mg, Ca, S, low Alet al. (2013) Weider et al. (2012-14); smooth plains outlines from Denevi et al (2013) Nittler Mg/Si Crustal Thickness (Smith et al. 2012) Mg/Si Mg-rich region Crustal Thickness (Smith et al. 2012) corresponds to thinner crust Mercury Geophysics Zuber et al. Science [2012] • Radio Science combined with topography (left, from laser altimetry) to infer gravity map (below) • Use to constrain interior structure Smith et al. Science [2012] Internal Structure • Model of interior based on gravity field – Based on millions of internal structure models (Smith et al. 2012, Hauck et al. 2013) – Top of liquid core at r=2020 ± 30 km [Rplanet=2440 km) • High density (FeS) layer at base of mantle not required but consistent with data and may be expected for highly reduced planet Magnetic Field Anderson et al. [2011] • Dipole field with 190 nT-RM3 moment • Magnetic equator is offset from planet equator (Z=490 km)! Crustal Magnetism? • Residual signal after subtracting dipole from low-altitude magnetic field measurements – Hint of crustal magnetism within northern volcanic plains – High-priority lower-altitude measurements toward end-of-mission will test Purucker et al. [2012] Mercury’s Magnetosphere • Very dynamic magnetosphere • Extremely High Reconnection Rates (3x – 10x Earth) • Frequent highly energetic bursts of 30-300 keV electrons – but no steady-state radiation belts. (Ho et al. 2012) Slavin et al. [2009] Mercury’s Exosphere • Na, Ca, Mg most abundant species (H, O also seen) • Asymmetries in distributions: different source mechanisms Sodium Calcium – Na uniformly distributed – Ca shows dawn enhancement – Both show seasonal variability Vervack et al., 2011, Killen et al., 2012, Burger et al., 2012, 2014; Merkel et al., 2012 What are the unusual materials at the poles? Earth-based Radar map (Harmon 1999) What are the unusual materials at the poles? Polar deposits (yellow) and areas of persistent shadow (red) in Mercury’s north polar region. MESSENGER data What are the unusual materials at the poles? • Neutrons, thermal modeling and MLA reflectance all point to radar bright materials being water ice + some organics • Delivery by comets? Lawrence et al., Neumann et al., Paige et al. Science [2013] What are the unusual materials at the poles? • Ice and organics can be found everywhere throughout solarand MLA Neutrons, thermal modeling reflectance all point to radar bright system materials being water ice + some organics • Delivery by comets? Lawrence et al., Neumann et al., Paige et al. Science [2013] The future? • MESSENGER is healthy and continuing to return great data • Enough fuel to last another 11 months – Natural evolution of orbit will lead to lower and lower altitudes and unprecedented resolutions (and eventual impact in March 2015) • High resolution images, chemistry, magnetic measurements (e.g., crustal magnetic anomalies) MESSENGER at Mercury • MESSENGER is an extraordinarily successful mission • Despite its small size, Mercury is a weird and wonderful world. – Different in fundamental ways from other terrestrial planets – May provide valuable information for extrasolar planets • 1 more year of orbits will both answer further questions and raise new ones Mg/Si on Mercury MDIS basemap; Nittler et al. [2014] – Especially opportunity for lowaltitude orbits Thank you!
© Copyright 2024 ExpyDoc
