NASAExplorerSchools XP

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MESSENGER: Mission to Mercury MESSENGER Sean C. Solomon Department of Terrestrial Magnetism Carnegie Institution of Washington NASA Explorer Schools Student Symposium Cocoa Beach, Florida 9 May 2004 MErcury Surface, Space ENvironment, GEochemistry, and Ranging

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Highest uncompressed density Highest diurnal variation in temperature Only solar system object in 3:2 spin-orbit resonance Geological history ended earliest among the terrestrial planets Smallest planet with global magnetic field Most Earth-like magnetosphere Mercury is a Planet of Extremes 2

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Mercury’s 3:2 Resonance 3

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Planetary System Context Orbital parameters of Mercury fall within those of known extrasolar planets. Mercury provides nearest laboratory for studying planetary system processes in the vicinity of a host star. 4

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5 Mercury is a difficult object to study by telescope or spacecraft.

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Mariner 10 6

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Mariner 10 mosaic of the Caloris basin. Mercury Exploration Imaged 45% of Mercury’s surface. Discovered Mercury’s magnetic field. Detected H, He, O in Mercury’s exosphere. Documented time-variable nature of Mercury’s magnetosphere. Contributions of Mariner 10 flybys 1974-1975 7

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Discovery of Mercury’s 3:2 spin-orbit resonance (1965) Discovery of Na (1985), K (1986), and Ca (2000) in Mercury’s exosphere Discovery of Mercury’s polar deposits (1992) Contributions of Earth-based Observations Mercury Exploration Discovery of Ca emission from the atmosphere of Mercury [Bida et al., 2000]. Top: Contour map of broadband image. Bottom: Extracted spectrum at one slit position, fitted continuum, and difference spectrum. 8

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7th Discovery Mission Discovery began as a NASA Program in FY 1994. Program goal is to launch frequent, small, scientifically focussed missions. Missions are led by a Principal Investigator. Mission proposals are competed and undergo rigorous scientific and technical reviews. 9 Deep Impact Mars Pathfinder Stardust NEAR Lunar Prospector Genesis

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Mission Timeline Mariner 10 image of Discovery Rupes. Selection as a Discovery Mission July 1999 Phase B (detailed design) January 2000 - June 2001 Phase C/D (fabrication, assembly, July 2001 - July 2004 and test) Launch July-August 2004 Earth flyby July 2005 Venus flybys October 2006, June 2007 Mercury flybys January 2008, October 2008, September 2009 Mercury orbit March 2011 - March 2012 10

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Guiding Science Questions 11

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“Standard Model” for Inner Planet Formation Formation of the Sun begins by collapse of a giant molecular cloud of gas and dust to a nebular disk. In the inner solar system, planetesimals accrete to kilometer size in 104 years. Runaway growth of planetary embryos up to Mars size accrete by accumulation of planetesimals in 105-106 years. Final terrestrial planets form by gravitational interaction of embryos in 107 years. “Solar System Origin” W. K. Hartmann 8 “Standard Model” for Inner Planet Formation 12

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Was Mercury’s high metal/silicate ratio the result of chemical gradients in the early nebula? Or was the high ratio a product of late-stage growth processes (e.g., a giant impact)? Could Mercury be a relict planetary “embryo”? Competing hypotheses can be distinguished on the basis of Mercury’s surface chemistry. Mercury’s Bulk Composition 13 Simulation of an off-axis collision of proto-Mercury with a protoplanet one sixth its mass [Benz et al., 1988].

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Are the plains of Mercury volcanic? Is the hemisphere of Mercury not seen by Mariner 10 geologically similar to the imaged hemisphere? Can differences in geological history among the terrestrial planets be related to planet size or initial conditions? Global high-resolution color imaging is needed to address these questions. Recalibrated Mariner 10 color image data show distinct units with boundaries suggesting a possible volcanic origin [Robinson and Lucey, 1997]. Geological History 14

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Not even dipole term well-resolved by Mariner 10 data. Competing hypotheses for the internal field predict different field geometries. Internal field can be separated from external field by repeated orbital measurements. Mercury’s magnetosphere provides an important comparison to that of Earth. Mercury’s Magnetic Field 15 Mercury’s magnetosphere (courtesy J. Slavin).

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Core radius ~ 75% of planet radius (from bulk density). Presence and thickness of a fluid outer core depends on the concentration of light alloying elements. Thickness of outer core affects dynamo stability and geometry. Amplitude of forced libration for a solid planet and one with a liquid outer core differ by a factor of 2. Mercury’s Core 16

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Polar Deposits Radar-bright polar deposits are confined to the floors of high-latitude craters. Cold trapping of ice in permanently shadowed crater floors is the leading explanation. Cold trapping of elemental S is a competing hypothesis. Chemical remote sensing and altimetry can distinguish among alternatives. Arecibo radar image of north polar deposits [Harmon et al., 2001]. 17

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UV spectrometry from orbit can measure profiles of known (H, O, Na, Ca) and expected (e.g., Si, Al, Mg, Fe, S, OH) species. In situ charged particle measurements can be diagnostic of sources and loss mechanisms. Measurement of temporal and spatial variability will be necessary. Volatile Budget Image of Na D2 emission at Mercury, McMath Solar Telescope [Potter and Morgan, 2002]. 18

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Map the elemental and mineralogical composition of Mercury's surface Image globally the surface at a resolution of hundreds of meters or better Determine the structure of the planet's magnetic field Measure the libration amplitude and gravitational field structure Determine the composition of the radar- reflective materials at Mercury's poles Characterize exosphere neutrals and accelerated magnetosphere ions MESSENGER Objectives 19

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Spacecraft Ceramic cloth sunshade Solar panels are 2/3 mirrors Low mass composite structure 3 large fuel tanks Phased array high-gain antenna 20

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Thermal Environment Qualification of Sun-facing spacecraft components (sunshade, solar arrays, antennae, thrusters, Sun sensor, solar X-ray monitor) involved a combination of thermal analysis and thorough testing. Testing to 11-Sun conditions validated both the thermal models and the subsystem and system designs.

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Mercury Dual Imaging System (MDIS) Gamma-Ray and Neutron Spectrometer (GRNS) X-Ray Spectrometer (XRS) Magnetometer (MAG) Mercury Laser Altimeter (MLA) Mercury Atmospheric and Surface Composition Spectrometer (MASCS) Energetic Particle and Plasma Spectrometer (EPPS) Radio Science (RS) Payload 22

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Trajectory 23

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Orbit at Mercury 234o Orbital 24 Orbit optimizes coverage and science requirements, while minimizing propulsion, thermal, and power demands Orbital

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What’s Next? 24 June Mission Readiness Review 8 July Mission Readiness briefing to NASA Associate Administrator Ed Weiler 26 July Flight Readiness Review 30 July Opening of launch window 10 December 2003, Applied Physics Laboratory. 25

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Mission to Mercury The journey begins July - August 2004 MESSENGER