Mercury’s Atmosphere: A Surface-bound ExosphereAtmosphere?What is an atmosphere?Mariner 10 in situ observationsWhat is airglow?OccultationGround-based identificationSix types of atoms, is that all?SourcesDirect to ExosphereDelivery to SurfaceRelease from SurfaceSink processesDistributionHydrogenHeliumMagnetosphere impactCalciumOxygenSodiumPotassiumMESSENGER GoalsMASCSMore instrumentsSummaryMercury’s Atmosphere:A Surface-bound ExosphereVirginia PasekPTYS 395Atmosphere?http://www.biocrawler.com/w/images/1/15/Edge_of_Space.pngWhat is an atmosphere?•Atmosphere - the gaseous envelope of a celestial body (as a planet)•Exosphere - the outer fringe region of the atmosphere of the earth or a celestial body (as a planet)•Surface-bound exosphere - an atmosphere where the atoms interact with the planet’s surface, but rarely with one another1.Merriam-Webster http://www.m-w.com/Mariner 10 in situ observations•H, He, and O–UV Airglow Spectrometer provided UV spectra, allowing for identification of H, He, and O–Occultation experiments set upper limit on atmospheric density at 10-12 bar, or 1/1,000,000,000,000 Earth’s 1-bar atmospheric pressureWhat is airglow?•Visible, infrared, and ultraviolet emissions from the atoms and molecules in the atmosphereSci-Tech Encyclopedia•Daytime airglow fluorescence processes as molecules and atoms are photodissociated and photoionized•Nighttime airglow predominantly due to recombination emissionsOccultation•An event that occurs when one object is hidden by another object that passes between it and the observer•Set upper limit on Mercury’s atmospheric pressure of 10-12 bar or 1/1,000,000,000,000 of Earth’s 1-bar pressureGround-based identification•Na, K, and Ca identified by their resonance scattering emission linesSix types of atoms, is that all?•Combined pressure of known constituents is much less than measured exospheric pressure•Unsuccessful searches for C, CO, CO2, Li, Ar, Ne, XeSources•Many•Complex interactions between exosphere, surface, magnetopause, and solar windDirect to Exosphere•Solar wind capture–H, He•Radiogenic decay and outgassing–He•Meteoroid volatilization–Na, K, CaDelivery to Surface•Diffusion–H, He, O, Na, K•Regolith turnover–H, He, O, Na, K, Ca•Magnetotail or ion recyclying–H, He, O, Na, K, CaRelease from Surface•Sputtering (physical and chemical)–Na, K, Ca, and OH from chemical only•Thermal desorption (evaporation)–H, He, O, Na, K•Photon stimulated desorption (PSD)–Na, K•Impact vaporization–allSink processes•Photoionization•Thermal escape•Surface implantation: adsorption•Surface implantation: chemical bondingDistribution•Highly variable, both temporally and spatially•Temperature dependant•High and low velocity componentsHydrogen•Two height distributions–Day side ~1330 km–Night side ~230 km•10x concentration found on Moon–Most likely attributed to magnetic fieldHelium•Strong solar wind source•Highest concentrations found over the dayside and above the sunward limb•He found at 3000 km above surface–Mercury is 4879 km diameter!Magnetosphere impact•Solar particles are pushed into magnetosphere, precipitate to surface where they are neutralized and thermalized, then released into exosphereCalcium•Enhanced over polar regions•Distribution not fully known•Very high temperatures•Large percentage is above escape velocity– Source of high-velocity is unknown•Found up to 3,000 km above surface!Spectroscopic intensity of Ca 422.6 nm emissionOxygen•Only an upper limit on abundance from Mariner 10–Recall column abundance of 7x1012 cm2•Can not detect from ground due to telluric atmospheric opacity in the UV•Possibly being vaporized from surface to form CaO–Dissociation leaves both Ca and O at high energySodium•Diurnal variation with latitude•Possible association with bright-ray craters and new regolith•Tail distribution controlled by solar photon pressurePotassium•More massive and mostly surface bound•Models use scale heights of ~150 km•Follows Na distributionsMESSENGER Goals•Map exospheric constituents and variations in column density with location and time•Map magnetic field as correlated to the solar plasma environment•Map plasma environment with time•Map elemental and mineralogical properties of the surfaceMASCS•Mercury Atmospheric and Surface Composition Spectrometer–UV / VIS will measure and map constituents within atmosphere–VIS / IR will map mineral composition of surface•Study the spatial distribution of known species and search for new species–S, Al, Fe, Mg, SiMore instruments•Magnetometer–Map the magnetic field•EPPS–Energetic Particle and Plasma Spectrometer–Observe and map the particle and plasma environment surrounding Mercury•XRS and GRNS–X-Ray Spectrometer and Gamma Ray Neutron Spectrometer–Map the surface elemental abundancesSummary•Lots of models to explain interactions between Mercury’s surface, exosphere, magnetosphere, and the solar wind–All poorly constrained•~35 years to analyze data and formulate questions–MESSENGER designed carefully to address shortfalls in current