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Mercury’s Atmosphere: A Surface-bound Exosphere

Mercury’s Atmosphere: A Surface-bound Exosphere. Virginia Pasek PTYS 395. Atmosphere?. http://www.biocrawler.com/w/images/1/15/Edge_of_Space.png. What is an atmosphere?. Atmosphere - the gaseous envelope of a celestial body (as a planet)

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Mercury’s Atmosphere: A Surface-bound Exosphere

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  1. Mercury’s Atmosphere:A Surface-bound Exosphere Virginia Pasek PTYS 395

  2. Atmosphere? http://www.biocrawler.com/w/images/1/15/Edge_of_Space.png

  3. What 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/

  4. 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 pressure

  5. What is airglow? • Visible, infrared, and ultraviolet emissions from the atoms and molecules in the atmosphere • Daytime airglow fluorescence processes as molecules and atoms are photodissociated and photoionized • Nighttime airglow predominantly due to recombination emissions Sci-Tech Encyclopedia

  6. Occultation • 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 pressure

  7. Ground-based identification • Na, K, and Ca identified by their resonance scattering emission lines

  8. Six 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, Xe

  9. Sources • Many • Complex interactions between exosphere, surface, magnetopause, and solar wind

  10. Direct to Exosphere • Solar wind capture • H, He • Radiogenic decay and outgassing • He • Meteoroid volatilization • Na, K, Ca

  11. Delivery 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, Ca

  12. Release 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 • all

  13. Sink processes • Photoionization • Thermal escape • Surface implantation: adsorption • Surface implantation: chemical bonding

  14. Distribution • Highly variable, both temporally and spatially • Temperature dependant • High and low velocity components

  15. Hydrogen • Two height distributions • Day side ~1330 km • Night side ~230 km • 10x concentration found on Moon • Most likely attributed to magnetic field

  16. Helium • 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!

  17. Magnetosphere impact • Solar particles are pushed into magnetosphere, precipitate to surface where they are neutralized and thermalized, then released into exosphere

  18. Calcium • 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 emission

  19. Oxygen • 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 energy

  20. Sodium • Diurnal variation with latitude • Possible association with bright-ray craters and new regolith • Tail distribution controlled by solar photon pressure

  21. Potassium • More massive and mostly surface bound • Models use scale heights of ~150 km • Follows Na distributions

  22. MESSENGER 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 surface

  23. MASCS • 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, Si

  24. More 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 abundances

  25. Summary • 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 knowledge

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