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 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 pressure 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 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 Ground based identification Na K and Ca identified by their resonance scattering emission lines 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 Sources Many Complex interactions between exosphere surface magnetopause and solar wind Direct to Exosphere Solar wind capture H He Radiogenic decay and outgassing He Meteoroid volatilization Na K Ca 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 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 Sink processes Photoionization Thermal escape Surface implantation adsorption Surface implantation chemical bonding Distribution Highly variable both temporally and spatially Temperature dependant High and low velocity components Hydrogen Two height distributions Day side 1330 km Night side 230 km 10x concentration found on Moon Most likely attributed to magnetic field 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 Magnetosphere impact Solar particles are pushed into magnetosphere precipitate to surface where they are neutralized and thermalized then released into exosphere Calcium Enhanced over polar regions Distribution not fully known Very high temperatures Large percentage is above escape velocity Source of high velocity is unknown Spectroscopic intensity of Ca 422 6 nm emission Found up to 3 000 km above surface 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 Sodium Diurnal variation with latitude Possible association with bright ray craters and new regolith Tail distribution controlled by solar photon pressure Potassium More massive and mostly surface bound Models use scale heights of 150 km Follows Na distributions 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 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 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 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