ercurys Atmosphere M A Surfacebound Exosphere Virginia Pasek
�ercury’s Atmosphere: M 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 another 1. 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 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 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 2, 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 • 10 x 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 7 x 1012 cm 2 • Can not detect from ground due to telluric atmospheric opacity in the UV • Possibly being vaporized from surface to form Ca. O – 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
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