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

Atmosphere?

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

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 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 Sci-Tech Encyclopedia Daytime airglow fluorescence processes as molecules and atoms are photodissociated and photoionized Nighttime airglow predominantly due to recombination emissions

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 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, 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 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 Found up to 3,000 km above surface! Spectroscopic intensity of Ca nm emission

Oxygen Only an upper limit on abundance from Mariner 10 –Recall column abundance of 7x10 12 cm 2 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