Transcript * Mercury’s Atmosphere: A Surface

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