Transcript power point

20 - 21 November 2006
MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING
Review of Physical Processes
and
Modeling Approaches
"A summary of uncertain/debated questions from a
modeler's point of view"
F. Leblanc
Service d'Aéronomie du CNRS/IPSL
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MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING
Most debated questions
based on
Mercury's observations
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Observed components of Mercury's exosphere
Species Subsolar column Near surface subsolar
density (cm-2)
density (cm-3)
Known
Species
Remarks
Na
0.1 - 10  1011
~ 104
From Earth
K
0.5 - 3  109
~ 102
From Earth
Ca
1.1  108
?
From Earth
H
3 109
~ 23 (hot) 230 (cold)
Mariner 10
He
3 1011
~ 6103
Mariner 10
O
3 1011
~ 4.4 104
Mariner 10
Mariner 10 Solar Occultation (Broadfoot et al. 1976)
At terminator: neutral density < 107 cm-3
Mariner 10 Radio Occultation (Fjelbo et al. 1976)
Electronic density around Mercury < 103 cm-3
 Which other species?
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MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING
Ground based observations of the Na, K, Ca components
• 1985: First Spectroscopic observation (Potter et al. 1985)
• 1986: observation of K, Na/K = 80-190
>> Moon (6), solar (20) (Potter and Morgan 1986)
Is the Na/K ratio always so large and why?
• Suprathermal component in Na line (Potter and Morgan 1987; Killen et
al. 1999)
Latitudinal, longitudinal, Mercury's position dependencies of
this suprathermal component?
• Sporadic spots of Na emission at high latitudes
(Potter and Morgan 1990, 1997; 2006)
Due to exospheric recycling and/or to solar wind sputtering?
• Local enhancement on Caloris of K emission (Sprague 1990)
Role of surface topography on the formation of the
exosphere?
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What spatial distributions?
North
West
Nightside
Terminator
N
Dayside
Terminator
E
W
Sun
S
Occultation of the Solar
Na D2 line
Observations of the Na D lines
by Mercury's exosphere
(Potter and Morgan 1997)
(Schleicher et al. 2004)
Role of the solar radiation pressure, of Mercury's orbit? 5
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MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING
What are the origins of the "short" term variation?
Potter et al. (1999)
214°
South
217°
• Is it a CME encounter with
Mercury?
North
220°
223°
• Is it a solar wind and UV
variation inducing this
observation?
• Role of Caloris?
229°
236°
• Other mechanisms?
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MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING
What drives Mercury's tail formation?
Related to the ejection process?
To the ionization frequency?
To the solar radiation pressure?
TAA = 23°
TAA = 83°
TAA = 125°
TAA = 190°
TAA = 261°
TAA = 315°
3D model
26/05/2001
Potter et al. (2002)
morning side
0.42 AU
D2 emission
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Variation with respect to TAA?
3D model
Aphelion
TAA
Aphelion
0.466 AU
Perihelion
The Sun
0.306 AU
Perihelion
Observation
Driven by Mercury's rotations?
Driven by the solar radiation
pressure?
From Potter et al. (2006)
Driven by the distance to the
Sun?
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2.0
104
Ca 4226 A
103 110 K
(Broadfoot et al. 1976)
Killen et al. (2005)
1.5
108  Ca/cm2
Emission (Rayleigh)
H 1216 Å
1.0
102 420 K
T = 12,000 - 20,000 K
0
0
200 400
600
Altitude (km)
800 1000
H and He: thermal desorption
and surface accomodation
Na: hotter than surface temperature
 Energetic processes (?)
Different energy distributions?
 Different release mechanisms?
2500
3000
3500
4000
Altitude (km)
4500
Ca meteoroid vaporization and photodissociation (+4 up to 6 eV)
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Na 5890 A
Intensity (MR/A)
10
-200
0.5
40
∆λ~6 mA
Data
1500 K
1100 K
750 K
20
0
0.056
Killen et al. (1999)
0.06
0.065
λ (5890 A)
0.07
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How could we
describe
Mercury's exosphere?
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• The surface: the first layers of grains or the regolith layer? Is it a
finite or infinite reservoir of ambient or/and source particles?
•The Interplanetary medium: the magnetopause? the bow
shock? The orbit of Mercury?
Exosphere
Regolith
Crust
Boundaries?
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• Surface absorption: is it a sink or/and just a recycling process?
• Neutral escape: what energy distributions for the ejecta?
• Ionization and Acceleration through the tail: what ionization
cross section and electric field?
Photo
ionization
Neutral
loss
Exosphere
Regolith
Crust
Absorption
of neutral and
magnetospheric
ion
Sinks?
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• Diffusion: through the grain and/or the regolith?
• Meteoroid supply: rate and spatial distribution ?
• Meteoroid gardening: how to constrain this mechanism?
• Solar Wind implantation: where, how much, which depth?
Sources?
Exosphere
Meteoritic
supply +
solar wind
implantation
Diffusion
Regolith
Meteoroid
gardening
Crust
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• Mechanisms of ejection:
- Acting on the same population or on different population?
(binding energy distribution, depth of implantation...)
- What kind of variability vs heliocentric distance, solar
activities (CME), surface temperature, radiative environment?
Solar
MicroWind
Photo sputtering Meteoritic
Stimulated
Impact Thermal
Desorption
Desorption
Regolith
Exospheric
production
Crust
Also chemical
sputtering,
is it negligible?
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In summary
Photo
ionization
Solar
Wind
Photo
Stimulated sputtering
Desorption
MicroMeteoritic
Impact Thermal
Desorption
Regolith
Meteoritic
supply + solar
wind
implantation
Diffusion
Neutral
loss
Absorption
of neutral and
magnetospheric
ion
Meteoroid
gardening
Crust
Adapted from Morgan and Killen (1997)
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Coordinate observing campaign
Mercury can be observed only the evening or the morning during
one hour
One hour = less than one Mercury minute
One Earth day = 1/176 of one Mercury day ~15 Mercury minutes
No possibility to observe simultaneously both evening and
morning sides
From telescopes located at different longitudes we can observe
the exosphere for few hours on the same day
 Access to new time scales
In particular of the solar wind variability time scale
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Conclusions
• Uncertainties on the real energy, density and composition
structure of Mercury's exosphere:
 Which mechanisms lead to ejection, with which intensity and
with which released energy (related to the boundaries)?
 What are the sources and sinks of Mercury's exosphere ?
• We can partially solve these questions by tracking variations:
 from day to night sides (global exospheric recycling)
 from perihelion to aphelion (ambient vs source populations)
 with respect to latitude (solar wind sputtering or topography)
 due to short and long time variations of the solar wind and
photon flux (relations with surface and magnetosphere)
 Access to new time scales should hightlight other
variabilities...
 Discovery of new exospheric species
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