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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. Most debated questions based on Mercury's observations. Observed components of Mercury's exosphere. Known Species. - PowerPoint PPT Presentation
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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. LeblancService d'Aéronomie du CNRS/IPSL
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20 - 21 November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING
Most debated questions
based on
Mercury's observations
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20 - 21 November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING
Observed components of Mercury's exosphere
H 3 109 ~ 23 (hot) 230 (cold)
He 3 1011 ~ 6103
O 3 1011 ~ 4.4 104
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
Species Subsolar column density (cm-2)
Near surface subsolar density (cm-3)
Na 0.1 - 10 1011 ~ 104
K 0.5 - 3 109 ~ 102
Ca 1.1 108 ?
Remarks
From Earth
From Earth
From Earth
Mariner 10
Mariner 10
Mariner 10
KnownSpecies
Which other species?
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20 - 21 November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING
• 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?
Ground based observations of the Na, K, Ca components
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20 - 21 November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING
West
North
Occultation of the Solar Na D2 line
by Mercury's exosphere(Schleicher et al. 2004)
DaysideTerminator
NightsideTerminator
What spatial distributions?
Observations of the Na D lines (Potter and Morgan 1997)
Sun
N
E W
S
Role of the solar radiation pressure, of Mercury's orbit?
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20 - 21 November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING
What are the origins of the "short" term variation?
Potter et al. (1999)
South
North
220°
214°
229°
223°
217°
236°
• Is it a CME encounter withMercury?
• Is it a solar wind and UV variation inducing this
observation?
• Role of Caloris?
• Other mechanisms?
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20 - 21 November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING
What drives Mercury's tail formation?
26/05/2001Potter et al. (2002)
morning side0.42 AU
D2 emission
TAA = 23°TAA = 83°TAA = 125°TAA = 190°TAA = 261°TAA = 315°
Related to the ejection process?
To the ionization frequency?
To the solar radiation pressure?
3D model
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20 - 21 November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING
Driven by Mercury's rotations?
Driven by the solar radiation pressure?
Driven by the distance to the Sun?
From Potter et al. (2006)
Aphelion
Perihelion
Variation with respect to TAA?
Perihelion0.306 AU
Aphelion0.466 AU
The Sun
TAA
3D model
Observation
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20 - 21 November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING
H and He: thermal desorptionand surface accomodation
Ca meteoroid vaporization and photo-dissociation (+4 up to 6 eV)
2500 3000 3500 4000 4500
2.0
1.5
1.0
0.5
0
108
Ca/
cm2
T = 12,000 - 20,000 K
Different energy distributions? Different release mechanisms?
Altitude (km)
Em
issi
on (
Ray
leig
h)
-200 0 200 400 600 800 1000
104
103
102
10
H 1216 Å(Broadfoot et al. 1976)
420 K
110 K
Ca 4226 AKillen et al. (2005)
Altitude (km)
λ (5890 A)0.056 0.06 0.065 0.07
Inte
nsit
y (M
R/A
)
60
40
20
0
Na 5890 A
Killen et al. (1999)
1500 KData
1100 K750 KNa: hotter than surface temperature
Energetic processes (?)∆λ~6 mA
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20 - 21 November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING
How could we
describe
Mercury's exosphere?
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20 - 21 November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING
Boundaries?
• 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?
Crust
Regolith
Exosphere
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20 - 21 November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING
• 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?
Sinks?
Absorption of neutral and
magnetospheric ion
Photoionization Neutral
loss
Crust
Regolith
Exosphere
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20 - 21 November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING
• 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?
Meteoroidgardening
Diffusion
Sources?
Meteoritic supply +
solar wind implantation
Crust
Regolith
Exosphere
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20 - 21 November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING
Photo StimulatedDesorption
SolarWind
sputteringThermal
Desorption
Micro-Meteoritic
Impact
Crust
Regolith
• 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?
Exospheric production
Also chemicalsputtering,
is it negligible?
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20 - 21 November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING
Adapted from Morgan and Killen (1997)
Photo StimulatedDesorption
SolarWind
sputtering
Absorption of neutral and
magnetospheric ion
Diffusion
Photoionization Neutral
loss
ThermalDesorption
Micro-Meteoritic
Impact
Meteoritic supply + solar
wind implantation Meteoroid
gardeningCrust
Regolith
In summary
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20 - 21 November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING
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
Coordinate observing campaign
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20 - 21 November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING
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