1 20 - 21 November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING Review of Physical Processes and Modeling Approaches "A summary of uncertain/debated

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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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Most debated questions

based on

Mercury's observations

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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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• 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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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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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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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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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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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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How could we

describe

Mercury's exosphere?

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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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• 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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• 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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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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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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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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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

Documents

1 20 - 21 November 2006 MERCURY OBSERVATIONS - JUNE 2006 DATA REVIEW MEETING Review of Physical Processes and Modeling Approaches "A summary of uncertain/debated