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Lunar and Hermean Exospheres Cesare Barbieri Department of Astronomy, University of Padova, vicolo Osservatorio 3, 35122 Padova,   : 

Lunar and Hermean Exospheres Cesare Barbieri Department of Astronomy, University of Padova, vicolo Osservatorio 3, 35122 Padova,  

The collaboration: 

The collaboration The work here presented results from a collaboration with: G. Cremonese (OA Pd), V. Mangano (UPOd and IFSI Rome), S. Verani (UPd) M. Mendillo, J. Baumgardner and J. Wilson (Boston University) A. Sprague and D. Hunten (LPL, Tucson) F. Leblanc (CNRS, Paris) C. Benn (ING)

What is an exosphere: 

What is an exosphere With the term exosphere, I mean the very low density, collision- less, non stationary atmosphere of a planet. In particular for the Moon and Mercury, the base of the exosphere (or exobase) is essentially coincident with the planet's surface. The exosphere is produced, lost and regenerated by a variety of processes. Being collision-less, each species can be treated independently from the others. Many considerations also apply to Io and Europa (moons of Jupiter), to asteroidal surfaces and to comets, and quite possibly to extra-solar planets, so that the study of exospheres has a very general appeal. In the following I’ll concentrate on the Moon and on Mercury, as seen via the yellow Na-D doublet.

Surface bounded exosphere: 

Surface bounded exosphere

Why Sodium Na-D is so important?: 

Why Sodium Na-D is so important? Na is NOT the most abundant gas in planetary exospheres, it is actually a minority species, but Na is an element easily vaporized. In addition, Na-D (D1=5890A, D2= 5896A) is easily excited by solar radiation, and well observable from the ground in emission. In the radiation coming from the thin lunar exosphere I(D2)  2 I(D1) while on Mercury the ratio is more like 1.4-1.7 The same considerations could be repeated for the Potassium (K-D around 7600A), however the intensity is lower, and the blend with the telluric O2 band complicates observations. Therefore we have not attempted as yet to observe it.

The lunar Na: 

The lunar Na We have observed the Moon in a variety of occasions and with many telescopes: imaging during eclipses with coronagraphic imaging (a 12-cm telescope with a coronagraph to suppress the Moon’ disk), The BU coronagraphic telescope at the TNG site in 1996 high resolution spectroscopy, with the 1.8m in Asiago and with the 4m WHT at the Roque. I’ll give more detail on data extraction when treating Mercury.

The strategic position of the Moon - 1: 

The strategic position of the Moon - 1 The Earth-Moon system is connected to the interplanetary medium, and the Moon’s surface samples a variety of phenomena which are present, with different intensities, on Mercury, on asteroids, on comets, on Jupiter.

The strategic position of the Moon - 2: 

The strategic position of the Moon - 2 The presence of the Earth (eclipses, shadow, magnetic field) modulates in a very predictable way the observations Many spacecraft are or have been or will be in the Earth-Moon system, and provide a wealth of useful additional data on the overall ambient.

The bright Na spot at opposition: 

The bright Na spot at opposition During new Moon, the all-sky monitor revealed a bright Na spot moving in opposition to the Moon. (Kindly provided by J. Wilson, BU)

The explanation: 

The explanation The explanation: at full Moon, during opposition, the Earth is inside the lunar long tail of escaping Na-D. The transit time from Moon to Earth is  2 days, so that the Na must remain neutral for 3-4 days. The intensity of the tail is highly variable

An animation of the Na lunar tail: 

An animation of the Na lunar tail (Kindly provided by J. Wilson, BU)

Imaging the Na lunar atmosphere during eclipses: 

Imaging the Na lunar atmosphere during eclipses The density of neutral Na is fairly low, say 10-100 atoms/cm3 , with radial and azimuthal dependence. There is some evidence of a suprathermal component with a kinetic temperature exceeding 1500 °K. Later on, we’ll see how eclipses are important!

The processes affecting the lunar Na: 

The processes affecting the lunar Na Surface sputtering by solar ions and photons is very effective. Some atoms reach escape velocities (tail), others are in ballistic trajectories. Some neutral disappear because of photoionizations, others appear because of charge-exchange mechanisms. Micrometeoroids can also contribute. The Na neutral atoms are produced and lost by a variety of processes.

Modulation processes - 1: 

Modulation processes - 1 The magnetopause shields the Moon from solar wind for ~4 days. However Micrometeor + UV remain. Eclipse improves data quality Moon in magnetotail ~2 days before eclipse Moon in umbra only ~1 hour Little effect on UV photon sources

Modulation processes - 2: 

Modulation processes - 2 Equinox Solstice The exosphere is  5 times dimmer at the solstice than at the equinox

Interaction Magnetopause - Solar Wind and Plasma Sheet : 

Interaction Magnetopause - Solar Wind and Plasma Sheet Solstice: Dim Exospheres, > 30 hours from plasma sheet passage to eclipse & observations Equinox: Bright Exospheres, Short time from plasma sheet passage to observations

What is the influence of the meteor showers on the lunar atmosphere?: 

What is the influence of the meteor showers on the lunar atmosphere? This is a question we tried to address with the WHT, obtaining conflicting evidence. The Leonids gave positive evidence of enhancement, but not the Quadrantidis and other showers Is this an effect due to impact velocity? More data would have been needed, but we in UPd stopped working on the Moon few years ago. The situation is changing, see next slide.

Future Work: 

Future Work Quite recently, the Italian Space Agency has launched a series of scientific and industrial studies about the Moon, to be completed by May 2007. We have resumed therefore work on the lunar exosphere, in a space-based perspective. International collaboration would be welcome, in case ASI decides to proceed to the next stage!

References to our previous papers: 

References to our previous papers Cremonese G.,Verani S. 1997, High resolution observations of the sodium emission from the Moon, Adv. Space Res. 19, 1561-1569 Hunten D.M., Cremonese G., Sprague A.L., Hill R.E., Verani S., Kozlovski R.W.H. 1998, The Leonid meteor shower and the lunar sodium atmosphere, Icarus 136, 298-303 Verani S., Barbieri C., Benn C.R., Cremonese G., Mendillo M. 2001, 1999 Quadrantids in the lunar atmosphere, MNRAS 327, 244-248, 2001 Barbieri, C., Rampazzi, F. (editors) 2001, Proceedings of the Conference Earth Moon Relationships, Kluwer Academic Publishers

Mercury basic data: 

Mercury basic data Radius = 2440 km No apparent tectonic Very large density (5.5) Intrinsic magnetic field Distance to the Sun ~0.385 AU Large orbital eccentricity and inclination Although Mariner 10 could image only less than 50% of the surface, it clarified in a definite way the diurnal rotation and the 3:2 ratio with the revolution period.

Mercury from Radar images: 

Mercury from Radar images Radar seems to show the presence of H2O ices around the two poles There are also radar bright spots over the surface

Observing from ground - 1: 

Observing from ground - 1 The difficulty of imaging Mercury from the ground in the visible are well known: the planet’s disk is  7”, the distance from the Sun is never larger than  26°. Here is an image obtained by careful selection of very short video frames taken at the old 60” Mount Wilson telescope


Two important telescope parameters : Minimum height above the horizon Straylight in FoV due to the Sun and bright sky 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 A partial remedy: From telescopes located at different longitudes one can observe the exosphere for few hours on the same day  Access to new time scales. Observing from ground- 2


(1) Cesare Barbieri and colleagues (Padova): TNG/SARG in the Canary Islands (16 - 19 June) (2) Alain Doressoundiram and colleagues (Paris): CFHT/ESPaDOnS in Mauna Kea (16 - 18 June) (3) Andrew Potter (NOAO) and colleagues: McMath-Pierce Stellar Spectrograph in Arizona (9 - 18 June) (4) Shoichi Okano (Tohuku University) and colleagues in joint observations with Jeff Baumgardner and BU team members new 50cm Japanese telescope on Maui (14 - 21 June) Five teams at four different locations International campaign of June 2006 Review meeting 20-21 November 2006 (BU)

Mercury’s Exosphere with the TNG: 

Mercury’s Exosphere with the TNG A Na-D 60 A wide filter is inserted before the slit to suppress unwanted orders, so we imagine 0.32x27 arccsec in the sky. The spectral resolution is 115.000. The observing period is very short, less than 60 min (only at sunset). Four TNG campaigns: 2002 (Barbieri et al. PSS, 2003) - 2003 (Leblanc et al. Icarus, 2006) - 2005 (Mangano et al. PSS 2006), 2006 (in preparation) Our program to observe with the High Resolution Spectrograph (SARG) of the TNG started in 2002.

Na-D emission extraction - 1: 

Na-D emission extraction - 1 Already on the raw spectrum, the Na-D emission is fairly conspicuous.

Na-D emission extraction - 2: 

Na-D emission extraction - 2 Left: raw spectra on Mercury and (below) adjacent sky only. Right: after sky removal, the spectrum of Mercury alone shows solar reflected light plus Na-D emission.

Na-D emission extraction - 3: 

S. Felice Circeo 2006 A Voigt profile is fitted to the continuum (left) anf finally the emission is isolated (right) Na-D emission extraction - 3

Profile of the line: 

S. Felice Circeo 2006 Here are the Na-D profiles along the slit, together with the continuum. A fairly crucial parameter in this procedure is the seeing value. Profile of the line

Hapke model fitting: 

S. Felice Circeo 2006 The Hapke bi-directional reflectance theory is fitted to the continuum, and used to put the observed counts on an physical scale (Rayleighs). Hapke model fitting

Known Components of Mercury's exosphere: 

Known Components of Mercury's exosphere

Mercury’s Exosphere: 

Mercury’s Exosphere In addition to the much smaller distance to the Sun, Mercury’s conditions differ from those of the Moon because of the vastly different orbital modulation and day/night thermal regime, and of the presence of a hermean magnetosphere. The Na density is much higher than lunar. Production and destruction mechanisms are similar to lunar ones.


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) Emission (Rayleigh) Ca 4226 A Killen et al. (2005) Altitude (km) λ (5890 A) 0.056 0.06 0.065 0.07 Intensity (MR/A) 60 40 20 0 Na 5890 A Killen et al. (1999) 1500 K Data 1100 K 750 K Na: hotter than surface temperature  Energetic processes (?) ∆λ~6 mA

Some observed features: 

Some observed features Asymmetry night-day: emissivity is strongly enhanced on the morning side Strong dependence from the orbital position (True Anomaly Angle , TAA) Localized high intensity regions; there is some evidence that the Na content is enhanced over radar bright spots A long, anti-solar Na tail has been reported, but no firm confirmation yet The dependence from solar cycle must be cleared by decade long observations


Dawn terminator 550 500 450 400 350 300 250 200 150 100 Surface temperature (K) at 0.35 AU Asymmetry in the Na surface  Strong release in the early morning Thermal desorption Solar Wind sputtering Photon desorption Day to night sides asymmetry


Dawn terminator 550 500 450 400 350 300 250 200 150 100 Surface temperature (K) 13.5 13 12.5 12 11.5 11 Upper surface density (log10 Na/cm2) Exosphere - upper surface relation Early morning degassing Day to night migration time/length of a day: Formation of high density in the colder high latitude regions

Local enhancements of surface density produce peaks of emission : 

N E S S E N N W S E W W Na column density from TNG data (Leblanc et al. 2003) 1010 Na/cm2 Morning Afternoon N W Local enhancements of surface density produce peaks of emission

Night to night variations: 

Night to night variations This composition shows the differences on 2 nights in the THN 2005 run. 29 June 2005 1 July 2005

Aphelion to perihelion variation: 

A day/night asymmetry of the surface density associated to the strong variation of the Sun rising velocity at Mercury  Variation with heliocentric distance of the total content of Mercury’s Na exosphere Aphelion Perihelion Perihelion 0.306 AU Aphelion 0.466 AU The Sun TAA Aphelion to perihelion variation

Solar Wind sputtering: 

Solar Wind Proton impact (Kallio and Janhunen 2003)  High latitude peaks in Na emission could be due to solar wind magnetospheric penetration  High variability related to high variability of IMF orientation (Potter and Morgan 1990) N E W S Solar Wind sputtering

Mercury from Space: NASA Messenger: 

Mercury from Space: NASA Messenger Launch 08/04/2004 Venus 1st flyby just happened Two Mercury flybys before insertion Orbital insertion at end 2010

The ESA/JAXA BepiColombo mission : 

The ESA/JAXA BepiColombo mission Dual spacecraft The orbiter will be Nadir pointing, with an elliptic orbit bringing it down to 400 km over the surface BC to be launched in 2013 for an arrival in 2019.

Two instruments on Bepi Colombo: 

Two instruments on Bepi Colombo I’ll illustrate two instrument of relevance to Hermean exosphere on board Bepi Colombo Mercury Planetary Orbiter (MPO): Phebus, the FUV-UV spectrograph Symbio-SYS, a stereoscopic plus hyperspectral imager

PHEBUS: A FUV-EUV spectrometer for the MPO platform: 

PHEBUS: A FUV-EUV spectrometer for the MPO platform E. Chassefière (SA/IPSL), S. Okano (Tohoku Univ.), O. Korablev (IKI), J.-P. Goutail (SA/IPSL) and the Phebus Team

Phebus schematic view: 

Phebus schematic view EUV spectrometer 55-155 nm FUV spectrometer 145-315 nm

Phebus: a EUV-FUV spectrometer : 

Phebus: a EUV-FUV spectrometer Spectral resolution : 1 nm (FWHM) Challenging species, requiring 1-1.5 nm resolution Ne He O Ar


A large dynamic of the expected signal Only discrete peaks but often very close Brightness of the emission (Rayleigh) Wavelength (A) 500 1000 1500 2000 2500 3000 104 102 100 10-2 Simulated emissions of Mercury's exosphere

Observation modes: 

Observation modes

PI: E. Flamini (ASI) Co-PI: Alain Doressoundiram (Observatoire de Paris) : 

PI: E. Flamini (ASI) Co-PI: Alain Doressoundiram (Observatoire de Paris) Spectrometer and Imagers for MPO BepiColombo Integrated Observatory SYStem SIMBIO-SYS

SIMBIO-SYS Package (1/2): 

SIMBIO-SYS Package (1/2) SIMBIO-SYS is an integrated package for the imaging and spectroscopic investigation of the Hermean surface. Capabilities: STC: STereo Channel for medium spatial resolution, global mapping, in stereo and colour imaging. VIHI HRIC STC HRIC: High spatial resolution imaging in a pan-chromatic and 3 broad-band filters VIHI: Visible Infrared Hyperspectral Imager in the spectral range 400  2000 nm


SIMBIO-SYS Science Surface geology: stratigraphy, geomorphology Volcanism: lava plain emplacement, volcanoes identification Global tectonics: structural geology, mechanical properties of lithosphere Surface age: crater population and morphometry, degradation processes Surface composition: maturity and crustal differentiation, weathering, rock forming minerals abundance determination Geophysics: libration measurements, internal planet dynamics


SIMBIO-SYS Package (2/2)


Synergies between PHEBUS and SIMBIO-SYS Correlation of exospheric data with topographic maps Identify variation in abundances related to surface features (e.g. Caloris basin, mountains…) search for exospheric anomalies related to the permanently shadowed craters (cold traps). Correlation of exospheric data with mineralogical maps Identify source minerals and regions Permanently shadowed craters (water ice or sulfur)


Open points for Mercury’s exosphere Uncertainties on the real energy structure of Mercury's exosphere: which mechanisms lead to ejection, with which intensity and with which released energy? what are the sources and sinks of Mercury's exosphere? Variations of the Mercury's exosphere: from day to night sides from perihelion to aphelion with respect to latitude and longitude (solar wind effects or surface density distribution) due to short and long time variations of the solar wind and photon flux New time scales and new species?


Acknowledgments I have utilized for this presentation excerpts from: Michael Mendillo and Jody Wilson, Sources of the Exospheres of the Moon and Mercury, European Geophysical Union General Assembly, Vienna, 7 April 2006 Alain Doressoundiram and Francois Leblanc, Mercury’s Exosphere: Observing Campaign of June 2006 , Hawaii June 2006 - Valeria Mangano, Cesare Barbieri (3). Gabriele Cremonese and Francois Leblanc, Osservazioni dell’Esosfera di Mercurio, VII National Meting of Planetology, S. Felice Circeo, Sept. 2006

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