Lunar and Hermean Exospheres Cesare Barbieri Department of Astronomy, University of Padova, vicolo O

1
Lunar and Hermean Exospheres Cesare
BarbieriDepartment of Astronomy, University
of Padova, vicolo Osservatorio 3, 35122 Padova,
cesare.barbieri_at_unipd.it 
2
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)

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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 Ill concentrate on the Moon and
  on Mercury, as seen via the yellow Na-D doublet.

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Surface bounded exosphere
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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 (D15890A, 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.

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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.
• Ill give more detail on data extraction when
  treating Mercury.

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The strategic position of the Moon - 1

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

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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.

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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)
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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

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An animation of the Na lunar tail
(Kindly provided by J. Wilson, BU)
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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, well
see how eclipses are important!
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The processes affecting the lunar Na
The Na neutral atoms are produced and lost by a
variety of processes.
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.
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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
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Modulation processes - 2
The exosphere is ? 5 times dimmer at the solstice
than at the equinox
Equinox
Solstice
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Interaction Magnetopause - Solar Wind and Plasma
Sheet
Solstice Dim Exospheres, gt 30 hours from plasma
sheet passage to eclipse observations Equinox
Bright Exospheres, Short time from plasma sheet
passage to observations
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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.

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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!
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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
20
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 32 ratio with
the revolution period.
21
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

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Observing from ground - 1

• The difficulty of imaging Mercury from the ground
  in the visible are well known the planets 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

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Observing from ground- 2
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.
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International campaign of June 2006
(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
Review meeting 20-21 November 2006 (BU)
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Mercurys Exosphere with the TNG
Our program to observe with the High Resolution
Spectrograph (SARG) of the TNG started in 2002.
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)
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Na-D emission extraction - 1
Already on the raw spectrum, the Na-D emission is
fairly conspicuous.
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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.
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Na-D emission extraction - 3
A Voigt profile is fitted to the continuum (left)
anf finally the emission is isolated (right)
S. Felice Circeo 2006
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Profile of the line
Here are the Na-D profiles along the slit,
together with the continuum. A fairly crucial
parameter in this procedure is the seeing value.
S. Felice Circeo 2006
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Hapke model fitting
The Hapke bi-directional reflectance theory is
fitted to the continuum, and used to put the
observed counts on an physical scale (Rayleighs).
S. Felice Circeo 2006
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Known Components of Mercury's exosphere
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Mercurys Exosphere
Production and destruction mechanisms are similar
to lunar ones.
In addition to the much smaller distance to the
Sun, Mercurys 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.
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2.0 1.5 1.0 0.5 0
Ca 4226 A Killen et al. (2005)
Emission (Rayleigh)
108 ? Ca/cm2
T 12,000 - 20,000 K
2500 3000 3500 4000 4500
Altitude (km)
Altitude (km)
Ca meteoroid vaporization and photo-dissociation
(4 up to 6 eV)
60 40 20 0
Data
Na 5890 A
1500 K
1100 K
Na hotter than surface temperature ? Energetic
processes (?)
??6 mA
750 K
Intensity (MR/A)
Different energy distributions ? Different
release mechanisms
Killen et al. (1999)
0.056 0.06 0.065
0.07
? (5890 A)
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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

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Day to night sides asymmetry
Photon desorption
Surface temperature (K) at 0.35 AU
550 500 450 400 350 300 250 200 150 100
Solar Wind sputtering
Thermal desorption
Dawn terminator
Asymmetry in the Na surface ? Strong release in
the early morning
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Exosphere - upper surface relation
Upper surface density (log10 Na/cm2)
Surface temperature (K)
13.5 13 12.5 12 11.5 11
550 500 450 400 350 300 250 200 150 100
Dawn terminator
Day to night migration time/length of a day
Formation of high density in the colder high
latitude regions
Early morning degassing
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Local enhancements of surface density produce
peaks of emission
N
Afternoon
Morning
W
1010 Na/cm2
N
N
N
W
W
W
E
E
E
Na column density from TNG data (Leblanc et al.
2003)
S
S
S
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Night to night variations
1 July 2005
29 June 2005
This composition shows the differences on 2
nights in the THN 2005 run.
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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
Mercurys Na exosphere
TAA
Aphelion 0.466 AU
Perihelion 0.306 AU
The Sun
Aphelion
Perihelion
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Solar Wind sputtering
N
Solar Wind Proton impact (Kallio and Janhunen
2003)
E
W
S
? 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)
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Mercury from Space NASA Messenger
Launch 08/04/2004 Venus 1st flyby just
happened Two Mercury flybys before
insertion Orbital insertion at end 2010
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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.
  

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Two instruments on Bepi Colombo

• Ill 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

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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

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Phebus schematic view

• EUV spectrometer
• 55-155 nm
• FUV spectrometer
• 145-315 nm

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Phebus a EUV-FUV spectrometer

• Spectral resolution 1 nm (FWHM)
• Challenging species, requiring 1-1.5 nm
  resolution

He
O
Ar
Ne
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Simulated emissions of Mercury's exosphere
104 102 100 10-2
Brightness of the emission (Rayleigh)
A large dynamic of the expected signal Only
discrete peaks but often very close
500 1000 1500 2000
2500 3000
Wavelength (A)
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Observation modes
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PI E. Flamini (ASI)Co-PI Alain
Doressoundiram(Observatoire de Paris)
SIMBIO-SYS
Spectrometer and Imagers for MPO BepiColombo
Integrated Observatory SYStem
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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.
HRIC
STC
VIHI
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
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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

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SIMBIO-SYS Package (2/2)
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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)

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Open points for Mercurys 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?

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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,
  Mercurys Exosphere Observing Campaign of June
  2006 , Hawaii June 2006
• - Valeria Mangano, Cesare Barbieri (3). Gabriele
  Cremonese and Francois Leblanc, Osservazioni
  dellEsosfera di Mercurio, VII National Meting of
  Planetology, S. Felice Circeo, Sept. 2006