Planetary Exospheres Cesare Barbieri Department of Astronomy, University of Padova, vicolo Osservato

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Planetary Exospheres Cesare
BarbieriDepartment of Astronomy, University
of Padova, vicolo Osservatorio 2, 35122 Padova,
barbieri_at_pd.astro.it 
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The collaboration

• The work here presented results from a
  collaboration with
• G. Cremonese (OA Pd) and Stefano Verani (Upd)
• M. Mendillo (BU)
• A. Sprague and D. Hunten (LPL, Tucson)
• R. Cosentino (TNG)

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What is an exosphere

• With the term exosphere, I mean the very low
  density, transient, atmosphere of a planet such
  as the Moon or Mercury, having the base
  essentially coincident with the planet's surface,
  and being produced, lost and regenerated by a
  variety of processes.
• An exosphere is collision-less, each species can
  be treated independently from the others.
• Many considerations derived for the Moon and from
  Mercury apply also to asteroidal surfaces and to
  comets, and quite possibly to extra-solar
  planets, so that the study of exospheres has a
  very general appeal.

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Why Sodium Na-D?

• 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 (D25890A, D1 5896A) is easily
  excited by solar radiation, and well observable
  from the ground.
• In laboratory, and also in the radiation coming
  from an exosphere I(D 2 5890) ? 2I(D1 5896)
• The same considerations could be repeated for the
  Potassium (K-D around 7600A), however the blend
  with the telluric O2 band complicates
  observations. We have not attempted as yet to
  observe it.

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The Boston University UPd overall project

• Moon
• Jupiter
• Hale Bopp and other comets
• (Earths mesosphere)
• and now, Mercury

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Past work

• We started working some 15 years ago on the thick
  Na atmosphere of Io, and then of Europa
• This figure shows an echèlle spectrum obtained
  with the Asiago 1.8m telescope in 1989

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Jupiters and Ios Na clouds

• Ios loses some 1027 Na atoms/s, but this amount
  is only say 1 of the total gases (mostly SiO2)
  lost by this moon to the magnetosphere of
  Jupiter, due to the strong volcanism.
• The interaction with Jupiters magnetosphere
  greatly complicates the interpretation
• For a detailed account
• Wilson et al., The Dual Source of Ios Sodium
  Cloud, Icarus 157, 476, 2002

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The lunar Na

• We have observed the Moon in a variety of
  occasions and with many telescopes, both in
• coronagraphic imaging (a 12-cm telescope with a
  coronagraph to suppress the Moon disk),
• and in
• high resolution spectroscopy, mainly with the WHT.

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Na-D emission extraction

• The observed spectrum contains a large amount of
  reflected and scattered sun-light, and many
  telluric absorptions, but if the S/N is good
  enough,Na-D emission is recovered
• The same technique is applied e.g. for Mercury

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The lunar Na processes
The Na neutral atoms are produced and lost by a
variety of processes (see next table). The
density of neutral Na is fairly low, say 10-100
atoms/cm3 , with radial and azimuthal
dependence. The kinetic temperature of the Na
atoms can exceed 1500 K, and there is some
evidence of a thermal and suprathermal components
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Imaging the Na lunar atmosphere during eclipses

• Some data were obtained from the Roque during the
  construction of the TNG

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How huge is the lunar atmosphere!

• The all-sky monitor revealed a bright Na spot
  moving in opposition to the Moon.

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An animation of the Na lunar tail
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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.
• Only the Leonids gave positive evidence of
  enhancement
• Why not the Quadrantidis and other showers?
• Is this an effect due to impact velocity? (see
  Carbognani and Cremonese, Astron. Astrophys.
  2002)
• More data are needed!

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The strategic position of the Moon

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

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The strategic position of the Moon

• The presence of the Earth (eclipses, shadow,
  magnetic field) modulates in a very predictable
  way the observations
• Many spacecraft are (or will be, like ESAs
  SMART-1) in the Earth-Moon system

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Why not from space?
We would gain many days of Moons visibility from
a low Earth orbit
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The UV spectrum of the lunar atmosphere

• From Space with could sample the UV spectrum,
  which should contain again, Na, and then Al and
  Mg, and also the OH coming from H2O ices via
  photodissociation of the molecule.

Too bad, at moment our proposal has not been
funded
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Mercury from Mariner

• Mariner 10 could image only less than 50 of the
  surface

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Mercury from the ground

• The difficulty of imaging Mercury from the ground
  in the visible are well known
• Here is an image obtained by careful selection of
  very short video frames taken at the old 60
  Mount Wilson telescope
• The TNG should do much much better! But up to now
  I could NOT convince the TAC!!!!

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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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The Hermean Na atmosphere

• We succeeded to pass a program to
  spectroscopically study, with the SARG-TNG,
  Mercurys exosphere, which contains some 105
  1012 atoms/cm3 of Na.
• The variable Na (and K, and possibly also Ca)
  content is explained by ion implantation during
  the long Hermean night, with subsequent diffusion
  during the day.

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Some observed features

• There is some evidence that the Na content is
  enhanced over radar bright spots
• Also the North pole seems anomalously bright,
  with Na thermal velocities up to 12.000 K!
• No firm data is however available on the origin
  above the surface or on the radial extent of the
  cloud, either in the solar nor anti-solar
  direction
• The dependence from solar cycle must also be
  cleared
• The long slit of the SARG (27) can provide
  decisive data, especially if the program can be
  carried out for a full solar cycle

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Mercury from Space

• Two missions are foreseen for the near future to
  Mercury, NASAs Messenger, and ESAs BepiColombo.
  Here is the possible Messengers timeline

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The BepiColombo mission

• The orbiter will be Nadir pointing, with an
  elliptic orbit bringing it down to 400 km over
  the surface
• The time frame for the mission would be some
  10-12 years from now (one full solar cycle).

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Memoris

• In the frame of the ESAs mission BepiColombo, we
  shall propose an imaging instrument (MEMORIS),
  with 3 FoV in order to provide a stereoscopic
  vision, and capable to observe the hermean
  exosphere during the limb passes.
• The observations with the SARG-TNG, if protracted
  for a sufficient number of years, would then
  provide an invaluable data base for better
  understanding the in-situ observations.

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Comets

• After Hale-Bopp, the Na tail has been looked for
  in other comets, e.g. Ikeya-Zheng with SARG

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Rosetta Mission

• We have included a Na filter in the WAC
  complement. Results from Space are expected in
  2014...

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Na-D from Mercury at the TNG

• Two runs have been secured, August 2002 and
  August 2003, practically in full day light
• The first set of data has been analyzed, and some
  results are reported here
• the data from the second one are even better

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Mercury, Venus and Spica
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Mercury aspect in Aug. 2002
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Spectra of August 2003