The composition of planetary atmospheres: a historical perspective

1
The composition of planetary atmospheres a
historical perspective
Emmanuel Lellouch
Observatoire de Paris, France
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Atmospheres of the Solar System

• Giant Planets
• Primary atmospheres (H2, He, CH4)
• Little evolution (no surface, little escape)
•  Terrestrial  planets (Earth, Venus, Mars,
  Titan)
• Secondary atmospheres (CO2 / N2, N2 / O2, N2 /
  CH4)
• Outgassed and strongly evolved (escape, surface
  interaction)
• Tenuous atmospheres (Pluto, Triton, Io,
  Enceladus)
• In equilibrium with surface ices or internal
  sources
• Exospheres (Mercury, Moon, other Galilean
  satellites)
• Solar flux or solar wind action on surfaces

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Overview

• Early times (1905-1970)
• The 1970s main concepts emerge
• The 1980s and 1990s accumulating molecules
• Recent spacecraft exploration (1995-2008)

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First detections the visible range
Wildt 1932
Identification of CH4 and NH3 in  visible spectra
of Jupiter and Saturn taken by Slipher in 1905
CH4 7260 A
CH4 8900 A
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First detections
Kuiper 1944
 The only reason why I happened to observe the
planets and the 10 brightest satellites was that
they were nicely lined up in a region of the sky
where I had run out of programs stars 
Detection of methane in Titan
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First detections
Spinrad et al. 1963

• Detection of H2 in Uranus

Identification of CH4 and NH3 in  visible spectra
of Jupiter and Saturn taken by Slipher in 1905
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First detections
1932
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Beyond photography the beginning of infrared
(courtesy Dale Cruikshank)
During the war, Kuiper learned about the
development of IR detectors (PbS) having
sensitivity up to 3 ?m
Kuiper 1947
CH4 in Jupiter
? CO2 in Venus
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The beginning of infrared
CO2 on Mars (Moroz, 1964)
Vassili Ivanovich Moroz
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Too much enthusiasm
Sinton et al. 1960
1960
Actually due to telluric HDO
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Mars discovery of atmospheric water in 1963
Water cycle on Mars
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Mars atmosphere basic chemistry
Detection of CO (1968) Detection
of O2 1.27 emission in 1976 O3 (1971), and O2
(1972) ? tracer of ozone (and not
vice versa!)
CO2 h ? ? CO O O O M ? O2
O2 O M ?O3 H2O h
?? OH H CO OH ? CO2 H (stability of
atmosphere) OH ? HO2 ? H2O2 (not detected
before 2005)
Noxon et al. 1976
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The solar reflected component of Venus

• Detection of HCl, HF and CO in Venus (above
  clouds)
• Michelson inteferometer R 20000
• Connes et al. 1967, 1969
• But
• H2O difficult to detect
• O2, O3 not detected
• How to probe below the clouds ?

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The 1970s The thermal infraredaccess to
physical concepts
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C2H6
In the thermal range

• Sensitive to temperature
• Sensitive to vertical distribution of gases

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Exploring the thermal range from Earth the 10 µm
window
Detection of strong hydrocarbon emission in outer
planets
C2H6
C2H6
Saturn Titan
Gillett et al. 1973, 1975 (R 60)
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Methane photochemistry in Giant Planets(a recent
view)
Moses et al. 2000 (Saturn)
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Methane photochemistry in Giant Planets(a recent
view)
Detection of C3H4 and C4H2 on Neptune IRS/Spitzer
, R600 Meadows et al. 2008
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Stratospheres
Warmer on Titan (170 K) than Saturn (140
K) Predicted due to haze (esp. Titan) and
methane heating
Pre-Voyager models of Titan - inversion only ? -
greenhouse also?
Hunten, 1973
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Equilibrium vs disequilibrium species in Giant
Planets
At the relevant T, NH3 is the thermodynamical
equilibrium form of N ? In principle NH3 / H2
gives the N/H ratio but PH3 is NOT the
equilibrium form of P Competition between
chemical destruction and vertical convective
transport Quench level where tchem
tdyn Occurs at T 1200 K for phosphine ?
Observed PH3 abundance still gives P/H ratio !
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Exploring the thermal range from Earth the 5-µm
window of the Giant Planets

• Hot radiation originating from 3-5 bar levels
  (due to low H2 and CH4 opacity)
• NH3, PH3
• New detections in 1973-1975 H2O (equilibrium)
• 
  CO (disequilibrium, much ltlt CH4)

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Vertical profile of NH3 in Jupiter physical
processes and deep abundance

• NH3 / H2 at 3 bar indicates N/H on Jupiter is
  enriched by a factor 2 over solar
• H2O Does not give O/H ratio because H2O
  condensation occurs deeper than levels probed
• NEED FOR DEEP IN SITU PROBE

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The 1970s First global views of the planet
infrared spectra
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Telluric planets from space a full view of the
thermal IR spectrum
MARS Mariner 9 / IRIS (1973) R 2.4 cm-1,
FTS Temperature, water vapor and dust in the
martian atmosphere VENUS Venera 15/ Fourier
Spectrometer (1983), R 2 cm-1 Temperature and
composition field at and above Venus clouds (H2O,
SO2, H2SO4)
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Full spectra of Giant Planets Helium
He/H in Giant Planets
H2-He
Saturn IRIS / Voyager R 4.3 cm-1 He (Jup) He
(Sat) lt He (U) He (N) He (protosolar) ?
Evidence for helium segregation in Jupiters and
Saturns interior Thermal balace of Giant
Planets (internal source)
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Full spectra of Titan chemistry
IRIS / Voyager R 4.3 cm-1
Voyager /UVS
N2 is dominant species in Titan
? Coupled photochemistry of N2 and CH4
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1980-2000 Accumulating molecules(the golden
age of infrared)
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From the ground the power of spectral resolution
Fourier Transform Spectrometer at
CFHT (1983-2000) 0.9 5.2 µm, InSb, InGaAs
detectors Best spectral resolution 0.01 cm-1
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Exploiting the 5-µm region
More disequilibrium species in Jupiter and
Saturn CO, GeH4, AsH3
Detection of arsine (AsH3 ) in Saturn FTS/CFHT,
R22000 Bézard et al. 1990 ?
As / H 5 times solar
Jupiter and Saturn are enriched in heavy elements
(C, N, P, As) Saturn more than Jupiter
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Deuterium in the Solar System
. Venus
Venus
Detection of CH3D in Neptune CFHT/FTS, R 1600
(de Bergh et al. 1990)
Owen et al. Nature, 1986. Deuterium in the
outer solar system Evidence for two distinct
reservoirs D/H enriched in Mars and Venus
H2O Evidence for H2O photolysis and atmospheric
escape
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A new, key, species
H3 on Jupiter
FTS/CFHT, R 15000 Maillard
et al. 1990
See J.P. Maillards and S. Millers talks
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Probing below Venus clouds
H3 on Jupiter
FTS/CFHT, R 25000 Bézard et al. 1989
The uppermost clouds form a curtain and by day
reflect sunlight back to dazzle us. By night,
however, we become voyeurs able to peep into
the backlit room behind D. Allen, Icarus, 1987
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ISO External water in outer planets
? external water
?internal water
ISO/SWS R1500 Feuchtgruber et al. 1997

• Interplanetary dust ?
• Planetary environments (satellites, rings?)
• Cometary impacts (e.g. Shoemaker-Levy 9)

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Comets are sources for atmospheres
HST Noll et al. 1995
1995
16-23 July 1994

JCMT 15-m Moreno et al. 2003
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Recent exploration fromspacecrafts (1995-2008)
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Spectroscopy from recent space missions the 3-D
view
Titan Cassini CIRS/(R0.5 cm-1)
Study of couplings between chemistry and
dynamics but no new detections (except many
isotopes)

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In situ measurements the chemical complexity of
Titans upper atmosphere from Cassini / INMS
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In situ measurements methane profile and
meteorology in Titans atmosphere from Huygens
Methane drizzle on Titan (Tokano et al. 2006)
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In situ measurements elemental abundances and
meteorology in Jupiter from Galileo
C/H, N/H, S/H are all 3 times solar Noble gases
are also 3 times solar. O/H is still not
measured
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Why even bother
to go there?
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Detection of J2O on Earth (Cambridge 2005 DPS
meeting)