Transcript The composition of planetary atmospheres: a historical

The composition of planetary
atmospheres:
a historical perspective
Emmanuel Lellouch
Observatoire de Paris, France
Atmospheres of the Solar System
•
Giant Planets
–
–
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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
Overview
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•
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Early times (1905-1970)
The 1970’s: main concepts emerge
The 1980’s and 1990’s: accumulating molecules
Recent spacecraft exploration (1995-2008)
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
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
First detections…
Detection of H2 in Uranus
Spinrad et al. 1963
Identification of CH4 and NH3
in visible spectra of Jupiter
and Saturn taken by Slipher in
1905
First detections…
1932
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
The beginning of infrared…
CO2 on Mars (Moroz, 1964)
Vassili Ivanovich Moroz
Too much enthusiasm…
Sinton et al. 1960
1960
Actually due to telluric HDO
Mars: discovery of atmospheric
water in 1963
Mars
Water cycle on Mars
R ~100000
Detection of H2O
on Mars (Spinrad
et al. 1963) at
0.82 micron:
“Watershed” discovery
Mars’ atmosphere: basic chemistry
* Detection of CO (1968)
O3 (1971), and O2 (1972)
* Detection of O2 1.27 emission in 1976
 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
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 ?
The 1970’s: The thermal infrared:
access to physical concepts
C2H6
In the thermal range:

I   B (T ( ))e d
0
• Sensitive to temperature
• Sensitive to vertical
distribution of gases
Exploring the thermal range from
Earth: the 10 µm window
Detection of strong hydrocarbon emission in outer planets
C2H6
Saturn
C2H6
C2H6
Titan
Gillett et al. 1973, 1975 (R ~60)
Methane photochemistry in Giant Planets
(a recent view…)
Moses et al. 2000
(Saturn)
Methane photochemistry in Giant Planets
(a recent view…)
Detection of C3H4 and C4H2 on Neptune
IRS/Spitzer, R=600
Meadows et al. 2008
Stratospheres
Warmer on Titan (~170 K)
than Saturn (~140 K)
Predicted due to haze
(esp. Titan) and methane
heating
Hunten, 1973
Pre-Voyager
models of Titan:
- inversion only ?
- greenhouse also?
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 !
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 << CH4)
Vertical profile of NH3 in Jupiter:
physical processes and deep abundance
10 µm + UV
5 µm
Photolysis
Condensation
“Bulk
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
The 1970’s: First global views of
the planet infrared spectra
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)
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) < He (U) ~ He (N) ~ He (protosolar)
 Evidence for helium segregation in Jupiter’s and Saturn’s interior
+ Thermal balace of Giant Planets
(internal source)
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

1980-2000:
Accumulating molecules
(the golden age of infrared)
From the ground: the power of spectral resolution
Jean-Pierre Maillard
Fourier Transform Spectrometer at CFHT
(1983-2000)
0.9 – 5.2 µm, InSb, InGaAs detectors
Best spectral resolution ~ 0.01 cm-1
Exploiting the 5-µm region
More disequilibrium
species in Jupiter
and Saturn
CO, GeH4, AsH3
Detection of arsine (AsH3 ) in Saturn
FTS/CFHT, R=22000
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
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
A new, key, species
H3+ on Jupiter
FTS/CFHT, R= 15000
Maillard et al. 1990
See J.P. Maillard’s
and S. Miller’s talks
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
ISO: External water in outer planets
Saturn
Jupiter
 external water
NH3
NH4SH
H2O
internal water
ISO/SWS
R=1500
Feuchtgruber et al. 1997
• Interplanetary dust ?
• Planetary environments (satellites, rings?)
• Cometary impacts (e.g. Shoemaker-Levy 9)
Comets are sources for atmospheres
HST Noll et al. 1995
1995
16-23 July 1994
JCMT 15-m
Moreno et al. 2003
Recent exploration from
spacecrafts (1995-2008)
Spectroscopy from recent space missions:
the 3-D view
Titan
Cassini CIRS/(R=0.5 cm-1)
Study of couplings
between chemistry
and dynamics
… but no new
detections (except
many isotopes)…
In situ measurements: the chemical
complexity of Titan’s upper atmosphere
from Cassini / INMS
In situ measurements: methane profile and
meteorology in Titan’s atmosphere from Huygens
Methane drizzle
on Titan
(Tokano et al. 2006)
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…
Why even bother
to go there?
Detection of J2O on Earth
(Cambridge 2005 DPS meeting)