Emmanuel Lellouch
Observatoire de Paris, France

Atmospheres of the Solar System
• Giant Planets
– Primary atmospheres (H
2
, He, CH
4
…)
– Little evolution (no surface, little escape)
• « Terrestrial » planets (Earth, Venus, Mars, Titan)
– Secondary atmospheres (CO
2
/ N
2
, N
2
/ O
2
, N
2
/ CH
4
)
– 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
• 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 CH
4 and NH in visible spectra of Jupiter
3 and Saturn taken by Slipher in
1905
CH
4
7260 A
CH
4
8900 A

First detections…
Kuiper 1944
Detection of methane in Titan
« 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 »

First detections…
Spinrad et al. 1963
Detection of H
2 in Uranus
Identification of CH
4 and NH in visible spectra of Jupiter
3 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
CH
4 in Jupiter

CO
2 in Venus

The beginning of infrared…
CO
2 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 H
2
O on Mars (Spinrad et al. 1963) at
0.82 micron:
“Watershed” discovery

Mars’ atmosphere: basic chemistry
* Detection of CO (1968)
O
3
(1971), and O
2
(1972)
*CO
2
+ h
  CO + O
*O + O + M

O
*O
2
*H
2
+ O + M

O
2
3
O + h
  OH +H
*CO + OH

CO
2
+ H
(stability of atmosphere)
*OH

HO
2

H
2
O
2
(not detected before 2005)
* Detection of O
2
1.27 emission in 1976
 tracer of ozone (and not vice versa!)
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:
- H
2
O difficult to detect
- O
2
, O
3 not detected
- How to probe below the clouds ?

In the thermal range:
I

 
0

B

( T (

)) e
  d

• Sensitive to temperature
• Sensitive to vertical distribution of gases
C
2
H
6

Exploring the thermal range from
Earth: the 10 µm window
Detection of strong hydrocarbon emission in outer planets
C
2
H
6
C
2
H
6 C
2
H
6
Saturn 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

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

Equilibrium vs disequilibrium species in Giant Planets
At the relevant T, NH
3 is the thermodynamical equilibrium form of N

In principle NH
3
N/H ratio
/ H
2 gives the
… but PH
3 is NOT the equilibrium form of P
Competition between chemical destruction and vertical convective transport
Quench level : where t chem
~ t dyn
Occurs at T ~1200 K for phosphine
 Observed PH
3 abundance still gives P/H ratio !

Hot radiation originating from ~ 3-5 bar levels (due to low H
2 and CH
4 opacity)
- NH
3
, PH
3
- New detections in 1973-1975: H
2
O (equilibrium)
CO (disequilibrium, much << CH
4
)

Vertical profile of NH
3 in Jupiter: physical processes and deep abundance
10 µm + UV 5 µm
Photolysis
Condensation
“Bulk abundance” ?
 NH
3
/ H
2 at ~3 bar indicates N/H on Jupiter is enriched by a factor ~2 over solar
H
2
O : Does not give O/H ratio because H
2
O condensation occurs deeper than levels probed
NEED FOR DEEP IN SITU PROBE

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 (H
2
O, SO
2
,
H
2
SO
4
)

Full spectra of Giant Planets: Helium
He/H in Giant Planets
H
2
-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
* N
2 is dominant species in Titan 
* Coupled photochemistry of N
2 and CH
4

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
Jean-Pierre Maillard

Exploiting the 5µm region
More disequilibrium species in Jupiter and Saturn
CO, GeH
4
, AsH
3
Detection of arsine (AsH
3
) 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 CH
3
D 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 H
2
O: Evidence for H2O photolysis and atmospheric escape

A new, key, species
H
3
+ on Jupiter
FTS/CFHT, R= 15000
Maillard et al. 1990 See J.P. Maillard’s and S. Miller’s talks

Probing below Venus’ clouds
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
H
3
+ on Jupiter
FTS/CFHT, R= 25000
Bézard et al. 1989

ISO: External water in outer planets
Saturn
Jupiter
 external water
ISO/SWS
R=1500
Feuchtgruber et al. 1997
NH
3
NH
4
SH
H
2
O
 internal water
•
Interplanetary dust ?
• Planetary environments (satellites, rings?)
• Cometary impacts (e.g. Shoemaker-Levy 9)

Comets are sources for atmospheres
HST Noll et al. 1995
16-23 July 1994
JCMT 15-m
Moreno et al. 2003
1995

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 J
2
O on Earth
(Cambridge 2005 DPS meeting)