the composition of planetary atmospheres: a historical perspective

The composition of planetary atmospheres:

a historical perspective

Observatoire de Paris, France

Emmanuel Lellouch

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

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 CH4 and NH3 in  visible spectra of Jupiter and Saturn taken by Slipher in 1905

CH4 7260 ACH4 8900 A

First detections…

Detection of methane in Titan

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 »

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 IRdetectors (PbS) having sensitivityup to 3 m

CO2 in Venus

CH4 in Jupiter

Kuiper 1947

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

Detection of H2Oon Mars (Spinrad et al. 1963) at 0.82 micron:

“Watershed” discovery

R ~100000

Mars

Water cycle on Mars

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

The solar reflected component of Venus

Detection of HCl, HF and CO in Venus (above clouds) Michelson inteferometer R ~ 20000Connes 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

deTBI

))((0

In the thermal range:

• 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

Gillett et al. 1973, 1975 (R ~60)

C2H6 C2H6

Saturn Titan

C2H6

Moses et al. 2000(Saturn)

Methane photochemistry in Giant Planets(a recent view…)

Detection of C3H4 and C4H2 on NeptuneIRS/Spitzer, R=600Meadows et al. 2008

Methane photochemistry in Giant Planets(a recent view…)

Stratospheres

Hunten, 1973

Pre-Voyagermodels of Titan:- inversion only ?- greenhouse also?

Warmer on Titan (~170 K)than Saturn (~140 K)

Predicted due to haze (esp. Titan) and methaneheating

Equilibrium vs disequilibrium species in Giant Planets

At the relevant T, NH3 is thethermodynamical equilibrium form of N In principle NH3

/ H2 gives the N/H ratio

… but PH3 is NOT the equilibriumform of P

Competition between chemical destruction and vertical convective transportQuench 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 probedNEED 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 MARSMariner 9 / IRIS (1973)R =2.4 cm-1, FTS

Temperature, water vapor and dust inthe martian atmosphere

VENUSVenera 15/ Fourier Spectrometer(1983), R = 2 cm-1Temperature and composition fieldat and above Venus clouds (H2O, SO2, H2SO4)

Full spectra of Giant Planets: Helium

Saturn IRIS / Voyager R = 4.3 cm-1He (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)

H2-He

He/H in Giant Planets

Full spectra of Titan: chemistry

IRIS / Voyager R = 4.3 cm-1

* N2 is dominant species in Titan

* Coupled photochemistry of N2 and CH4

Voyager /UVS

1980-2000: Accumulating molecules

(the golden age of infrared)

From the ground: the power of spectral resolution

Fourier Transform Spectrometer at CFHT(1983-2000)0.9 – 5.2 µm, InSb, InGaAs detectorsBest spectral resolution ~ 0.01 cm-1

Jean-Pierre Maillard

Exploiting the 5-µm region

More disequilibriumspecies in Jupiterand Saturn

CO, GeH4, AsH3

Detection of arsine (AsH3 ) in SaturnFTS/CFHT, R=22000 Bézard et al. 1990 As / H ~ 5 times solar

Jupiter and Saturn are enriched in heavyelements (C, N, P, As); Saturn more than Jupiter

Deuterium in the Solar System

Detection of CH3D in NeptuneCFHT/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 andatmospheric escape

. VenusVenus

A new, key, species

H3+ on Jupiter

FTS/CFHT, R= 15000 Maillard et al. 1990

See J.P. Maillard’sand S. Miller’s talks

Probing below Venus’ clouds

H3+ on Jupiter

FTS/CFHT, R= 25000Bézard et al. 1989

The uppermost clouds form a curtainand by day reflect sunlight back todazzle 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

ISO/SWSR=1500Feuchtgruber et al. 1997

Jupiter

Saturn

NH4SHH2O

NH3

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

internal water

external water

Comets are sources for atmospheres

JCMT 15-mMoreno et al. 2003

HST Noll et al. 1995

199516-23 July 1994

Recent exploration fromspacecrafts (1995-2008)

Spectroscopy from recent space missions: the 3-D view

Study of couplings between chemistry and dynamics

… but no new detections (except many isotopes)…

TitanCassini CIRS/(R=0.5 cm-1)

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 drizzleon 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)