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The composition of planetary atmospheres: a historical perspective Emmanuel Lellouch Observatoire de Paris, France 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 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)
