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What Are the Habitable Worlds in the Outer Solar System?

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Exploration of planetary atmospheres (and search for habitable conditions within Horizon 2000) Athena Coustenis, Thérèse Encrenaz LESIA, Paris Observatory, France Our Solar System A highly diversified environment… Why two classes of planets in the solar system? A consequence of their formation scenario: the Primordial Nebula Model (Kant & Laplace, XVIIIth century) Venus Venus • Our closest neighbour… -Distance to the Sun: 0,7 ua -Mass: 0.8 x M(Earth) -Obliquity: 177° -> retrograde rotation -Surface temperature: 457°C = 730 K -Surface pressure: 93 bars -CO2: 96%, N2:4% -Water vapour: < 0.01% • …but very hostile conditions! – Very high pressure and temperature, opaque cloud deck of sulfuric acid The space exploration of Venus • The Venera missions (USSR) (1970-80s) – Descent probes -> half-failures (hostile conditions) but returned the first images of the surface – Vega (Venera-Halley) -> atmospheric balloons in Venus Venera 9&10, 1975 • The NASA missions – Pioneer Venus (1978), Galileo (flyby, 1990) – Magellan (1992) -> Radar cartography • Venus Express (ESA)(2006 -> 2015), orbiter The surface of Venus Images of the surface Synthetic radar image Venera 13 & 14 (USSR), 1982 Magellan (NASA), 1992 The Venus Express mission (2005-2015) Venus Exploration Venus Express habitable worlds? •Since 2006 and until December 2014 •Goals : •Venus Express is a satellite optimised for studying the atmosphere of Venus, from the surface right up to the ionosphere. • It arrived at Venus in April 2006 and continued operating for more than eight years. •End of mission: December 2014 Aerobraking manœuvre : VEX orbiting at 130 km from 18 June to 11 July 2014 Venus as seen by VEX ESA/MPS, Katlenburg-Lindau, Germany (ESA/INAF/LESIA/U. Lisbon/U. Evora) VMC (UV-2006) VIRTIS (IR-2011) Probes the top of the cloud deck Probes below the clouds Evidence for a polar vortex at the South Pole (April 2011) (similar to the vortex observed by Pioneer Venus at the North Pole) Evidence for O2 airglow on the night side Day side: CO2 photodissociation -> Transport of O atoms through Subsolar-Antisolar circulation -> Recombination of O atoms into O2 on the night side - Observation with VIRTIS at 1.27 µm (Piccioni et al. 2009) Evidence for temporal changes in the thermal emissivity on 3 hot spots -> A signature of recent volcanic activity (age < 2.5 105-2.5 106 years) VIRTIS-M (1.02 µm): probes the surface Smrekar et al. 2010 Long-term variations of SO2 at the cloud top (z = 65 km) A peak of activity in 2007 Pioneer. Venus +. Venera Venus Express-SPICAV Esposito et al. 1984 Marcq et al. 2011 Zasova et al. 1993 Venus runaway greenhouse effect - harsh conditions on the surface, - but upper atmosphere the most Earth-like area in the Solar System (50 -65 km) - very little water on Venus It is speculated that the atmosphere of Venus up to around 4 billion years ago was more like that of the Earth with liquid water on the surface. The runaway greenhouse effect may have been caused by the evaporation of the surface water and subsequent rise of the levels of other greenhouse gases. Thus most of the original water was transferred to the upper atmosphere -> O2, H2 and was lost to interplanetary space. The search for habitable conditions in our solar system Habitability: four requirements essential chemical water elements energy stable (CHNOPS...) environment Habitability 1. Why is Ganymede an habitable world Habitability in the Solar System: extended HZ Are icy satellites like Ganymede, Europa, Titan or Enceladus habitable worlds ? Liquid water Stable environment Deep habitats Surface habitats Essential elements Energy Deep habitats The habitable zone is not restricted to the Earth’s orbit… Univ. Nantes, O. Grasset What are the habitable worlds? Class II : habitable environnement in the past but evolution from the Earth’s case habitable worlds? Lammer et al., 2009 Time dimension is crucial •migration due to star evolution •Location within the HZ •Weak magnetic fields •Dynamical evolution •Atmospheric loss processes Mars The exploration of Mars • A planet that has similarities with the Earth with a CO2 tenuous and dry atmosphere -Distance to the Sun: 1,5 UA -Mass: 1/10x M(Earth) -Obliquity: 25° -> seasonal effects -Temperature at the surface: -40 (+/- 40)°C -Surface Pressure: 6 millibars -CO2: 96%, N2 & Ar:2% -Water vapor: < 0,01% • Most explored planet due to its proximity and theorized habitability : search for extinct or extant life • Viking (1976) did not discover life -> the NASA Martian program was stopped for 20 years! 1996: New start in Martian space exploration Mars Odyssey (2001) Mars Pathfinder (1996-97) Mars Global Surveyor (1996-97) MARS EXPRESS Mars Express: Mars planetary physics mission, launched 2003 / ESA 1. Hydrated minerals – evidence of liquid water on Mars #2. Possible detection of methane in the atmosphere #3. Identification of recent glacial landforms #4. Probing the polar regions #5. Recent and episodic volcanism #6. Estimation of the current rate of atmospheric escape #7. Discovery of localised auroras on Mars #8. Mars Express discovers new layer in Martian ionosphere #9. Unambiguous detection of carbon dioxide clouds #10. Mapping and measuring Phobos in unprecedented detail N↑ Worcester Crater in Kasei Valles / MEX HRSC / ESA / DLR / FU Berlin / CC BY-SA IGO 3.0 20km The search for liquid water in the past history of Mars Viking (1975): Evidence for valley networks Mars Odyssey/GRS (2001): Water detected under the poles Mars Express/OMEGA (2006): Detection of clays in the ancient terrains -> Liquid water was present on Mars in the past! The Rover MSL/Curiosity on Mars (Since August 2012) Search for organic molecules and favorable habitable conditions for life in the past MSL: Discovery of a stratified terrain near Mont Sharp (Yellowknife Bay) -> Evidence for the presence of a lake in the past -> Mars has been « habitable » 4 billion years ago : - neutral pH, weak salinity - C, H, O, N, P, S -Fe, S in different states of oxydation Mars runaway greenhouse effect From Mars Express : Mars is thought to have lost its atmosphere to space. When Martian volcanoes became extinct, so did the planet’s means of replenishing its atmosphere turning it into an almost-airless desert. Geological observations suggest rivers and seas dotted the martian surface 3.5 billion years ago. No liquid water is detected on the surface today. Via ion and neutral molecular loss, Mars could have lost a 10-meter-deep layer of water from its surface over the last 3.5 billion years. Mars loses its atmosphere due to the solar wind The results of the NASA MAVEN mission show that in the absence of a magnetic field, Mars has seen its atmosphere disappear against the action of a solar wind consisting of a stream of electric particles arriving at full speed (1,500,000 km / h on average) and interacting with the particles of the atmosphere dragging them into a sort of cometary tail that escapes from the red planet. Jakosky et al., Science, March 2017 The paradox of the Early Mars Geologic Age Geologic Minearologic Environment Evolution - 3.3 Ga - 0 Amazonian Cold and Dry Ferric oxide Volcanism - 3.7 – 3.3 Ga Hesperian Outflow channels Sulfates - 4.1 – 3.7 Ga Noachian Valley networks Clay formation -> - 4.6 Ga Pré-Noachian Cold and dry or Clay formation temperate and humid? Wordsworth et al. 2015) Primitive Sun 30% weaker than today : Teq < 200 K! -> How were 273 K attained as required to form valleys? A possible explanation: Surface warming through a series of large-scale volcanic eruptions over 10s-100s years, thawing the ice -> A primitive cold and dry climate with temperate episodic periods The terrestrial planets : A comparative approach Venus Earth Mars Mars 93 bars, 457°C 1 bar, 15°C 6 mbar, - 40°C Primitive atmospheres of similar compositions (CO2, N2, H2O) but very different destinies... The reason : the different phases of water Comparative evolution of the terrestrial planets: The role of water and the greenhouse effect • At the beginning: Similar atmospheric composition (CO2, N2, H2O) but different temperatures • On Venus: H2O gaseous-> runaway greenhouse effect - > Ts = 730 K (457°C)! • On Mars: H2O liquid -> CO2 trapped in the oceans -> moderate greenhouse effect -> Ts remained approximately constant (288 K = 15°C) • On Mars: H2O solid(today) and the planet is small -> weak internal activity -> the greenhouse effect vanishes -> Ts = about 230 K (-40°C) Giant planets Two classes of giants • Jupiter(5 AU) & Saturn (10 AU) : 318 and 95 ME – > mostly made of protosolar gas (H2, He) - > gaseous giants • Uranus (19 AU) & Neptune (30 AU): 14 and 17 ME – > mostly made of their icy core - > icy giants • A possible explanation: U&N formed at the time of disk dissipation (about 10 My) The space exploration of the giant planets • Pioneer 10 & 11 • Voyager (NASA) (NASA) Launch : 1977 (V1, V2) - First flyby of Jupiter A series of successful flybys: by Pioneer 10 (1973) - Jupiter 1979 (V1, V2) - Saturn 1980 (V1), 1981 (V2) - First flyby of Saturn - Uranus 1986 (V2) by Pioneer 11 (1979) - Neptune 1989 (V2) From flybys to orbiters and probes : The Galileo mission to Jupiter (NASA) Launch : 1989 In the jovian system : 1995-2000 From flybys to orbiters and probes : The Galileo mission to Jupiter (NASA) Juno images of Jupiter (NASA/JPL/SWRI) JunoCam different aspects of Jupiter Launch: August 2011 (NASA) Orbit insertion: July 2016 Mission duration: > 3 years Objectives: Southern storms - Investigate the formation mechanisms - Study the internal structure - Study the auroras Saturn through ages Pioneer, 1977 Voyager 1, 1980 NAOS/VLT (ESO), 2000 The exploration of Saturn’s system with Cassini-Huygens (NASA-ESA) Launch : 1995 Saturn encounter: 2004 Titan descent: 2005 Saturn explored by Cassini: A very complex meteorology Saturn’s troposphere (5 µm) VIMS/Cassini Saturn: The huge storm of December 2010 Cassini (visible) Cassini ESO/VLT/VISIR Visible & Infrared Infrared Uranus: from Voyager to HST Voyager, 1986 HST, 2005 > Neptune from 1989 to 2017 Voyager, 1989 Keck July 2017, Molter et al.
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