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. 2017 Giant planets: Open questions

• What is the origin of the planetesimals which formed Jupiter? • Galileo probe data suggest a very low formation temperature • For future investigation: the JUNO mission (2016 ->) • What is the enrichment in heavy elements for Saturn, Uranus and Neptune? • No descent probe yet! • Why is Neptune bigger than Uranus although more distant? • A possible effect of migration? • Why are the internal structures of Uranus and Neptune different? • Uranus has no internal energy source, why?

• -> A need for future space missions Habitable conditions in the Outer Solar System What are the habitable worlds in the outer solar system?

habitable worlds?

Lammer et al., 2009 Cassini-Huygens Two categories of icy moons as possible habitats : oceans in contact with the silicate core or not.

How do we explore them?

Galileo Cassini-Huygens Possible habitable worlds around Jupiter Three large icy moons to explore

Ganymede – class IV • Largest satellite in the solar system • A deep ocean • Internal dynamo and an induced magnetic field – unique • Richest crater morphologies

• Best example of liquid environment trapped between icy layers

Callisto – class IV • Best place to study the impactor history • Differentiation – still an enigma • Only known example of non active but ocean-bearing world • The witness of early ages

Europa – class III • A deep ocean • An active world?

• Best example of liquid environment in contact with silicates What are the possible habitable worlds around Jupiter? Class III : subsurface oceans in contact with silicates - Europa habitable worlds?

Europa-like • Water:

– Warm salty H2O ocean. • Essential elements:

– Accretion of CO2? – Impactors. – But radiation destroys organics in upper ~10s cm of ice. • Chemical energy:

– Radiation of H2O ⇒ oxidants. – Mantle contact: serpentinization and possible hydrothermal activity. • Relatively stable environment: – Large satellite retains heat. – But activity might not be steady-state. About the existence of deep oceansLiquid water: GANYMEDE

Galileo evidences Ganymede-like • Induced magnetic field from •Liquid water interaction of jovian magneto Observedwithbut conducting not characterisedlayer •Chemistry: silicate (ocean?) needed…? •Energy: heat transfer ? •Stable environment

Geologic activity Indications for young surface from water flooding

• Own internally-driven dipole magnetic field • Interaction of Ganymede’s mini- magnetosphere with Jupiter’s Saturn’s satellites

Saturn SOI 1/7/2004 The Cassini-Huygens mission 1997-2017

12 instruments on the orbiter 6 instruments on the probe The Cassini-Huygens mission (1997-2017) Roger-Maurice Bonnet, ESA Science Program Director at the time when the Huygens probe was conceived, played a fundamental role in the success of this important mission.

E.H. Levy D.M. Hunten H. Masursky F.L. Scarf S.C. Solomon L.L. Wilkening

Frank Press T.M. Donahue U.S. NAS  Joint Working Group on Cooperation in Planetary Exploration ESF H. Curien J. Geiss

H. Fechtig H. Balsiger J. Blamont M. Fulchignoni S.K. Runcorn F.W. Taylor 1982 Cassini and Huygens : of men and … missions

Toby Owen, Daniel Gautier, Wing Ip and others came on board to promote and develop the mission 1988

73 Cassini-Huygens (2004-2017) reveals Titan and Enceladus

Titan Enceladus Activity in Enceladus

April 2017 : The discovery of hydrogen gas and the evidence for ongoing hydrothermal activity on Enceladus, along with the other organics and the water, suggest that habitable conditions could exist beneath the moon's icy crust

75 Athena Coustenis Predecisional - for discussion and planning purposes only Hsu et al. 2015 Cassini INMS

Cassini CIRS

Cassini CIRS

Huygens GCMS Huygens GCMS

79 Titan : images from Cassini with VIMS VIMS Cassini/RADAR/Titan

Rivers and lakes in the North Pole : the missing CH4 reservoir? Canterbury, 1991 Titan’s surface with Huygens Titan is alive : undersurface water ocean

Titan’s internal structure (Tobie et al. 2006) Titan’s spin and large tides on the surface indicate the presence of an internal liquid water ocean between ice layers (Iess et al., 2012)

Huygens measures radio waves at extremely low frequency suggesting a Active regions on the surface (Solomonidou et al., 2016) subsurface ocean Titan as an astrobiological object

• The physical conditions • The organic chemistry • The methane cycle • The undersurface water ocean • Climatology/season al effects Seasons on Titan

During its 13 year mission, Cassini will have monitored only Cassini extended two seasons... mission

We witness the onset of winter in the South pole with strong enhancement of gases and steep drop in temperature ISO (1997) The exploration of the Saturnian system : habitats Saturnian Environment

Titan Enceladus

Terrestrial planet Composition of climate evolution material from Active organic Life which Titan formed chemistry Sources of heating on icy worlds

The Saturnian system is rich in worlds that could bring insights on important aspects of Earth’s . climate, . organic chemistry and . emergence of life. Cassini-Huygens Mission Overview

93 Cassini’s Final Days

Peggy

Titan Saturn and Rings

Earhart

Enceladus setting behind Saturn Propellers Saturn last mosaic taken by CASSINI on September 13, 2017

Observatoire de Paris January 14, 2006 Direct sampling of Saturn’s atmosphere

Dynamics, composition, structure and more after the final 5 periapses and till the end! 15 Sept. 2017: Final moments Galileo Cassini-Huygens From the icy moons to extrasolar planetary systems

Waterworlds like Ganymede or Titan: If Europa or Enceladus-like: If habitable, the habitable, the liquid layers are trapped between liquid layers may be in contact with silicates as on two icy layers Earth

Occurrence: Occurrence: Largest moons, hot ice giants, ocean-planets… Europa, Enceladus Most common habitat in the universe ? Only possible for very small bodies

Key question: Key question: Are these waterworlds habitable ? How are the surface active areas related to potential deep habitats?

What JUICE will do: What JUICE will do: Via characterisation of Ganymede, will constrain Pave the way for future landing on Europa the likelihood of habitability in the universe Better understand the likelihood of deep local habitats From the Jovian system to extrasolar planetary systems Waterworlds and giant planets Habitable worlds Astrophysics Connection By studying Ganymede, we can characterise an entire family of exoplanets: the waterworlds.

Jupiter system Ganymede Three waterworlds One giant planet

Jupiter

Exoplanets Waterworlds Five families Earth-like > 1800 planets Iron-rich

B1257 +12A Kepler 22b GJ1214b >140 >530

SUPER-EARTHS INTERMEDIARY GIANTS Future exploration exomars Europe’s new era of Mars exploration What are the habitable worlds? MARShabitable and EXOMARSworlds? ExoMars 2016: an orbiter and a descent module: Schiaparelli Launch : 14 March 2016

•Mission from December 2017 until end of 2022. Orbiter will serve as relay for the 2018 rover mission • will study the Maritan atmosphere for evidence of biological gases (CH4, etc) The TGO has already started its science invesitgations.

•The EDLM Schiaparelli module has provided technology validation for entry and descent but not for landing …due to premature end of the descent sequence following software problem EXOMARS : Trace Gas Orbiter

TGO Payload : • ACS (Atmospheric Chemistry Suite), • CaSSIS (Colour and Stereo Surface Imaging System) • NOMAD (Nadir and Occultation for MArs Discovery). • FREND (Fine Resolution Epithermal Neutron Detector)

Science : • ACS and NOMAD spectrometers with complementary frequency range will provide for the atmosphere: o Inventory of Mars trace gases o monitor seasonal changes in the composition and temperature o detect minor constituents • CaSSIS will image and characterise features on the martian surface that may be related to trace- gas sources such as volcanoes. • FREND will map subsurface hydrogen to a depth of 1m to reveal deposits of water-ice hidden just below the surface, EXOMARS : 2020 ExoMars rover and surface science platform

2020 Rover advanced rover that will carry out the first sub- surface investigations of Mars (down to 2 m) in order to answer questions about whether life could or ever did exist on the Red Planet

•Two low-level ancient landing sites have been selected in March 2017 for the 2020 ExoMars rover and surface science platform: Oxia Planum and Mawrth Vallis. JUICE: JUpiter Icy moons Explorer JUICE Science Goals • Emergence of habitable worlds around gas giants • Jupiter system as an archetype for gas giants Water

Habita bility Chemistry Energy

Cosmic Vision Themes • What are the conditions for planetary formation and emergence of life? • How does the Solar System work? JUICE : the 1st Large CV mission concept • Single spacecraft mission to the Jovian system • Investigations from orbit and flyby trajectories • Synergistic and multi-disciplinary payload • European mission with international participation JUICE Payload Acronym PI LFA Instrument type Remote Sensing Suite JANUS P. Palumbo Italy Narrow Angle Camera MAJIS Y. Langevin France Vis-near-IR imaging spectrometer G. Piccioni Italy UVS R. Gladstone USA UV spectrograph SWI P. Hartogh Germany Sub-mm wave instrument Geophysical Experiments GALA H. Hussmann Germany Laser Altimeter RIME L. Bruzzone Italy Ice Penetrating Radar 3GM L. Iess Italy Radio science experiment PRIDE L. Gurvits Netherlands VLBI experiment Particles and Fields Investigations PEP S. Barabash Sweden Plasma Environmental Package RPWI J.-E. Wahlund Sweden Radio & plasma Wave Instrument J-MAG M. Dougherty UK Magnetometer Exploration of the Jupiter system JUICE The biggest planet, the biggest magnetosphere, and a mini solar system

Jupiter • Archetype for giant planets Magnetosphere Natural planetary-scale laboratory • • Largest object in our Solar System for fundamental fluid dynamics, • Biggest particle accelerator in the Solar chemistry, meteorology,... System Window into the formational history • • Unveil global dynamics of an of our planetary system astrophysical object

Coupling processes Hydrodynamic coupling Gravitational coupling Electromagnetic coupling

Satellite system • Tidal forces: Laplace resonance • Electromagnetic interactions to magnetosphere and upper atmosphere of Jupiter Future Saturnian system exploration

TSSM: Balloon, lander & orbiter (Coustenis et al. 2009)

AVIATR /plane (Barnes et al. 2010)

TIME: Lake lander (Stofan et al. 2013) Dragonfly (Turtle et al., 2018) The big questions… • Why is the formation scenario of the Solar system different from the ones of exoplanetary systems? – Giant exoplanets are often close to their stars -> Migration ? – Why was the migration moderate in the Solar system ?

• Can we hope to find traces of life in the Solar system? – Mars : Liquid water was present at the surface in the past (When? For how long?) – Outer satellites: Liquid water is present under the surface (Europa, Ganymede, Enceladus, Titan) -> JUICE mission and Europa Clipper – Titan: ions and complex molecules -> prebiotic life? Coustenis, A., Encrenaz, Th., 2013. Life beyond Earth. • Can we hope to find life on exoplanets? Cambridge Univ. Press THE FUTURE OFFrom EXPLORATIONdiscovery to characterisation of deep habitats Rich future for exploration of habitable worlds in the outer solar system with JUICE as L1 and more : missions to Europa, Titan, Enceladus, and exoplanets

Credit N. Powell, Imperial College Thank you for your attention