Titan and Enceladus after 7 years of. discoveries by Cassini

Transcription

1 Titan and Enceladus after 7 years of 1. Titan Organic cycle Geological features Deep ocean 2. Enceladus Jets of vapor on the southern hemisphere Origin of the jets 3. New questions and future missions - JET: Journey to Enceladus & Titan High resolution imaging Mass spectroscopy discoveries by Cassini Christophe Sotin Jet Propulsion Laboratory (Pasadena, CA) and Université de Nantes (France)

2 Some characteristics of Titan and Enceladus 6371 km 1822 km 2575 km km kg kg kg kg 5525 kg/m kg/m kg/m kg/m 3 2/3 Silicates and 1/3 iron Silicates Ice and silicates Ice and silicates 42 TW 80 TW (2 W/m 2 ) 750 GW 6 GW in the South Pole area Radioactive power is proportional to the mass Other internal heat sources include tidal dissipation, cooling, and latent heat

3 Titan Increasing atmospheric density at surface Increasing size Titan can be compared to the Earth-like planet in terms of global processes implying exchanges between the interior and the atmosphere and erosion processes driven by meteorological conditions It can also be compared with the largest icy moons Ganymede and Callisto for its interior structure.

4 Suite of instruments includes VIMS, INMS, Radar, CIRS Infrared wavelength range of the Visual and Infrared Mapping Spectrometer (VIMS)

5 Kraken Mare Dune fields Bright plateaus carved by rivers Mountains

6 Titan s surface 100 km 50 km

7 Atmospheric composition Total mass of C in the atmosphere: kg

8 Lakes Total mass of C in the lakes: m 3

9 River channels Huygens panorama réalisé entre 13 et 8 km d altitude Huygens landing site Erosion from River channels : 8,000 km 3

10 Surface temperature Without the greenhouse effect caused by methane, the surface temperature would be around 70 K.

11 Methane stability Data by Choukroun et al. (2008, 2009) suggest that methane clathrates could be destabilized at much smaller temperature in the NH3-H2O-CH4 system.

12 Cloud cover: From Rodriguez et al. (2010)

13 Carbon cycle: transformation of methane (CH 4 )

14 Carbon cycle: transformation of methane (CH 4 ) Lunine and Atreya (2008)

15 Organic cycle on Titan Methane is extracted during differentiation processes and by evaporation of lakes In the atmosphere, methane is transformed into ethane and other heavier hydrocarbons Methane and Ethane eventually condense and precipitate forming the rivers and lakes. They can be stored as clathrates in the icy crust. Heavy hydrocarbons form an aerosol layer. The aerosols fall and accumulate at the surface where they can be remobilized and transported by winds to form dunes Are there places on Titan where organics and H 2 O can react? impact craters? Cryovolcanism? No evidence for recycling of organics in the deep interior (deep ocean)

16 CH 4 on Titan H 2 O on Earth

17 The triple point of methane (water) is in the (Pressure, Temperature) domain of Titan (Earth) s atmosphere. Methane on Titan, like water on Earth, evaporates at the surface, condenses in the atmosphere, rains, runs at the surface and fills topographic depressions. Methane is a very effective greenhouse gas. BUT CH 4 on Titan and H 2 O on Earth: similarities and differences methane irreversibly transforms into ethane If there is no replenishment (from the interior or from meteorites), the methane would disappear, the greenhouse effect would vanish, the surface temperature would drop, and nitrogen would freeze. All this would happen in less than 100 Myrs (likely 30 Myrs). Titan would become Triton like. WHAT IS THE SOURCE OF METHANE?

18 Geological features CH 4 has a short lifetime (10 to 100 Ma) according to the H 2 escape rate. 2 CH 4 C 2 H 6 + H 2 Gradient in concentration from 1.67% at high altitude to 5% at the surface. 40 Ar is present it implies exchange processes between the interior where silicates are leached and released the Argon produced by the decay of 40 K. And there is very little 36 Ar. The isotopic ratio 15 N/ 14 N is higher than the terrestrial value and suggests that nitrogen escaped. The lack of 36 Ar suggests that N2 was formed from the photolysis of ammoniac Heavy organic molecules are being synthesized in the upper atmosphere.

19 Impact craters The density of impact craters is low. On average, the age is 200 Myr or 1 Byr depending on the choice of the law describing impact flux (Wood et al., 2010). Sinlap crater is one (only) place on Titan where dielectric constant and IR spectra are correlated and suggest enrichment in H2O ice. (Le Mouelic et al., 2008) Everywhere else, the dielectric constant is much smaller than the value for water ice (Janssen et al., 2009)

20 Mountains? Linear features observed by radar and VIMS (Visual & Infrared Mapping Spectrometer) at different locations. Same origin?

21 Cryovolcanic features? Wall et al. (2009): images obtained by the Cassini Titan Radar Mapper (RADAR) reveal lobate, flowlike features in the Hotei Arcus region that embay and cover surrounding terrains. A sedimentary alternative cannot be rejected. Nelson et al. (2009) observed variations in the albedo Lopes et al. (2007) and Le Corre et al. (2009): small vent with deposits to the East of the crater.

22 Cryovolcanic features?

23 Bright 5 microns areas Barnes et al., 2006 Tui Regio - red = 1.55 µm, green = 2.69 µm, blue = 5µm. Data cube from T12 flyby at a distance of km from Titan. The Tui region covers about 4, km².

24 Electric field in Titan s atmosphere (Beghin et al., 2009, 2010) The decrease with altitude of the strength of the horizontal component of the 36 Hz electric field is consistent with the presence of a Schumann resonance triggered and sustained by ionospheric currents (Beghin et al., 2009 and 2010). It requires the presence of a conductive layer at 45 km depth: it could be a deep ammonia-rich ocean.

25 Deep ocean Moment of inertia implies that the satellites are differentiated. Induced magnetic field suggests the presence of a deep ocean within Europa, Ganymede and Callisto Ganymede Europa

26 Conclusions on the deep ocean So far, the Schumann resonance is the only observation that points toward the existence of a deep ocean. It suggests the presence of a deep conductive layer that could be a salt-rich layer. Additional information could come from the determination of the periodic Love number. Satellites cool down very quickly. Maintaining an ocean requires: Additional heat source (tidal dissipation) Anti-freezing material (ammoniac)

27 Enceladus Characteristics of the jets Modeling of the interior dynamics

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29 Tidal forces Jupiter and Saturn create very large tidal forces Eccentricity can be sustained by resonance between satellites Satellites have synchronous rotation

30 Tidal dissipation within Enceladus

31 Enceladus The amount of heat is difficult to reproduce in the numerical simulations Transient process What is the lifetime of the jets. Reconcile INMS and UVIS data

32 Extension of the Cassini mission Seasonal variations, in particular for Titan Complete the radar coverage and the VIMS coverage at resolution from 1 to 10 km/pixel More gravity passes for determining the periodic Love number Monitoring the evolution of Enceladus plumes Equinox: May 14, 2025 Summer Solstice: May 23, 2017 Winter Solstice: Oct 30, 2002 Equinox: Aug 9, 2009 Cassini Prime Mission XM

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34 JET: Two instruments plus radio-science High resolution mapping Mass spectroscopy Radio science

35 From Cassini to JET - Titan Cassini has discovered that JET will rivers and valleys are carved into plateaus and mountains Titan s surface is covered by organic molecules in solid and liquid phases valleys, riverbeds, and lakes are present at Titan s surface some geological features may be of cryovolcanic origin Titan s surface is shaped by proces-ses operating at seasonal, Milanko-vitch, and/or geological timescales search for sedimentary layering in valleys to determine the history of the flows map the distribution of solid organics and organics in smaller lakes determine if any of the rivers are actively flowing because better special resolution should reveal major sedimentary units high-resolution color imaging allows determination of surface heterogeneities within geologic units better spatial resolution will show inner portions of valleys and specular reflections; color capabilities will detect liquid hydrocarbons test hypotheses of cryogenic origin better spatial resolution distinguishes alluvial fans from volcanic flows characterize erosional and depositional features in lacustrine and fluvial environments better spatial resolution allows for detection of benches, deltas, etc.

36 From Cassini to JET: Enceladus Cassini has discovered that JET will Enceladus plume has organic species Enceladus plume is produced by jets emanating from faults known as Tiger Stripes heavy molecules (m>100 Da) are produced in Titan s upper atmosphere mass 28 in Enceladus plume has possible CO, N 2 and/or hydrocarbon components molecule(s) or elements with mass 40 is (are) present in Enceladus plume quantify the organic inventory to high molecular weight precisely determine the energy output and the lifetime of the jets determine the nature of these molecules confirm what fraction is CO vs. N 2 vs. hydrocarbons determine whether mass 40 truly is argon because it has 10 times better mass range,100 times better mass resolution, and three orders of magnitude better sensitivity thermal mapping can be done at resolutions adequate to see individual jets the mass resolution and range with JET is sufficient to see heavy species its mass resolution and sensitivity are three orders of magnitude better than Cassini its mass resolution and sensitivity are three orders of magnitude better than Cassini

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38 Comparaison DISR VIMS: importance de la resolution spatiale Synthesis to show the difference between a 1.2 km resolution image (VIMS) and the few meter resolution images obtained by DISR. OR How the resolution turns a boring area into an active place where rivers and erosion processes have operated. It emphasizes the importance of Huygens observations.

39 Comparaison Radar VIMS: deux vues très différentes T8 and T41 radar images from the PDS have been used to compare the view of the HLS with the two instruments. Comparisons between the Radar images and VIMS images have already been presented (Soderblom et al., 2007 for the HLS and other areas, Le Mouelic et al., 2008 for Sinlap crater). The radar team has some very nice stereo of this area on top of which the VIMS highres images can be overlaid.

40 Entre 2 et km 2 couverts par flyby (4 à 10 fois la superficie de la France) avec une resolution spatiale de 50 m/pixel

41 Larger mass range (x10) Better mass resolution (x100) Higher sensitivity (x1000)

42 Future missions Equinox: May 14, 2025 Summer Solstice: May 23, 2017 Winter Solstice: Oct 30, 2002 Equinox: Aug 9, 2009 Cassini Prime Mission XM

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44 Conclusions Titan and Enceladus exhibit a large diversity of geological features that challenge our terrestrial analogs (relationships convection / tectonics / plumes / mountains, tidal dissipation, very active small body, dense atmosphere, ). Higher spatial resolution is required to address the geological questions. Titan is the only moon with a dense atmosphere, only body besides Earth with liquid bodies at the surface. Titan organic cycle allows us to witness formation of heavy organics. Are there places where organics and H2O can react? Enceladus is one of the three places in the solar system where water is in contact with silicates (Europa and Earth). The jets provide a window to the interior processes. Can life start and develop in the deep oceans under the crust of icy moons? Understanding the interior structure and the evolution of satellites bring constraints on the formation of the solar system. Such satellites should be present around exoplanets. Future: Cassini Solstice Mission ( 2017), JET, TiMES, TAE.