Life in the outer Solar System. AST 309 part 2: Extraterrestrial Life

Transcription

1 Life in the outer Solar System AST 309 part 2: Extraterrestrial Life

2 Prospects for life on: Overview: 1. Europa (Jupiter moon) 2. Titan (Saturn s moon) 3. Enceladus (Saturn s moon)

3 Life on Europa? NASA s JUNO mission to Jupiter: Juno arrives at Jupiter in 2016 Science goals: 1. abundances (e.g. ratio of O/H) 2. determine core mass 3. map the gravitational and magnetic field 4. map the variation in atmospheric composition, temperature structure, cloud opacity and dynamics

4 Life on Europa? Europa is the sixth of Jupiter's known satellites and the fourth largest; it is the second of the Galilean moons. Europa is slightly smaller than the Earth's Moon. Galileo Galilei Europa 1610!

5 Life on Europa? Spacecraft exploring the Galilean satellites: Voyager 1 & 2 Galileo

6 Life on Europa? Basic Properties of Europa: Semimajor axis = km Orbital period = days Heliocentric Distance = AU Rotational Period Synchronous Orbital inclination = degrees Eccentricity = Radius = 1565 km Mass = 4.797E22 kg Mean density = 2.99 g/cm3 Surface Gravity = of Earth's Escape Velocity = 2.02 km/s Geometric Albedo = 0.6 Surface Temperature = 128 K (-145 C) Surface Composition = Water Ice Tenuous O2 Atmosphere = Surface Pressure about Earth's

7 Europa: Europa s surface is smooth and young (no craters), and is covered with cracks: Europa is tidally heated like Io (just less) and has maybe the youngest surface in the outer solar system! Icebergs on the surface moved by liquid water that later froze or by slushy warmer ice beneath?

8 Europa: Europa is heated by tidal forces from nearby massive Jupiter and has forced orbital eccentricity of from the gravitational interactions with the other Galilean moons: That is exactly the same process that drives Io s intensive volcanism: Europa is tidally heated like Io (just less) and has maybe the youngest surface in the outer solar system!

9 Europa: 2 models of Europa s interior: Artist s conception of the icy surface of Europa

10 Europa: Cycloidal features ( flexi ) near Europa s south pole. These cycloidal cracks form in Europa's solid-ice surface with the daily rise and fall of tides in the subsurface ocean (Gregory V. Hoppa, Randall Tufts, Richard Greenberg and Paul Geissler of the Luna and Planetary Laboratory, University of Arizona). This image shows what appears to be the most convincing evidence yet for a global ocean under Europa's icy crust.

11 Europa: Reasons why Europa is so interesting: the likely presence of a sub-surface ocean of liquid water (perhaps as much as 150 km deep) which could provide a medium and solvent for life. intense radiation from Jupiter's magnetosphere striking ice on Europa's surface and releasing oxygen, which if it finds its way into ocean could provide a fuel for life; the possible presence of undersea volcanic vents, which could furnish energy and nutrients for organisms.

12 Europa: Reasons why Europa is so interesting: Chaotic features seen in many images of Europa's icy surface are probably created by Europa's tides, and are believed to be evidence of melt-through needed for exposing the oceans. The mixing of substances needed to support primative life may be driven by the tides on Europa, with maximum heights of 500 meters (much larger than Earth tides). Circulation of liquid water through cracks produced by tidal forces could bring salts and organic compounds dissolved in the water up to Europa's surface. This circulation also brings biologically useful chemicals, such as formaldehyde (as well as organic compounds dumped on Europa's surface by cometary impacts) down to the subsurface ocean. Other chemicals, formed by radiation near the surface, such as sulfur, hydrogen peroxide, and free oxygen, would also provide primative life with sources of energy and nutrients. Hydrothermal vents would produce organic compounds (seen as dark material coloring cracks?) and provide a heat source. Undersea volcanism could also lead to large melt-throughs, and tidal heat, created by internal friction could also melt the ice. The melted-through ice provides light and surface chemicals to the oceans. Any creatures inhabiting these oceans could use photosynthesis for energy.

13 The question really is: is the ice thin or thick? Tides and undersea volcanoes could play a role too!

14 Europa: Let s go there and find out! Before we drill, we would send a radar mapping probe to measure the thickness of the ice crust! Indirect evidence from large craters suggest a thickness of 19 to 25 km. On Europa, larger craters become smoothed out with gentle concentric rings. They are shallower than their counterparts on other moons. This is because of the influence of a very thick cushion of ice (Schenk, 2002, Nature)

15 Titan: The biggest moon of Saturn Cold! 75 K (-180 C) Thick atmosphere! ~ 1.5 x pressure of Earth mostly nitrogen (95%) and methane (CH4) Origin? Probably outgassing after formation by accretion of icy planetesimals/comets and subsequent evolution Methane from cryovolcamism

16 Titan: Exploring Titan: Voyager 1 & 2 Cassini / Huygens

17 Titan: Titan as seen from space: visible 938nm methane window composite

18 Titan: Titan is big! 2 nd largest moon in our solar system: Size comparison: our Moon, Earth and Titan Simple model of internal structure

19 Titan: Titan s thick atmosphere: Layered structure in Titan s atmosphere

20 Titan: Chemical composition determined by spectroscopy:

21 Titan:

22 Titan:

23 Titan: Source of Titan's atmosphere? Internal - original planetesimals contained dissolved/trapped volatiles -> interior and are now outgassing to provide persistent atmosphere External - delivery of comets (same as Earth, Venus, Mars) - evidence in high altitude H2O and CO Losses of Titan's atmosphere? Thermal escape - particularly H, also other atoms produced by dissociation of molecules at high altitudes Ionization, stripping by surrounding magnetosphere and solar wind Condensing -> ocean Recycling? Methane is recycled in a similar way to water in the Earth's atmosphere through the hydrological cycle...

24 Titan: Why is this important for astrobiology? Titan s atmosphere is remarkably similar to the Urey-Miller experiment and early Earth (except temperature) Is Pre-biotic chemistry possible on Titan? But life? Remember that Titan is cold and all chemical reactions slow down, but still. Ok, so let s land on Titan!

25 The Huygens lander: Titan:

26 The Huygens lander s descent: Titan:

27 Titan: The Huygens lander, impact data: Surface is like wet clay, packed snow or sand

28 Titan: The Huygens lander: Evidence for liquid methane on the surface Heating of the surface by the probe caused methane outgassing

29 Titan: The Huygens lander: Surface reflectivity measured on the landing site with a lamp turned on (red line). The visible portion of the spectrum is consistent with laboratoryproduced tholins, thought to be analogs of Titan's photochemical aerosols (black curves). Water ice is likely responsible for the absorption seen at nm. The decrease of the reflectivity with wavelength beyond 830 nm is due to an unidentified material.

30 Enceladus:

31 Enceladus: Enceladus has the highest albedo (>0.9) of any body in the solar system. Its surface is dominated by fresh, clean ice. At least five different types of terrain have been identified on Enceladus. In addition to craters there are smooth plains and extensive linear cracks and ridges. At least some of the surface is relatively young, probably less than 100 million years. This means that Enceladus must have been active until very recently (and perhaps is still active today). Enceladus is much too small (500km) to be heated solely by the decay of radioactive material in its interior at present. But briefly after its formation 4.5 billion years ago short-lived radioisotopes may have provided enough heat to melt and differentiate the interior. That combined with modest present day heating from long-lived isotopes and tidal heating may account for the present day activity on Enceladus.

32 Enceladus: Enceladus orbits in the densest region of Saturn s E-ring Is it the source? Enceladus is currently in a 2:1 mean motion orbital resonance with Dione, completing two orbits of Saturn for every one orbit completed by Dione. This resonance helps maintain Enceladus's orbital eccentricity (0.0047) and provides a heating source for Enceladus's geologic activity

33 Enceladus: Cassini discovers jet-like plumes rising from the south polar regions. During fly-throughs the spacecraft detected mostly water vapor, as well as minor components like molecular nitrogen, methane and CO2.

34 Enceladus: Cassini measures mean density: 1.6 g/cm3 => some % of silicates & iron Now two heat sources: 1. Radioactive decay 2. Tidal forces Possible molten core or magma pockets that drive volcanism!

35 Enceladus:

36 Enceladus: The Tiger Stripes in the south polar region. Thermal map of cracks near south pole of Enceladus. High temperatures around 180 K (-135 F), normal = 72 K

37 Enceladus: Pockets of water of salty ocean? Postberg et al. (2011, Nature): "Alkaline salt water, together with the observed organic compounds and the thermal energy obviously present in the south polar region, could provide an environment well suited for the formation of life precursors. Cassini's Cosmic Dust Analyser instrument found that about 6% of the grains forming the E-ring were rich in sodium salts. Saturn's tiny moon Enceladus may be the best place to look for life elsewhere in the Solar System. Bob Brown (Cassini mission)

38 Summary: Europa likely has a liquid sub-surface ocean and is heated by tidal forces Titan has complex organic chemistry and a thick nitrogen atmosphere (early Earth) Enceladus is geological active, has lots of water, plus some organic material and a heated interior