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Titan: First Views of an Alien World Jonathan I. Lunine The University of Arizo

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Titan: First Views of an Alien World

Jonathan I. LunineThe University of Arizona

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Titan is…

•…a natural satellite of Saturn.•… one of a triad of giant moons with bulk densities of 1.8 g/cm3, and radii ~ 2500 km (Ganymede, Callisto, and Titan).•… the second-densest atmosphere of the solid bodies of the solar system—nitrogen-methane composition organic chemistry.•Surface P,T=1.5bars, 95K. Methane 2-10% near-surface, 2% in stratosphere.•Target of 45+ Cassini flybys and the successful Huygens probe descent

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The three great questions:1. Why does Titan have an atmosphere?2. How much methane is, and has been, in the surface-atmosphere system over Titan’s history?3. How far has organic chemistry gone on the surface of Titan?

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methane

ethane

acetylene

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methane

ethane

acetylene

nitrogen HCN

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ISS : high sensitivity but images of surface are affected by the haze extending from the high altitudes to the lowermost atmosphere

Optical Near-IR (0.93 microns)

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Credit: C. J. LunineCredit: G. Nierenberg

Haze scattering limits the information in an image…….

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Credit: C. J. LunineCredit: G. Nierenberg

Haze scattering limits the information in an image…….

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Each strip covers 1% of the surface at a wavelengths of a few cm’s and resolution ½-1 km.

In radar images, dark is smooth or oriented away from beam; bright is rough or oriented toward the beam

Radar probes the surface unaffected by the haze.

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Volcanism

These features are tectonic in nature, and/or might be associated with volcanism

Data from Ta (October 04)Smallest features ~ 500 meters

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Weaker evidence for volcanic flows in T3 data

~ 30

0 km

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Titan—Heat flow today 6 milliWatts/m2

Earth mean heat flow 80 milliWatts/m2

Kargel et al., 1991

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~440 km

Impact craters are seen in the most recent radar data. But the surface is still devoid of craters relative to the other Saturnian satellites.

~60 km

Impacts

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This portion of Tethys is roughly equal to the surface area covered by the radar on Titan pass Ta. Resolution is ~ 2 km.

A direct example of cratering in the Saturn system from ISS

(Titan’s dense atmosphere screens out impactors < 1 km diameter (craters < 10 km diameter). Relaxation or flooding of craters > 200 km may remove topographic clues.)

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The bright plain in upper right of the T3 radar pass might be akin to the Huygens landing site

But other areas look like none else—not cryovolcanic, fluvial, nor impact-related:

~200 km

Longest channel ~ 200 km

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PROBE RELEASE December 24, 2004PROBE DESCENT January 14, 2005

The probe was designed only for the atmospheric descent, with the possibility of another 3-30 minutes of transmission on the surface. The probe returned over 80 minutes of data on the surface.

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Complex, dendritic patterns carved by liquid methane and shaped by topography. Some of the patterns imply rainfall; others could be liquid methane liberated from subsurface.

Descent Imager and Spectral Radiometer

~10 km

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Huygens landed near horseshoe near lower right. Surface has a hard crust below which it is the consistency of wet sand (SSP)

~10 km

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Where are the large liquid deposits of

methane and liquid/solid deposits

of the products of methane

photochemistry?

•Paucity of small craters hints at burial by substantially thick deposits•Huygens saw methane coming from the immediate subsurface…

•Large parts of the surface appear bright in optical/near-IR, yet show no evidence of specular reflection. Optical depth effect?

•Liquids exist subsurface in porous crust? (Stevenson)•Polymers—polyacetylenes (ρ ~ 0.42 g/cm3) floating on liquid?•Methane outgassing has been limited and late? Where is ethane?

~15 cm

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Possible history of methane degassing (consistent with isotopes of

carbon, nitrogen measured by Cassini-Huygens): Tobie et al. 0 0.5 1.0 2.0 3.0 4.0 Ga

Accretion + heavy bombardment

Clathrate dissociation at the interface icy layer/ocean.

Core overturn + Formation of an isolating MH layer

+ Heating of the ocean

Cooling of the ocean

+ Formation of Ice I and HP ice layers

Onset of convection

Clathrate dissociation within the conductive lid

Massive hot NH3-CH4 atmosphere

N2 atmospherewith traces of CH4

N2 –CH4 atmosphere(5-10% of CH4)

Escape-photolysis-condensation

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The evidence for ammonia as a key player in Titan’s evolution

•Radar bright areas suggest internal activity and cryo-volcanism, implying both a liquid layer in the interior and an agent to lower the melting point, density and mobility of liquid water.

•Tidal dissipation models and the observed eccentricity of Titan’s orbit (Tobie et al., 2004) require either that Titan’s interior always have been completely solid or have maintained a liquid layer to the present (that is, freezing is not allowed). Maintenance of this layer requires ammonia or similar antifreeze.

•Chemical evidence from two mass spectrometers on Cassini and Huygens: See presentation by Toby Owen.

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Craters are being obliterated by resurfacing, by burial, by flooding

•Viscous relaxation or flooding of >200 km-sized craters if a liquid layer exists below Ice I crust•Burial by solid/liquid organics sedimenting out of the atmosphere (the products of methane photolysis)•10 km diameter crater 0.1-1 km depth, could be completely obliterated by hydrocarbon deposits •We have sampled 2% of the surface with radar

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Water and ammonia

Atmospheric UV-driven chemistry

Amino Acids

Impact / volcanic melt of icy crust

HCN oligomers

Purines, Pyrimidines

Sugars

Hydrocarbons (C-H), Nitriles (H-C-N)