charm 050222
TRANSCRIPT
Titan: First Views of an Alien World
Jonathan I. LunineThe University of Arizona
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
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?
methane
ethane
acetylene
methane
ethane
acetylene
nitrogen HCN
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)
Credit: C. J. LunineCredit: G. Nierenberg
Haze scattering limits the information in an image…….
Credit: C. J. LunineCredit: G. Nierenberg
Haze scattering limits the information in an image…….
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.
Volcanism
These features are tectonic in nature, and/or might be associated with volcanism
Data from Ta (October 04)Smallest features ~ 500 meters
Weaker evidence for volcanic flows in T3 data
~ 30
0 km
Titan—Heat flow today 6 milliWatts/m2
Earth mean heat flow 80 milliWatts/m2
Kargel et al., 1991
~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
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.)
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
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.
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
Huygens landed near horseshoe near lower right. Surface has a hard crust below which it is the consistency of wet sand (SSP)
~10 km
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
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
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.
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
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)