Class 23 - Titan

Reading: One purpose of this class is to use Titan to review the course content. So, the reading is class notes - Classes 1 to 22.

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Latest images - "backside" - Natural light, 938nm methane window, false color composite

Basic Properties

E.g. Table of Titan properties

a=9.5 AU, R=2575 km, g=1.36 m/s2, Vesc=2.6 km/s

P=Po exp -(z/H) where H=kT/mg ~19 km and Po=1.5 Bars

To=95K, <m>=28 amu

Po=no kTo -> no= 10^26 m-3 ( 4 x earth)

rho=no <m> = 5.4 kg m-3 (1.2 x earth)

Mc = rho H = 10^5 kg m-2 (10 x earth)

Composition

NOTE: We do not know the Argon abundance - critical for understanding the origin of the atmosphere - nor the isotope abundances....

Altitude Structure - Layers

WHY this profile? Heat budget

Adiabatic lapse rate = g/cp - at the very bottom, where non-adiabatic processes are important

Non-adiabatic processes: latent heat, absorption, radiation, scattering...

If you want to look at the surface at what wavelengths do you look?

Determination of composition comes from high-resolution spectroscopy such as this:

Detection of water in the upper atmosphere - shows water being brought into the upper atmosphere by comets. Similar argument for CO2 and CO.

Solar flux = So / (a in AU)^1/2 - divide by 4 to get average over planet - at Titan F=1368/(4 x 9.5^2)=3.8 W m^-2

Tequilibrium = To (1-A)^1/4 / (a in AU)^1/2 where To= (So/4 sigma)^1/4 = 280K (for So=1368 W m^-2)

Visible Albedo A=0.27 -> Eeq=83K

NOTE:

• Absorption of visible sunlight by hazes in stratosphere, inefficient radiation of IR by haze - ANTI-greenhouse
• Absortption of out-going IR in troposphere by GH gases
• Tg=Teq + 12K of GH heating - this is about HALF the GH effect of the earth - despite Titan having a thicker atmosphere
• Only 10% of visible sunlight gets to the ground!

NOTE: Because of all the scattering there are few shadows - permanently heavily overcast - this is what it looks like:

Photochemistry

Methane photochemistry:

Methane photochemistry with nitrogen:

Here's the "lite" version:

Prebiotic chemistry???

Latitude distribution - seasonal?

Sources, Losses & Recycling

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
• Need Ar and isotope ratios to tell - Cassini, please!

Losses of Titan's atmosphere?

• Thermal escape - particularly H, also other atoms produced by dissociation of molecules at high altitudes
• Ionization, stripping by surroundinng 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......

French version...

Ideas of how organic raindrops develop and grow as thy slowly drop.....

Fat raindrops (up to centimeter) caused by slow descent.

Dave Grinspoon's version - with biology....

Source of these "warm underground lakes"?

• Note: add ammonia to water as antifreeze - stays liquid to ~120K
• Note: add similarly, ethane to methane ocean - increases vapor pressure over oceans
• Volcanism? Cryovolcanism? i.e. in ice. heat source?
• Exothermic reaction C2H2 + 3H2 ->2CH4 - liberates methane
• Impact craters?

 In upper atmosphere UV dissociates CH4

Meteorology / Dynamics

NOTE: Persistent longitudinal variations and significant temporal variations - cloud outburst

Eccentricity of Saturn = 0.056 = 5.6% (compare with Earth = 1.7%).

Looking at the solar flux over the year we see a similar asymmetry that we saw at Mars due to the orbital eccentricity.

Units of Watts m^-2 at outer atmosphere

Winds?

• Superrotation - similar to Venus
• Consistent with E-W wind streaks on surface
• Hadley cells? Slow rotation (16days) would suggest similar to Venus - 2 cells

Here's the picture overview