METO 637

Lesson 23

Titan

Titan• A satellite of Jupiter. • Titan has a bulk composition of about half water ice and

half rocky material.• Although similar to the other satellites of Jupiter it is

denser because it is so large that its gravity slightly compresses the interior.

• Titan has no magnetic field. Hence the solar wind can ionize and carry away some of the molecules from the top of the atmosphere.

• There is some evidence for precipitation, erosion, mechanical abrasion, and other fluvial activity.

• Few if any craters visible – surface must be young. However it could be that ‘oceans’ have filled some of the voids.

The atmosphere of Titan

• Has an atmosphere that is largely composed of Nitrogen.• The temperature at the surface is 94.5K and the surface

pressure is 1.5 bar. • The instrument IRIS on Voyager detected a suite of

hydrocarbons and nitrogen compounds in addition to methane.

• Satellite is covered by colored clouds. Clouds have been identified as arising from the gaseous organic compounds – Titan’s equivalent of photochemical smog!

• Clouds extend from the surface to 200 km.• Above this is a thinner haze layer of aerosol particles.

Heating in this layer leads to a temperature inversion.

Altitude profile of temperature and total number density on Titan (Model)

Titan’s emission spectra in the ultraviolet

Composition of Titan’s atmosphere

Schematic of temperature profile on Titan

Photochemistry of Titan

• The minimum temperature at the tropopause is about 70 K.

• For many of the organic compounds found on Titan the measured abundance is above the saturated vapor pressure at the tropopause.

• The source of these species cannot therefore be the troposphere, but must be the stratosphere. Must be derived from volatile parent molecules.

• The absence of H2 and the presence of abundant N2 modify the chemistry considered previously for Jupiter and Saturn. On the planets radicals such as CH3 or NH2 derived from CH4 and NH3 react by abstraction of H from H2 or by three body combination with H-atoms to return to the parent molecule.

• So-called ‘do nothing’ cycle

Photochemistry of Titan

• This cannot happen on Titan, so the less hydrogen-rich hydrocarbons are favored.

• Mixing ratios of C2H6 are four times greater on Titan than on Saturn, those of C2H2 are 27 times greater, and C2H4, which cannot be detected on Saturn, is clearly detected on Titan.

• The key differences between Titan and the planets are the absence of back-reactions involving H and H2, the presence of processes involving N and N+, and the quenching of 1CH2 to 3CH2 by N2

• This leads to the formation of C2H2, C2H4 and C3H4 from the triplet state.

Atmospheric chemistry on Titan

Photochemistry of Titan

CH4 + hν + N2 → 3CH2 + H2 (or 2H) + N2

3CH2 + 3CH2 → C2H4+H2 (or2H) Followed by:

C2H2 + hν → C2H + H

C2H + CH4 → C2H2 + CH3

These then lead to the formation of the other organic compounds

3CH2 + CH4 → C2H4 + H

CH3 + CH3 + M → C2H6 + M

C2H + C2H6 → C2H2 + C2H5

C2H5 + CH3 + M → C3H8 + M

Theoretical altitude profiles of H2 and

hydrocarbons on Titan

Theoretical altitude profiles of H2 and hydrocarbons on Titan

Altitude profiles of carbon species and H (Model)

Io

Io

• Very few, if any, impact craters on the surface. Surface is young

• Hundreds of volcanic calderas. Some are still active. Striking photographs have been taken from Voyager 1 of actual eruptions.

• Vapor from the vents of the volcanoes appears to be SO2 or S. Optical emissions have been observed from atomic sulfur and oxygen

• Atmosphere is tenuous – pressure at surface about 10-7 atmospheres.

Io, Sulfur saturation vapor pressure

Io

• Sulfur dioxide has a vapor pressure of 10-9 bar in the polar regions (<98K) and on the night-side. But at the sub-solar point the pressure could be as high as 10-7 bar, ~130K).

• Simplistic view of the atmosphere is a relatively dense atmosphere near the volcanoes and the sub-solar point, which becomes thin near the poles and on the dark side.

• Microwave observations show 4-35x10-9 bar of SO2 covering 3-18% of the surface – consistent with SO2 being in equilibrium with the surface temperature.

• It has been suggested that O2 at a pressure of 20x10-9 also exists, but there is no direct evidence.

Io photochemistry

• Primary path for the dissociation of SO2 is as follows:

SO2 + hν(λ<221 nm) → SO + O

SO2 + hν(λ<221 nm) → S + O2

• Followed by:

SO + SO → SO2 + S

S + O2 → SO + O

SO2 number density (N) and temperature (T) for Io (Model)

Distribution of major constituents on Io (Model)

Europa

Europa

• Satellite of Jupiter• Similar in composition to Io, but unlike Io has a thin outer

layer of ice.• Very few craters on Europa, suggesting a young and

active surface. Images of Europa surface strongly resemble images of sea ice on Earth.

• Has a very tenuous atmosphere 10-11 bar, composed of oxygen. Almost certainly not of biogenic origin. Most likely source is the bombardment of the icy surface by UV radiation, and charged particles in the solar wind.

• Has a weak magnetic field, which varies periodically as Europa moves through Jupiter’s massive magnetic field.

• Interpreted as signifying that Europa has a conducting layer beneath the surface – probably a salty ocean.

Callisto

Callisto

• The satellite of farthest from Jupiter.

• Surface is covered entirely with craters. Is very old. Callisto has the oldest and most cratered of any body yet discovered in the solar system (4 billion years).

• Has a very tenuous atmosphere composed of carbon dioxide.

• Has a weak magnetic field.

• Little evidence of tectonic activity