Planetary Chemistry

Titan's Atmosphere

Smog Composition of Saturn's Moon



Titan, a moon of Saturn, has the only atmosphere in the solar system similar to that of Earth, containing a Nitrogen-rich mix of gases.  Titan, 1.5 times the diameter of the Earth's moon, is both cold enough and massive enough to retain an atmosphere.  The photograph at left was taken by the Cassini spacecraft July 3, 2004, one day after Cassini flew past Titan.  Because of its thinness, the high haze layer is only visible where dark space is a background.  (Note the layering.)  The camera's sensor recorded both visible and ultraviolet light.  Since ultraviolet light isn't visible to the human eye, that portion of the light has been colorized in this image as bluish-purple.  The pale orange of Titan's smog is shown as the color it appears to the human eye.  The visible haze extends 250 miles (400 kilometers) above the surface.

Data collected using a plasma and electron spectrometer carried by the Cassini spacecraft has been analyzed and reported in 11 May 2007 issue of Science.  The data reveals a cloud of very heavy ions 1000 km above Titan's surface.  These ions are complex organic molecules formed from methane and nitrogen when exposed to the energy carried by the shorter wavelengths of ultraviolet sunlight.  CH4 and N2 react to form simple organic molecules such as acetylene, ethylene, and HCN.  Then, reactions between ions and neutral molecules link those components into larger molecules, including benzene.  Previously it was thought that reactions would occur predominantly by radical processes lower in the atmosphere.  But now, it appears that ion-neutral reactions occur in the upper atmosphere eventually producing polycyclic aromatic hydrocarbons and similar compounds containing nitrogen.  It is believed these gradually combine to form more and more complex molecules, reaching masses of 8,000 Daltons.

tholinsA team led by J. Hunter Waite Jr. of Southwest Research Institute used data from Cassini's mass and plasma spectrometers to identify a range of compounds in Titan's upper atmosphere.  They used data from the ion-neutral mass spectrometer to identify and quantify amounts of N2, methane, cyanogen, benzene, and other organic molecules up to 100 Daltons.  They used the plasma spectrometer to identify larger molecules such as naphthalene, anthracene derivatives, and an anthracene dimer.  The plasma spectrometer also revealed the presence of extremely heavy negative ions, with masses up to 8,000 Daltons.

The large molecules do not behave as gases, but produce aerosols which, due to their greater densities, gradually sink towards Titan’s surface.  It is now thought that lower in the atmosphere they forming a layer of tholin compounds which produce the visible smog like haze.  Tholins were first observed in 1953 in an experiment attempting to determine if organic molecules could be formed from inorganic precursors such as was proposed for explaining the development of life on Earth.

Andrew Coates, co-author of the report in Science, said: It’s humbling to think that, with our instrument at Titan, we may be seeing processes which were at work in Earth’s early atmosphere and which eventually led to life on Earth.  It turns out that Titan’s atmosphere is an organic chemical factory on a grand scale.  To see such heavy negative ions was a big surprise for us, and is a key finding linking processes in Titan’s atmosphere to the surface of Titan itself and perhaps to dark, PAH (polycyclic aromatic hydrocarbons), related deposits on Saturn’s other moons.


To better understand the role of ultraviolet light (Latin ultra = beyond) on chemical reactions in the atmosphere of Titan, it should be possible to conduct an investigation of ultraviolet light on chemical reactions here on Earth.

Outdoor exposure to sunlight on Earth results in skin sun tanning and sunburn as well as bleaching or colored pigments and disinfecting.  The sunlight passing indoors through glass generally exhibits less of these effects.  This suggests that ultraviolet light in outdoors sunlight, but blocked by glass and other housing materials, is capable of causing at least some chemical reactions.  Visually distinct skin areas under clothing which are not darkened give further evidence that the ultraviolet light which causes such reactions is easily blocked.

Light with shorter wavelengths than violet light have bundles carrying more energy.  Since ultraviolet light is beyond the range visible to human eyes, it is sometimes called black light  Exposure of skin to ultraviolet light induces the production of vitamin D.  Deficiency of vitamin D causes rickets and the adult equivalent, osteomalacia, which can result in bone pain, difficulty in bearing weight and sometimes bone fractures.  It has been suggested that vitamin D deficiency can result in a range of cancers.  But exposure to ultraviolet light can also cause mutations as a result of double bond formation between adjacent thymine bases in DNA, some of which initiate skin cancer.  So exposure of skin to ultraviolet light has both beneficial and potentially harmful reactions.  Ultraviolet light can also cause cataracts and other damage to eye tissue.  Take appropriate precautions if alternate sources of ultraviolet light are used.

Many commercial products include ingredients to block or retard the effects of ultraviolet light.  So the trick will be to find materials which have not been provided such protection.  Perhaps colored paper, inks, or stretched rubber bands might provide suitable material to begin research.

So it should be possible to create experimental procedures to investigate whether ultraviolet light available in outdoor sunlight can promote a variety of other chemical reactions.  Control comparisons can be conducted by similar arrangements either indoors or outdoors with suitable shielding.

Once chemicals are identified which respond to ultraviolet light, further investigation might involve materials intended to block exposure such as sunscreens and various fabrics.

Some mathematical consideration should be given to differences between the effects on Titan and at the Earth's surface.  Titan's greater distance from the Sun should decrease the intensity of ultraviolet light by the inverse square law.  And absorption by the Earth's atmosphere probably attenuates a portion of ultraviolet light compared to that which interacts with the upper Titan atmosphere.

If you need course credit, use your observations recorded in your journal to construct a formal report.



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created 18 May 2007
revised 19 May 2007
by D Trapp
Mac made