astrochemistry

AN APPROACH TO ASTROCHEMISTRY

-Arif Faisal Ahmed

Tholins: The Complex Organic Molecules Of Space,

Which May Be The Raw Materials For Life.

  The word ‘Tholin’ was derived from the

Greek word ‘tholόs’, meaning ‘hazy’ or ‘muddy’. They are a

wide variety of organic compounds formed by Solar

ultraviolet irradiation or cosmic rays from simple carbon

containing compounds such as carbon dioxide (CO2),

methane (CH4) or ethane (C2H6), often in combination with

nitrogen gas (N2) or water (H2O).

Tholins are complex polymer-like materials made of repeating chains of linked subunits and complex combinations of functional groups,including heteropolymers that give a reddish, organic

surface covering on certain planetary surfaces, like the

reddish-brown cap of the North Pole of Charon, the largest

of five moons of Pluto.

 Tholin formation: 

Tholins may be a major constituent of

the interstellar medium. Tholin is a polycyclic aromatic

hydrocarbon formed by photolysis. On Titan ( the largest

moon of Saturn), their chemistry is initiated at high

altitudes, around 1000 km above Titan’s surface. At this

altitude, a concentration of methane and nitrogen

molecules is the defining characteristic of the atmosphere.

These molecules are bombarded with UV radiation, cosmic

rays and other energetic particles which ionizes the

nitrogen and methane molecules. The charged molecules

undergo a series of chemical reactions with the molecules

already present in the atmosphere and form distinct

species of organic compounds which are further

photolyzed by UV rays. This results in a polymerization of

C2H2 and HCN producing benzene. Benzene continues to

react with other complex organic compounds and their

polymerization leads to the formation of aerosols of

heavier molecules. These aerosols are then further

processed into Tholin particles, which begin to form at

about 400 to 500 km above the surface. The suggested

formula for Tholin is (C11N4H14)n .

 Do Tholins form on Earth?

Tholins do not form naturally on modern-day Earth,

because of the highly oxidizing atmosphere. But, Tholins

might have formed on the primitive Earth (having reducing

atmosphere), as a constituent of the Earth’s primitive

oceans and therefore as relevant to the origin of life.

 Occurrence:

Tholins occur as a component of red aerosols

in the atmospheres of the outer planets , dwarf planets

like Pluto, Ceres, Makemake and moons like Titan, Europa,

Rhea, Triton, etc., present in comets, asteroids, preplanetary

solar nebulae, many icy moons and Kuiper belt

objects in the outer Solar System are rich in deposits of

Tholins.

 Tholins beyond the Solar System: 

Tholins might have also

been detected in the stellar system of the young star

HR4796A using the Near-Infrared Camera and Multi Object

Spectrometer (NICMOS) aboard the Hubble Space

Telescope. The HR4796 system is approximately 220 light

years from Earth.

 Biological significance: 

Some theories have suggested that

Earth may have been seeded by organic compounds early

in its development by tholin-rich comets, providing the raw

materials necessary for life to develop.

 As Tholins do not exist naturally on present day Earth due to the oxidizing

atmosphere, so laboratory experiments show that Tholins

near large pools of water that might persist for thousands

of years might facilitate the formation of prebiotic

chemistry to take place and has implication of the origins

of life on Earth and possibly, on other planets.