- The Cassini spacecraft has flown through the upper atmosphere
of Saturn's giant moon, Titan, and detected a huge number of complex organic
chemicals. Scientists believe that similar processes may have built organic
molecules in the atmosphere of early Earth.
- Cassini's closest encounter with Titan so far came on
16 April, as it skimmed within about 1000 kilometres of the moon's surface.
During the flyby, an instrument called the Ion and Neutral Mass Spectrometer
captured and analysed gases from the upper atmosphere. INMS uses a beam
of electrons to ionise these gases before subjecting them to electric fields
to measure the telltale atomic mass of different molecules.
- Some complex organic molecules - such as benzene and
diacetylene - had already been picked up on an earlier approach to Titan,
but the latest encounter has yielded an even wider range.
- It includes nitriles and scores of different hydrocarbons,
some with up to seven carbon atoms. And the results suggest that Titan's
upper atmosphere holds even heavier and more complex organics, which are
beyond the instrument's mass range. Reactive radicals
- The INMS team were not expecting to find such a rich
soup of chemicals so high up - it is something of a mystery why these heavy
molecules do not rapidly condense in Titan's cold atmosphere and rain down
to the surface.
- Scientists do have some idea about how they are formed,
however. Methane and molecular nitrogen are thought to be smashed apart
by ultraviolet radiation from the Sun and by high energy particles trapped
in Saturn's magnetic field. That creates highly reactive radicals that
can combine to form more complex molecules.
- Similar processes might have operated on the Earth a
few hundred million years after it formed, generating the raw materials
for life. On the other hand, Earth's organics may have been created in
deep space and then delivered by comets.
- The Huygens probe - which landed on Titan in January
after hitching a ride with Cassini - had previously taken gas readings
as it plummeted through the atmosphere, but only began those measurements
as it fell beyond an altitude of 150 km.
- The Cassini team are hoping to learn more about Titan's
atmosphere over the next few years - at least 39 further flybys are planned.
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