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Dust particles from space falling onto the early Earth provided an important
source of the molecules that gave rise to life, according to a study by
researchers at Cornell University, New York. Until now, most scientists
believed that the molecules from which the first living things formed were
made almost entirely in the Earth’s atmosphere.

It has long been known that meteorites and comets contain carbon-based
‘organic’ molecules. But these bodies either burn up while falling to Earth
or hit the ground with so much energy that any organic molecules would be
destroyed. Now the scientists from Cornell, Christopher Chyba and Carl Sagan,
have suggested a new source of organic material – the continuous ‘drizzle’
of very small dust particles that reach Earth from space (Nature, 9 January,
p 125).

Particles less than 1 micrometre across do not burn up in the atmosphere,
but fall gently to the ground. Many of these particles come from comets
and are rich in organic compounds. At present, the Earth is sweeping up
3000 tonnes of this interplanetary dust per year, providing about 300 tonnes
of organic material.

In the early years of the Solar System, interplanetary space was filled
with much more debris than now. It is possible to calculate the abundance
of the larger objects by looking at the craters they created in the Moon’s
surface. Assuming that the number of dust particles was larger by the same
ratio, Chyba and Sagan estimate that the Earth was then accumulating organic
material from space at a rate of 60 000 tonnes per year.

Previous theories about the origins of the precursor molecules to life
on Earth are based on research that began in the 1950s, when Stanley Miller,
an American chemist, passed an electric spark through a mixture of gases
thought to resemble the Earth’s early atmosphere. This mixture was modelled
on Jupiter’s present atmosphere, and included methane, ammonia and hydrogen.
The spark was intended to simulate lightning.

Miller’s experiments produced a cocktail of amino acids and other molecules
found in living cells. According to Miller and his colleague Harold Urey,
these substances would have been washed into the Earth’s early oceans, where
they formed into the first living cells.

Miller and Urey later suggested that ultraviolet radiation from the
Sun would be just as effective in welding atmospheric gases into organic
molecules. This idea gained support in 1980, when NASA’s space probe Voyager
1 found that Saturn’s moon Titan has a thick layer of orange clouds, undoubtedly
made of organic compounds formed from the simple molecules in its atmosphere.

Another theory was that the blast from a meteorite hitting the Earth
synthesised organic molecules in the atmosphere.

The Cornell scientists say that if the Earth’s atmosphere had the same
composition as the gases in Miller’s laboratory flask, ultraviolet radiation
would have been by far the most important agent in making organic molecules
from the atmosphere. It would have produced about 200 million tonnes of
organic material per year. The blast from falling meteorites would have
created another 20 million tonnes annually from the atmosphere, and lightning
some 3 million tonnes.

But scientists studying the Earth’s history now believe that the Earth’s
atmosphere was never like that of Jupiter. While Jupiter’s powerful gravity
drew in hydrogen and other gaseous molecules from space, the Earth’s early
atmosphere consisted of gases belched out by volcanoes. Studies of the oldest
rocks, theories of our planet’s formation and analysis of gases from volcanoes
all show that the original atmosphere was made mainly of water vapour and
carbon dioxide.

In such an atmosphere, the production of all organic molecules is much
smaller. Chyba and Sagan have computed the results for an atmosphere that
has 10 times as much carbon dioxide as hydrogen and its compounds. They
found that little organic material was produced: ultraviolet radiation produced
only 300 000 tonnes a year, lightning 30 000 tonnes and meteorite shocks
less that 1 tonne. They conclude that the 60 000 tonnes of organic material
that fell to Earth from space each year made up about 15 per cent of the
Earth’s total budget of organic material.