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Seeding the Earth
- The Molecules that Fell to Earth

Further away from the sun (more than 5AU), the environment was not so harsh and the icy mantles would not have sublimed away. These larger particles formed agglomerations larger and faster than the grains nearer to the sun. Thus, the snowball-like cores of the 'gas giants' Jupiter, Saturn, Uranus and Neptune formed more quickly than the masses of the rocky planets. When the cores became sufficiently large to capture the gas in the accretion disc, the gas giants were formed. An offshoot of this process was the development of a huge number of smaller, snowball- like planetesimals - the comets - which came to inhabit the Kuiper belt (orbiting just beyond Neptune) and the Oort cloud (a spherical collection of comets that extends to about 50000AU from the sun).

Because the accretion process would have been much less severe at the distances where comets formed, scientists hope that any complex organic molecules may have been able to survive incorporation into a comet intact. Tantalising glimpses have been obtained by studying passing comets, such as Halley and, more recently, Hale-Bopp, using radio and IR telescopes.

Almost all the parent molecules detected in dense molecular clouds have been identified in these comets, including carbon dioxide, water and carbon monoxide. However, it is the spectra for methanol and cyanide (-CN) detected in comets that are most important in terms of complex organic molecules. Methanol is a precursor for many of the biologically-relevant molecules thought to reside in the interstellar clouds, while the CN could well be related to the XCN feature detected in dense molecular clouds. Experiments have already shown that the ultraviolet photolysis of HMT frozen in H2O ice produces this XCN band.

The evidence so far does support, albeit tentatively, the idea that comets may contain complex, biologically-relevant molecules, like HMT. But scientists still don't know this for sure, and cannot prove it, until they have a bit of comet to study in the laboratory. HMT would be a very interesting molecule to discover in a comet: a whole host of biogenically interesting species are produced along its synthesis pathway and by its photolysis, hydrolysis and thermolysis (see Fig 3).

Fig 3. The proposed hexamethylenetetramine (HMT) interstellar ice photochemical synthesis route and decomposition products
(Source: Scott Sandford)

But assuming that comets do contain biologically-interesting molecules, there are two ways in which these molecules could arrive on Earth. The first, and most dramatic, way is by crashing into it.

At the dawn of the solar system, comets are thought to have formed in the region of the gas giants, but over time their orbits are believed to have been perturbed by the gas giants such that they either left the solar system altogether, became part of the Kuiper belt or Oort cloud, or were sent careering into the inner solar system, occasionally colliding with one of the forming planets. For the accreting Earth this period of heavy bombardment, as it is known, would have lasted for the first 1000m years of its existence (roughly 4500m-3500m years ago).

Most scientists now accept that it was this cometary bombardment that gave the Earth its atmosphere and water. What is more contentious is the suggestion that it also provided the organic material needed to kick- start life on Earth.

The alternative, and maybe ancillary, way for cometary material to reach the Earth is in the form of cometary dust. As comets come closer to the sun, ice sublimes away, forming the distinctive tail. This process leaves a trail of dust in the comet's wake. If the Earth's orbit then passes through that dust, particles may reach the Earth's surface.

These dust particles are so small that they are unlikely to be heated too much by collision with the Earth's atmosphere - indeed, samples of interplanetary dust (from both asteroids and comets) have been collected from the upper atmosphere and shown to contain PAHs.

PAHs, although biologically-interesting, are only a small part of the story; a lot more than one species of organic molecule would be needed to initiate life. Thus comets, potentially containing a whole host of 'prebiotic' molecular species, could be a major part of the story. But to determine how much of part would require a comet sample. As Sandford says, 'if you want to really understand in detail the composition of something, and how things are in relation to each other, nothing beats having a sample in hand.' Getting hold of a sample is what Nasa hopes Stardust will do.

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