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Stardust's seven year mission is to collect samples of the interstellar dust …
Seeding the Earth
|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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