Figure
2. Traditional views of comet formation hold that materials in the
outer parts of the solar nebula aggregate into larger cometary components
in a “one-way trip” as the material spirals in on its orbit around
the protosun. The process begins with a grain of interstellar dust
(a)—made of an amorphous silicate core and a “varnish” of
organic molecules—which is present in the molecular cloud that collapses
to form the solar nebula. Many of these dust grains acquire a coating
of ice (b). Once inside the solar nebula, they form larger,
fractal-like dust particles (c) when the icy grains collide
and stick together. Aggregation of the particles eventually produces
cometesimals of ever-increasing size (d) and (e) until
a comet-sized object forms (f). Ultimately, the materials
in a comet are collected from a wide swath of its orbital radius.
Recent research suggests that traditional models may be too simple.