But researchers have long
wondered whether they were created by a localized gravitational collapse of
material that yielded a single object somewhat like a giant icy boulder, or
if they were built piece-by-piece as smaller building blocks stuck together
over time.
The way Comet LINEAR came
apart provides the first direct evidence for one latter scenario, confirming
a suspicion researchers had shortly after the breakup last summer.
Detailed study shows that
the comet was ready to come apart because it was composed of separate pieces
held together only loosely. Weaver's international team used the Hubble Space
Telescope and the Very Large Telescope to spot at least 16 chunks roughly 110
yards (100 meters) across.
Weaver thinks these pieces
represent "primordial building blocks" that became a comet as the result of
"a continual buildup of bodies of increasing size via mutual collisions and
sticking."
In one of the other studies,
the gases emitted by LINEAR were found to be nearly identical before and after
the breakup, indicating that the entire comet, inside and out, was formed at
roughly the same distance away from the Sun. This means the comet evolved to
maturity before it was tossed out to the Oort Cloud, a distant reservoir of
comets.
Like LINEAR, other comets
in the Oort Cloud are sometimes drawn into the inner solar system after billions
of years. Others that have been studied in detail were all found to be related
in how they were composed.
"But Comet LINEAR is an
exception," said Mumma, the NASA researcher.
Mumma and his colleagues
found four organic compounds, including carbon monoxide and methanol, to be
scarce in LINEAR. The lack of these compounds indicates that the comet formed
in a warmer region of the solar nebula.
"We think it formed much
closer to the planet Jupiter than the other [studied comets] did," Mumma said.
Because mighty Jupiter would
have had the gravitational capability to toss comets like LINEAR directly at
Earth, this new view supports an old notion that life on Earth was seeded by
comets.
Next
Page: Seeding
Life on Earth
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