For
millennia, comets were believed to be omens of doom. Instead, solving the
mysteries regarding these "dirty snowballs" could help reveal the
part they played in the birth of life on Earth, as well as secrets concerning
the rest of the galaxy.
Did
comets help create Earth's seas?
For years
scientists thought comets slamming against the newborn Earth helped
deliver water to a once dry planet. But roughly a decade ago this view was
shaken by the discovery that the water in comets and Earth's oceans did not
match up in terms of hydrogen isotopes.
Calculations
then showed it was highly improbable that enough icy rocks from the suspected
homes of comets — the Kuiper belt past Neptune and the Oort cloud past that — could
have collided with Earth to supply its oceans.
In the last
two years, however, researchers have discovered comets in the outer part of the
asteroid belt. These "main-belt
comets" may have the right levels of hydrogen isotopes, and are
perhaps close enough to Earth to have realistically brought us the seas that
life emerged from.
"No one
knows for certain yet where Earth's oceans came from," said University of Hawaii astrophysicist David Jewitt. "Earth's oceans are likely a mixture
of water from all sorts of places, but the main-belt comets are very likely one
of them."
Where do
comets come from?
The
suspected homes of comets include the Oort cloud, the Kuiper belt and now the
asteroid belt. But are there more reservoirs
of comets yet to be found?
The Oort
cloud is a theoretical cloud of icy rocks roughly 4.6 trillion miles (7.5
trillion kilometers) from the sun thought to be the source of long-period
comets — that is, ones that take more than a few centuries to complete their
orbits. It was once thought the original home of short-period comets as well,
until calculations suggested that was impossible.
About 20
years ago, the Kuiper belt roughly 4.6 billion miles (7.5 billion kilometers)
from the sun was then proposed to be the home of short-period comets. "But
measurements taken in the last few years raise some doubts about that,"
Jewitt explained. "Maybe there are other reservoirs of comets yet to be
discovered."
Secrets
regarding the birth of the solar system?
Comets were
long thought to be primordial relics, pristine leftovers from the
protoplanetary disk that once surrounded the newborn sun. As such, it was
supposed they might hold secrets untouched for billions of years regarding the
birth of our solar system.
Increasingly,
however, it looks as if the comets we see are anything but unspoiled. Instead,
"there is good evidence that many of them are nearly burned-out
hulks, with neither the size, mass, shape nor spin they might have had
before entering the solar system," Jewitt said.
Still,
"since comets are icy, they're not entirely cooked, and we may learn a lot
regarding the formation of the solar system from chemicals trapped in their
ice," he added.
Comets
so close to the sun?
The
main-belt comets are themselves
a mystery. Until their discovery, researchers had largely supposed no
comets could have lasted that close to the sun without getting baked away after
a few centuries or millennia.
Dirt
coatings on main-belt comets could have protected them from sunlight for
billions of years. Every now and again boulders a yard or larger tumbling around
the asteroid belt might hit these comets, uncovering their ice and triggering
the plumes of gas and dust that got them discovered in the first place.
"We
expect to soon find many hundreds or thousands of main-belt comets,"
Jewitt said.
Interstellar
comets?
As our
solar system formed, calculations predict the gravitational pull of the planets
would have scattered 90 to 99 percent of all comets that once orbited the sun
away toward the stars, never to be seen again. "If every star does that,
you would expect some of their comets to come toward us, but no such object has
ever been seen," Jewitt said.
Still, as
astronomical telescopes and techniques improve, Jewitt remains optimistic that
such interstellar comets will be detected fairly soon. These comets would prove
quite distinctive, zipping at great speeds and following trajectories
completely unlike the orbits our comets follow.
"We
could see interstellar comets for the first time in the next few years,"
Jewitt predicted. "It would be great if we saw one, especially so if we
had the wherewithal to launch a mission to one, to get samples and study the
diversity of comets in an interstellar and galactic context. But we have to
find one first."
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