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A
DYING COMET'S KIN MAY HAVE NOURISHED LIFE ON EARTH
A
comet that shattered on its approach to the Sun breathed new
life into the theory that comet impacts provided most of the
water in Earth's oceans. The same NASA observations of the
comet, designated C/1999 S4 LINEAR (LINEAR), also support
the idea that comet impacts furnished a significant amount
of the organic molecules used in life that later arose on
Earth.
LINEAR
was the first comet with a chemistry that indicated its water
had the same isotopic composition as the water actually found
on Earth.
"The
idea that comets seeded life on Earth with water and essential
molecular building blocks is hotly debated, and for the first
time, we have seen a comet with the right composition to do
the job," said Dr. Michael Mumma of NASA's Goddard Space Flight
Center. Mumma is lead author of a paper about this research
to appear in the May 18 issue of Science.
A
separate announcement, also to appear in the May 18 Science,
is a unique observation that reveals just how much water comets
of this type can carry. LINEAR, with a nucleus estimated at
2,500 to 3,300 feet (about 750 to 1,000 meters) in diameter,
carried about 3.6 million tons (3.3 billion kilograms) of
water within its bulk, according to astronomers who used the
Solar Wind Anisotropies instrument on the Solar and Heliospheric
Observatory spacecraft to observe water vapor released from
the comet as it fragmented.
Using
telescopes sensitive to infrared light, Mumma and a team of
astronomers studied comet LINEAR before its dramatic breakup
last July and determined that its unusual chemistry points
to an origin near Jupiter's orbit. Comets that formed in this
region are expected to have the same ratio of normal water
to "heavy" water as found in Earth's oceans.
Although
it would appear that all water molecules are identical --
two atoms of hydrogen joined to one oxygen atom -- this isn't
the case. Hydrogen comes in different types (isotopes) that
behave the same way chemically but are heavier due to an extra
component (one or more neutrons) in their nuclei. One such
heavy cousin of hydrogen is called deuterium (one extra neutron).
Based on very low-temperature experiments of gas chemical
reactions, water ice incorporated in comets that formed far
from the Sun (near Neptune's orbit, for example) should have
a greater deuterium to hydrogen (D to H) ratio than the water
found on Earth.
Recent
observations of comets Halley, Hyakutake, and Hale-Bopp confirm
this, leading researchers to believe that these comets formed
further from the Sun than LINEAR. Pinpointing the origin of
these comets was remarkable, but it provided no support for
the cometary origin of water on Earth.
The
chemistry of LINEAR, however, indicated that it formed in
warmer regions closer to the Sun. For example, it had much
less carbon monoxide (CO), methane (CH4), ethane (C2H6), and
acetylene (C2H2) than typical remote-origin comets like Halley.
These volatile organic molecules freeze at extremely cold
temperatures, so it appears that LINEAR formed in a place
where it was too warm to incorporate a great deal of these
volatile molecules into its ices.
However,
the same low-temperature experiments that successfully predicted
the correct D to H ratio in remote-origin comets predict that
a comet forming in a warmer Jupiter orbit region should have
the same D to H ratio as Earth's water. LINEAR broke up before
this could be confirmed, but its low amount of volatile organic
molecules provides a strong indication that it carried the
same kind of water that comprises terrestrial seas.
LINEAR
is believed to have arrived from the Oort cloud, a vast comet
swarm surrounding the frigid distant regions of the solar
system, trillions of miles from the Sun. According to theories
of the solar system's formation, these comets formed from
the same gas and dust cloud that gave rise to the planets
and the Sun. They accumulated in the colder regions where
the gas giant planets are found today (Jupiter - Neptune).
Gravity from the gas giants kicked the comets out of the solar
system, either to interstellar space or to the Oort cloud
region. Occasionally, the Oort cloud is perturbed, perhaps
by the gravity of a passing star, returning some comets to
the inner solar system. The amount of various molecules incorporated
into a comet's ices depends on temperature, so determining
a comet's chemistry reveals where in the gas giant region
the comet formed.
As
the most massive planet in the solar system, Jupiter's gravity
was so powerful that it shoved most comets near it into interstellar
space, while the lesser gravity from the smaller gas giants
gave comets near them a gentler push, landing a greater portion
in the Oort cloud.
Consequently,
comets that formed near Jupiter are rare today, but they would
have been in the majority during the solar system's formation,
simply because the Jupiter orbit region had most of the material
in the pre-planetary gas and dust cloud. Therefore, scientists
expect that the primordial Earth would have intercepted more
comets formed near Jupiter's region than those formed elsewhere.
Because
Jupiter's region was closer to the Sun than the other gas
giant planets, it received more light and was warmer, so more
reactions occurred in the gas. Thus, greater amounts of complex
organic molecules were available to wind up in a comet. Also,
Jupiter's powerful gravity kept collision speeds between comets
near it high, preventing them from growing very large. Both
factors may have given a boost to life on Earth.
"It's
like being hit by a snowball instead of an iceberg," said
Mumma. "The smaller comets from Jupiter's region impacted
Earth relatively gently, shattering high in the atmosphere
and delivering most of their organic molecules intact. Also,
these comets would have had a greater portion of life's building
blocks -- the complex organic molecules -- to begin with.
This means life on Earth did not have to start completely
from scratch. Instead, it was delivered in kit form from space."
The
team used infrared-sensitive instruments on telescopes at
the W. M. Keck Observatory and the NASA Infrared Telescope
Facility, both on Mauna Kea, Hawaii, to make the observations.
Heat and light from the Sun caused material from LINEAR to
evaporate into space and form a gas cloud around the comet
as it entered the solar system. Sunlight energized molecules
in the gas cloud surrounding LINEAR, allowing the team to
identify the comet's chemistry by the unique types of infrared
light emitted by its various molecular components. Comet LINEAR
was named for the observatory that first spotted it, the Lincoln
Near Earth Asteroid Research (LINEAR) program.
MEDIA
CONTACT LIST
Dolores
Beasley/Space Science PIO/NASA Headquarters
(202)
358 1753
Nancy Neal/SEU PIO/Goddard Space Flight Center
(301) 286 0039
Bill
Steigerwald/Technical Writer/Goddard Space Flight Center
(301) 286 5017
Wade Sisler/Executive Video Producer/Goddard Space Flight
Center
(301) 286 6256
Rachel Weintraub/Associate Video Producer/Goddard Space
Flight Center
(301) 286 0918
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