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 and a significant amount of the organic molecules used
in the machinery of life. The comet, designated C/1999 S4 LINEAR
(LINEAR), is the first comet observed to have a composition that
would allow it to carry the same type of water as found in Earth’s
oceans.
“The idea that comets seeded life on Earth with water and essential
molecular building blocks is dramatic, 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, approximately 100 feet (almost 31 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 as the comet
fragmented.
Mumma and a team of astronomers using telescopes sensitive to
infrared light studied comet LINEAR before its dramatic breakup last
August and determined that its unusual chemistry points to an origin
around 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 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 (around 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, indicates that it formed in warmer
regions closer to the Sun. For example, it has 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 predicts
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 nether regions of the solar system,
trillions of miles from the Sun. According to theories of the solar
system’s formation, comets formed from the same gas and dust cloud
that gave rise to the planets, but comets came together in the 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.
Jupiter’s massive 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. Because this region
was closer to the Sun, 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 a boulder,” 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.
For more information and pictures, refer to:
http://www.gsfc.nasa.gov/GSFC/SpaceSci/origins/linearwater/linearwater.htm
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