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.
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