On 25 July 2000 astronomers on Earth noticed that the central condensation of the comet, now known as Comet C/1999 S4 Linear (aka Comet LINEAR) was becoming very elongated compared with the roughly oval form one would expect to see. Over the coming nights, the comet's brightness continued to fade and its central region's length increased. It soon became obvious that the comet had been blown apart into a swarm of mini-comet fragments.

This slow motion explosion was watched by a variety of Earth-based telescopes such as the European Southern Observatory's Very Large Telescope in Chile, the W. M. Keck Observatory and the NASA Infrared Telescope Facility, both on Mauna Kea, Hawaii as well as observatories in space (the Hubble Space Telescope, the Chandra X-Ray Observatory, and SOHO, the Solar and Heliospheric Observatory).

This comet's death throes are now providing insights into cometary composition and the role its ingredients may have played in the origin of Earth's oceans, and perhaps, the origin of life on Earth. Results of international research efforts will be published in this week's editions of both Science and Nature magazine.

Comet LINEAR was discovered on 27 September 1999 by the Lincoln Laboratory Near-Earth Asteroid Research project (LINEAR) when it was roughly the same distance from the Sun as Jupiter. This program uses a robotic telescope to search comets and Near Earth Asteroids (NEAs) whose orbits bring them close to Earth. Based on observations as it approached the sun, it was clear that this was the comet's first approach to the inner solar system. Comet LINEAR eventually passed 114 million kilometers from the Sun on 26 July 2000 and 56 million kilometers from Earth on 22 July 2000.

Indications of the comet's imminent demise began months before the final blatant fragmentation seen by large Earth - and space-based telescopes. Observations made by SOHO Over time it became clear that the comet was rather flimsy and its exterior was more or less of the same composition as its interior.

Comets are thought to have formed in the region of the solar system now dominated by the gas giants Jupiter, Saturn, Uranus, and Neptune. Over time, these small bodies were flung into the outer regions of the solar system (or beyond) via repeated gravitational perturbation by the giant planets.

Cometary composition is a function of what portion of the solar system wherein they originally formed. Comets that formed near the smaller gas giants Saturn, Neptune, and Uranus managed to remain under the sun's influence albeit in the distant Oort cloud. However, most of the comets that formed nearest to Jupiter are thought to have been thrown clear out of the solar system into interstellar space by the planet's massive gravitational presence. As such, the population of this group of comets, while not zero, is rather rare compared to those that formed further out.

An analysis of Comet LINEAR indicates that it formed in warmer regions closer to the Sun. Compared to comets such as Comet Halley which formed further out, Comet LINEAR has much less carbon monoxide (CO), methane (CH₄), ethane (C₂H₆), and acetylene (C₂H₂). These volatile organic molecules freeze at extremely cold temperatures such as are found in the outer solar system. Given the small amounts of these volatiles, it would seem that Comet LINEAR formed in a location closer in to the sun where it was too warm to incorporate large amounts these volatile molecules.

According to estimates being announced by NASA, the nucleus of Comet LINEAR was approximately 100 feet (31 meters) in diameter and carried about 3.6 million tons (3.3 billion kilograms) of water. These estimates were made from data obtained from SOHO's Solar Wind Anisotropies Instrument (SWAN) as it observed the release of water vapor as the comet fragmented.

NASA scientists state that "the same low-temperature experiments that successfully predicted the correct Deuterium to Hydrogen 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. Comet 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."

The significance of cometary composition? Many planetary scientists and astronomers think that a large portion - perhaps the majority - of the water on Earth (and at one time on the other terrestrial planets Venus and Mars) was delivered by cometary impacts as these planets formed. Earth (and to a much lesser extent Mars) managed to retain its water while Venus (closest to the sun) lost most of it over time. Compared to the violent days of the early solar system, the impact rate for comets and asteroids we see today is rather sedate - although planetary impacts still occur - Comet Shoemaker Levy's impact with Jupiter a few years ago being one spectacular example.

Cometary surfaces - as well as those on a number of outer solar system bodies - have been shown to be strewn with simple and complex organic compounds as well as an abundance of water. Moreover, over the life of the solar system repeated heating and interaction with solar radiation can cause some rather complex organic chemistry to occur on - and within comets. Studies of comet impacts and the impacts of meteorites (which may be either fragments of comets or formed in similar regions of the solar system) show that many of these organic compounds can survive a fiery impact with a planet such as Earth without being torn apart by the violence of impact.

Theoretically, over time, the impact of these bodies could be responsible not only for Earth's inventory of water, but perhaps a substantial portion of its initial supply organic compounds as well. Many of the chemical precursors from which life is speculated to have arisen on Earth are found in comets and presumably could have been delivered over the course of hundreds of millions of years.

According to NASA: "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."

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