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Comets: Ancient Relics of the Solar System

Optical Image of Comet

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What are Comets and Where do They Come From?

Within a sphere one light year across, and centered on the Sun, lies the Oort Cloud. Filled with rocky debris originating from the birth of the solar system. Making up the debris are thousands, perhaps millions of comets, icy chunks of rock and dust in all shapes and sizes, most spanning less than 300 kilometers (180 miles) in diameter.

Another belt of icy debris is thought to reside within the so-called Kuiper Belt, which unlike the Oort Cloud, lies in the plane of the solar system, 500 times further out than the distance between the Sun and Earth.

In contrast to the planets, comets orbit the Sun along highly elliptical paths. Some "short-period" comets take up to 200 years to orbit the Sun. Other "long-period" comets exhibit far longer orbital periods, lasting up to hundreds of thousands, even a million years.

Occasionally passing stars outside the Solar System tug at the icy objects, hurtling them inwards into orbit around the Sun and its cohort of planets. Only last year Comet Shoemaker-Levy slammed into the giant planet, Jupiter: a spectacle recorded in extraordinary detail thanks to the far-reaching gaze of the Hubble Space Telescope.

What are Comets Made of?

Dubbed "dirty snowballs" by some astronomers, comets are composed of dust grains, chunks of dirt, and ice. Apart from water, cometary ice also contains carbon dioxide, ammonia, and methane. While residing in the Oort Cloud and Kuiper Belt, much of the ice remains frozen, preserving the dust trapped inside since the birth of the solar system. Such pristine material can reveal much about the interstellar matter out of which the Sun and planets condensed.

As a comet nears the Sun, its icy layers begin to evaporate, releasing dust and dirt to form a curved dust tail. The liberated gases glow as they become heated, producing the coma, a luminous envelope surrounding the solid core. Ionized by the Sun's radiation and pushed backwards by the solar wind, the heated gas trails behind and also glows, forming a second, straight tail.

Comets and the Origins of the Solar System

Current theory holds that our sun formed when a giant molecular cloud collapsed under its own gravity. Some material was pulled by the nascent Sun into orbit around it, eventually coalescing into planetesimals that included comets, then into the planets and asteroids.

When approaching closer to the Sun, a comet's nucleus heats up. As gas and dust molecules previously bound inside the nucleus are released into the coma, they emit millimeter radiation. Much of what astronomers learn about the composition of comets will come from spectral line studies rather than through images. In order to map the distribution of gas and dust molecules in comets, researchers are now beginning to use the BIMA array's high spectroscopic resolutioncapabilities.

Lew Snyder, University of Illinois, on-camera
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Knowing the distribution of gas and dust within comets might also indicate how these same materials were distributed the region of space in which they formed. In turn, this information could yield clues on the composition of the giant gas cloud from which the solar system is thought to have condensed some 4.5 billion years ago.
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Comet and the Origin of Life on Earth

Of particular interest to researchers studying comets are the carbon-rich compounds they contain. Detailed measurements of the molecular make-up of comets could indicate what types of chemical reactions were responsible when they formed. The results might provide clues to the evolution of carbon-based chemistry on Earth early in its history. Some scientists believe that comets bombarded the young Earth, perhaps seeding it not only with the water, but also with organic compounds that would later evolve into living matter.

Lew Snyder, on-camera
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QuickTime Movie (1.5 MB); Sound File (899K); Text

Lew Snyder, on-camera continued
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NCSA.Last modified 11/11/95