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A small comet will often be referred to as ‘a snowball in space’, a name that is given because of the composition of these comets. In contrast to the larger, more well-known comets, such as Hale-Bopp and Halley’s that posses a bright nucleus and tail, produced by the reflection of sunlight off the dust and gases that are released, their smaller cousins don't show these characteristic comet features. The existence and influence of these small comets is still a much debated issue.
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Origin and Fate
A snowball in space is thought to be formed in the Kuiper belt, an inner belt of cometary material beginning just beyond the orbit of Neptune. The small comets mostly weigh between 20 and 40 tons, although some might be smaller or larger.
The proposed structure of these snowballs is, just as their name suggests, snow, quite possibly surrounded by a carbon shell that is responsible for their structural integrity. As they near the Earth’s atmosphere, they tend to break into pieces at an altitude of about 800 miles due to electrostatic stress. When these pieces have descended to 600 miles, they will have been turned to their gaseous form by the Sun's rays. This vaporization gives rise to clouds of water vapor that will be further dispersed by winds.
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A Source of Water?
The proponents of the ‘small comet hypothesis’ suggest that these small objects are responsible for much of the water on earth. Recently, the amount of deuterium has been measured in large comets, and based on the results, it was concluded that they could not have been a source of water on earth. Smaller comets may, however, differ in composition from their larger relatives, meaning that they could still be candidates for the source of water on our planet.
It is even speculated that these comets could contain organic material. If so, they might be adequate vehicles for carrying such material though the atmosphere, since their icy inside might be able to protect the organic material long enough to get through the atmosphere.
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Some scientists believe that the observations of ‘a snowball in space’ are caused by instrument artifacts. However, new results from NASA’s Polar satellite seem to confirm the original images from over a decade ago. On these images, holes in the atmosphere can be noticed. The proponents of the 'small comet hypothesis' claim these are caused by small comets entering the atmosphere. Supporting their claim is the fact that these holes can be noticed in a sequence of frames. Furthermore the current computer technology can compensate for instrumental artifacts, so, as time passes and technology improves, it seems that the small comet hypothesis receives additional support from the available evidence.
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- Frank, L.A. & Sigwarth, J.B. (1993). Atmospheric Holes and Small Comets. Reviews of Geophysics. 31 (1), pp. 1 – 28.
- NASA: http://www.nasa.gov/worldbook/comet_worldbook.html
- The University of Iowa: Small Comets: http://smallcomets.physics.uiowa.edu/
- The University of Uppsala: Comets – Relics from the Birth of the Solar System: http://www.astro.uu.se/~bjorn/eng_comet.html