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This is a picture of a Brownlee particle, which may be what a comet is made of.
Click on image for full size
JPL

Brownlee Particles

This example of Interstellar Dust is a perfect example of the kind of rocky material comet may be made of. The grains themselves seem to be made of smaller grains. There are many holes, or pores. In a proper comet, these holes would be filled with ice.

This example of Interstellar Dust is called a Brownlee Particle after D. E. Brownlee. Dr. Brownlee first detected these particles in the Earth's atmosphere and speculated that they may be the left over remnants of comets which passed through the Earth's orbit. He postulated this theory because of the unusual construction of these rocky particles.

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