Why are comet nuclei thought to be porous rubble piles?

Comet nuclei are widely considered to be porous "rubble piles" or "fluffy aggregates" due to the conditions under which they formed in the solar nebula 4.56 billion years ago. As icy planetesimals collided at low velocities, they lacked the energy required to compress or melt into a single solid body, resulting in structures with very high porosity—estimated at 60 percent or more.1

Key evidence supporting this model includes:

• Low Bulk Density: Estimates of nucleus bulk density typically range from 0.2 to 1.0 gram per cubic centimetre, with preferred values between 0.3 and 0.6 gram per cubic centimetre, indicating a highly porous internal structure.1
• Disruption Observations: Comets are frequently observed breaking apart, either randomly or due to gravitational tidal forces when passing near large planets like Jupiter or the Sun. The behavior of Comet Shoemaker-Levy 9, which fragmented into a string of smaller "cometesimals" upon its close approach to Jupiter, provides strong evidence for a weakly bound, aggregate composition.1