What are comets and where did they come from?

    What are comets made of?

    The first modern description of a comet’s nucleus was proposed by Fred Whipple in the 1950s. In his model, the comet was a more or less homogeneous object, which he called an “ice conglomerate”, consisting of various ices mixed with dust. The press called it the “dirty snowball” premise. While this could explain the activity of comets, it does not fit with our emerging understanding of comets as bodies nearly devoid of inner force that sometimes disintegrate on their own when only warmed by the Sun.

    In 1986, Paul Wiseman suggested that comets were instead “piles of rubble” of smaller, loosely interconnected bodies. These pieces of debris had to collide at very low speeds and be in fairly similar orbits to avoid destroying each other.

    Modern models of comet formation are based on one of two ideas. The first is that when the Sun was still forming, an instability developed in the surrounding disk of matter. This instability may be the result of a variety of processes, but the end result is pockets of high density where groups of small bodies are attracted by the force of gravity to each other and unite.

    The second is radial drift, which occurs when the particles formed grow to meters in diameter. At this point, the clouds cause them to drift slowly inward toward the sun, attaching themselves to other small bodies as they go.

    Both of these mechanisms can form comet nuclei from a rubble heap with the characteristics we observe, including a low overall density and constituent particles that have almost no affinity among themselves. However, the properties of the components depend on assumptions about the conditions under which they were formed. Some models predict a nucleus consisting of a uniform mixture of smaller particles of similar sizes, while others suggest a mixture of sizes ranging from 1 to hundreds of meters in diameter. Another model predicts that comet nuclei form when high-velocity impactors collide, compressing each other to form an inner layer rather than a rubble mound.

    Unfortunately, it is difficult to tell the difference between different formation models even when we visit a comet up close. However, we are sometimes presented with clues that we try to interpret. In 1992, Comet Shoemaker-Levy 9 had a close encounter with Jupiter as tidal forces tore it into 21 smaller fragments, which famously impacted Jupiter two years later. But in between, we were able to watch fragments scattered along the comet’s orbit. The sizes of the individual segments varied greatly, with estimated diameters ranging from 330 feet (100 m) to 2.5 miles (4 km). These sizes may be a clue to the comet’s original internal structure. However, it is also likely that they resulted from a different process related to tidal disruption.

    Subsequent missions to comets Hartley 2 (103P/Hartley) and 67P/Churyumov-Gerasimenko may have revealed evidence of a different formation process. The crowded halo of debris surrounding 103P contained objects up to 1 foot (30 cm) in diameter; Some researchers have suggested that these represent the small pebbles from which the comet’s nucleus aggregates. Rosetta found a similar case at Churyumov-Gerasimenko, where meter-sized “goose bumps” features stacked along the walls of craters eroded at the surface are examples of the primitive bricks that make up the comet. Also at Churyumov-Gerasimenko, observers mapped what appeared to be a series of layers on the surface that were subjected to the comet’s evolution, leading some to suggest that they were evidence of its formation through compressive effects.

    In the end, the only definitive way to understand a comet’s interior is to measure it directly. Penetration aside, the best way we have to do this is to map the core with radar. We’ve come very close to doing this at Churyumov-Gerasimenko. The orbiting Rosetta spacecraft had a detachable lander, Philae, that was supposed to serve as half of the dipole radar mapping experiment. Unfortunately, that opportunity was ruined when Philae rested on the roof in a shaded area where she was unable to recharge her batteries. Only one measurement was made before the probe lost power.

    Despite this, we did not give up. Scientists continue to develop new radar experiments that we hope will one day fly to another comet.


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