When the Rosetta spacecraft entered orbit around comet 67P/Churyumov-Gerasimenko, first images captured deep circular depressions among the other surface features. Now that the orbiter has taken significant observations of the comet’s surface, the science team has concluded that these depressions are actually sinkholes that form by the same processes that form sinkholes 500 Million kilometers away here on Earth.
Sinkholes on Earth form when subsurface material is eroded away. Though not initially visible on the surface, the material falls deep underground leaving a circular cavern. Once enough material has eroded away, the top of the cavern collapses, leaving a large depression in the surface.
The scientists behind the study noticed that there are two distinct types of pits on the surface, deep ones with steep sides and shallow ones similar to depressions observed on other comets. The team also noticed that the deep pits seemed to be outgassing material, a phenomenon not observed in the case of the shallower depressions.
One of the hypotheses for formation if the pits was that explosive outbursts of material seen by Rosetta could be the outward rupturing of a depression. The issue is that when the amount of material released in the outburst was quantified, it didn’t add up to the amount of material that would ‘fill the hole’ so to speak.
Instead, astronomers suggest that the pits are the result of heating of subsurface ice. The ice sublimates (changing state from a solid to a gas) and leaves gaps beneath the surface, which collapse to form the deep depressions.
As we have slowly gained a scientific understanding of the behavior of comets and specifically their nuclei, we have seen that they are far more than simple dirty snowballs in the sky. They are complex geological structures with life cycles governed by the same Sun that governs our own on Earth.