What Is a Comet Nucleus? A Few Kilometres of Very Dark Ice

The nucleus is a mixture of ices, dust and rocky material, and the old description of a dirty snowball has been revised towards something closer to an icy dirtball, since the dust fraction is higher than early models assumed. The ices include water, carbon dioxide, carbon monoxide, methane and ammonia, with more volatile components driving activity further from the sun and water dominating closer in. The density is remarkably low, generally well under that of water, which means the interior is extremely porous with a large fraction of empty space, and the material is mechanically weak. The surface is among the darkest known, reflecting only a few percent of incoming light, which was surprising for an object made largely of ice and is explained by a layer of organic-rich dust left behind as ice sublimates away. That dark surface absorbs sunlight efficiently and reaches temperatures well above the ice beneath it, which is part of why activity is concentrated in particular spots rather than occurring uniformly.

Several spacecraft have reached nuclei and each returned unexpected results:

• •Giotto at Halley in 1986, which returned the first images of a nucleus and showed a very dark irregular body with jets issuing from discrete active areas rather than the whole surface
• •Deep Impact in 2005, which fired a projectile into a nucleus and analysed the ejected material, finding a fine weakly bound surface layer
• •Stardust, which flew through a coma and returned dust samples to Earth, finding minerals that form at very high temperature and therefore imply mixing from the inner solar system outward
• •Rosetta, which orbited a comet for two years from 2014 and landed a probe on it, watching activity increase and decline through the approach to the sun
• •The two-lobed shape of Rosetta's target, joined at a narrow neck, which is now known to be common and is interpreted as a gentle merger of two bodies early in the solar system
• •Measurement of the hydrogen isotope ratio in the comet's water, which differed from Earth's oceans and weakened the case that comets delivered most of the planet's water

Approaching the sun, ice near the surface sublimates directly from solid to gas, and the escaping gas carries dust with it, producing the coma and the tails. The process is not uniform. Activity concentrates at specific regions and produces collimated jets, apparently where the insulating dust layer is thin or where pits and cliffs expose fresh ice. It varies with the rotation of the nucleus, so a given region switches on when it turns into sunlight and off when it turns away. It varies with distance, with different ices driving activity at different points in the orbit. Outbursts occur, some associated with cliff collapses observed directly, which expose fresh material abruptly. Each passage removes a layer of material, so a comet has a finite active life measured in hundreds or thousands of passages, after which it either exhausts its volatiles and becomes a dark inert object or breaks apart, and both outcomes have been observed.

These bodies are of scientific interest disproportionate to their size because their material has stayed cold since the solar system formed. Planetary material has been heated, melted, differentiated and reprocessed, destroying the original composition, while a small body far from the sun preserves it, which makes a nucleus a sample of the starting material. Their organic content is particularly interesting, since amino acids and other prebiotic molecules have been detected, which bears on how such material could have reached the early Earth. Their isotopic composition constrains where they formed and how much mixing occurred in the early disc. Their structure records how small bodies accumulated, with the merged two-lobed shapes indicating gentle low-speed collisions. And they pose a practical question, since a body of this kind on an intersecting orbit is a genuine if remote hazard, and understanding the strength and structure of a nucleus determines whether any deflection method would work on one.

The solid body is a few kilometres of porous ice and dust reflecting only a few percent of light, darkened by organic-rich residue left as ice escapes. Activity comes from discrete spots that switch on as the nucleus rotates into sunlight rather than from the whole surface. Rosetta found a two-lobed body formed by a gentle merger, and its water isotopes did not match Earth's oceans.