Far from the sun: the quiet nucleus
Far from the sun, at distances greater than 3 AU (three times the mean distance from Earth to the sun), a comet’s nucleus is relatively cool, “quiet,” and “bare,” meaning that it hasn’t developed a visible coma . It is still characterized as a dusty snowball hurtling through space, with no visible coma or atmosphere surrounding it, and no ion or dust tails visible either. It is, however, directly exposed to the solar wind , so we know that something must be happening on the surface. The sun’s UV light is probably shaking loose molecules from the nucleus surface that will interact with each other and the UV light. There may be low-level activity in the nucleus, creating a small coma that isn’t visible from great distances. That’s why Rosetta is designed to join comet C-G at aphelion, or its furthest position from the sun. Way out there, far from the sun, Rosetta’s ALICE instrument will map the nucleus for pockets of both dust and ice. MIRO and ROSINA will look for water coming off the nucleus. MIRO will do it remotely, and ROSINA will wait for particles to actually hit its detectors. IES will explore any interaction of the solar wind and the nucleus.
Approaching the sun: The active nucleus and emergence of the coma
As the comet approaches to within 1.5 AU of the sun, solar radiation begins to heat the nucleus. As the nucleus warms up, it becomes “active.” The ices in the nucleus, which are mostly frozen water but also frozen volatile gases, begin to sublimate, meaning that they pass instantly from ice to gas without becoming a liquid. They boil off the surface, and in some cases jet from within the surface, creating a coma or atmosphere around the nucleus. As the coma grows, it begins to provide a parabolic shield against the solar wind.
Closest to the sun: full development of the coma, bow shock, and the ion and dust tails
As the comet moves toward perihelion, its closest position to the sun, its coma expands rapidly and begins to deflect the solar wind and the interplanetary magnetic field so that it wraps around the comet. This clash between the solar wind and the coma is an extremely complex physical and chemical process. The closer the comet comes to the sun, the more intense this process becomes.
A myriad of chemical and physical processes occur simultaneously. The mostly neutral molecules emerging from the nucleus are blasted by UV light and solar radiation, causing them to fragment and recombine into new chemical species, become ionized (charged negatively or positively), or burst out of the region as x-rays. Meanwhile the incoming ions from the solar wind begins to “pick up” or collect the slow-moving species from the coma, causing additional chemical and physical reactions. All of this material erupting from the comet loads mass to the solar wind, slowing it down.
When the pressure of the comet's coma becomes about the same as that of the solar wind, boundaries and regions begin to form in space around the comet. The solar wind and interplanetary magnetic field lines confronting the comet begin to pick up particles expelled from the comet. This mass loading of the solar wind begins to slow it down from supersonic speed to subsonic speed. As the decelerating solar wind confronts the growing coma, a bow shock forms, similar to the reactive wave that forms in front of a ship (the bow) as it plows through the oncoming waves of a current at sea.
Against the solar wind, the coma grows increasingly dense and develops into distinct interior layers or regions, each of which embraces different chemical and physical activities. (See illustration.) As the solar wind encounters the denser gas in the inner coma, the solar plasma and interplanetary magnetic field lines are compressed. At the cometopause, they are pushed out and around the cometosphere, in which the cometary ions are themselves piling up against each other. If the comet is outgassing fast enough to produce a large amount of ions , the interior region of the coma will become free of any magnetic field and form an ionopause that pushes most of the cometary ions out and around the nucleus, into the streaming ion tail . If this ionosphere is expanding supersonically, it is deflected by the inner shock , where the neutral molecules outgassing from the nucleus still hold sway against the powerfully charged ionosphere .
The final, but most mysterious layer, lies right on the surface of the nucleus where the coma is generated. This layer is little known because it is so difficult to observe through the dynamic coma from long distances. Rosetta, however, will have a front-seat view, and its lander Philae will be at center stage, sitting right on the surface of the comet’s nucleus. Scientists assume that the irregular shape and different elements of the nucleus will cause the gas jets erupting from the nucleus to vary in chemical composition and pressure. As a result, different parts of the coma near the nucleus may also vary.
In the regions of the coma closest to the nucleus, the outflowing gas first cools as it expands outward, but is then heated by solar radiation and chemical reactions with other elements, including the reactive radicals created by chemical activity, as well as the hot ions and dust particles. In the outer coma, this “soup” of radicals, molecules, and ions becomes thicker and full of extremely hot electrons. New chemical reactions, especially proton transfer reactions among the elements in the coma produce entirely new species that in turn react with the other species. Photodissociation by solar radiation also breaks down the organic materials emerging from the nucleus.
A particle leaving the surface of the comet may fly out, accelerate through the inner shock, interact with light from the Sun, become fragmented by photodissociation, interact with other molecules or photoelectrons, and possibly even continue accelerating out toward the bow shock and even beyond. Meanwhile incoming ions from the solar wind collide with outgoing cometary ions, neutral molecules outgassing from the nucleus, fragmented molecules, and even dust particles dislodged from the nucleus. This cauldron of chemical and physical processes called the coma is a fascinating mystery that the Rosetta mission hopes to solve.