In case you forgot, human beings currently have a spacecraft escorting a comet along its solar-system-touring orbit and reporting back everything it sees. The latest piece of research published from that data helps solve a puzzle: comets release lots of water vapor (enough to form most of the cloud-like coma that surrounds them), but water ice is barely present on their surfaces.
A team led by Maria Christina De Sanctis of the Italian National Institute of Astrophysics used observations of infrared light reflected off the surface of comet 67P in August 2014 to look for water ice at the skinny neck between the comet's two main lobes. Because water ice absorbs specific wavelengths of infrared radiation, these measurements can identify the ice amid the other materials that cover the surface. Generally, ice accounts for less than one percent of that material, even though jets of dust and water vapor have been observed coming from that part of the comet.
But as the comet rotates, parts of the comet's skinny neck find themselves in the shadow of the sunward lobe. Each time the portion the researchers were studying emerged from the shadow, the infrared signature temporarily looked significantly different. There was stronger absorption at the wavelength of ice. Using a model, the researchers estimated that the proportion of ice at the surface appeared to increase to as much as ten to fifteen percent while diminishing back to the usual amount as the surface warmed in the sunlight.