The results, published Wednesday in Nature, add to our growing understanding of comets. Comets are mostly made up of water ice. Their inner cores (called nuclei) are full of the stuff, and scientists had already noted that 67P's coma (the gassy cloud that surrounds it) was full of water vapor. It follows that the water vapor is probably released as the frozen water in the nucleus heats up, but it's rare for scientists to catch a glimpse of frozen water at the surface.
You'd think that ice in the core and ice in the coma would mean there must be ice in the middle, sitting on the surface — but scientists have found surprisingly little evidence of that. It's likely that water ice has trouble staying frozen when exposed to the sun.
"First, not finding ice was a surprise; now, finding it is a surprise," study author Murthy Gudipati of the Jet Propulsion Laboratory told the LA Times. "It is exciting because now we are starting to understand the upper dynamic layers of the comet and how they evolved."
Rosetta’s VIRTIS infrared instrument revealed a surface that was mostly dark and dry, but with two large patches that included small amounts of highly reflective ice in the dusty mix. By analyzing the way the patches reflect light, the researchers were able to identify two different grain sizes of pure water ice.
The team believes that the ice — which was found on cliff walls and in debris falls — was exposed as the top layer of the surface was eroded or damaged.
“The various populations of icy grains on the surface of the comet imply different formation mechanisms, and different time scales for their formation,” lead author Gianrico Filacchione said in a statement. Over at the comet's "neck", scientists had previously found tiny grains of ice that formed daily as frost. The newly found ice, which comes in larger grains (of two distinct sizes) probably forms more slowly than that.
The largest grains, which are about 2 millimeters in size, may have formed as secondary ice crystals when smaller grains were forced together (a process called sintering). It's also possible that the "large" grains form during sublimation, when the heat of the sun causes ice at the comet's core to go straight from solid to gas. Some of this gas may cool back down and settle just beneath the surface as liquid that quickly refreezes.
“The thin ice-rich layers that we see exposed close to the surface may be a consequence of cometary activity and evolution, implying that global layering did not necessarily occur early in the comet’s formation history," VIRTIS principal investigator Fabrizio Capaccioni told the ESA. Figuring out which aspects of the comet are new and which are left over from its formation is an important task. Comets like 67P were formed in the very early days of the solar system, so their icy cores could contain the molecular building blocks available 4.6 billion years ago. Revealing these could help us understand how the entire solar system — including our own planet — was formed.