0916 GMT June 24, 2019
The finding solves a long-standing mystery about water ice in comets. Scientists already knew that the coma — the expansive cloud of gas surrounding the comet's nucleus — is dominated by water molecules. They also knew that water ice is one of the main components of the nucleus. But until now, traces of water ice on the surface of the comet had been difficult to detect, phys.org said.
"First, not finding ice was a surprise; now, finding it is a surprise," said Murthy Gudipati a planetary scientist at the Jet Propulsion Laboratory in La Canada Flintridge, Calif., and an author on the paper. "It is exciting because now we are starting to understand the upper dynamic layers of the comet and how they evolved."
The surface of comet 67P, like most comets, is primarily covered by dark organic materials that appear almost black. That's because as comets fly toward the sun, they are exposed to warm temperatures that cause volatiles like water ice on their surface to sublimate — or go directly from solid to gas.
What remains on the crust are what are known as refractory materials. These include silicates similar to rocks, sand and dirt on Earth and carbonaceous materials. Because these materials do not sublimate, the comet's surface becomes increasingly organic and silicate rich over time, said Michael Combi, who studies comets at the University of Michigan and coauthored the paper.
The surface water ice on comet 67P Churyumov-Gerasimenko was discovered in two places several tens of feet across in a region known as Imhotep, on the bottom part of the main lobe of the comet. It was found using the VIRTIS infrared instrument, which scanned the area looking for water ice spectra signals not long after the Rosetta orbiter caught up with 67P in the fall of 2014.
In both cases the ice appeared on cliff walls and debris falls, and appeared as noticeably bright patches in visible light.
"It looked like there was a breakage, or something fell down on the surface of the comet, and a large, new inside area that had water ice was exposed," Gudipati said. "Although we knew water ice had to be in the nucleus, this was our first direct detection of that interior ice."