For the first time ever, scientists from the Institute for Space Astrophysics and Planetology have discovered patches of water ice on the surface of a comet using the technology on the European Space Agency's (ESA's) Rosetta orbiter. Although scientists already knew that water molecules make up most of comet comas, which are the clouds of gas surrounding the comet's nucleus, the new finding is the first time that traces of water ice have been detected.
"First, not finding ice was a surprise; now, finding it is a surprise," Murthy Gudipati, co-author of the paper, said in a press release. "It is exciting because now we are starting to understand the upper dynamic layers of the comet and how they evolved."
The water ice on the surface of 67P, the comet in question, was found to exist in two places across the region known as Imhotep, located on the bottom part of its main lobe. The discovery was made using the VIRTIS infrared instrument aboard the Rosetta, which picked up the water ice spectra signals shortly after the orbiter gained ground on 67P in late 2014. In both cases, the ice was located on cliff walls and debris falls and were easily noticeable.
"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."
Upon further examination, the team discovered that water ice grains come in two sizes - small grains within the micrometer range and larger grains that are just a few millimeters across. The smaller grains seem to be associated with the thin later of frost, whereas the larger grains likely stem from the cooler lower surface layers.
"Keep in mind that comets are very porous, like cotton candy," Gudipati said. "Seventy percent of this comet is a void, and because of that, the heat from the surface does not go that deep."
Furthermore, the water ice was only a small portion of what was detected by VIRTIS, suggesting that this material exists among the refractory materials located in the upper layers of the comet.
The findings give scientists plenty to work with and Gudipati and his team are in the process of examining how ice exposure changed as the comet moved closer to the sun.
"We knew water ice made up the majority of the comet, but we didn't know how deep or in what condition it was," he said. "This shows that it not very deep at all - perhaps just a few feet beneath the surface."
The findings were published in the Jan. 13 issue of Nature.
