Rosetta provides unique insight into comet shedding its dusty coat

ESA's Rosetta mission has enabled scientists to watch the life cycle of a comet, 67P/Churyumov-Gerasimenko, as it sheds the dusty coat accumulated over the past 4 years.

The COmetary Secondary Ion Mass Analyzer, or COSIMA, is one of Rosetta's three dust analysis experiments. It started collecting, imaging and measuring the composition of dust particles shortly after the spacecraft arrived at the comet in August 2014.

The study covers August to October, when the comet moved along its orbit between about 535 million kilometers to 450 million kilometers from the Sun. Rosetta spent the most of this time orbiting the comet at distances of 30 km or less.

The scientists looked at the way that many large dust grains broke apart when they were collected on the instrument's target plate, typically at low speeds of 1-10 m/s. The grains, which were originally at least 0.05 mm across, fragmented or shattered upon collection.

The fact that they broke apart so easily means that the individual parts were not well bound together. Moreover, if they had contained ice, they would not have shattered. Instead, the icy component would have evaporated off the grain shortly after touching the collecting plate, leaving voids in what remained.

By comparison, if a pure water-ice grain had struck the detector, then only a dark patch would have been seen.

The dust particles were found to be rich in sodium, sharing the characteristics of 'interplanetary dust particles.' These are found in meteor streams originating from comets, including the annual Perseids from Comet 109P/Swift-Tuttle and the Leonids from 55P/Tempel-Tuttle.

Lead author Rita Schulz of ESA's Scientific Support Office said that they found that the dust particles released first when the comet started to become active again are 'fluffy.' They didn't contain ice, but they did contain a lot of sodium. We have found the parent material of interplanetary dust particles.

The scientists believe that the grains detected were stranded on the comet's surface after its last perihelion passage, when the flow of gas away from the surface had subsided and was no longer sufficient to lift dust grains from the surface.

The comet is on a 6.5-year circuit around the Sun, and is moving towards its closest approach in August of this year. At that point, Rosetta and the comet will be 186 million kilometers from the Sun, between the orbits of Earth and Mars.

"Rosetta's dust observations close to the comet nucleus have been crucial in linking together what was happening at the very small scale with what was visible at much larger scales, as dust was lost into the comet's coma and tail," said Matt Taylor, ESA's Rosetta project scientist.

"For these observations, it really is a case of 'watch this space' as we continue to watch in real time how the comet evolves as it approaches the Sun along its orbit over the coming months."

Results from the first analysis of data are reported in the journal Nature.