While in orbit around Comet 67P/Churyumov-Gerasimenko, the European Rosetta mission has reached out to grab particles being flung into space from the cometary surface and taken microscopic photographs of the comet's ‘snowflakes.' However, in results published today in the journal Nature, these snowflakes are anything but - they are devoid of ice and are instead composed of old dust rich in sodium.
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Captured by the COmetary Secondary Ion Mass Analyser, or COSIMA instrument, these specimen dust samples were collected as the ESA mission started to orbit the comet's nucleus in October 2014, around the time when 67P started to become more active as sunlight heated its surface.
"We found that the dust particles released first when the comet started to become active again are ‘fluffy,'" said lead author Rita Schulz of ESA's Scientific Support Office. "They don't contain ice, but they do contain a lot of sodium. We have found the parent material of interplanetary dust particles."
Schulz's team studied the COSIMA measurements and found that, typically, the large dust grains hit the instrument's target plate at speeds between 1-10 meters per second (2.2-22 miles per hour) and shattered on impact. The fact that the grains, that measure up to 0.05 millimeters across, broke apart so easily suggest that they lack the ice to bind them together.
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This has an interesting implication - the researchers believe that the grains collected by COSIMA are in fact old dust grains that collected on the comet's surface the last time the comet made close approach (perihelion) with the sun. During the comet's last solar orbit (67P completes one orbit of the sun every 6.5 years), as it raced away from the sun's heat after perihelion, gas pressure from subliming ices would have subsided. Any dust grains jettisoned at this late phase would not have had enough energy to escape the comet's gravity, falling back onto the surface.
Now, over 6 years later, this top coat of old dust has been blown off by the renewed gas pressure generated by perihelion; Comet 67P/Churyumov-Gerasimenko has basically shed its old coat of dust that has long since ‘dried out.' "We believe that these ‘fluffy' grains collected by Rosetta originated from the dusty layer built up on the comet's surface since its last close approach to the Sun," said Martin Hilchenbach, COSIMA principal investigator at the Max-Planck Institute for Solar System research in Germany.
"This layer is being removed as the activity of the comet is increasing again. We see this layer being removed, and we expect it to evolve into a more ice-rich phase in the coming months."
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Comet 67P/Churyumov-Gerasimenko will make its next close approach to the sun in August this year, so astronomers expect Rosetta to collect comet particles with increasingly different properties and ‘fresher' material is ejected into space.
"Rosetta's dust observations close to the comet nucleus are crucial in helping us to link together what is happening at the very small scale with what we see at much larger scales, as dust is lost into the comet's coma and tail," said Matt Taylor, ESA's Rosetta project scientist.