Sen—Even before the Philae probe landed on comet 67P/Churyumov–Gerasimenko in November, to international acclaim, Europe's Rosetta spacecraft that carried it was already studying the dust being shed by it.
Between August and October 2014—before Philae's famous 12 November landing—Rosetta was orbiting the four km-wide comet at a distance of 30 km. Dust emanating from the comet due to solar radiation (there's no air to blow particles around) effectively "blew" into Rosetta's eyes. But this was a good thing.
Scientists observed the dust disintegrating on impact with an instrument sensor. This told them that the dust grains weren't bound together well, and that there was also no ice present.
These were dry, "fluffy" grains—which should be unusual as comets are typically composed of up to 50-per cent water ice. But scientists found the grains were instead made of sodium, which is a major constituent of interplanetary dust.
The comet has an orbital period of 6.5 years. As it nears perihelion (closest approach to the Sun) in August 2015, it will warm up and the vapour from evaporating water-ice will lift the dry dust into the coma that surrounds all comets that approach the Sun.
Eventually, there will be enough solar energy to remove all the old dust altogether—dust which the Philae lander found could be up to 20cm thick.
Dr Martin Hilchenbach of the Max Planck Institute said in a statement: “We believe that these fluffy grains collected by Rosetta originated from the dusty layer that built up on the comet’s surface since its last close approach to the Sun.”
Hilchenbach is Principle Investigator of the COSIMA instrument (COmetary Secondary Ion Mass Analyser) on-board Rosetta that studied the dust. He expects to see more ice-rich debris again in the months to come.
In fact on Thursday NASA reported a "significant increase" in the amount of water that 67P/Churyumov–Gerasimenko is releasing (about 1.2 litres in vapour form). Once it passes perihelion and speeds away from the Sun again, it should cool down, its watery coma should recede, and it will start to pick up fine interplanetary dust once again.
Rosetta is a European Space Agency mission and was launched in 2004 to answer questions about comets, because they may hold clues about the formation of our Solar System.
In December 2014, it discovered that 67P/Churyumov–Gerasimenko has three-times more of a heavy version of hydrogen in its water than Earth's oceans do. Comets are thought to be the origins of Earth's water, so this discovery cast doubt on that.
The results of the new dust analysis appear in the journal Nature.