GIADA’s dust measurements: 3.7-3.4 AU

Based on inputs from GIADA team members Alessandra Rotundi (instrument PI) and Marco Fulle, following the publication of “Dust Measurements in the Coma of Comet 67P/Churyumov-Gerasimenko Inbound to the Sun Between 3.7 and 3.4 AU” in the journal Science today. GIADA is Rosetta’s Grain Impact Analyzer and Dust Accumulator. Our published research so far focuses on two aspects of the dust around Comet 67P/C-G: the different populations of dust grain found either in bound orbits or out-flowing from the nucleus, and the dust-to-gas ratio in the coma. First let’s discuss the grain population, considering measurements made with both GIADA and OSIRIS on 4 August 2014, when we were still at 275 km from the comet. These observations allowed us to count about 350 grains in bound orbits around the comet nucleus, and 48 fast, out-flowing grains that were ejected about a day before the observations. These two families of detected grains – out-flowing and bound – do not overlap in space. Out-flowing grains were not detected farther than 20 km from the spacecraft, whereas bound grains were not detected closer than 130 km from the spacecraft (that is, they were found within about 145 km of the comet). The space density of bound grains is at least 100 times lower than that of out-flowing grains and, in general bound grains are much bigger than out-flowing grains. Indeed, based on the observed brightness range, we infer that the bound grains varied from 4 cm to 2 m*, whereas the out-flowing grains seen in the images were less than 1.7 cm. And in fact the largest grain detected directly by GIADA is on the order of 0.1mm. We don’t see so many larger grains outflowing from the comet, because the gas density at the nucleus surface was still unable to lift larger […]



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