EVOLUTION OF THE DUST SIZE DISTRIBUTION OF COMET 67P/CHURYUMOV–GERASIMENKO FROM 2.2 au TO PERIHELION

Abstract

The Rosetta probe, orbiting Jupiter-family comet 67P/Churyumov-Gerasimenko, has been detecting individual dust particles of mass larger than 10 −10 kg by means of the GIADA dust collector and the OSIRIS Wide Angle Camera and Narrow Angle Camera since 2014 August and will continue until 2016 September. Detections of single dust particles allow us to estimate the anisotropic dust flux from 67P, infer the dust loss rate and size distribution at the surface of the sunlit nucleus, and see whether the dust size distribution of 67P evolves in time. The velocity of the Rosetta orbiter, relative to 67P, is much lower than the dust velocity measured by GIADA, thus dust counts when GIADA is nadir-pointing will directly provide the dust flux. In OSIRIS observations, the dust flux is derived from the measurement of the dust space density close to the spacecraft. Under the assumption of 1 radial expansion of the dust, observations in the nadir direction provide the distance of the particles by measuring their trail length, with a parallax baseline determined by the motion of the spacecraft. The dust size distribution at sizes >1 mm observed by OSIRIS is consistent with a differential power index of −4, which was derived from models of 67P's trail. At sizes <1 mm, the size distribution observed by GIADA shows a strong time evolution, with a differential power index drifting from −2 beyond 2 au to −3.7 at perihelion, in agreement with the evolution derived from coma and tail models based on ground-based data. The refractory-to-water mass ratio of the nucleus is close to six during the entire inbound orbit and at perihelion.

The Agilkia boulders/pebbles size–frequency distributions: OSIRIS and ROLIS joint observations of 67P surface

“…9). The index of −2.8 also matches (within the index accuracy) the incremental index of −3 inferred during the inbound comet orbit (Rotundi et al 2015;Fulle et al 2016a) for the ejected dust of size larger 6 than 1 mm. By assuming that 5 Taking into account that all the dust from 10 −10 to 10 2 kg has a water mass content of about 5 per cent, as discussed in Fulle et al (2016b), the ratio between the dust loss rate and the water loss rate from the southern hemi-nucleus provides a dust-to-water mass ratio in the nucleus ranging from 10 to 15.…”

“…1 The comparison between the results obtained from previous works on this topic (Pajola et al 2015(Pajola et al , 2016aPommerol et al 2015;Vincent et al 2016) provides fundamental hints to understand if similar boulder/pebble formation and degradation processes occur in such wide range of dimensions. Moreover, our studied size range overlaps the ones described in the 67P coma by Rotundi et al (2015), Davidsson et al (2015) and Fulle et al (2016a); therefore, comparisons between our SFD analysis and the one inferred for coma particles can yield important information regarding the dust lifting processes occurring on the cometary surface.…”

“…9) may point out intrinsic differences between northern and southern dust size distributions. A mixture of these two size distributions, the first shallower in the airfall deposits and the second steeper in the exposed northern hemi-nucleus, may explain the index −3 retrieved from coma observations during the inbound orbit (Rotundi et al 2015;Fulle et al 2016a), steeper than that observed in smooth plains (−2.8) and the one retrieved from coma observations at perihelion (−2.7). The much shallower index of −2.2 observed by ROLIS in rough terrains from 0.05 to 0.4 m may point out a selection effect occurring during the inbound orbit, when the gas density also at noon is too low to lift up dust larger than 0.4 m: the smaller the dust, the easier to eject these pebbles, the shallower the size distribution of pebbles remaining in rough terrains.…”

“…By analysing the data shown in fig. 6 of Fulle et al (2016a), we obtained that a constant power law with incremental index of −2.7 is the best data fit over the entire observed mass range from 10 −10 to 10 2 kg. In the mass range from 1 to 100 kg, the observed size distribution is probably polluted by submetre-sized chunks in metastable orbits which are going to populate the chunk cloud orbiting 67P during the following aphelion (Rotundi et al 2015).…”

“…By assuming that 5 Taking into account that all the dust from 10 −10 to 10 2 kg has a water mass content of about 5 per cent, as discussed in Fulle et al (2016b), the ratio between the dust loss rate and the water loss rate from the southern hemi-nucleus provides a dust-to-water mass ratio in the nucleus ranging from 10 to 15. 6 Smaller ejected dust has a much shallower power index, close to −1 (Rotundi et al 2015;Fulle et al 2016a), and is out of ROLIS detection limits. This knee in the size distribution has been explained in terms of coma gas dust deposits are characterized by the incremental index of −2.8, different index values are pointing out different dust and chunk populations.…”