Comet 67P/Churyumov-Gerasimenko sheds dust coat accumulated over the past four years

Abstract

Comets are composed of dust and frozen gases. The ices are mixed with the refractory material either as an icy conglomerate, or as an aggregate of pre-solar grains (grains that existed prior to the formation of the Solar System), mantled by an ice layer. The presence of water-ice grains in periodic comets is now well established. Modelling of infrared spectra obtained about ten kilometres from the nucleus of comet Hartley 2 suggests that larger dust particles are being physically decoupled from fine-grained water-ice particles that may be aggregates, which supports the icy-conglomerate model. It is known that comets build up crusts of dust that are subsequently shed as they approach perihelion. Micrometre-sized interplanetary dust particles collected in the Earth's stratosphere and certain micrometeorites are assumed to be of cometary origin. Here we report that grains collected from the Jupiter-family comet 67P/Churyumov-Gerasimenko come from a dusty crust that quenches the material outflow activity at the comet surface. The larger grains (exceeding 50 micrometres across) are fluffy (with porosity over 50 per cent), and many shattered when collected on the target plate, suggesting that they are agglomerates of entities in the size range of interplanetary dust particles. Their surfaces are generally rich in sodium, which explains the high sodium abundance in cometary meteoroids. The particles collected to date therefore probably represent parent material of interplanetary dust particles. This argues against comet dust being composed of a silicate core mantled by organic refractory material and then by a mixture of water-dominated ices. At its previous recurrence (orbital period 6.5 years), the comet's dust production doubled when it was between 2.7 and 2.5 astronomical units from the Sun, indicating that this was when the nucleus shed its mantle. Once the mantle is shed, unprocessed material starts to supply the developing coma, radically changing its dust component, which then also contains icy grains, as detected during encounters with other comets closer to the Sun.

GIADA: shining a light on the monitoring of the comet dust production from the nucleus of 67P/Churyumov-Gerasimenko

“…They are charged, fragmented, and decelerated by the spacecraft negative potential and enter GIADA in showers of fragments at speeds <1 m s −1 (Fulle et al, 2015a). We suggest that these fluffy particles resemble the 67P particles collected by COSIMA (Schulz et al 2015) having a tensile strength such that they could break apart on impact. This behavior is consistent with fluffy particle fragmentation (Fulle et al 2015a).…”

CONSERT suggests a change in local properties of 67P/Churyumov-Gerasimenko’s nucleus at depth

“…Nuclei typically have very low albedo values ranging from 0.03 to 0.06, indicating the presence of refractory materials (Lamy et al 2004) in agreement with the fact that only limited patches of the surface are active icy regions. Indeed, VIRTIS, OSIRIS, and COSIMA observations during the second half of 2014 suggest that the nucleus of 67P is covered by an almost ice-free layer or mantle of dust particles (Capaccioni et al 2015;Schulz et al 2015;Thomas et al 2015). For an ice-free and porous dust mantle at 300 K, the dielectric constant at the frequency of CONSERT is expected to be about two, decreasing from about 2.41 to 1.94 for porosities increasing from 70% to 80% (Brouet et al 2015).…”

Negatively charged nano-grains at 67P/Churyumov-Gerasimenko

“…The Rosetta dust instruments have observed very fluffy small grains (see Schulz et al 2015;Rotundi et al 2015). It was also noted that >50 μm grains are mainly composed of refractory material (Schulz et al 2015).…”