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 grains at these distances from the Sun (about 553 million km to 508 million km).

(*Editor’s note: In a follow-up discussion, I asked Alessandra and Marco about the possible impact – literally – of ‘grains’ up to 2 metres in size on the spacecraft, but there is no need to panic: according to their space density, Rosetta needs to travel for many centuries at a speed of 1 m/s before impacting even just one of them!)

While we expect more outflowing grains as we get ever closer to perihelion in August, the bound grains also have an interesting story to tell. We think they are left over from the comet’s last excursion around the Sun, left suspended around the comet after the gas flow had decreased such that it could no longer perturb the orbits. The number and range of size of dust particle observed requires several years to build up, so they could not be ejected during the outburst observed at the end of April 2014, for example – all of that dust escaped into the tail of the comet without any collision with the bound grains.

Will the cloud dissipate once activity increases? Well, this is an example of where orbiting very close to the comet actually puts us at a disadvantage! Even at 30 km distance the images are so full of out-flowing grains, that to identify the few hundreds of tiny dots of the far bound grains is practically impossible. We’ll have to wait a few more months to check in on this population when we are flying further from the comet nucleus, when conditions will be again favourable to the detection of bound grains.

Dust-to-gas ratio

Now let’s look at the dust-to-gas ratio. By considering data from GIADA, OSIRIS, MIRO and ROSINA, this was determined to be 4 ± 2 for Comet 67P/C-G. Why should we care about this ratio and what does it really mean? Well, it provides important information about where comets formed and how they evolve – it actually helps us to say how much the classical view of a "dirty snowball" is correct. In that case, we’d expect a dust-to-gas ratio in the range of 0.1-1, so you can see that really they are more like “snowy dustballs” than dirty snowballs! For another comparison, if we assume only water is being emitted from the comet then the derived dust:gas ratio would be 6 ± 2. This ratio decreases to 4 ± 2 if we include CO and CO2.

The dust-to-gas ratio is also important to infer the internal porosity. Since dust has a bulk density larger than ice, the larger the dust-to-gas ratio, the larger the internal nucleus porosity we need to explain the measured low bulk density of the nucleus.

An estimated value of 3 has been made for comet 67P/C-G at perihelion, so we’re looking forward to seeing if/how the value changes in the coming months.

It has the potential to change because the value reflects an average composition of the material being lost from the nucleus, which currently is very dusty. The value also assumes that we can separate solids (dust) from volatiles/ices (gas). We use the GIADA data to check the bulk density of the grains so that any ice-rich grains would be excluded from the dust count. But, at the time of the observations at least, the out-flowing grains are very small (less than 2 cm as we discussed in the previous section). In grains of these sizes, ice would take very little time to sublimate. When the comet starts to eject bigger clumps (metre-sized clumps at next perihelion, as already observed in the bound cloud as being remnants from the last perihelion), then the ice content in these clumps could contribute to the gas flux, and so will have to be taken into account when we calculate the ratio again in August.

It’s a complicated and ever-changing story!