*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 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!

## Discussion: 11 comments

subdirectory_arrow_rightvery interesting; and yes, I did just subscribe to science for that.

finally some things are released.

It does travel within it’s “fan club” of a dust cloud (with ‘grains’ to several meters…).

I would not be surprised if there were some larger ‘brothers’ travelling alongside at larger distances, holding microgravity orbits of years or decades…..

subdirectory_arrow_rightHas there ever been considered to visit one of these large bound gains to take close-up images, from some reasonable safety distance?

It’s almost like tiny moons, and what we know as meteors at the same time.

subdirectory_arrow_rightRight: Perseid, Leonid and other meteor showers.

subdirectory_arrow_rightThe bound grains are circa 145Km out. Rosetta is being put into a different pattern of flight (I’m trying not to use “orbits”) which will involve going further out to swoop past 67P at less than 10KM. This is because they expect material leaving the comet to become so intense that continued close observation becomes dangerous. So we might get to see these bound grains more closely, though how close its possible to get (how do you find them?) is dubious.

subdirectory_arrow_rightI think the conjecture that larger chunks up to ten metres may be ejected from the surface is very interesting. They may not reach escape velocity, but return as giant hailstones, as Jacob N. suggested last summer. So maybe Cheops and the large boulders in Imhotep were once flying debris. It is suggested lumps as large as 150 metres can be expelled from the surface by the build up of gas pressure below the surface. Apparently a detailed search for bound satellites/moons was made and nothing larger than 2m was found.

This raises questions about a required variability in the permeability of the comet material. We have seen walls, ridges, flat plates, outcrops and caves, with apparently far less porous, or more consolidated, less volatile material. The mechanisms for this “internal skeleton” have not been addressed, other than to place some limits on the original size of 67P, or the object it may have been a part of, 20 to 200Km was mentioned. This means gravitational pressures would be insufficient to create differentiation. This tends to point to accretion energy, maybe some initial radioactivity, creating convection within the comet, or impact shock/energy from later impacts. The porosity places limits on the size of impacts that have occurred too, so this will be an interesting conundrum for the science teams to solve.

It would seem things might get quite “explosive” come August time!

subdirectory_arrow_rightJust what is escape velocity for a chunk of material as big as Manhattan Island.?

A friend at NASA says that for something as big as a couple of football fields it’s about five inches per second or one third of a mile an hour.

subdirectory_arrow_rightThe escape velocity for spherical, non-rotating objects depends on the mass and the distance to its center (radius). The Wikipedia formula looks correct, v = sqrt(2 G M / r)

https://en.wikipedia.org/wiki/Escape_velocity

For M you may take the mass of a sphere of radius r and density rho. Since mass M = rho V, with V the volume, for a sphere V = 4 pi r³ / 3, you get M = 4 pi rho r³ / 3.

https://en.wikipedia.org/wiki/Density

https://en.wikipedia.org/wiki/Sphere#Enclosed_volume

Together

v = 2 sqrt(2 G pi rho r² / 3)

with G the gravitational constant.

https://en.wikipedia.org/wiki/Gravitational_constant

Inserting all constants you get

v = 2.364e-5 r sqrt(rho), with r in meters and rho in kg/m³

[Correct units would be.

v = 2.364e-5 r sqrt(rho) sqrt(m/kg) m/s].

See also https://en.wikipedia.org/wiki/Escape_velocity#Calculating_an_escape_velocity

For a five inch per second escape velocity, equals 0.127 m/s, and say a density of 3,000 kg / m³, you’ve to solve

2.364e-5 r sqrt(3 kg/m³) sqrt(m/kg) m/s = 0.127 m / s,

or

r = 0.127 m/s / (2.364e-5 sqrt(3,000 kg/m³) sqrt(m/kg) m/s)

= 0.127 / (2.364e-5 sqrt(3,000)) m

= 98.08 m.

Hence a sphere with reasonable assumptions for solid rock, and a diameter of a little less than 200 m should result in an escape velocity of 5 inches per second at the surface, if non-rotating.

subdirectory_arrow_rightReplace the line

2.364e-5 r sqrt(3 kg/m³) sqrt(m/kg) m/s = 0.127 m / s,

by

2.364e-5 r sqrt(3,000 kg/m³) sqrt(m/kg) m/s = 0.127 m / s,

subdirectory_arrow_right“[Dust-to-gas ratio] 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…”

“…this was determined to be 4 ± 2 for Comet 67P/C-G.”

Wow! The scientists are about to make a completely objective statement regarding the quite large discrepancy in their model! Here we go…

“…so you can see that really they are more like “snowy dustballs” than dirty snowballs!”

[shaking my head] The dirty snowball model was inferred upon indirect observations. The predicted and described “icy body” is not icy! ESA provides an incredible opportunity to directly observe the comet and we’re dragging this dead theory along for the ride until the inevitable dead end.

The D/G does not describe a snowy dustball, there is no snow; it describes a dusty rock producing gas electrochemically. Water exists around comets just as water has been directly observed on the lunar surface produced by interactions with the solar wind. This is your answer. Ockham’s razor applies, please stop looking for what isn’t there.

subdirectory_arrow_rightThe electrochemical portion is negligible with respect to outgassing. There will be some photochemistry at the surface and in the coma, instead, but not remotely enough to explain the outgassing.

Sublimation of frozen gasses and ice is the main source for gas and water vapor in the coma.

subdirectory_arrow_rightWould it be possible for Rosetta (when the time comes) to settle like a hen on a nest of eggs on Comet 67P rather than crash land it as some have suggested?