Rosetta orbiter shows comet 67P is full of ‘powdery grains of dust and ice’

The ESA’s Rosetta mission has solved yet another longstanding scientific mystery about the comet, conducing a series of measurements and clearly demonstrating that there are no large caverns inside of Comet 67P/Churyumov-Gerasimenko, officials at the space agency announced this week.

So what is inside the comet? Dust, and lots of it, according to a new study published Wednesday in the journal Nature. In that paper, a team of researchers led by Martin Pätzold from Rheinische Institut für Umweltforschung an der Universität zu Köln in Germany reviewed data collected by Rosetta and found that, despite its low density, the comet is not a porous object.

Comets, Gizmodo explained, are made up of a frozen mixture of water ice and rock left over from the formation of the solar system. Their low density had led scientists to ponder whether or not they had caves and caverns beneath their surface, but the new study indicates that this is not the case. While 67P is solid throughout, it is actually filled with extremely light, powdery grains of dust and ice.

Pätzold and his colleagues reported that their findings are consistent with earlier results obtained by Rosetta’s CONSERT radar experiment, which had revealed that the “head” of the twin-lobed comet had a relatively homogenous structure on spatial scales of a few tens of meters.

Lack of changes to acceleration rules out presence of big caves

As Gizmodo pointed out, the study authors used a rather ingenious technique to arrive at their conclusion. They analyzed the effect that the comet’s gravity had on radio signals received on the ground. A cavernous interior would have meant that 67P’s gravitational pull on the orbiter would have been stronger at some points in the orbit than others.

This would have resulted in a change in Rosetta’s acceleration, and thus would have produced a Doppler shift in the frequency of the spacecraft’s radio signals. Since no sizable shifts in speed were detected, however, it allowed them to conclude that the comet has a homogeneous structure comprised of three-fourths dust and one-fourth water ice.

According to the ESA, Rosetta is the first probe to successfully record this kind of measurement for a comet, and by removing the pressure of solar radiation and the escaping gas, they were able to determine the mass of the comet. They found that 67P has a mass of just under 10 billion tons, and images from the OSIRIS camera instrument place its volume at an estimated 18.7 square km, meaning that the density of the comet would be about 533 kg/m3.

There is a possibility that the comet contains small caves that have thus far escaped detection by the orbiter, Gizmodo explained. Should such caverns exist, they will likely be detected later this year, when Rosetta is guided to a controlled impact on the comet surface. The spacecraft will continue to make observations right up until impact, and those more precise readings could lead to the discovery of caves just a few hundred meters in size, the agency noted.

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Image credit: ESA