A surface covered by a thermally insulating layer. Bizarre landscapes which could not be more different. A low density comparable with that of cork, and jets of dust and gas which are ejected into space to their own beat: Measurement data recorded by the scientific instruments aboard ESA’S Rosetta space probe are drawing a more and more accurate picture of comet 67P/Churyumov-Gerasimenko, which it has been accompanying on its journey towards the Sun.
A special issue of the magazine Science provides an overview of the current state of knowledge. Scientists from the Max Planck Institute for Solar System Research in Göttingen have made crucial contributions to five of the seven published articles.
When comet 67P/Churyumov-Gerasimenko woke up in spring 2014 and started to eject dust and gas into space, the Rosetta space probe was already in position: many of the ten instruments on board recorded initial measurement data while it was still on the approach. 67P/Churyumov-Gerasimenko has been under constant observation ever since the space probe arrived at “its” comet in August 2014, if not before. Researchers were able to see not only a surprising variety of surface structures, but also extremely dynamic processes which feed the atmosphere of the comet.
The researchers want to derive one thing in particular from the cometary mosaic which they have assembled step by step from the great variety of measurement results: How did comet 67P/Churyumov-Gerasimenko form? Was it originally a single, larger piece of rock that lost material during its earlier orbits around the Sun and thus achieved its current bipartite shape? Or did two smaller pieces of rock which today form the head and the body of the comet merge into one? This could help to understand how comets formed in the infant phase of the solar system.
The scientists hope to obtain crucial information by comparing the different parts of the comet. Whereas only a few fundamental differences have so far been evident between the head and the body, the neck region stands out. It was not only the main starting point of the comet’s gas and dust emissions in the last few months, it could also differ in other characteristics.
Comet’s nucleus and activity
In the past few months, researchers have used images from the OSIRIS camera system to compile a three-dimensional model of the comet and have thus been able to measure it accurately: while the smaller part, the so-called head, measures 2.6 x 2.3 x 1.8 kilometres, the larger part, the body, is 4.1 x 3.3 x 1.8 kilometres. Taking these dimensions together with the mass of the comet, determined by the RSI instrument, results in a density of 470 kilogrammes per cubic metre – roughly comparable with the density of cork. This is the first direct measurement of the density of a cometary nucleus.
“We assume that the comet consists of ice and dust, materials which both have a much higher density,” says Holger Sierks from the Max Planck Institute for Solar System Research, head of the OSIRIS team. “The measured value therefore leads us to conclude that the comet has a porosity of 70 to 80 percent. Our current understanding of the comet is as a type of loose accumulation of ice and dust particles with many, many cavities,” adds Sierks.
67P’s colouring is surprisingly unassuming. Unlike asteroids, for example, almost no colour variation can be detected on the surface. The images reveal that only the neck region, which connects the larger with the smaller part, and individual rocks on the surface are brighter than the surroundings.
Moreover, a large part of the comet’s activity originates from this area: almost all the dust jets which could be seen especially during the first few months have their roots here. “We see that this region differs greatly from the rest of the comet,” says Sierks. Calculations done by the OSIRIS team have shown that the neck by no means absorbs more thermal energy from the Sun than other regions, as had been assumed initially. On the contrary: in recent months, 67P was oriented towards the Sun in such a way that the neck received less energy than other regions. “The ice in the neck region could contain carbon monoxide or carbon dioxide, or simply be closer to the surface,” says Holger Sierks.
On the part of the back close to the neck there are also further sources of activity: cylindrical depressions with a diameter of up to 300 metres and a depth of up to 200 metres that provide an insight into the vertical structure of the comet. The formations could provide information on the early evolutionary phase of the solar system.
The OSIRIS scientific camera system has already taken images of around 70 percent of the surface of 67P/Churyumov-Gerasimenko and compiled detailed topographic maps from the images. These show bizarre landscapes that could not be any more different: even regions alternate with rugged and furrowed ones or those possibly covered with a one-metre layer of dust. The researchers have identified a total of 19 morphologically different regions and divided them into five categories. These regions have been named after Egyptian gods.
“The neck region of the comet differs significantly from other regions from a morphological point of view as well,” says Sierks. In contrast to the regions on the head and the body of the comet, the surface here is even: with no craters, furrows or cliffs whatsoever. It has a long fissure, however, which indicates mechanical stress in the cometary nucleus.
Other regions – particularly in the northern hemisphere of the comet – are covered by a loose layer of dust which appears to have drifts and dune-like structures in some places. “The images almost resemble images familiar from desert regions on Earth,” says Sierks. The scientists assume that this landscape was formed by dust that could not escape from the gravitational field of the comet and fell back to the surface.
The vast temperature differences to which the celestial body has been subjected on its journey from the icy depths of space to a distance of 180 million kilometres from the Sun leave their marks: some places exhibit bizarre landscapes that are dominated by grooves, fissures and furrows.
There are also areas where the surface material could be relatively hard and compacted, and a large bowl-shaped depression on both head and body of the comet.
The MIRO instrument investigates the electromagnetic radiation which comet 67P/Churyumov-Gerasimenko emits into space, in the wavelength ranges around 1.6 and 0.5 millimetres. This radiation comprises not only the thermal radiation originating from the comet, but also contains characteristic fingerprints of water molecules.
The MIRO team, headed by Sam Gulkis from the Jet Propulsion Laboratory (USA), was thus able to detect water vapour in the cometary atmosphere back in June 2014. At this stage, the Sun was causing the comet to “sweat out” around 300 millilitres of water per second. This rate had increased to 1.2 litres per second by the end of August.
Particularly noticeable: The majority of the measured water molecules are moving towards the Sun, evaporating namely primarily from the dayside of the comet. Where sunlight reaches the surface, it heats it so much that ice can sublime. On the nightside, the temperatures remain below the sublimation temperature, however. “The temperature measurements also confirm large temperature differences between dayside and nightside,” explains Paul Hartogh from the Max Planck Institute for Solar System Research and member of the MIRO team. Seasonal variations also occur during one orbit around the Sun.
Overall, the data leads the researchers to the conclusion that although the dusty surface layer of the comet reacts rapidly to temperature changes, it is a very poor heat conductor. It forms a type of thermal insulation that protects lower-lying layers from the effects of the Sun. “This could explain why 67P and other comets which penetrate into the inner solar system are so long-lived and survive many orbits around the Sun,” says Hartogh.
Gases from the depths
The ROSINA mass spectrograph has also taken a detailed look at the gaseous surroundings of the comet in recent months. The team headed by Katrin Altwegg from the University of Bern was able to identify not only water vapour, but also carbon dioxide and carbon monoxide. “The occurrence of the gases is very different, however – both in respect of their spatial distribution and also in the course of one rotation of the comet about its own axis,” says Urs Mall from the Max Planck Institute in Göttingen, a member of the ROSINA team.
During the two measurement periods, the ROSINA team found that the quantity of the gases emitted by the comet depended greatly on the rotational direction of the comet relative to the space probe. Further measurements will show whether water vapour is emitted primarily from the neck region, while carbon dioxide is released more by the underside. Similar behaviour is known from the 103P/Hartley2 comet namely, the destination of the EPOXI mission in 2010.
It is still unclear whether it is possible to conclude from this that the frozen gases are distributed unevenly across the cometary nucleus. Seasonal effects could also play a role here. The underside of the comet is currently only weakly irradiated by the Sun. It is winter there. It is conceivable that during the warmer summer months more water vapour forms there as well.
“What we are observing, however, is that the emission of carbon dioxide and carbon monoxide does not vary as much during a cometary rotation as the emission of water vapour,” says Mall. This could possibly indicate that these gases evaporate from greater depths where the temperature differences between day and night do not have such a pronounced effect.
Dust particles in orbit
The GIADA particle instrument has spent the last few months investigating the mass and size of the dust particles in the comet’s surroundings, the OSIRIS camera system their flight speed and direction. The researchers not only discovered particles which move away from the cometary surface, but also those in a stable orbit. The bound lumps of dust dwell at a distance of up to 145 kilometres or so from the surface of the comet.
The scientists assume that these particles have been accompanying the comet since it last flew by the Sun. When 67P’s gas and dust activity subsided again after it had passed by the Sun, outgassing material was no longer able to disturb the motions of the lumps and they remained bound on stable orbits. When the comet approaches the Sun again over the coming months and the emission of dust and gas increases drastically, these lumps will probably be lost into the vastness of space.
In the data obtained from GIADA, OSIRIS and MIRO, the researchers also found indications that 67P ejected four times more dust than gas into space during recent months. Earlier measurements on other comets, in contrast, showed a higher proportion of gases by mass. It is to be expected, however, that the gas production of 67P will increase significantly during the coming months.