Rosetta: Scrutinizing the nucleus Print
Thursday, 30 October 2014 11:17

The Rosetta spacecraft continues to move in close vicinity of the nucleus of comet 67P/Churyumov-Gerasimenko, and it has reached distances less than 10 kilometers from the surface. Images have been taken with an amazing resolution. Even so, it is not immediately clear what it is that we are seeing. This discussion is ongoing and will certainly continue for some time. Actually, even the large-scale features are sometimes hard to interpret, and this makes it difficult to reach conclusions about the most fundamental questions about the origin of the nucleus.

The surface energy budget

What makes a comet nucleus surface interesting is that it is an ever-changing document of ongoing, hyperactive geologic processes. These are caused by the presence of very volatile materials that vaporize due to the heating of sunlight. When the orbital motion makes the nucleus approach the Sun, as is presently the case for the Rosetta target, the heating gets stronger. The volatiles wake up and the gases exert a higher pressure, trying to get out into the vacuum of space.

This causes an erosion of the surface and a loss of material that in the long term is bound to make the nucleus shrink. The surface is certainly shaped by the erosion caused by previous and ongoing outgassing, and one of our tasks is to infer from the images, how this process has worked. Doing this, we may also learn about the structure of the nucleus in chemical and physical terms, since this also influences the resulting surface shapes and formations.

When we try to read this puzzle, we have to use all the knowledge we have gained about the seasons on the comet nucleus. There are very different from our seasons on the Earth for two reasons. First, the orbit is so elongated that the intensity of sunlight varies by more than twenty times between the innermost and outermost parts. Second, the spin axis of the nucleus is much more inclined to the orbital plane, making an angle of 52 degrees instead of the Earth’s 23 degrees between the rotational and orbital poles. As a result, large parts of the nucleus experience midnight Sun or total darkness during large parts of the comet “year”.

But further complications exist. Due to the complicated shape of the nucleus, there are lots of ways for light to travel from one part of the surface to another. Therefore, the illuminated side may cast reflected sunlight onto parts that the sunlight does not reach, and the same goes for the thermal radiation that the surface emits. In summary, the energy budget of any surface element as the comet orbits around the Sun is a complicated affair, and we have to study it carefully in order to understand how erosion works in detail.


Picture 1. Global view of the OSIRIS shape model for the comet nucleus, showing the basic structure with two main lobes connected by a narrow neck region.



In general terms, the nucleus appears to be built of two main parts, as shown by the adjacent picture, which illustrates a model of the nucleus shape representing all the pictures obtained. This shape is colloquially interpreted as a duck. The two main lobes are referred to as the body and the head, and the narrow, connecting part is the neck.

It has been found that the North Pole is situated at the base of the neck, where it joins the body. A large region surrounding this has been constantly illuminated for a long time, as the comet traversed the most distant parts of its orbit. These parts are well explored, while the opposite side has not been seen at all because it has remained in darkness. When the comet reaches the innermost part of its orbit, the roles will have changed, and the now hidden region will experience the largest illumination ever seen on the surface.

The surface up-close

The OSIRIS pictures have shown the surface morphology to be diverse with very different units covering different parts. But there is no obvious difference between the two major parts of the nucleus – the body and the head of the duck. The different types of topography tend to appear on both of them, and the only part that stands out is the neck region connecting the two. The whole nucleus has a low reflectivity to sunlight (that is, it is very dark), similar to other comet nuclei that have been studied in detail before. There are only small variations from place to place, amounting to about 20%. The color is reddish and does not differ between the two main lobes.

Certain regions on the nucleus appear very smooth. They are likely covered by “air-blown” dust – small particles of sold material that have been lifted by the outflowing gas from one place and landed elsewhere on a less active surface. Dune-like features have been observed, which are evidence of lateral transport of dust, driven by gas streams. The most prominent of these smooth regions is called Hapi and covers part of the neck. It is shown on the adjacent picture.

Beneath this dust there is a surface layer that appears consistent and brittle but very weak. There are many cracks and fractures in it, and we have seen places where it has collapsed and broken up because of strains that cannot be much larger than 10 Pa (an almost vanishing pressure by normal standards). Some regions exhibit many circular pits and larger depressions with very flat floors. These have likely been carved out by the local escape of gas and thus bear witness of surface erosion. 


Picture 2. To the left, the smooth Hapi region on the neck of the nucleus. The fine dust is strewn with large boulders. To the right, the head starts with a steep cliff called Hathor, where long, vertical fractures are seen. The imaged area measures several hundred meters.



But while most fractures are rather short and randomly oriented, there are also systems of parallel features hundreds of meters in length. The most prominent such system is seen on a giant cliff bordering on the neck on the smaller lobe, as shown on the adjacent picture. The region in question is called Hathor. It is very interesting, and finding a unique and convincing explanation for it would be a major step forward, but we are not there yet.

Reading the history of the comet

Let us assume that the main lobes that form the nucleus are two big building blocks that came together by a collision, when the nucleus was formed. If so, it seems natural that the neck region is special and appears different from either of the two lobes. The collision should have left its traces in the material even if it was very gentle, since the material is so weak.

However, it is not likely that the surface of Hathor stems from the formation of the nucleus. Most likely, the comet has made a very large number of orbits around the Sun, and this would have led to a large amount of surface erosion. Thus, it is possible and even likely that some surface structures that we see were once hidden deep in the interior of the original nucleus.

But which ones? Most features are certainly recent and caused by the surface erosion and the strains caused by gravity and rotation. If we could nonetheless identify and interpret those that date back to the origin, they would yield a precious clue to comet formation.

Other findings already provide some rather clear indications. One is the density of the nucleus. Its mass has been accurately measured using the radio signal from the Rosetta probe, and the volume comes from the OSIRIS pictures via the shape model that we have derived. Since part of the nucleus cannot be clearly seen because it remains in permanent shadow, but their contour can be traced from the measurements of their weak thermal radiation by the MIRO instrument. The result is so low that we infer a large porosity (more than 50%).

It has been estimated that such a large porosity could not exist, if the material had been subject to compression by more than 10 kPa. This means that the accretion process whereby the nucleus was built cannot have involved velocities larger than 10 m/s. Theoretical models of comet formation during the very beginning of the solar system predict such low velocities, and they are hence supported by our results.

Moreover, the ROSINA instrument on Rosetta has detected some extremely volatile substances in the comet. The fact that these have survived the whole history and formation of the nucleus implies that very little heating has taken place apart from what is currently caused by sunlight. Such a gentle history is also consistent with the comet being a true witness of the birth of the solar system.

The activity intensifies

In the beginning of the rendezvous, the images already showed signs of material being shed from the nucleus in spite of the large distance from the Sun. The first signs were luminous jets reaching out from the nucleus. This light was actually sunlight scattered from small grains that had been lifted off the nucleus by the pressure of released gases. Later on it became clear that the OSIRIS images also showed single grains of larger size moving away from or around the nucleus.

Those moving away are the smallest, and they are leaving the comet for good. The larger ones have the size of usual snowballs, and they appear to have left the nucleus during its previous approach to the Sun and stayed in orbit ever since. There are probably such chunks spreading along the comet orbit after the comet entered into it in 1959 after a close approach to Jupiter. In due time, there may be a continuous stream of grains, and if the Earth’s orbit crossed this stream, we might see a beautiful meteor shower emanating from the Rosetta comet on certain nights every year. But alas, this is not the case, and we have to be content with the Perseids and other showers that other comets invite us to.

The first jets of dust were relatively weak, and they came mostly from sources in the Hapi region. It is not yet clear, which was the driving gas. Water vapor is a good candidate. However, the ice is not abundant on the surface. This has been shown by the VIRTIS instrument, which has so far failed to detect the spectral signature of water ice. Thus, wherever one looks, no more than a few percent of the surface material can be ice, and the rest has to be of rocky or carbonaceous composition. The ice is probably more abundant just a little below the surface, from where the gas can easily escape.


Picture 3. Dust jets emanating from the nucleus. The latter is intentionally overexposed in order to bring out the much fainter jets.



Recently, some real changes have been seen, as illustrated by the adjacent picture. The activity has intensified, and jets are now coming out of the nucleus practically everywhere. This is no surprise, since the comet has come much closer to the Sun, and the increase of activity is expected to continue. Now starts a new phase of the investigation, which deals with the monitoring of this process. From this we hope to learn much more about the structure and composition of the material and the way erosion works in comets.





 Hans Rickman












































































Last Updated on Tuesday, 04 November 2014 12:02