Rosetta: Rendezvous with the comet Print
Thursday, 25 September 2014 14:42

The Rosetta spacecraft has now been moving in an orbit around the nucleus of its target comet, 67P/Churyumov-Gerasimenko, since several weeks. The orbit is very close – just a few tens of kilometers away – and different from the perfect ellipse that would result, if the target were regular and spherical. By contrast, the early pictures already showed that the overall shape of the nucleus is quite weird, and hence, Rosetta has to travel in a complex, distorted gravity field. But in spite of such difficulties, everything goes as planned, and as we shall now describe, the touchdown site for the Philae lander has been identified.


The outgassing from the nucleus

We continue to see narrow jets of dust grains leaving the nucleus in specific directions, coming from small, local spots on the nucleus surface as far as we can tell. While the tracing of these jets down to the surface is very difficult, the OSIRIS pictures indeed show features that may be witnesses of local ice sublimation. They are holes, where the surface seems to have sunk down with respect to the environment, probably due to local sublimation. Why this happened is not clear, but obviously, the surface must be heterogeneous with characteristics that vary from place to place.

This heterogeneity is clearly seen on the pictures as well. The surface is layered. On top there is a sheet of non-volatile materials, which are probably rich in carbon and therefore very dark.  It is possible that this consists of grains that were lifted off the surface at some other place, where the gas flow was strong, and rained down after being cut off from the flow. Below this surface deposit there may be another sheet of local material that has been evacuated by the sublimation and escape of its most volatile components. Only further down, one might find the unaltered, pristine material.

The OSIRIS images show an abundance of “cliffs” or steep slopes, where this layering can be studied like geologists study the layers in the Earth’s bedrock. But the layering in the comet is not comparable to that in the Earth. It may simply be due to the sublimation of ices and escape of gases from the surface layers of the nucleus. However, it is still too early to pass a definitive judgment!

Why the surface is so varied with so much structure both laterally and vertically is another issue that will require further study. One possibility is that we see evidence that the comet nucleus was formed of small parts (so-called cometesimals) with different composition leading to different abundance of the volatile species, and that we see a patchwork of such cometesimals, which contribute to the surface of the nucleus. Another idea is that the shape of the nucleus focuses the solar energy to some spots, where sublimation becomes more intense, and yet another one is that impact cratering causes local heating and compaction of the surface.


Picture 1. The OSIRIS narrow-angle camera took this dramatic picture on Sept. 5, 2014, from a distance of 62 km. The side of the larger part is in the foreground, while the smaller part (the head) is beyond, facing the camera.



The origin of the comet

In the OSIRIS camera team, we are still struggling with attempts to decide how the nucleus was formed based on its appearance. We also have access to preliminary results from the mass determination that comes from monitoring the radio signal from the probe as it orbits the nucleus. This seems to indicate that our team members, Davidsson and Gutiérrez, were right in their analysis and that the density of the comet is as low as about 0.4 grams per cm3.

From this value we conclude that the nucleus is highly porous. The question is if it is a homogeneous body made up of very porous material like freshly fallen snow, or if it has large voids in its interior between cometesimals that may be less porous internally. Hopefully, Rosetta may later on bring other data, which help to answer the question, but at present one can only guess.

When it comes to how the comet was formed, we work with two competing ideas. One is that the nucleus came together by the merger of many different building blocks, when the solar system was extremely young and planets had not yet been formed. This merger was very gentle, and the aggregate became very porous for that reason. But there is an alternative, according to which those mergers only produced very large objects, hundreds of kilometers across. These would later on collide with each other at large speeds, whereby pieces were torn off and reassembled into new aggregates. Comet nuclei would then be such aggregates – much younger and less pristine than in the first scenario.

Looking at the OSIRIS images, some get the impression that the first idea is the winner, but others get the opposite impression. Therefore, at the moment, the issue is open.


A map of the nucleus

The OSIRIS images have shown that the nucleus hosts a multitude of surface features. There are craters, smooth areas, ragged terrain, large depressions, small pits, excavations, cliffs, and so-called boulders, just to name a few. But it is also clear that different parts of the surface look different, being dominated by different types of features. This lays the foundation for drawing a geologic map of the nucleus, where the borders between different regions are indicated.



Picture 2.   Here, the body is the closer part, and the head (on top) is further away. Different regions with different surface structure are marked by colors.



Such a map is shown on the attached color picture. It is based on detailed analysis of the OSIRIS images, and each color stands for a specific type of terrain as defined by its morphology. However, it is worth keeping in mind that the classification does not reflect a deep understanding of how the morphologies came to be. Geologic maps of the Earth were made before the shaping of our planet by plate tectonics was understood, and we may face a similar situation with the comet. Hopefully, by following the action of the comet as it approaches the Sun, we may reach some understanding in due time.

Awaiting this, we can at least put names on the regions, and this has been done. It may take some time before these names get official status, but the agreed principle is to use the names of Egyptian deities. The reason is of course that the space mission is named after the Rosetta stone, which was found in Egypt and helped clarifying the meaning of the hieroglyphs.


The landing site

During the weekend, 13-14 September, scientists and flight dynamics experts of ESA met in Toulouse to narrow down the choice of the Philae landing site. The landing is scheduled for November 11. A pre-selection had already identified half a dozen candidates, and now the unanimous choice between them was a place on the smaller of the two main parts of the nucleus. This is sometimes called the head, while the bigger part is the body or the belly. The site carries the designation “J” and is marked by a cross on the attached image.


Picture 3.   The cross marks the selected landing site, situated on the “head” of the nucleus. But the uncertainty region covers a large part of the picture.



We have to keep our fingers crossed, because the landing will not be easy. The comet presents us with several unforeseen difficulties. One is the irregular shape of the nucleus, which complicates its gravity field. Another is the unexpectedly strong outgassing that has recently become evident. The motion of the lander on approach to the surface will be affected by the outflowing gas, which acts like a wind carrying the lander sideways. In fact, the uncertainty of the spot of touchdown is estimated to be hundreds of meters, so the cross is just an aiming mark in the midst of a large target area.

When selecting the site, several constraints had to be met. Because of the ragged appearance of the nucleus, safety was a very important issue. From the scientific aspect, perhaps the most important aim is to place the lander either on or very close to the pristine material of the comet. Since the surface has been altered by ice sublimation, leading to the above-described layering, this means landing at a place where the altered layer is very thin. However, this may actually be an unrealistic goal, and we can only hope for the best.








Hans Rickman



Last Updated on Friday, 26 September 2014 12:23