A comet turns into the Rosetta stone
The European space probe’s successful mission ended on the surface of 67P/Churyumov-Gerasimenko
It is the end of a long journey: launched on 2 March 2004, the Rosetta space probe swung into an orbit around the comet 67P/Churyumov-Gerasimenko in August 2014 and set down the Philae lander onto the cometary surface in November of the same year. The mother probe itself now landed on the comet’s nucleus – and thereby completed the mission. However, scientists will be working on all the recorded images and measurement data for a while yet. The initial findings already promise to provide a wealth of new knowledge.
Text: Helmut Hornung
As Rosetta approached the nucleus of the comet 67P/Churyumov-Gerasimenko more than two years ago, a strange shape appeared on the images it transmitted back. What they revealed was not the expected “space potato” measuring only a few kilometres across; instead, a dumbbell-shaped body with two distinct parts gradually took shape, and reminded the researchers of a rubber duck. The comet’s surface turned out to have a structure which was by no means smooth, but covered by several terraces. These are obviously several hundred metres deep and structured like the skin of an onion.
Although the scientists are still puzzling over their origin, they have a few ideas as to how the cosmic duckling could have formed. There are two fundamental possibilities: the nucleus was “carved out” in one piece from the original body; or it consists of two individual bodies held together by gravity. Most researchers tend towards the latter possibility – especially since the two ends of the dumbbell have different compositions and the densities of the two parts differ by around ten percent. This seems to support the theory that the two bodies came together after each had formed.
The nucleus measures around 4 x 3.5 x 3.5 kilometres and rotates about its axis once every 12 hours and 24 minutes. Rosetta found mainly water (in the form of ice, of course) as well as carbon monoxide, carbon dioxide and oxygen – which was surprisingly the fourth most abundant substance – on the comet. The space probe also detected ethane, methane, nitrogen and argon. The chemical analysis of the material making up the nucleus leads the researchers to conclude that it formed at a temperature of minus 250 degrees Celsius.
This extremely low temperature means the comet must have been born far away from the Sun, which is in strong agreement with the customary views of the origin of these cosmic drifters, which say that they formed at the edge of a protoplanetary disc that orbited the infant Sun just under 4.6 billion years ago and gave birth to the planetary system. The dust and gas particles in this primeval cloud stuck to each other and formed larger and larger crumbs. According to one theory, they grew into cometary nuclei within a few million years in the cold outer regions of the cloud.
In another scenario, the cometary nuclei are evidence of collisions between larger rocks which had already formed according to the aforementioned process. These “accidents” produced a great many pieces of debris a few kilometres in diameter. Many of them continued their haphazard journey and suffered collisions themselves. Computer simulations show that they could remain stuck to each other if they had the right impact speed – and form the twin body observed with 67P/Churyumov-Gerasimenko, and several asteroids as well.
One of Rosetta’s aims was to follow the comet as it got increasingly closer to the Sun and observe the activities this triggered on the nucleus. These activities occur because the solar rays heat the surface of a comet as it approaches our Sun. The nucleus literally “melts” and the volatile substances undergo a direct transition from the solid to the gaseous state without first going through the liquid state. This process is called sublimation.
The emanating gas pulls dust particles along with it. This material finally envelops the nucleus, forming the so-called coma in the process. When the comet gets even closer to the Sun, the ultraviolet radiation ionizes the gas in the coma, and the solar wind yanks it away with it – a gas tail forms that is usually very linear. In addition, the pressure of the particles of light (photons) from the Sun acts on the dust and forms the usually fan-shaped dust tail.
All the activities described here and observed by Rosetta only take place on just over five percent of the total surface of 67P/Churyumov-Gerasimenko. This is where jets of gas and dust are ejected, blowing mainly steam from the interior of the nucleus into space. Some of these jets were “firing” only for a few minutes before disappearing again.
And talking about water: billions of years ago, many comets are thought to have crash-landed on the infant Earth, bringing a large proportion of the water into the oceans. But how high is this proportion? Researchers obtain their information here by comparing the ratio of heavy to normal hydrogen in terrestrial oceans and in comets. If the comets are the main supplier of the water, this ratio should be roughly the same. Rosetta measured a clear deviation by a factor of three for 67P/Churyumov-Gerasimenko.
As already mentioned, water ice is one of the main components of comets, yet no pure ice is in evidence on the images of the surface. Instead, the nucleus is covered by a dark layer of dust. This layer is approx. 10 to 20 centimetres thick in some places; the researchers estimate, however, that it could even be several metres thick in other places.
In this dust layer, spectrometers on board Rosetta and the Philae lander found not only various hydrocarbons but also several dozen different organic molecules, including glycine. The Stardust space probe had already detected this amino acid in the dust of the Wild 2 comet back in 2004. This is not really surprising, as, in interstellar clouds in particular, the astronomers have so far identified more than 150 molecules in space, including many organic compounds. Did comets bring the seeds of life to Earth in the distant past? This question remains unanswered, even after Rosetta.
The Moon has only the hint of an atmosphere and is scarred by craters gouged by rocky debris crashing onto its surface. The surfaces of asteroids have a similar appearance. It could therefore be possible that cometary nuclei are also covered by craters. Rosetta revealed structures that at first sight do actually resemble craters, but which turn out to be something different on closer inspection: almost circular pits or vents several hundred metres in diameter.
Unlike craters, which are formed from the outside, the pits appear to form from the interior of the nucleus outwards. And they are evidently connected to the aforementioned activity on the comet as it approaches the Sun, as gas and dust stream out of some comets. The explanation: When the nucleus heats up, gases such as carbon monoxide and carbon dioxide are formed by sublimation beneath the surface. Material is thereby lost and cavities form. In the course of time, the nucleus is “hollowed out” from the inside, as it were. Finally, the material above gives way, the ground caves in, pits and vents are formed.
According to this scenario, the nucleus of a comet should be ridden with large numbers of cavities and be as porous as a sponge. Rosetta confirmed this; around eighty percent of the nucleus apparently consists of such cavities. The researchers are discussing the question of whether cavities were even encapsulated into the nucleus during the birth of the comet – for example by large numbers of small bodies several dozen metres in size sticking together – and that they then expanded during the phases of activity.
In August 2015, 67P/Churyumov-Gerasimenko was the closest it came to the Sun, and Rosetta’s orbit had to be increased to a height of more than 400 kilometres due to the comet’s increasing activities. It was decreased again during the following months and was less than ten kilometres in summer 2016. On 2 September, the eye of the Osiris onboard camera immediately discovered the Philae lander in the shadow of a rock ledge from a distance of only 2.7 kilometres.
The vehicle had maintained contact with ground control for around 56 hours after the unsuccessful landing on 12 November 2014 – the anchoring mechanism failed – before switching off. In June 2015, Philae then transmitted several data packets of measurements before finally falling silent.
The mother probe will meet this fate as well: on 30 September, Rosetta will descend into the Ma`at region and will hopefully continue to send a great deal of images and data until shortly before touching down. This will not, however, be a controlled landing by any means, as the probe is not designed for this. Rosetta will thus continue its journey around the solar system as a heap of scrap metal on the surface of the comet. And the researchers on Earth will be evaluating the valuable data for a few more years yet.