Rosetta’s views of Comet 67P

Mauro Pirarba BSc, a Planetary Sciences Graduate Certificate student and Fellow of the Royal Astronomical Society, discusses Dr Ramy El-Maarry’s recent talk on the geology of Comet 67P/Churyumov-Gerasimenko.

Figure 1 – Five comets have been photographed at close range during flyby missions, but only one, comet 67P, has been studied closely for an extensive period of time (image credit: El-Maarry et al., 2019, https://link.springer.com/article/10.1007%2Fs11214-019-0602-1)

Comets are temperamental, often breaking all rules, suddenly appearing out of nowhere, occasionally getting close enough to the Sun and the Earth to display long tails that make us marvel at their beauty and diversity. They have been studied by astronomers for centuries and yet we still do not understand them fully.

What can Dr. Ramy El-Maarry, a geologist at Birkbeck College, possibly tell about one of them, Comet 67P/Churyumov-Gerasimenko, at a special meeting organized by the Royal Astronomical Society?

Does that C-something – G-something sound vaguely familiar? Perhaps you are more likely to remember another name, Rosetta, the probe of the European Space Agency (ESA) tasked with deciphering its mysteries, a few years ago. Launched in 2004, Rosetta reached Comet 67P in August 2014 and spent two years observing it closely.

“I have 80,000 images from that mission and 20 minutes to go through them…”, Ramy started his talk, to everybody’s laughter.

In fact, such a big figure hints at an unprecedented and extraordinary achievement. Twelve spacecrafts have sent back to Earth data about eight different comets and images of six of them (see figure.1). What makes Rosetta stand out is the length of the observation, two years, and its closeness, on average a few tens of kilometres. You may also remember that the mission included a lander, Philae, which failed to anchor itself to the ground and bounced a few times, before settling down and sending back images and data. The greatest feat though was achieved by the “mothership”, Rosetta, which accompanied the comet for most of its orbit around the Sun, taking images that show details as small as a fraction of a meter. These images have allowed scientists for the first time to observe geological processes, as they happened, on the surface of a comet.

We are all familiar with the effects of water, ice, temperature excursions and wind in weathering, transporting and depositing sediments, reshaping the landscape on the Earth. We’ve seen images of craters and the effects of earthquakes and volcanic eruptions on our planet. What geological processes has ESA’s spacecraft uncovered on Comet Churyumov-Gerasimenko?

This is a tiny world with a miniscule gravity and an irregular shape (approximately 4 x 4 x 2 km), made of porous and light material rich in different ices. The agent driving most geological processes on P67 is solar radiation. As the comet orbits the Sun along a very elongated orbit, which takes it further away from our star than Jupiter and then brings it not much closer to the Sun than Mars, insolation varies dramatically and seasons become extreme. Autumn and winter last about 5.5 years in the northern hemisphere, while the southern spring and summer last nearly a year and are relatively hot, causing the icy surface to sublimate copiously, creating a coma, a tenuous atmosphere. Activity is patchy, occasionally “violent”, jets of gas burst into space taking dust and larger particles with them. Significant amounts of gas and some of the dust are lost, but part of the solid material is transported by “winds” to the northern hemisphere, blanketing it with dust and coarser grains. Over time, the material moves down gentle and steep slopes, “pushed” by the weak gravity, forming a variety of terrains. These give the northern hemisphere a very different look from that of the southern half, which is quite rugged (see figure.2). One of the most striking images shown by Ramy was one where aeolian ripples appeared on an otherwise smooth terrain in the neck of the comet, a narrow region connecting the two main “lumps” that make up 67P. No one had ever thought such features could occur on a comet.

Figure 2 -Dr. El-Maarry shows how different the northern and southern hemispheres of comet 67P appear (image credit: El-Maarry et al., 2019, https://link.springer.com/article/10.1007%2Fs11214-019-0602-1 and El-Maarry et al., 2016, https://www.aanda.org/component/article?access=doi&doi=10.1051/0004-6361/201628634).

The rate of ices’ sublimation is so high in some areas that several meters of materials are removed during the summer, uncovering a variety of features. Some of these are circular and resemble craters, others look like depressions that wax and wane over time. Their origin is baffling geologists, as that of other so-called transient surface features. Somewhere else on the surface of the comet new pits (probably sinkholes) appear and cliffs collapse. Outbursts of activity, driven by the Sun’s heat, propel jets that like rockets push the ground in the opposite direction, varying the speed of rotation of the comet. The resulting forces cause tension, leading to the formation of tectonic fractures, as several photos shown by Dr. El-Maarry clearly prove. Picture after picture a tiny complex word emerges, revealing to us in detail what we had been able to observe only from very far or for very brief instants, in the case of other comets.

In August 2016, Rosetta ended its mission and landed softly on Comet Churyumov-Gerasimenko. The legacy of the mission is not just what it has taught us about Comet P67, but it goes beyond. As Ramy summed up at the end of his talk, the number of images and other data collected in situ is helping scientists not only to understand 67P, but also interpret previous and future mission to other comets. A long time is likely to pass, before a probe like Rosetta will be launched.

In the meantime, Comet Interceptor, a new exciting cometary mission is taking shape at ESA. We’ll ask Ramy to bring a few thousand pictures next time…

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