A special issue of the journal Science highlights seven new studies that delve into the data that has been collected by ESA’s probe Philae on 67P/Churyumov-Gerasimenko.
This false-color four-image mosaic comprises images taken from a distance of 28.7 km from the center of Comet 67P/Churyumov-Gerasimenko on 3 February 2015. The mosaic measures 4.2 x 4.6 km. Image credit: ESA / Rosetta / NAVCAM / CC BY-SA IGO 3.0.
In a research article by Dr Fred Goesmann from the Max Planck Institute for Solar System Research in Germany and his colleagues, the team analyzes the composition of Comet 67P/Churyumov-Gerasimenko using the COmetary SAmpling and Composition (COSAC) instrument, designed to identify organic compounds in the comet and thus contribute to a deeper understanding of the history of life on Earth.
The instrument collected molecules from 10 km (6.2 miles) above the comet surface, after the initial touchdown, and at the final site.
16 organic compounds were identified, divided into six classes of organic molecules (alcohols, carbonyls, amines, nitriles, amides and isocyanates). Of these, four organic compounds were detected for the first time on a comet (methyl isocyanate, acetone, propionaldehyde and acetamide).
Almost all the compounds detected are potential precursors, products, combinations or by-products of each other, which provides a glimpse of the chemical processes at work in a cometary nucleus, and even in the collapsing Solar Nebula in the very early Solar System.
COSAC identified a large number of nitrogen compounds but no sulfur compounds, contrary to what the ROSINA instrument on board Rosetta had observed. This suggests that the chemical composition varies depending on the area sampled.
In a related study, Dr Ian Wright of the Open University, UK, and co-authors analyzed organic compounds on the comet but used a gas analysis instrument called Ptolemy.
The instrument detected compounds containing carbon, hydrogen and oxygen – all of which are key elements in the formation of water and simple sugars.
“The compounds detected are not biogenic in nature and therefore do not indicate signs of life,” Dr Wright said.
“The compounds found are elements that will have gone into the mix that led to the formation of the life on Earth.”
Not only has Ptolemy’s first analysis given scientists insight into what comets are made of, it has also revealed more about what chemical reactions occur on the surface.
“We now know more about the surface of 67P/Churyumov-Gerasimenko that we ever did before. Findings such as the fact that its surface is soft and dusty, but beneath that is hard layer of ice, will play an important part to inform plans for future comet landings and space exploration,” said co-author Dr Andrew Morse, also from the Open University.
In a separate paper, Dr Jens Biele of the German Aerospace Center and his colleagues describe the critical moments where the probe descends on the comet, only to bounce off the soft, intended landing area and finally settle on a harder surface farther away.
Analysis of the different compressive strengths of the two surfaces, based on the bouncing trajectory, sheds more light on the evolution of comets and could improve the design of future comet missions.
Previously, researchers trying to understand comet surface material strength had to rely on indirect observations, which have ranged widely, and include some very low values that have raised questions about whether a comet could successfully dock on such weak material.
After analyzing the depth profile of the lander footprint features with imaging tools, the team believes that the Philae’s feet first came in contact with a soft granular surface, which was approximately 0.82 feet (0.25 m) thick, with a harder layer below.
This layering creates a compressive strength of about 1 kilopascal, whereas the compression strength of Philae’s final, much harder landing site exceeded 2 megapascals (2,000 kilopascals), possibly contributing to why only one leg was able to anchor to this latter surface, and partially at that.
In a study by Dr Wlodek Kofman from the Université Grenoble Alpes in France and co-authors, the scientists found the composition of the head of 67P/Churyumov-Gerasimenko to be fairly homogenous.
To get a better idea of the comet’s interior, they directed electromagnetic signals through the nucleus of the comet to Rosetta on the opposite side. The signals Rosetta received lacked a scattering pattern, indicating that the interior of the comet is uniform throughout.
The scientists used these electromagnetic measurements, which analyze the permittivity (resistance of the electrical field), to further determine that the comet has dust/ice ratio of 0.4 to 2.6 and a very high porosity of 75 to 85 percent.
In a study by Dr Jean-Pierre Bibring of CNRS/Université Paris Sud in France and colleagues, the surface of the comet is analyzed in panoramic images taken by a set of seven cameras as part of the Comet Infrared and Visible Analyser (CIVA).
The collection of images, taken just after Philae’s initial bounce and final touchdown, reveal a fractured surface with a variety of grain scales and reflective rock structures, offering unprecedented insights into this type of primitive space matter.
As Philae approached the comet, far-field sequence and the near-field sequence images revealed a clearer picture of the comet’s geography.
An analysis of Rosetta Lander Imaging System (ROLIS) descent images by Dr Stefano Mottola of the German Aerospace Center and co-authors suggests that the comet’s landscape is shaped by erosion.
Boulders jutting out of granular areas are surrounded by depressions that are reminiscent of the wind tails observed on Earth, the result of wind erosion and deposition.
The scientists speculate that some of the erosion occurs from ‘splashing,’ the ejection of one or more soil particles by the impact of an incoming projectile, which they confirmed using models.
Finally, to determine thermal and mechanical properties of the comet, a team of scientists led by Dr Tilman Spohn from the German Aerospace Center analyzed data from Multi Purpose Sensors for Surface and Subsurface Science (MUPUS) thermal and penetrating sensors aboard Philae.
Due to the lander’s unintentional resting spot, the sensors were unable to penetrate the hard surface to attain subsurface temperature readings. However, the data reveals that the comet’s daytime surface temperature varies between 90 and 130 Kelvin.
By analyzing thermal inertia and soil composition, the scientists found that the surface at the landing spot is covered with a highly compact, microporous, dust-ice layer with a porosity of 30 to 65 percent.
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Fred Goesmann et al. 2015. Organic compounds on comet 67P/Churyumov-Gerasimenko revealed by COSAC mass spectrometry. Science, vol. 349, no. 6247; doi: 10.1126/science.aab0689
I. P. Wright et al. 2015. CHO-bearing organic compounds at the surface of 67P/Churyumov-Gerasimenko revealed by Ptolemy. Science, vol. 349, no. 6247; doi: 10.1126/science.aab0673
Jens Biele et al. 2015. The landing(s) of Philae and inferences about comet surface mechanical properties. Science, vol. 349, no. 6247; doi: 10.1126/science.aaa9816
Wlodek Kofman et al. 2015. Properties of the 67P/Churyumov-Gerasimenko interior revealed by CONSERT radar. Science, vol. 349, no. 6247; doi: 10.1126/science.aab0639
J.-P. Bibring et al. 2015. 67P/Churyumov-Gerasimenko surface properties as derived from CIVA panoramic images. Science, vol. 349, no. 6247; doi: 10.1126/science.aab0671
S. Mottola et al. 2015. The structure of the regolith on 67P/Churyumov-Gerasimenko from ROLIS descent imaging. Science, vol. 349, no. 6247; doi: 10.1126/science.aab0232
T. Spohn et al. 2015. Thermal and mechanical properties of the near-surface layers of comet 67P/Churyumov-Gerasimenko. Science, vol. 349, no. 6247; doi: 10.1126/science.aab0464
