Abstract
On 12 November 2014, the Philae lander descended towards comet 67P/Churyumov–Gerasimenko, bounced twice off the surface, then arrived under an overhanging cliff in the Abydos region. The landing process provided insights into the properties of a cometary nucleus1,2,3. Here we report an investigation of the previously undiscovered site of the second touchdown, where Philae spent almost two minutes of its cross-comet journey, producing four distinct surface contacts on two adjoining cometary boulders. It exposed primitive water ice—that is, water ice from the time of the comet’s formation 4.5 billion years ago—in their interiors while travelling through a crevice between the boulders. Our multi-instrument observations made 19 months later found that this water ice, mixed with ubiquitous dark organic-rich material, has a local dust/ice mass ratio of \({2.3}_{-0.16}^{+0.2}:1\), matching values previously observed in freshly exposed water ice from outbursts4 and water ice in shadow5,6. At the end of the crevice, Philae made a 0.25-metre-deep impression in the boulder ice, providing in situ measurements confirming that primitive ice has a very low compressive strength (less than 12 pascals, softer than freshly fallen light snow) and allowing a key estimation to be made of the porosity (75 ± 7 per cent) of the boulders’ icy interiors. Our results provide constraints for cometary landers seeking access to a volatile-rich ice sample.
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Data availability
All OSIRIS, VIRTIS, RPC-MAG and ROMAP calibrated data are publicly available through the European Space Agency’s Planetary Science Archive website (https://archives.esac.esa.int/psa/). The Supplementary Information contains additional supporting images, data and explanatory text with the aim of allowing readers to understand what we have done and how we have done it.
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Acknowledgements
B.G. and J.B. thank Deutsches Zentrum für Luft- und Raumfahrt (DLR) for continuous support and Deutsche Forschungsgemeinschaft for their support under grant Bl 298/24-2 in the framework of the Research Unit FOR 2285 ‘Debris disks in planetary systems’. OSIRIS was built by a consortium led by Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany, in collaboration with CISAS, University of Padova, Italy, Laboratoire d’Astrophysique de Marseille, France, Instituto de Astrofisica de Andalucia, CSIC, Granada, Spain, the Scientific Support Office of the European Space Agency, Noordwijk, The Netherlands, Instituto Nacional de Tecnica Aeroespacial, Madrid, Spain, Universidad Politechnica de Madrid, Spain, the Department of Physics and Astronomy of Uppsala University, Sweden, and Institut für Datentechnik und Kommunikationsnetze der Technischen Universität Braunschweig, Germany. The support of the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), Sweden (SNSB), and the ESA Technical Directorate is gratefully acknowledged. Those authors who are part of the VIRTIS and GIADA teams wish to thank the Italian Space Agency (ASI, Italy; contract number I/024/12/2) and Centre National d’Études Spatiales (CNES, France) for supporting their contribution. The contribution of the ROMAP and RPC-MAG teams was financially supported by the German Ministerium für Wirtschaft und Energie and the Deutsches Zentrum für Luft- und Raumfahrt under contract 50QP1401. This research has made use of the scientific software shapeViewer (www.comet-toolbox.com). Video rendering was powered by PRo3D, a viewer for the exploration and analysis of planetary and smaller body surface reconstructions. It was developed by VRVis Zentrum für Virtual Reality und Visualisierung Forschungs-GmbH in close collaboration with Joanneum Research and Imperial College London; see http://pro3d.space for more details. Trajectory and instrumental information relevant to the observations performed on Rosetta was based on the use of SPICE kernels. We acknowledge the important role played by the Rosetta Science Ground Segment, the Rosetta Mission Operations Team and the Philae Lander team(s) in the running of the Rosetta mission and Philae Lander Operations.
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Contributions
Identification of the skull-top crevice ice and other touchdown points was made by L.O’R. The lead writer of the paper is L.O’R. Contributions to Methods and sections of Supplementary Information were made by L.O’R., P.H., S.F., H.V.H., G.F., A.R., M.C., J.B., B. Gundlach, A.R., M.d.P.C.P. and G.P. The ROMAP and RPC-MAG data analysis was performed by P.H., K.-H.G. and H.-U.A. in close collaboration with L.O’R. The compressive strength and porosity analysis was performed by J.B. and B.G. The OSIRIS image analysis was performed by L.O’R., S.F. and H.V.H. The VIRTIS data analysis was carried out by G.F., A.R., M.C. and F.C. The skull-top crevice ice and dust analysis was carried out by A.R., D.B.-M., M.K., O.G. and N.O. The OSIRIS image processing was performed by S.F., H.V.H., G.K., C.T. and H.S. Trajectory data analysis was performed by B. Grieger, L.O’R., R.A.B., P.H., J.-B.V., C.T. and H.S. Shape model support and analysis were provided by L.J., J.-B.V., R.A.B., M.d.P.C.P. and G.P. Figures (in support of analysis) were generated by L.O’R., P.H., R.A.B., J.B., B. Gundlach, S.F., H.V.H., G.F., A.R. and M.C. All authors have participated in the review of the paper and its contents.
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The authors declare no competing financial interests. One of the authors, N.O., is currently an editor at Nature Communications, but was not in any way involved in the journal review process.
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Extended data figures and tables
Extended Data Fig. 1 Comparison of pre-landing and post-landing images.
a, b, Comparison of the location of the skull-top ridge (yellow rectangle) as it looked in a pre-landing OSIRIS image (a; 22 October 2014) and in a post-landing image (b; 14 May 2016). The yellow dashed lines outline the skull-top boulders (see Supplementary Video 1 for a detailed comparison), while context terrain similarities between images are shown in blue dashed lines. Although the spacecraft distance is the same (about 8 km), the solar illumination and the Rosetta viewing geometry differ between the images. An arrow points at the Philae lander location in b.
Extended Data Fig. 2 OSIRIS colour composites of skull-top boulders.
(In a–h, the boxed feature in the main image is shown magnified in the inset. In i, j, no magnification is required.) a, b, Pre-landing images of skull-top boulders. RGB setting; ‘green’ = F24 (480.7 nm), ‘red’ = F22 (649.2 nm), ‘blue’ = F16 (360.0 nm). c–e, Skull-top boulders observed in December 2014. RGB setting; ‘green’ = ‘red’ = F22 (649.2 nm), ‘blue’ = F24 (480.7 nm). f–i, The skull-top boulders observed in March 2016. RGB setting; ‘green’ = ‘red’ = F22 (649.2 nm), ‘blue’ = F24 (480.7 nm). j–m, Colour composite of the skull-top boulders in early and mid-June 2016 with the brightly lit ice feature visible in k–m specifically. RGB setting; ‘green’ = F24 (480.7 nm), ‘red’ = F22 (649.2 nm), ‘blue’ = F16 (360.0 nm).
Extended Data Fig. 3 Illumination geometry of the skull-top crevice.
a, OSIRIS NAC image of the Abydos region on 12 June 2016 with three overlaid azimuth plots. The central point of each plot is positioned on the skull-top crevice. The two skull-top boulders are outlined by the green ellipse and the crevice by a tiny purple ellipse inside it. At this central point the viewer is at a 90° angle, that is, an overhead view; each concentric circle represents a decrease in viewing angle of 10°. The large red box encloses an azimuth plot overlaid on the image and the green line shows the horizon. The red area in the two left-hand azimuth plots shows the track of the Sun over this region during the full period of the Philae landing and to the end of the Rosetta mission, while the yellow line overlay is the crevice’s own horizon mask (created as explained in b). b, Example azimuth plot for 21 August 19:19–19:24 ut with Rosetta’s position and the Sun’s position at this time marked by the very short green and red lines. These short lines are then connected by long blue lines to the central crevice. This gives the line of sight for the OSIRIS camera and for solar illumination of the crevice during this period. We then look at the OSIRIS image (inset) to see if the crevice is illuminated. We find that it is, which is why it lies inside the red-lined horizon mask. The dot-dashed line passes right through the crevice, meaning that the right of that line is the right side of the crevice and the left, the left side.
Extended Data Fig. 4 VIRTIS water-ice analysis plots.
a, b, Average radiance factor of the skull-top boulder location (blue curve) and of the nearby dark terrain (red curve) before (a) and after (b) normalization at 550 nm. c, Theoretical abundance of water ice as a function of the spectral slope in the visible (VIS) spectral channel44. Black dots and dashed lines indicate a water-ice-rich region observed by VIRTIS to calibrate the theoretical curves. The black line represents a solely areal mixing case and the red line a solely intimate mixing case. The blue and green lines represent a merge of both, with different mixing ratios of 1 and 10, respectively. The x-axis values correspond to slope values scaled to the dark terrain unit viewing conditions (Fig. 3b, panel b1, red box) for observation V1_00424522185.QUB.
Extended Data Fig. 5 Combined ROMAP and RPC-MAG magnetic-field and boom measurements.
The three components of the magnetic field observations (Bx, By, Bz, respectively top, middle and bottom) are shown starting before TD2 at 17:23:00 utc and ending shortly after the boom movement was detected. This figure also shows the concurrent orbiter RPC-MAG observations as a reference. See Methods and Supplementary Methods section 7 for a detailed description.
Extended Data Fig. 6 Philae interaction geometry and boulder volume-filling factor.
a, Interaction geometry of Philae with the ice/dust at the moment of deepest penetration in TD2c. b, Philae superimposed on an OSIRIS image (2 September 2016), showing an interaction geometry equivalent to that in a. c, Compressive stress curves for a boulder consisting of pebbles. Note that the volume-filling factor Φ denotes packing of porous pebbles only; these are then further packed to make up the whole boulder. See Methods for a detailed explanation of this panel.
Supplementary information
Supplementary Information
This file contains fourteen sections describing the engineering and scientific detective story that serves as the backdrop to this paper. While some sections are high-level and descriptive, others are very detailed as they expand the contents of the main paper Methods section. Fifteen Supplementary Figures are provided; four are videos.
41586_2020_2834_MOESM3_ESM.mov
Video 1 Animation showing comparison between pre and post-landing images - This video allows the viewer to see a comparison being made between the image from 22nd October 2014 and that taken on the 14th May 2016. The comparison shows the similarities (yellow boxes) and also the differences (red boxes).
41586_2020_2834_MOESM5_ESM.mov
Video 3 Animation of Fig. 2 of main paper - Philae is shown flying through Skull-top crevice in (b) with the link to the data observed by ROMAP (a). As Philae impresses on TD2a, b, c and d, the corresponding markings are highlighted. Images (c), (e) and (f) are not present in Fig. 2 of the main paper but Philae is shown also flying through these images.
41586_2020_2834_MOESM6_ESM.mov
Video 4 Animation showing the TD2b Dust wall - This animation shows a repeated set of images (going forward in time and then in reverse order) from the back side view of the skull-top boulders where a structure separate from the crevice is presented i.e. the dust wall (highlighted in yellow).
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O’Rourke, L., Heinisch, P., Blum, J. et al. The Philae lander reveals low-strength primitive ice inside cometary boulders. Nature 586, 697–701 (2020). https://doi.org/10.1038/s41586-020-2834-3
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DOI: https://doi.org/10.1038/s41586-020-2834-3
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