Large heterogeneities in comet 67P as revealed by active pits from sinkhole collapse

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Abstract

Pits have been observed on many cometary nuclei mapped by spacecraft1,2,3,4. It has been argued that cometary pits are a signature of endogenic activity, rather than impact craters such as those on planetary and asteroid surfaces. Impact experiments5,6 and models7,8 cannot reproduce the shapes of most of the observed cometary pits, and the predicted collision rates imply that few of the pits are related to impacts8,9. Alternative mechanisms like explosive activity10 have been suggested, but the driving process remains unknown. Here we report that pits on comet 67P/Churyumov–Gerasimenko are active, and probably created by a sinkhole process, possibly accompanied by outbursts. We argue that after formation, pits expand slowly in diameter, owing to sublimation-driven retreat of the walls. Therefore, pits characterize how eroded the surface is: a fresh cometary surface will have a ragged structure with many pits, while an evolved surface will look smoother. The size and spatial distribution of pits imply that large heterogeneities exist in the physical, structural or compositional properties of the first few hundred metres below the current nucleus surface.

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Figure 1: Location of the pits considered in this study.
Figure 2: Jet-like features in the Seth region.
Figure 3: Depth-to-diameter ratio as a function of pit diameter.
Figure 4: Pit formation mechanism by sinkhole collapse.

Change history

  • 02 July 2015

    The values in the second column of Extended Data Table 2 were corrected.

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Acknowledgements

OSIRIS was built by a consortium of the Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany, the CISAS, University of Padova, Italy, the Laboratoire d’Astrophysique de Marseille, France, the Instituto de Astrofísica de Andalucia, CSIC, Granada, Spain, the Research and Scientific Support Department of the European Space Agency, Noordwijk, The Netherlands, the Instituto Nacional de Técnica Aeroespacial, Madrid, Spain, the Universidad Politéchnica de Madrid, Spain, the Department of Physics and Astronomy of Uppsala University, Sweden, and the 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 acknowledged. This work was also supported by NASA JPL contract 1267923 to the University of Maryland (M.F.A’H. and D.B.). M.F.A’H. is a Gauss Professor at the Akademie der Wissenschaften zu Göttingen and Max-Planck-Institut für Sonnensystemforschung (Germany). This research has made use of NASA’s Astrophysics Data System Bibliographic Services. We thank H. J. Melosh for reviews and criticism.

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Authors

Contributions

J.-B.V. led the study, identified the pits and measured their global parameters. D.B. analysed outbursts and phase change transitions and prepared the sinkhole model. S.B. performed the detailed morphology analysis. H.S., C.B., P.L., R.R., D.K. and H.R. are the lead scientists of the OSIRIS project. The other authors are co-investigators who built and ran this instrument and made the observations possible, and associates and assistants who participated in the study.

Corresponding author

Correspondence to Jean-Baptiste Vincent.

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The authors declare no competing financial interests.

Additional information

All data presented in this paper will be delivered to the ESA’s Planetary Science Archive and NASA’s Planetary Data System in accordance with the schedule established by the Rosetta project and will be available on request before that archiving.

Extended data figures and tables

Extended Data Figure 1 Perihelion distance of comet 67P as a function of time.

Solid line, mean value of the orbits integrated according to a Monte Carlo method. Dashed lines, standard deviation of the mean value. a, Perihelion distance over the last 270 years, when comet 67P experienced several close encounters with Jupiter. b, The long term integration over the full dynamical lifetime of the comet (10,000 years).

Extended Data Figure 2 Multiple views of the Seth_01 pit observed by the OSIRIS camera.

a, Southern part of the pit wall; b, western part of the pit wall; c, d, eastern part of the pit wall with different illumination conditions; and e, southeastern part of the pit wall observed in the shadow. In all the images, the green arrow points to the same boulder and the blue arrow to the same ridge inside the pit. The orange arrows point to terraces within the pit. The Seth_01 pit is 220 m in diameter.

Extended Data Figure 3 Multiple views of the Ma'at_01, Ma'at_02 and Ma'at_03 pits observed by the OSIRIS camera.

a, b, Side views of the pits with different illumination conditions; c, opposite viewing conditions highlighting the other side in the shadow; and d, e, detailed views of Ma’at_02 (d) and Ma’at_01 (e) from light reflection in the shadow. Note the clear cross-cutting fractures on the wall in e. In c, the white line is an artefact due to stretching of the image to highlight the shadowed part. The Ma’at_02 pit is 130 m in diameter. The blue, green and oranges arrows point to the same features in each image.

Extended Data Figure 4 Additional views of the Seth_01 and Ma'at_01 pits.

a, The floor of Seth_01 shows no accumulation of boulders; the same is true for Seth_02 and Seth_03 (not shown). b, The floor of Ma’at_01 shows a few boulders that have accumulated; note the activity located at the bottom. c, The floor of Ma’at_02 shows an asymmetric accumulation of boulders that could be the result of upper wall collapse.

Extended Data Figure 5 Boulder counts in Ma'at_01 and Ma'at_02.

We counted boulders on the floor of Ma’at_01 and Ma’at_02. We used OSIRIS narrow angle camera (NAC) images with a resolution of 1.2 metres per pixel, acquired at 67 km from the comet nucleus centre. a, b, The illumination conditions are such that almost 80% of the floor of Ma’at_01 (a) and 95% of the floor of Ma’at_02 (b) are illuminated and the pits are facing the observer, which ensures an unbiased boulder count. We identified 23 boulders inside Ma’at_01 and 68 on the floor of Ma’at_02. The diameter of the boulders (in metres) is indicated by the coloured circles; see inset. Despite the 1.2 metres per pixel resolution, we were able to identify some boulders with a diameter between 1.5 m and 2.5 m (9 in Ma'at_01 and 15 in Ma'at_02), owing to the presence of elongated shadows. The maximum boulder diameter is 4.3 m in Ma’at_01 and 9.0 m in Ma’at_02.

Extended Data Figure 6 Cumulative boulder-size distribution for Ma’at_01 and Ma’at_02.

This distribution has a power index of for Ma’at_01 (left) and for Ma’at_02 (right), for boulder diameters greater than 3 m; the corresponding power laws are indicated the by the solid (fit) lines. Boulders smaller than 3 m in diameter are at the edge of our detection limit, meaning that the counts for these boulders are less reliable than the other counts; consequently, they were not included when fitting the power law. The higher number of boulders in Ma’at_02 is consistent with the theory that boulders are debris that falls from the walls as the pit erodes, long after the initial formation of the pit. Error bars are defined as the square root of the cumulative number of boulders to reflect the increasing diameter uncertainty for small boulder sizes.

Extended Data Figure 7 RGB view of the Seth pits and the Hapi region.

The red–blue–green components of this colour map represent colour ratios between the reflectance signals measured at different wavelengths: red, 989 nm/649 nm; green, 480 nm/649 nm; blue, 649 nm. The colour map is overlaid onto a grey image showing the comet surface. The Hapi region and part of Seth appear with a blue hue, indicative of a bluer spectral slope than other regions of the nucleus, which are typically red. The interior of Seth_01, Seth_02 and Seth_03 have the same blue hue that is characteristic of the active Hapi region.

Extended Data Figure 8 Modelled critical ceiling thickness for increasing cavity diameter and different tensile strengths.

We predict the average tensile strength of a collapsed layer using the dimensions of a pit (Methods). For example, a pit of 220 m in diameter and 185 m in depth (such as, Seth_01) suggests that the collapsed layer had an average tensile strength of 50 Pa.

Extended Data Table 1 List of pits considered in this paper
Extended Data Table 2 List of images used

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Vincent, J., Bodewits, D., Besse, S. et al. Large heterogeneities in comet 67P as revealed by active pits from sinkhole collapse. Nature 523, 63–66 (2015). https://doi.org/10.1038/nature14564

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