Within the MiARD project, the team members at Heriot-Watt University on Scotland are using terrestrial rocks as analogues for the porous material expected at the cometary surface when they carry out numerical simulations of gas flow through such porous material.
The link below is to a file from an X-ray computer tomography or ‘CT’ scan of a porous rock. (On some platforms the file will be viewable in your browser, on others you will have to download the file and use a viewer for .stl files, such as ‘Preview’ on Macintoshes or ‘Meshlab’ on Macintosh, Windows or Linux platforms.). The image is taken from the same file, and shows the extremely high porosity of this rock sample. Estimates of the porosity of the cometary surface vary, but it may be as high as 75%.
Two recent scientific publications indicate the dynamic nature of comets as they pass near the Sun. An analysis of images taken from the Rosetta spacecraft show that a large crack between the two halves of the comet has grown, thought to be because the spin of the comet was altered as large amounts of gas and dust (to depths of several meters per day) were ejected in jets heated by the Sun. Furthermore, a different team of authors has shown that a 100 m high cliff on the comet collapsed (in July 2015, shortly before the comet came closest to the Sun in August 2015). Amazingly, the OSIRIS camera onboard Rosetta actually captured the debris plume of about one thousand tonnes of material associated with this collapse. The supplementary figures to this paper include anaglyphs of the cliff region – 3D images that can be viewed through red and cyan filters, and a video illustrating the event.
Although this work was not done as part of the MiARD project, many of the authors of the two studies linked to above are also contributing to the project work, and a major goal of the detailed shape models being produced by the MiARD project team is to enable the study of the geomorphology of the comet, including changes over time.
Various shape models and 3D renderings of comet 67P can be seen at the websites maintained by Matthias Malmer and Greg Frieger. In particular, the Frieger site allows different shape models to be compared for 67P, and also shows the shape models for several other comets. Very soon, the MiARD project will have a new higher resolution shape model which will be displayed on this website, and probably also made available to others.
The Laboratoire d’Astrophysique de Marseille, one of the partners in the MiARD project, have just released this rendering of their digital terrain model of the Agilkia area of the comet where the Philae lander first touched down. The name Agilkia was selected by the European Space Agency from suggestions by the public, and is taken from the name of an island in the Nile river in Egypt.