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%.