Four years after the end of the European Space Agency’s Rosetta mission, detective work combining data from several instruments aboard the Philae lander and the Rosetta orbiter has identified the second Philae landing site on comet 67P/Churyumov-Gerasimenko, also nicknamed "Chouri".
Dropped on November 12, 2014, Philae landed well at the originally planned site, Agilkia, but was unable to anchor there because of a failure of its harpoons. So the lander bounced and began an adventurous flight over the comet for two hours. Its trajectory was meticulously reconstructed.
Philae flew over the Hatmehit Depression above the small lobe of the comet before hitting the edge of a cliff. This impact threw him towards a second landing site, identified in the present study, where he spent 2 minutes before bouncing one last time, and stopping his trajectory at the Abydos site. It is in this sheltered place, poorly illuminated by the Sun, that Philae was found on September 2, 2016, 22 months after being lost and only a few weeks before the end of the Rosetta mission.
The second landing site consists of a set of rocks whose shape is reminiscent of a skull.
It is thanks to the magnetometers of the ROMAP instrument, whose 48 cm boom hit the ground, that the beginning and the duration - two minutes - of this landing could be established.
A discovery by a mechanical effect of Philae
ROMAP also determined that Philae did not simply land, but sank about 25 cm into the ground on a crevice.
Its multiple contacts with the surface then produced local morphological changes that could be identified using the OSIRIS imaging system, by comparing high spatial resolution images taken before and after landing.
Philae’s contact lifted the dark dust covering the surface and uncovered buried matter, much more primitive, 6 to 8 times brighter than the comet.
Joint analysis of the data and images from OSIRIS and Rosetta’s VIRTIS spectrometer revealed that this bright area was freshly exposed water ice with a surface area of about 3.5 square meters.
Involvement of researchers from Paris Observatory - PSL
At Paris Observatory - PSL, scientists from the Laboratory for Space Studies and Instrumentation in Astrophysics - LESIA (Paris Observatory - PSL / CNRS / Sorbonne University / University of Paris) worked in particular on the identification and composition of the subsurface matter exposed by Philae.
They found that the abundance of water ice is 46%, one of the highest values already measured on the surface of the comet.
The derived rock/ice mass ratio of 2.3 is consistent with values already obtained on freshly exposed ice at the Aswan site, following the collapse of a cliff due to a surge of cometary activity, while the overall value for the comet nucleus is greater than 3.
The simple fact that Philae left his imprint on the wall of a crevasse allowed to measure in situ different physical properties :
- a sub-surface composition rich in water ice;
- the compressive strength of this mixture of ice and dust several billion years old is extraordinarily low (< 12 Pascals); in other words, the consistency of this material is lighter than that of freshly fallen snow ;
- a porosity of the rocks which is locally very high (75%).
These results on composition, hardness and porosity are very important not only to understand the nature of comet 67P, but also for the design of future missions for the in situ study of comets, and for their sampling.
Bibliography
The study "The Philae lander reveals low-strength primitive ice inside cometary boulders," by O’Rourke L, Heinisch P., Blum J., Fornasier S., Filacchione G., et al. appears in the journal Nature, October 28, 2020.
DOI :10.1038/s41586-020-2834-3.
URL : https://www.nature.com/articles/s41586-020-2834-3