The Philae Lander reveals the presence of low strength primitive ice inside cometary boulders

On the 12 November 2014, the Philae lander descended towards comet 67P/Churyumov–Gerasimenko, bounced twice off the surface, finally arriving under an overhanging cliff in the Abydos region. In this study (published in Nature on 28th Oct 2020), we present the results of our investigation of a 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 +0.2/−0.16 : 1 , matching values previously observed in freshly exposed water ice from outbursts and water ice in shadow. At the end of the crevice, Philae fell forward and left 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 that seek to access a volatile-rich ice sample.