The Philae lander reveals low-strength primitive ice inside cometary boulders

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关键词: EXPOSED WATER ICE;    67P/CHURYUMOV-GERASIMENKO;    NUCLEUS;    SURFACE;    MAGNETOMETER;    DENSITY;    PHYSICS;   

DOI  :  10.1038/s41586-020-2834-3

来源: SCIE

【 摘 要 】

On 12 November 2014, the Philae lander descended towards comet 67P/ChuryumovGerasimenko, bounced twice off the surface, then arrived under an overhanging cliff in the Abydos region. The landing process provided insights into the properties of a cometary nucleus(1-3). Here we report an investigation of the 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(-0.16)(+0.2), matching values previously observed in freshly exposed water ice from outbursts4 and water ice in shadow(5,6). At the end of the crevice, Philae made 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 seeking access to a volatile-rich ice sample.

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