Thursday, 9 July 2015

Do micro-organisms explain features on comets?

So was the RAS’s provocative headline on the Press Release for our presentation on 6th July Icy structures and terrain in comet 67P.
    The thrust of the talk was that the comet appears more habitable for microorganisms than the Arctic and Antarctic regions, with its organic-rich surface warming in sunlight, shielding an icy environment below a protective crust which traps gases.  A second point linked the surface crust with particles from comets collected in the stratosphere.  These comprise consolidated aggregates as well as ‘fluffy’ smoke-like particles, both high in carbon.  The former could be identified with fragments of the surface crust - some of these studied in Cardiff University labs show embedded Si-rich fibres, unlike crystalline astrophysical silicates.  The fibres were suggested to be spines from terrestrial  ice diatoms (Miyake, EPSC 2013).

What of the refutation no-alien-life-on-philae-comet ? Stuart Clark ignored the substance, approving instead of Uwe Meierhenrich (co-investigator on Philae’s COSAC chemical analyser) telling him a comet’s black surface crust was a prediction made in 1986 by J. Mayo Greenberg (Nature 321, 385), who had modelled what would happen to naturally occurring organic molecules on the comet when they were struck by cosmic rays and light. “These explanations seem to be valid, also with regard to new data of the cometary Rosetta mission,” wrote Meierhenrich, though the chemistry is indecisive.

Hoyle & Wickramasinghe did publish the prediction of a black surface (Earth Moon Planets 36, 289) before Greenberg in 1986.  Significantly, the Vega/Giotto missions to comet Halley proved Greenberg’s "bird’s nest" structure of rod-particles and UV-processed condensates on mineral cores to be wrong. The PIA and PUMA dust analysers found carbonaceous (CHON) particles, mineral particles and mixed ones.  The carbonaceous CHON component is highly complex, more than the UV-induced compounds of Greenberg’s experiments.  
   Cometary science needs to include past discoveries over comet Halley as well as newer ones on interplanetary dust particles.

Relevant papers in the RAS session Comets: Rosetta results and related science
Cometary dust properties measured by Rosetta GIADA:   Simon F. Green (OU) + GIADA Science Team.  GIADA has detected two distinct particle populations emitted from the nucleus: compact, 0.03-1 mm grains analogous to the processed mineral grains detected by Stardust, and ultra-low density fluffy aggregates of sub-micron grains with diameters 0.2-2.5 mm.

Comet nucleus surface properties in-situ, in the lab and on the computer: Axel Hagermann, DPS, The Open University

The picture of the physical nature of cometary nuclei has been changing dramatically since Giotto's encounter with 1P/Halley. The talk used information from laboratory experiments, computer models and in-situ measurements. The comet is rugged with firm surface and dark surface crust.  How is this compatible with low density?  Philae’s drill failed to penetrate when on low power (before shut down).  The OU group have repeated laboratory simulation of building up a dust-crust from a snow-dirt mix, as in the COSI experiments in the ’90s (Gruen et al).  Heat transport via vapour convection dominates over thermal convection when T>205K.  Fluffy snow converts to hard but porous ice under vapour transport, as modelled by Kossacki (Icarus 2015, applied to comet Tempel-1).  Thus, sintering of the snow-dirt mix increases the contact between ice grains with no change in porosity, so creating the firm and coherent surface crust. But the model contains mineral particles only, not the carbonaceous component which chemically degrades near the surface.

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