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Rosetta Images of Comet 67P/Churyumov–Gerasimenko: Inferences from Its Terrain and Structure

Wallis MK and Wickramasinghe NC

The Rosetta mission has given us remarkable images of comet 67P/C-G both from the orbiter, and recently from the Philae lander during its brief days before running out of power. Though its crust is very black, there are several indicators of an underlying icy morphology. Comet 67P displays smooth, planar ‘seas’ (the largest 600 m x 800 m) and flat-bottomed craters, both features seen also on comet Tempel-1. Comet 67P’s surface is peppered with mega-boulders (10-70 km) like comet Hartley-2, while parallel furrowed terrain appears as a new ice feature. The largest sea (‘Cheops’ Sea, 600 x 800 m) curves around one lobe of the 4 km diameter comet, and the crater lakes extending to ~150 m across are re-frozen bodies of water overlain with organic-rich debris (sublimation lag) of order 10 cm. The parallel furrows relate to flexing of the asymmetric and spinning two-lobe body, which generates fractures in an underlying body of ice. The mega-boulders are hypothesised to arise from bolide impacts into ice. In the very low gravity, boulders ejected at a fraction of 1 m/s would readily reach ~100 m from the impact crater and could land perched on elevated surfaces. Where they stand proud, they indicate stronger refrozen terrain or show that the surface they land on (and crush) sublimates more quickly. Outgassing due to ice-sublimation was already evident in September at 3.3AU, with surface temperature peaks of 220-230 K, which implies impure ice mixtures with less strongly-bound H₂O. Increasing rates of sublimation as Rosetta follows comet 67P around its 1.3 AU perihelion will further reveal the nature and prevalence of near-surface ices.