Rosetta mission discovers the wet-comet model Rob Sheldon Aug 11, 2015

Comet 67P/Churyumov-Gerasimenko

Comet 103P/Hartley2 P/Hartley2 Smooth ice? This picture of comet P/Hartley2 shows a dumbbell shaped comet (similar to Borrelly, and explainable with the wet comet model of Sheldon 2005, 2006), where rough terrain is outgassing, while smooth terrain is free of jets. Two possibilities: 1) if the rotation of the comet is a prolate tumbler, then the middle section is “up” in the gravitational field. So if a small amount of liquid water is on the comet, it will escape out the ends, but not in the middle. Of course, this also suggests that the gas doesn't escape out the middle either, so the gas pressure is also low in this scenario. 2) or, if the comet has enough liquid water so that the middle section is wet, it may be sealed by a more impervious material—such as concrete or magnetofluid. Certainly it's smoother surface seems to suggest that it has a different crustal composition or morphology. But the wet comet theory would put most of the concrete on the ends at the lowest point on the gravitational potential well. So perhaps a ferrofluid sealant would explain the lack of jets in the central section, whereas internal pressures might be large enough (or crustal magnetism low enough) that ferrofluid doesn't seal the ends. P/Hartley2

Comet 19P/Borrelly

Temperature on 9P/Tempel-1 Most of comet hovers just above freezing point ice Sunshine

Geysers from spin axis Deep Impact Giotto DS-1 Stardust All three imaged comets have asymmetric jets or geysers. Note that Wild-2 appears to have geysers even on the night side. DS-1 Stardust

A Comet’s Life: “Wet” Model b) c) Ice Liquid Vapor Spin Axis Spin Flip Melting snowball Pristine Splitting Cement d) g) e) f) This cartoon indicates the sequence of events that can lead to a Borrelly type prolate comet with a gas jet at the pole. Gray is surface, brown is coalesced dust. Dark blue is liquid water, light blue is water vapor. White is ice or dirty snow. Red line is spin axis. If geysers contain liquid water rather than just vapor (e.g. Yellowstone Park), then flash-freezing of boiling liquid is likely to produce fluffy aggregates that are observed in Brownlee particles and s/c flybys. It also imparts a large tangential velocity to the jet by liquid boiling (as at Halley), so narrow jets are probably gas- dynamic (Borrelly), versus wide jets by liquid-boiling. Finally, the higher viscosity of water permit relatively larger dust particles to escape in water geysers. Geysers far from the spin axis (Halley) will have a higher probability of being wet (water runs downhill) as well as muddy (high dust content), whereas geysers near the pole (Borrelly) will be most likely dry (gaseous) with smaller particulates and less dust. Variable gas/dust ratios then can have a single explanation. When enough erosion at equator reduces the moment of inertia, water facilitates a change in rotation axes to a prolate spinner (2nd row). This may precipitate breakup. If the vapor pocket makes it to the pole, one would expect the thinner crust there to produce a geyser (Borrelly). This suggests that Borrelly jets may be significantly “older” than Halley type jets. Extinguished comets may then be hollow, resulting in collisional fragmentation, and the apparent lack of a large number of asteroidal-type bodies. They would also look a lot like carbonaceous chondrites. Prolate tumbler Rubble pile Polar jet Eggshell

Comet P/C-G

Wurz et al., H20 on P/C-G Note the location of the water observed from sputtered solar wind—directly impacting the comet.

Wurz et al. Rosetta rock composition

P/C-G looks like CI meteorites

Conclusions Rosetta confirma the wet comet model: rotation rate, prolate tumbler, ice content near poles, resurfacing, axis flip, organics... In addition, Rosetta shows that CI carbonaceous chondrites are indeed remnants of comets Rosetta's Philae lander failed to deploy harpoons, demonstrating the rigidity of the crust, as predicted in 2005 (but Nature didn't publish) If they had anticipated the wet-comet model, the anchors would have held, and Philae would have returned far more data.