Saturday, June 28, 2014

Liquid water in comets

Conditions required for liquid water is a temperature and pressure above the triple point of water. The surfaces of comets are certainly warm enough, based on measurements taken by probes and earth based. Pressure only needs to be .006 Atmospheres. Since jets are powered by the thermal energy absorbed by the surface, there must be some gas pressure being held in the surface- ie. a subsurface atmosphere. Thus liquid water could flow in the microgravity environment. Some Stardust probe sample return minerals found can only be formed in liquid water. Deep impact probe excavated clays which are suggestive of liquid water. Capillary action, microgravity, centrifugal forces and internal wind would affect the movement of water and material inside the comet.

Long live the wet comet model.

Sunday, June 01, 2014

How does a comet outgass?

From the latest direct evidence from robotic satellites, comets mainly outgass from "jets". Most of the volatiles, dust and larger particles come out of the jets. Some out gassing of volatiles happens in areas with no jets see Hartley. 

The energy for the sublimation of ices happens near the surface (in the top metre or so) based on temperature readings. Thus the volatiles travel from where they are heated under the black surface to the jets, picking up solid particles as they go. There is considerable mass loss in each orbit, while the position of the jets remains relatively stable. Thus the black "skin" needs to shrink with the comet, in the places where the volatiles are sublimated. 

The jets appear not to collapse in on themselves, and thus have a more solid structure around them.

The combination of observations would contradict the models of "fissures" ie cracks. Cracks causing the out gassing cannot be reconciled with the observations of discrete jets and temperature profile. 

Models of regularly collapsing and new jets cannot be reconciled with non gravitational accelerations for Halley class comets that are consistent from apparition to apparition despite substantial mass loss. Jets need to be in the same configuration from apparition to apparition for the adjustments to work the way they do for Halley.