viscount aero wrote:
Moreover, if water ice has been confirmed to exist on Mercury then why do comets need to go to some light year distant "cloud"?
http://www.space.com/18687-water-ice-me ... overy.html
"It's time to add Mercury to the list of worlds where you can go ice-skating. Confirming decades of suspicion, a NASA spacecraft has spotted vast deposits of water ice on the planet closest to the sun.
Temperatures on Mercury can reach 800 degrees Fahrenheit (427 degrees Celsius), but around the north pole, in areas permanently shielded from the sun's heat, NASA's Messenger spacecraft found a mix of frozen water and possible organic materials."
It sounds like they're talking about a comet. Even if comets have very little to zero actual ice on or within them, finding ice on Mercury and the Moon makes a theoretical Oort Cloud unnecessary.
I've been putting together some notes on this over the last couple of days and landed on Mercury also:
Mercury has a Sodium Tail
. Along with other processes the following paper notes that "(photosputtering, particle sputtering, and meteoroid impact) are not sufficient to allow the atoms to escape from the planet"
- It has been assessed that solar radiation is "adding enough energy to whatever source energy they initially possessed to allow them to escape into the tail." Observations of Mercury’s Sodium Tail
: Andrew Potter (NSO) & Rosemary Killen (University of Maryland)
In light of VIRTIS characterizing Comet 67P as being "dehydrated" note that this observation is constrained to the "outer surface" leaving subsurface hopes alive:
The team have also been poring over the spectra to search for hints about the chemical makeup of the surface of comet 67P/C-G. Among the preliminary results reported today was no evidence of water ice on a global scale, confirming that the outer surface is generally dehydrated.
… the following contrast is interesting :
The innermost planet Mercury has been photographed spouting a faint comet-like tail. Unlike a comet, which is a sublimating ball of dusty ices, Mercury is bone dry. This tail is created by a gusher of solar radiation accelerating sodium atoms off Mercury’s surface.
They absorb enough energy to escape the planet’s gravitational pull and zoom off into space. - Discovery
Further again, to underscore a point being made by member Viscount Aero there is this:
But there’s a catch. For water ice to remain stable inside these craters, an insulating layer of regolith
— the thin, pulverized rock dust that forms on planetary bodies after eons of meteorite impacts — needs to blanket the surface, keeping the ice in a frozen state (preventing it from sublimating into space). For the ice to be preserved
, around 20-30 centimeters (8-12 inches) of overlaying regolith needs to be present
. - Mercury Not Too Hot For Polar Water Ice?
As of late they think the ice is residing in shadowed craters at Mercury's north pole. Makes me wonder why Philae didn't snap a photo of ice in the shadowed crevasse where it has landed unable to get sunlight to its solar panels. Is that little factoid to be ignored? Anyways...
In light of electric field induced "Leaping Lunar Dust
" (possibly electrostatically cleaning the Mars Rover) which, on the one hand says that dust doesn't leave the surface but that at the termination (the transition between day side and night side) the charged dust is electrostatically 'bouncing'/leviated due to being attracted and repelled between the two differently charged regions. For a smaller body such as asteroid or Comet 67P rotating with a remarkably uneven surface this picture of the termination region isn't going to be so neat and spherically tidy as to present a backlit haze of electrostically levitated dust over comet horizon.
In the following reference it is to be noted that "theory show that electric field strengths increase rapidly towards the surface":
Renno et al.  show theoretical and observational evidence that martian dusty phenomena are electrically active. This has important implications for dust lifting and atmospheric chemistry [9, 10, 11].
The large electric fields produced by dusty phenomena can substantially reduce the critical wind speed necessary to produce saltation, and even directly lift dust particles from the surface . Saltation is the process by which sand particles are forced to move by the wind and bounce on the surface, ejecting the smaller, harder to lift, dust particles into the air . Electric fields might facilitate saltation and dust lifting on Earth, Mars and elsewhere. Both field measurements  and theory ,  show that electric field strengths increase rapidly towards the surface  - A miniature sensor for electrical field measurements in dusty planetary atmospheres N O Renno1, 2, J F Kok3, H Kirkham4, and S Rogacki1
What might this imply about the lowest surface areas of a comet such as the neck of 67P? It would imply that the lowest surface areas might be where the electric field strength is strongest as opposed to an electric field having an even distribution over an idealized spherical surface. Very interesting.
However, there is a problem with the "radiation pressure" idea as well. When comets have mysterious "flare ups" at remarkable distances from the Sun the "radiation pressure" idea fails to account for the increased activity. It seems crazy but having a copy of photoshop myself and fiddling with the contrast and brightness of recent 67P images from ESA it appears that the bright spots from adjusting just these two settings to bring out more detail in black and white images of darkened comets may have contributed to the ice interpretation. Especially with images of comets from quite some distance.
"Our laws of force tend to be applied in the Newtonian sense in that for every action there is an equal reaction, and yet, in the real world, where many-body gravitational effects or electrodynamic actions prevail, we do not have every action paired with an equal reaction." — Harold Aspden