Starship Asterisk*

APOD Robot wrote:has shown that water being expelled by 67P has a significant difference with water on Earth, indicating that Earth's water could not have originated from ancient collisions with comets like 67P. [/b]

isoparix wrote:
Comet tails are supposed to be blown away from the comet by the 'solar wind', ie in the direction opposite to the Sun. The coma in this image appears to be emanating from the solar-illuminated side (fair enough, as expected), but then to be heading towards the Sun. At what point does it get turned round and obey conventional wisdom and head off into head off into outer space?

• At the point that it can no longer take any more buffeting from the Sun's radiation pressure & solar wind.
  (Comet ion tails are supposed to be blown away from the comet by the 'solar wind')

http://en.wikipedia.org/wiki/Comet#Coma wrote:

[color=#0000FF][size=150]Comet 17P/Holmes and its blue ion tail (taken on November 4, 2007)[/size][/color]

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<<The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the "coma", and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous "tail" to form pointing away from the Sun.

The coma is generally made of  H₂O and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within 3 to 4 astronomical units of the Sun. The  H₂O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization, with the solar wind playing a minor role in the destruction of water compared to photochemistry. Larger dust particles are left along the comet's orbital path whereas smaller particles are pushed away from the Sun into the comet's tail by light pressure.

Although the solid nucleus of comets is generally less than 60 kilometres across, the coma may be thousands or millions of kilometres across, sometimes becoming larger than the Sun. For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun. The Great Comet of 1811 also had a coma roughly the diameter of the Sun. Even though the coma can become quite large, its size can actually decrease about the time it crosses the orbit of Mars around 1.5 astronomical units from the Sun. At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, enlarging the tail. Ion tails have been observed to extend one astronomical unit or more.>>

isoparix wrote:Comet tails are supposed to be blown away from the comet by the 'solar wind', ie in the direction opposite to the Sun. The coma in this image appears to be emanating from the solar-illuminated side (fair enough, as expected), but then to be heading towards the Sun. At what point does it get turned round and obey conventional wisdom and head off into head off into outer space?

CURRAHEE CHRIS wrote:That's a pretty significant development or am I behind the times? Everything Ive heard claims that water got on Earth due to collisions with comets. Curious what everyone thinks.

Dad iswatching wrote:Great photo...

Again, we are wondering about the surface and subsurface temperatures. With the rate this object is rotating in its BBQ role, why does it not crumble at the surface because of the freeze-thaw cycle, and end up more spherical from gravitational collapse instead of that 'rubber duck' shape? Is this the first time around the sun for the two parent bodies?


Dads Note: Last time the kids asked about this, the response they received was somewhat less that enlightening and was far from polite. Fortunately the moderator deleted it, but not before it was seen. Anyway, if you can enlighten the kids, that would be good and keep them thinking...

APOD Robot wrote:

Comet 67P, shown in a crescent phase in false color

CURRAHEE CHRIS wrote:

APOD Robot wrote:has shown that water being expelled by 67P has a significant difference with water on Earth, indicating that Earth's water could not have originated from ancient collisions with comets like 67P. [/b]

That's a pretty significant development or am I behind the times? Everything Ive heard claims that water got on Earth due to collisions with comets. Curious what everyone thinks.

isoparix wrote:Comet tails are supposed to be blown away from the comet by the 'solar wind', ie in the direction opposite to the Sun. The coma in this image appears to be emanating from the solar-illuminated side (fair enough, as expected), but then to be heading towards the Sun. At what point does it get turned round and obey conventional wisdom and head off into head off into outer space?

Dad iswatching wrote:Again, we are wondering about the surface and subsurface temperatures. With the rate this object is rotating in its BBQ role, why does it not crumble at the surface because of the freeze-thaw cycle, and end up more spherical from gravitational collapse instead of that 'rubber duck' shape? Is this the first time around the sun for the two parent bodies?

Ann wrote:What would 67P look like in "true" (RGB) color?

Chris Peterson wrote:

Ann wrote:What would 67P look like in "true" (RGB) color?

Probably not all that different from what we'd see in a B&W image. A nearly monochrome object with some unsaturated browns typical of many silicates.

Ann wrote:Thanks, Chris. I, too, would expect the chunk of dirt itself to be very, very dark with minimal variations in hue. But what about the gaseous outflows? Would they, too, be colorless at this distance from the Sun?

Chris Peterson wrote:

Dad iswatching wrote:Again, we are wondering about the surface and subsurface temperatures. With the rate this object is rotating in its BBQ role, why does it not crumble at the surface because of the freeze-thaw cycle, and end up more spherical from gravitational collapse instead of that 'rubber duck' shape? Is this the first time around the sun for the two parent bodies?

This comet has made many orbits.

The sort of "freeze-thaw" cycle you're thinking of as we observe on Earth requires the transition between liquid and solid states. The materials that the comet is made of can't exist in liquid phase in a vacuum. So material leaves the surface of the comet by sublimation processes. This does, however, result in a weathered, crumbled surface in many places. But it's very shallow. The comet loses a little of its surface with each pass through the inner part of the system. But just a few meters below the surface it stays very cold.

Dad is watching wrote:I was doing some reading and found this article (http://www.space.com/11307-comet-sample ... rdust.html) that evidence of liquid water on comets exists (or has). I understand the process of sublimation, but am a bit confused about the interpreted results from the Stardust probe (Comet Wild 2?). Is it a comet size thing? Or a process that comets go thru while forming from ... primordial matter/elements/molecules? I figured that these comets would be frozen solid after many many years in deep interplanetary space, but would their formative stages account for the Wild2 results?

mickwilson wrote:Examining the hi-res version of this image one can see - at about the 3 o'clock position and quite near the main body, what appears to be a small but extended, and apparently roughly-spherical, secondary body exhibiting the same phase of solar illumination as the main body. The smaller body does not exhibit any of the motion smears that the other specks and spots (background stars?) do.

Any ideas?