APOD: Comet Churyumov Gerasimenko in Crescent (2015 Apr 29)

[APOD Robot]

Comet Churyumov Gerasimenko in Crescent

Explanation: What's happening to Comet 67P/Churyumov–Gerasimenko? As the 3-km wide comet moves closer to the Sun, heat causes the nucleus to expel gas and dust. The Rosetta spacecraft arrived at the comet's craggily double nucleus last July and now is co-orbiting the Sun with the giant dark iceberg. Recent analysis of data beamed back to Earth from the robotic Rosetta spacecraft 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. Additionally, neither Rosetta nor its Philae lander detected a magnetic field around the comet nucleus, indicating that magnetism might have been unimportant in the evolution of the early Solar System. Comet 67P, shown in a crescent phase in false color, should increase its evaporation rate as it nears its closest approach to the Sun in 2015 August, when it reaches a Sun distance just a bit further out than the Earth.

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[/b]

[Lars]

Why assume all comets are equal

[isoparix]

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?

[starsurfer]

What an age we live in when we can have exquisite detailed closeups of comets from a space probe!

[CURRAHEE CHRIS]

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.

[neufer]

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')

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

---

<<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.>>

[Dad iswatching]

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...

[Steve Dutch]

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?

This is probably an "anti-tail," a perspective effect. The Sun is behind (WAY behind) the plane of the photo off the right side. The tail is coming toward us (i.e. away from the sun). Given that the comet is farther from the sun than we are, and the orbiter is only a few tens of kilometers away, the angular separation of the Sun and comet is probably only a few degrees. The tail fans out in the plane of the comet's orbit but viewed edge-on, it looks narrow. Comet Arend-Roland (1957) was famous for its bright, sharp anti-tail.

[Steve Dutch]

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.

As a geologist, I for one never really took that idea terribly seriously. There was plenty of water ice and hydrous silicate material in planetismals. Sure, comets could contribute, but as the primary source? An answer to a nonexistent problem.

[BMAONE23]

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...

The mass of any object tends to surround the specific center of gravity which roughly correlates to the Center of Mass. when the Mass and thereby gravity is sufficient, the mass is forced into a round shape to surround the center of gravity. In the case of this comet, it has several centers of gravity. Each lobe has it's own center of mass and gravity and there is a third where they interact within the density of the neck. The density of the comet is sufficient to support the shape and overpower the effect of gravity.
Much like any rock you pick up is dense but not round the Comet is somewhat dense but not round.
Basically, it is more dense than the crushing effect of its slight gravity can overcome

[Ann]

APOD Robot wrote:

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

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

Ann

[Chris Peterson]

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.

It's still very likely that the Earth received a lot of water from comets. The idea that most of it came from comets dates back to when we had a much poorer understanding of Hadean chemistry. We now understand better the ways that water can be maintained in large amounts by deep molten material (and why it wouldn't simply boil away to space, as once believed).

Water on Earth is a mixture of stuff that was part of the original formation, as well as stuff that was later added by cometary and asteroidal bombardment. The information about water on this particular comet places some constraints on where water came from on Earth, but that's all it does.

[Chris Peterson]

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?

The material in dust trails remains in very nearly the same orbit as the nucleus. Drag effects slow down the particles slightly, causing them to move outward and lag the parent slightly, so the tail sweeps back along and outside the parent orbit a bit. But definitely not pointing away from the Sun. You're thinking of the ion and neutral gas tail, which is very tenuous with this comet at the moment, and wouldn't be visible at any time from Rosetta's perspective.

[Chris Peterson]

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.

[Chris Peterson]

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]

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.

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?

Ann

[Chris Peterson]

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?

Neutral gas is invisible. We see the glow of ionized gas, of course, but I don't think there's any way we could see it from the comet (except, possibly, as a diffuse background glow when looking out into space directly along the ion tail path). We see an ion tail from a distance because we are looking through thousands of kilometers of gas. From the comet, we are only going to see outflows that are meters or tens of meters wide.

[Ann]

Thanks again, Chris.

Ann

[Dad is watching]

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.

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?

[rpnman]

So I don't think the apparent 'tail' off to the right in this picture is a valid indication of directionality in the comets coma. Instead what I think we're seeing here is differential lighting by the reflected light of the lit face of the comet.

[Chris Peterson]

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?

I think this could either be evidence of primordial heating, or of water deep in the comet's interior, created by a collision or tidal stress. Inside the comet the pressures could be high enough to allow water in its liquid phase for long enough that chemical processes could occur.

[mickwilson]

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?

[DavidLeodis]

It's a terrific image .

Talking of images, this is a link to one that is brought up through the 'Comet 67P' link:- http://blogs.esa.int/rosetta/files/2015 ... 4x1024.jpg. With our (well OK, my) tendency to see things in images that one looks to me like a profile of a face, but beware of the backward facing eye(s?) .

The image is in a Rosetta blog where in its caption it states "Comet 67P-C-G on 16 April 2015 at 01:06 UT from a distance of 146 km".

[geckzilla]

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?

What you are perceiving as the dark side of the smaller body is actually just what image sharpening and JPEG compression artifacts when sampled up (enlarged). Artifacts like these are not obvious until one looks closely.