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Rosetta - Early Orbital Operations at Comet 67P C-G, August 6, 2014 - November 13, 2014
wildespace
post Oct 23 2014, 03:38 AM
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My stitch of the recent mosaic: http://www.pictureshack.us/images/49436_ES...1018_stitch.jpg

I think those round features are sublimation sites rather than impact craters.


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nprev
post Oct 23 2014, 04:25 AM
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I'm leaning that way as well. The odd rims look as if they may be encrusted with material, perhaps precipitates from volatilized material abruptly re-freezing.


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JohnVV
post Oct 23 2014, 05:47 AM
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QUOTE
Some of these structures are reminiscent of fumaroles, with encrustations around the edges.


they do

but
if a low speed impact happened then the material around the crater would be of higher density , and have a higher temp of evaporation
that might leave the area around it but allow the surrounding area to evaporate .
[attachment=34086:1.jpg]
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Phil Stooke
post Oct 23 2014, 02:39 PM
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http://blogs.esa.int/rosetta/2014/10/23/co...n-the-increase/

As the URL says, comet activity is on the increase. Also, it's stinky:

http://blogs.esa.int/rosetta/2014/10/23/th...fume-of-67pc-g/

Phil



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tanjent
post Oct 23 2014, 02:42 PM
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John W's suggested mechanism sounds like one that could apply not only to low-speed impacts into ice and snow, but also to higher-speed impacts into sand, dust, or rocky material. But have we seen similar structures forming elsewhere? I am trying to think of other examples on earth or extraterrestrial bodies where an impact "shell" survives as the surrounding material erodes away. For instance, sometimes when a big, isolated raindrop lands on sand and then evaporates, it can leave a cup-shaped structure behind, but this is a poor example because no actual melting is involved.

Maybe the experimental types who create controlled impacts by firing projectiles in the lab could tell us more about when a fused shell structure is likely to remain behind.
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scalbers
post Oct 23 2014, 03:51 PM
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QUOTE (Phil Stooke @ Oct 23 2014, 02:39 PM) *
http://blogs.esa.int/rosetta/2014/10/23/co...n-the-increase/

As the URL says, comet activity is on the increase.


In the third image down I like the appearance of shadowing onto the jets. Helps to give some 3-D perspective.


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Marvin
post Oct 23 2014, 04:15 PM
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QUOTE (Phil Stooke @ Oct 23 2014, 10:39 AM) *
As the URL says, comet activity is on the increase. Also, it's stinky:

http://blogs.esa.int/rosetta/2014/10/23/th...fume-of-67pc-g/

Phil


Look at all those organic molecules.

If comets like this crashed into a young Earth, they could have kick-started organic chemistry.
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Gerald
post Oct 23 2014, 05:04 PM
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Nice Miller-Urey-like cocktail.
I'm feeling somehow, that this list will grow much longer at higher temperatures.
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charborob
post Oct 23 2014, 05:37 PM
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The point of origin of the jets isn't obvious yet, at least to me. Are they really caused by matter spewing out through identifiable vents, or is it just a general sublimation of the surface? I hope it is one question that will find an answer as the mission advances and as the comet activity increases.
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TheAnt
post Oct 23 2014, 05:48 PM
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QUOTE (Phil Stooke @ Oct 23 2014, 04:39 PM) *
As the URL says, comet activity is on the increase. Also, it's stinky:

Phil


Yes stinky indeed, it's a solar system leftover that's been left basking in the sun for a handful of billion years. So of course it is stinky! =)
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anticitizen2
post Oct 24 2014, 02:56 AM
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In the Comet Activity is on the Increase blog entry, they include two WAC exposures.

Over the dark region in the overexposed image (18.45 second exposure), there are long, faint lines (almost horizontal, tilted with the left side up). Could these be grains floating by, as had been mentioned in the blog a couple times?

Does anyone want to make a guess about their size and distance from Rosetta?
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Gerald
post Oct 24 2014, 10:34 AM
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Since the lines are roughly parallel, CRs are almost ruled out. They are not perfectly parallel, and a bit random, hence camera artifacts are unlikely.
Therefore I'm sharing your suggestions, that grains are a good explanation.
The short exposure image contains at least one such line (near pixel pos (1440;400) with (0;0) as the (left;top) corner).
This one line is statistically questionable in terms of length, but it may be taken as a hint for the angular velocity of the supposed dust particle. The line is about 35 pixels long; this corresponds to 0.2 degrees for WAC, using 20.5 arcsec / pixel.
The longest line I could find in the long-exposure image is about 230 pixels, or 1.3 degrees; that's only about 6.5-times the length of the short-exposure line, less than the 18-fold, as would be expected due to the more than 18-fold exposure time. This provides a hint, that some of the long-exposure lines show the respective particle along all the path it's visible for the camera.
Statistics is poor with only one grain in the low-exposure image, but it might provide a hint, that some of the grains even float into the shadow of the camera or the probe, with a very low relative velocity of roughly between one mm to one cm / second.
The size would be in the order of a few micrometers.
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charborob
post Oct 24 2014, 01:49 PM
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Four new navcam images today. On one of the images, we can see scallop-shaped features (see arrows). Sublimation zones?
Attached Image
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Gerald
post Oct 24 2014, 02:18 PM
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QUOTE (Gerald @ Oct 24 2014, 12:34 PM) *
Statistics is poor with only one grain in the low-exposure image...

... Hipass-filtering, brightness stretching, rgb-channel combine, closer inspection, and annotation connects the two OSIRIS WAC images:

At least four grains can be identified in both images. This reveals the longest line in the low-exposure image as an outlier in terms of angular velocity, and hence weakens the constraints for distance, grain size and relative velocity by discarding the shadowing hypothesis.
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Phil Stooke
post Oct 24 2014, 03:56 PM
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"On one of the images, we can see scallop-shaped features (see arrows). Sublimation zones?"

I had been thinking of these as avalanche scars, places where a patch of the dusty surface slipped and fell off the nearby cliff.

Phil



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