67P/C-G-How to measure comet geological structures using shadows

While composing the various images of comet 67P/C-G, I noticed a number of sharply angled structures that when viewing their shadows, appeared to be much more jagged and higher then appeared. It came to me how interesting how it might be to understand the scale of many of the objects and structures of craters and mounds that we generally either see overhead or at an angle as observed from the Rosetta orbiter.

To measure distance of an object you cant directly measure, like on Earth when seen from orbit, you need a couple pieces of information. Using trigonometry, the phase angle of the sun, and the known distance in pixels of an image, this can happen. After requesting it on the Sep 11 composite post on the ESA Rosetta forums, the first inclusion of the phase angle "angle the sun is pointing at the comet" was included in the Sep 14 image post.

To show you how this works, I am using the Sep 14 composite image I stitched together from the three images captured by Rosetta.

So, what are our initial measurements?

We are told that the phase angle is 61.5 degrees
We are observing 2.5 meters per pixel. It is roughly .400m/pixel

We now need to determine the sun's azimuth. Making up North on the image. Measuring the angle of the shadow to the E/W line, it can be determined the azimuth to be 270 degrees.

How do we apply all this information to determine depth/height?

1. First measure the distance from the top of a feature, to the end of the shadow.

This feature is measured at 67.7 pixels. If you multiply it by 2.5m/pix, you arrive at a distance from top of feature to end of shadow as 169.25m.

2. Now plug in the numbers to the equation.

d=169.25m*tan(61.5degrees)
d=308.5
d=309m

If we then apply the same method to the little hill to the upper right of this formation we get a height of 107m.

Now using this method, knowing the phase angle of 61.5 degrees, and measuring the pixel distance you can determine the estimated depth of craters and height of features. As long as the Rosetta forum mods include the phase angle in each of their image submissions, this can be done.

Cassini capturing storm on Saturn

NASA/JPL/Space Science Institute/Errol Coder

Captured at approximately 1,624,665 miles (2,614,645 kilometers) away from Saturn, this image composed of 3 other images using the MT2, CB2, and IR filters of the Cassini orbiter, this image seen in false color details the different cloud structures of the Atmosphere. The  cloud walls are clearly visible rotating around the central storm that resembles the one on Jupiter a great deal.

Comet 67P/C-G - Possible evidense of dust vent?

click image to enlarge

On August 17, 2014 the left image was captured. What appears to be a discoloration or shadowing in the crater stretching across a number of different boulders seems to no longer appear on the new image captured on September 10, 2014. The Sep 10 image doesn't seem to show any ridge or anything that may have caused this line of darkness. I attempted to color code a version of the above image to pinpoint which lines were in line, but it was a no go. Too many differences in stone orientation.

I wonder if this could perhaps be evidence of some sort of linear vent? The area along the line in the Aug 17 images appears rougher and more stones seems to be along the discoloration. Yet, when we see it again in the Sep 10 image, the area seems to be a lot more smoother, perhaps the smaller stones/boulders having been covered by the dust? Though, is there enough of a centripetal force for dust to settle?

Additionally, in the lower left of the right image, there does appear to be a round O shaped feature like a "Cheerio." The way the shadowing is seems to appear it has a pit veres the top or jagged edge of another boulder. Could this also be a vent? It does also seem to be there on the left as of the "Aug 17" image.

Update 1 - Sep 12
I rewatched the arrival conference video. http://wpc.50e6.edgecastcdn.net/8050E6/mmedia-http/download/public/videos/2014/08/010/1408_010_AR_EN.mp4

When they approached the comet and was testing out VIRTIS, they detected that the minimum mean temperature of the comet seems to be coming from the neck. We must then ask, why? We have seen images of direct sunlight, and it was getting direct sunlight at the time of the measurement. The max temperature came from a section on the "Body", the mid temp was on the "head", and minimum on the neck. The areas void of volatiles and are pores are the locations with the max temp. Does this mean there is either more volatiles and the area is less pores at the neck?

 The Sep 2 image did show out gassing at the neck, which would seem to make sense if there were more volatiles there.

We also saw jets at the plains area on the underside of the "body". Im interested in measurements there, and at the large plains area on the crown of the "head" as seen in my comparison. It would seem due to the smoothness that there has been activity there.

Now, since they did post the VIRTIS map on the 8th. It does show that the top of the "head" which is A site landing target is very cool, so is the area around the neck, as well as the bottom of the "body" as very cool. The large plains on the body which Bill and I had recently conducted comparisons is also the other cool location.
http://celestialimaging.blogspot.com/2014/09/67pc-g-comparing-shadows-and-angles.html

These three locations seem to show the evidence of activity geologically. While these areas would be great landing sites terrain wise, they are areas of cold conditions. The only one of these where a landing was considered was "A", which may have lost out due to the revelation of the temperature. The landing sites seem to be in areas other then these active locations, which for a success of the mission, would make sense, though the question stands, will there be any data regarding the outgassing if you are not in an area not as active? Come November perhaps those sides of the body will be more active. But, as they are the hot areas, it would show that not only is it porous there, but either low or no volatiles. Which goes to show, IF there are volatiles at the dark regions, what do they composed of? We wont get this answer unless we are there to drill and find out.

Possible Landing Site "A" on the crown of the "head"
http://celestialimaging.blogspot.com/2014/09/comet-67pc-g-has-ridge-gone-missing.html

Update 2 - Sep 12
After further research,

The line/discoloration is visible on the arrival images as showing during the arrival webcast. To orientate you, he image compared to the above images are upside down.The O cheerio stucture is now seen at the upper right of the image now.

Cassini image composite of Saturn's North Pole

Image Credit: NASA/JPL/Space Science Institute/Errol Coder

Traveling at a distance of approximately 1,699,200 miles (2,734,597 kilometers) away from Saturn, the Cassini orbiter captured a sequence of images focused at an angle above Saturn down towards it northern pole. Using its CB2/IRPO, CB3/IRPO, and MT3(Methane)/IRPO filters, I combined the three images into a false color image. The false color, while not true colors, allows the viewer to see the different divisions, features, and differences between the different characteristics of Saturn.

67P/C-G Comparing Shadows and Angles from Aug 15 and Sep 7

The time of day and angle of an image in photography can play a tricky part in the process of imaging. It can also reveal important information about a subject. Capturing images at different times of day which results in different shadow angles, as well as capturing them at slightly different angles changes the image significantly. The same goes for the Rosetta orbiter as it captures its images of comet 67P/C-G.

On August 15, 2014 an image of the comet that appears to be the end of the section typically designated as the "body." The sun was at an angle nearly straight on causing short shadowing on the surface. Only in areas with high crater walls were the effects of the shadows drastically present. A part of the comet typically called the "head" of the comet is seen in the distance past the "body" section on the left side of the lower left of the image.

But, then today Rosetta again captured an image of the same area, but the sun itself was at a drastically different angle. This caused some interesting shadowing and made some areas that originally looked smoother, to appear rougher and more jagged. Additionally, due to the angle of illumination the "head" section that was visible on the August 15 image was now hidden.

 

Some interesting observations can be taken from these two slightly different images.

#1. On the Aug 15 image, the 8 boulders that are present in the oblong crater appeared to be low to the surface with broader structures. But, in the Sep 7 image made them appear to be taller then expected. While the field of view (FOV) of the NAVCAM is slightly more over head, their shadows just light with the local peaks of the craters are quite long. Although, their shapes seem to indicate the rocks to be a bit less round at their tops. While the shadows do elongate the shape of the object, they do help aid in identifying their basic shape. In fact one, the lower left of the right three boulders appear to be more wedge shaped.

#2. The crater on Aug 15 appears to have a shallower depression. While the crater wall on the left shows shadowing, the remaining area of the crater doesn't appear to be much lower. But, when you compare it to the Sep 7 image, a few things stick out. A few features on the lower section of the crater while appear smooth, indicate three raised ridges. Additionally, a single ridge on the upper left of the crater on Sep 7 shows a wall high enough to cut through the edge of the crater, and high enough to capture an illuminated slope on the sunward side cutting through shadowed crater floor.

#3.Yet again, areas that while are on the edge and would begin to wrap around the edge of the comet, seem to appear more level ground. But, due to the angle of the shadowing, crater edging seems to appear as the image wraps around the side.

#4. Using a bit of trigonometry by knowing the angle of the sun to the object, and knowing some bit of measurements such as the length of the shadow itself, you may be able to calculate the height of a peak from a crater. But, some of that information may not be available. If someone with a bit more trig experience wants to give it a go, I think it would be possible with a bit of digging for angle and distance numbers to find its height. What is interesting is the drastic change between the length of the Sep 7 peak (left) on the left and the peak shadow on the right as well as the second step the appears nearly halfway down the slope. Now, as the area of illumination on the side of the formation side seems to be relatively the same amount of area on both days, the height may be quite shorter, relatively then observed. But as we are looking nearly straight down it would be hard to really visualize the the height. But, as observed, the shadow on Sep 7 seems to give hint to the real topography of the area.

#5. Just like with #4, a lack of shadowing can be deceiving in regards to land structure. Apparently, the edge of this crater/field is higher then originally observed, as it casts a quite uneven edge shadow hinting at the structure of this edge. What is quite interesting is the deep in the middle showing two tall peaks that are completely invisible in the Aug 15 (left) image.

#6. While it may simple be a visual illusion, there seems to be a boulder propped on the edge of the hill/crater edge. You can see it in the Aug 15 image both on the top of the wall, and rising higher then the top of the rim in the shadow. The shape and protrusion with a squarish top to it would seem to be an independent object from the shape of the rim edge itself.

#7. This section of the crater field may give indication to the comet activity. It doesn't seem to give the typical signs of showing a crater edge. Although, visually, the right side is the higher smooth ground, while the crater evenly dips down into the other smoother field. My question is, what would make for smooth surface? I would suspect it is similar to what occurs on the moon for the Mare. The Mare are flooded (by volcanic erupted basalt) impact craters. While the interior of the Moon was still hot an molten, asteroid or comets hit the moon and created impact craters. Lava from the Moon's interior then welled up to flood these craters, making the Mare. But, would a comet once have an interior like this? If so, these comet mare would be much older then the surrounding overlaying caters. Instrument investigations indicate that there is very little near surface ice, so these Mare would not seem to be formed by ice. Is this a remnant of the comets ancient hotter period?


#8. Even such a drastic angle of the sun, the "head" of the comet disappears. The only surface that remains is the "body" as its wrapping edge ends at the terminating shadow edge. You would not guess that with just a bit of change in the suns angle that a completely other piece of terrain exist nearby.

30 Days of comet 67P/C-G

Launching in 2004, and traveling for the last 10 years of an estimated 12 year voyage, the Rosetta orbiter has conducted gravity assist after gravity assist flyby maneuvers with the Earth and Mars as it traveled around the sun during its long mission to eventually rendaveau with, orbit around, and land "the Jupiter-family comet 67P/Churyumov-Gerasimenko with a combination of remote sensing and in situ measurements". On August 6th, 2014 it finally arrived at the comet and immediately got to work. As it approached the comet on August 1, 2014 it began a sequence image capture that included a composite animation of 101 of them, using its NAVCAMs (Navigation Cameras) as it continued to approach. 

On August 6th as it arrived in orbit about the come, it began its Global Mapping of the surface which continued through August and into September. The images, using OSIRIS aboard Rosetta were captured in full from as it slowly orbited closer and closer to the tumbling mass of cosmic collisions. It is observed of course that the comet itself was formed by the collision of two other objects, that connected and fused at the "neck" area of the comet itself which gives it its odd "duck bill look."

Unfortunately, as Rosetta has moved closer to the comet, 50km as of August 23, instead of being able to capture a full frame of the comet, it began capturing only 1/4 segments to still allow for mapping of the object. These new segments were released on September 1st, allowing the image processing community the ability to create composites of a few that had been captured on August 31st. The second of these four-part images were released on September 4th of a sequenced captured on the 2nd.

On the August 31st images, a plume of what appears to be dust or some sort of out-gassing is seen originating from the "neck" narrow section of the comet.

The following animation is a complete sequence of the released full frame images from August 6 to the 23rd, and the composite images I have been splicing together from September 1st through the 4th.

While these images are captured days apart, it is interesting to see how the comet itself continues to tumble about its wobbly rotation.

ESA/Rosetta/NAVCAM/animation by Errol Coder

Venting captured from comet 67P/C-G

The European Space Agency operates the ROSETTA spacecraft that is currently enroute towards comet 67P/C-G. Due to is close proximity to the comet, the NavCam, which was capturing full frame images of the comet, it now can only capture a smaller field of view. As a result, ESA has put a task out to the imaging community to create composite images of the quarter sections of the comet they release each week.

Their first set of images were released September 4th of the images captured on the 2nd. At a distance of 56kms, four images were captured, approximately 20 minutes apart as the capture changes its angle. The comet is also rotating on its axis at this time. So not only are the images captured minutes apart, but the view of the surface features also change position.

It takes a bit of twisting and adjusting to stitch the four images together to create a cohesive composite of the combined images. But, when done correctly, you can reveal some great features on the surface, and also other interesting occurrences.

This image was combined using the 4-images, stitched together and enhanced to help the features come out. While there are currently clearer composite images completed by other individuals, this image that I processed is targeted to help highlight the current out gassing of the comet itself. Out gassing occurs when parts of the comet, has contact with the direct sunlight, causing the surface to heat up and react with trapped gases beneath the surface in the rock and ice that compose the comet. In this image, it was processed to have  +10 brightness and -12 contrast to reveal the out gassing jet that forms a plume in the "neck" of the dirty snowball. It seems to reach quite a distance, nearly the same distance as the comet is long.

click image to enlarge

To be able to see the more detail on the comet itself, the brightness was reduced -72 from the above settings. While the out-gasing plume is lost, the surface features popout and become more clear.

click image to enlarge