Tuesday, September 9, 2014

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.

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