JA Lens

Setting aside the impressive achievements of the NASA teams that successfully designed and delivered this feat of engineering so precisely to its intended destination, what is most interesting about this mission isn't the prospect of solving the mystery of whether there was once surface water on Mars. What interests us most is very much grounded in the present day. Seen above is the first clear publicly documented glimpse of Bonneville crater by the robotic geologist Mars Exploration Rover Spirit (MER-A) on sol 66 (Martian day) of its mission. Notable objects seen in the image are the lander's heat shield on the far side of the crater and two prominent rocks on it's near edge. Would you like to make a quick guess as to which of these objects is most important?

I assume that you have already read the introduction on the home page Hiding in Plain Sight, right? Good, I thought so! Because without that information this article will be of little interest to you.

You have already seen a particularly interesting photograph of something or possibly someone nicknamed Kilroy peeking around a rock. One of the challenges this find presents us with is figuring out what we are looking at. To do this — to decide whether this is merely an interesting illusion or something more significant we must look for all of the evidence we can find. In this context it means looking for all photographs taken while the rover was within site of the rock. We must analyze all of the images carefully and identify as much detail as we can. Images that appear to show the rock over time and/or from different angles must be verified to be the same rock. In a monochromatic landscape filled with rocks notable landmarks need to be found. We need to be able to place the rock in positional context within its surroundings and because of what is visible in that interesting first image we need to be able to also understand the position of the rover when pictures are taken and from which angle are we seeing the rock. e.g., are looking at it's 6 o'clock or from it's 9 o'clock.

As you will see, the appearance of the rock on sol 66 (see above and below) is different than how it looks on sols 67, 68, 69 and 70 for instance. The task at hand is to figure out why. I think it is best to present this in order following the way in which Spirit encountered the rock.

Please note that for ease of reference I will sometimes refer to the large light colored rock as Kilroy's rock — they are the same thing. This is the same rock you have seen in the lead image on the home page. Make a mental note of what it looked like then.

Crater within Sight

After investigating the area near its landing site the robotic geologist Mars Exploration Rover Spirit (MER-A) headed in a northeasterly direction towards the 210 meter Bonneville crater located approximately 300 meters away. Figure 2. shows the path Spirit took towards the impact crater. We are mainly interested in sol 66 through sol 70. Of interest is that Spirit changed direction after sol 66.

Figure 2. Topographic map of Spirit's traverse to Bonneville. Various colors correspond to changes in elevation with yellow being the highest and dark blue the lowest. Note the increase in elevation towards the crater rim which is marked with a dashed line. Illustration Credit: NASA/JPL/Malin Space Science Systems/ASU/New Mexico Museum of Natural History

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Keep in mind that the resolution of the illustration showing the path taken to the rim implies that Spirit did not move laterally and did not investigate its immediate area during sols 66-70. There is evidence that Spirit covered more ground than is shown and more than is documented in photographs available on NASA's public servers. This is not to imply that data is being withheld from public view. On the contrary, the real limits of onboard data storage coupled with limited bandwidth for transmissions back to Earth are real constraints. Also, keep in mind that this was a robotic geologist in design and there would have been a priority given to reserving bandwidth for the examination of rocks, etc. that fit within experiment design criteria.

Rock and Landmark Identification

Image analysis confirms that as one nears the crater's edge the mostly smallish ejecta (rocks displaced upon meteor impact) are joined by increasingly larger rocks. The rock-size distribution along the crater's rim demonstrates an exponential increase in number with decreasing size. In other words, as the size of the rocks increase their number decreases significantly.^{[1] [2] [3]}

An awareness of the rock-size frequency distribution is is useful because it increases the confidence level of the accuracy in identifying these two large rocks in other photos. Surrounded by substantially smaller ejecta and prominently situated on the raised rim of the crater they are presented almost as if on display. Pictures taken beginning on sol, or Martian day, 66 feature these same two distinctive rocks seen at top and below.

In the Figures 3 & 4 you can see Bonneville in the distance. The first is the full composite black & white version from which the lead photo was taken, the second is an anaglyph or 3D version of the same picture. To view the 3D images you will need your red & blue (cyan) glasses. The full image provides a better perspective as to what engineers on Earth saw while they were planning their path to the crater's edge. As always, maximize your browser's width and click on the images for a larger pop-up to see more detail. 

All images unless otherwise indicated are from NASA/JPL. Cropped and/or enhanced images are from KilroyOnMars.com.

Figure 3. Mosaic panorama assembled from indvidual images captured by the mast-mounted Navigation Camera. The camera is located 1.54 meters (~5' ½") above the Martian surface providing an eye level view of the area.

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Figure 4. 360º stereo panorama using the Navigation Camera pair. One of the primary objectives of the Navcam camera system is to acquire an "end of day" panorama of the local terrain after a rover traverse. This provides a rover traverse planning function.^[4] Use your red & cyan 3d glasses to view this.

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The following is a more closely cropped and enlarged version of Figure 3 which better highlights the two landmark rocks. 

Figure 5. This is a cropped and enhanced version of Figure 3. It highlights the two large rocks on the near side of the rim of Bonneville crater. We are most interested in the one on the right. Notice the distinct lack of anything protruding beyond its left edge and the clear view behind it to the crater floor. Note too, a diagonal line emanating from the left edge running down towards the ground to the right, a vertical line bisecting the rock and a view of some of the top surface of the rock can be seen in this image.

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The two landmark rocks are easily identified by their general appearance, shape, relatively large size and their positions relative to each other. This makes their identification in other more narrowly focused images straightforward. Notice that the large rock on the left is quite dark relative to other nearby rocks and that there is a small impact crater visible within Bonneville crater and behind the large, lighter colored and somewhat trapezoidal rock on the right. This right hand rock is of particular importance and is the one in which we are most interested.

Direct your attention the left edge of this rock and you see little of note with the exception of a slightly curved diagonal line. With the help of the increased resolution from later images it can be seen that this is an accumulation of windblown sediments which forms a triangular section against the rock's left side. 

More detail can be seen in the following enhanced anaglyph image. To view this you will need your red & cyan 3D glasses. Details are a bit more clear owing to the enhanced resolution from the use of two separate images to create this composite image.

Figure 6. Enhanced anaglyph from Figure 4. Cropped to highlight the two rocks. Use your red & cyan 3d glasses to view this.

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Next, I have isolated Kilroy's rock and presented two versions, one in black & white and the other an anaglyph. For an example of the kind of enhancements I have typically made to NASA's images you can compare the differences between the enhanced anaglyph directly above and the minimally processed version directly below. All images share part of a common source.

Figure 7. Isolating Kilroy's rock in black & white and anaglyph. Compare the detail provided by the enhanced anaglyph in Figure 4. with the unenhanced version here.

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An interesting detail that can just barely be discerned in the above black & white version and is a bit more visible in the anaglyph is a small rock partially obscuring the large rock's lower right side. While image resolution is somewhat lacking here, later it will become clear that this rock has two intersecting perpendicular lines resembling a plus sign "+" on one end. This small rock is a good reference for validating that we are seeing the same rock in subsequent images.

Of course at this point it wouldn't be unreasonable to ask why should I care about a rock? After all it doesn't seem like there is anything particularly interesting or significant to be seen.

There are two reasons, the first is that it gives us a baseline to use in comparison with later images. The second is that Spirit, as shown in our cropped version of NASA's annotated image below, is headed right towards it. This means that this rock should look pretty much the same or exactly the same on sol 66 as it did on sol 67 or on sols 68-71, accounting for differences due to increased resolution from being closer and as well as from changes in the rover's position relative to the rock and from the differing heights of the source camera used.

Additionally, referring back to the illustration, the path taken between sols 66 and 67 demonstrates that Spirit adjusted it's course to position itself quite close to Kilroy's rock. Additionally, the annotated image created by NASA below shows where Spirit was headed next after sol 66 consistent with that illustration. This provides positional context to compare images taken during the next few days. You should now be able to easily recognize the two large rocks.

Figure 8. Illustration created by NASA to show where Spirit on sol 66 was heading next. This is a cropped version of the original illustration. Note the rock ahead of and to the right of the rover's wheel.

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As you saw in Hiding in Plain Sight this is an important rock because of what's next to it. In the images from sol 66 you will notice that the important bit is largely missing. I say largely because there is actually a tiny amount of something just barely visible behind the rock in the enhanced anaglyph although it is not sufficiently distinct in the black & white version.

Now that you have had a chance to see the two rocks more closely and get the sense of where Spirit was and was headed next take a look at the real life version of the illustration you saw near the top of the page. Shown here is the satellite image from the Mars Orbital Camera (MOC) with the actual tracks left by Spirit over the first roughly 90 sols of the mission. The dark spot on the northern rim of the crater is a result of the impact from the lander's heat shield.^[5] 

Figure 9. Satellite image from the Mars Orbital Camera (MOC) with the actual tracks left by Spirit over the first roughly 90 sols of it's mission. The arrow pointing to the large dark rock is to its location and not to the actual rock. The limits of image resolution and lack of contrast do not enable direct imaging of a 77 cm object. Interestingly it does image a pair of 10 cm wide tire tracks. Image Credit: NASA/JPL/Malin Space Science Systems. Cropped and annotated version KilroyOnMars.com

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Link to unannotated version of above: R1502643_cropped.jpg

Link to original file: https://science.nasa.gov/resource/spirit-lander-and-bonneville-crater-in-color/

A careful examination of the satellite image will reveal several items of interest. You will notice that the actual track appears to deviates from that portrayed in the illustration most notably near the craters edge. The large dark rock (seen at top and in images below) is visible just beyond the end of the initial northeasterly track. The resolution of the image is .5 meters per pixel so that it is visible but lacking in detail. Positioned where the large light colored rock should be there is instead a white isosceles triangle or arrow head pointing in a northeasterly direction. I have no conclusion at this point as to what the arrow represents. It could be an artifact of image processing by Marlin Space Science Systems and thus somewhat random or it could have been inserted in the image as a reference point by someone — though not by me. I would suggest that the explanation is likely a simple one but those of you with a conspiracy bent, have fun with this.

Here is a much higher resolution satellite image of Bonnevile taken years later. By this time the tire tracks from Spirit have apparently been erased by wind but the lander is clearly visible as is the heat shield and both of the rocks we are interested in. I'm not certain this adds much to the discussion but I think it is quite interesting.

Figure 10. Spirit lander and Bonneville in high resolution color satellite image, January 29, 2012. From the High Resolution Imaging Experiment or HiRISE camera onboard the Mars Reconnaissance Orbiter. Original image scale range: 26.3 cm/pixel (with 1 x 1 binning) so objects ~79 cm across are resolved. Image: NASA/JPL/University of Arizona

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Link to source image page: https://www.uahirise.org/ESP_025815_1655

Summary

What I'd like you to take away from this is that this large light colored rock is sufficiently distinctive to make its identification in later images not only possible but, aided by the presence of its equally distinctive neighbors, somewhat easy.

You have seen that on sol 66 the rock has:

• a lack of anything protruding beyond its left edge or above above it
• a diagonal line emanating from the left edge running down towards the ground to the right
• a vertical bisecting line
• some of its top surface visible

Now that you know what it looks like on sol 66 you should be able to spot any differences on subsequent days.

Continue reading Part Two of this series where I will show you this same rock a few days later with far greater detail and start to expand on what I mentioned in the introduction. 

References and Notes

1. Figure 21. Golombek, M. P., et al. (2006), Geology of the Gusev cratered plains from the Spirit rover transverse[sic], J. Geophys. Res., 111, E02S07, doi:10.1029/2005JE002503

2. Grant, J. A., et al (2004) Surficial deposits at Gusev crater along Spirit rover traverses, Science, 305, 807—810, doi:10.1126/science.1099849  

3. Grant, J. A., et al. (2006), Crater gradation in Gusev crater and Meridiani Planum, Mars, J. Geophys. Res., 111, E02S08, doi:10.1029/2005JE002465.

4. Maki, J. N., et al., Mars Exploration Rover Engineering Cameras, J. Geophys. Res., 108(E12), 8071, doi:10.1029/2003JE002077, 2003.

5. Figure 6. Grant, J. A., et al. (2006)