After the shape was fit, a vertical line was projected from Point L1A up to Point H1 (the length of this line equals the previously derived height).  The procedure was repeated for sun incidence angles of 75 degrees and 60 degrees, drawing the lines tangent to the shape and projecting vertical height lines from Points L2A and L3A.  As can be seen, height H2 and H3, are considerably higher than the true height of Shape 1.

Repeating the graphical method for Shape 2 and Shape 3 provides a rough method of determining the profile of the shadow causing shapes.  By comparing the height to width (H/W) ratios of the three shapes above to the H/W ratio derived using measured data, the third shape is the closest match. 

5 Zone of Disturbance

Although direct images of the shadow producing shapes do not appear to be visible, a close examination of the area around the ring shadows does reveal subtle evidence of the source.  Dune slope has a very smooth appearance (either light or dark depending on sun angle in relation to slope), while areas of flat ground have a distinctive mottled pattern.   Surrounding some rings is an area that I will call the disturbance zone, in which the dune and flat ground patterns are both disrupted.  This is not easy to see in a printed image, but the NASA MOC image downloaded as a 4,714 KB .gif file and imported into Microsoft Excel was clear enough to view it.  In the middle image of the series below, phantom lines have been circled around the disturbed zone.  And in the third image, contrast has been increased to 85% and brightness decreased to 38%, which has the effect of causing the disturbed ground pattern to turn dark.​

Using ring shadows to determine shape

​Because upper layers would block the reflected sunlight on the sunny side of the tree, the resulting reflection would look like a "C".  The gap in the "C" would face the sun if the MOC viewpoint were directly overhead, but the gap would align more towards the viewpoint if it were at an angle.  

10 Tree Trunk Slant Due to Wind

Barchen dunes form in areas where the winds blow consistently from one direction.  Trees on earth exposed to steady wind tend to grow/lean with the wind.  If trees on Mars slant with the wind, the tree trunk would appear to point above the prevailing wind direction when viewed from the MGS MOC camera.  As can be seen in the image below, that is exactly the case.​​

In the Mars Orbital Camera images, as reported by NASA:  “Many of the dark spots [in image SP2-50805] have thin, dark streaks that radiate in several different directions.

Although it was theorized that the streaks were caused by passing winds that mobilized some of the dark sand at each location.  The thin, upward-pointing rays run perfectly straight over both flat ground and curved dune surfaces.  Even if the wind blew the sand in a straight line over the dune surface, from space the line would appear to curve (bend this piece of paper and you'll see how the line above becomes distorted).  It is more likely that the thin streaks are digital noise, which indicates that the inner structure of the Martian trees is similar to trees on earth.

9 Reflected Ring vs. Shadow Ring

In MOC image M19-01495 two circular white rings are visible at the base of one dune.  As well as being white rather than black, these rings are not distorted by the dune slope in the manner of shadow rings.  That is because these white rings are most likely formed by sunlight reflecting off frost on the foliage, and since the rings are not shadows on the ground, the dune slope does not affect their shape.​

​One measure of the ring shadow that can be determined with some certainty is that sunlight travel-length through the absorption layer must be equal at the I.D. and O.D. of the ring shadow.  With the actual boundary of the absorption layer being XOD and XID, the light paths LID and LOD (red lines) must be equal for the shadow intensity to be equal.​

The apparent angle between wind direction and tree trunk direction (q) depends on the MGS viewing angle and degree of tree slant.  Assuming MOC viewpoint is 90 degrees to wind direction, then

TAN(q) = TAN(camera angle) * TAN(tree slant angle)

If camera angle is 15 degrees and tree slant is 19.4 degrees, the apparent angle (q) equals 37.5 degrees, which matches the measured angle for q.   This is an oversimplification, but demonstrates the principle.

11 Tree Trunk Size

In the images where tree trunks can be clearly seen, the tree trunk diameters appear to be large in relation to crown diameter, at least in comparison to common Earth trees.   If we consider that image resolution ranges from 1.42- to 5.24-m/pixel, the trunk diameters must be at least 5.2 meters for them to appear in the lowest resolution image.  

Although the margin of error is high due to measurement fidelity, two trunks diameters were directly measured.   The MOC2-072 image was chosen to do this because at 1.42 meters/pixel it has the highest resolution of the four images with visible trunks.   The resulting average trunk diameter was 5 meters.

As a third method of roughly determining trunk diameter, rings in image SP2-53807 could be lined and scaled.  As shown in the image below, the boundaries of the trunk were determined and the percent of overall ring diameter estimated.  In this case it was 20%, and considering the average ring diameter for image SP2-53807 is 25.5 meters, the average trunk diameter would then be 5.1 meters.

This fact allows Lmin, which occurs at the center of the ring, to be compared to Lmax, which occurs at the tangent to XID.


LOD = 2*SQRT((XOD/2)2 - a2)

LID = 2*SQRT((XOD/2)2 - b2) ñ 2* SQRT((XID/2)2 - b2)

Lmax = 2* SQRT((XOD/2) 2 - (XID/2)2 )

Lmin = (XOD ñ XID)

Setting the equations for LOD and LID equal gives us:       

XID = 2*SQRT(( SQRT((XOD/2)2 - b2) - SQRT((XOD/2)2 - a2))2 + b2 ).

Since XOD (the zone of disturbance) was measured in the prior section, and was found to be 1.67 * Ring OD, then XID, Lmin and Lmax can be solved and the ratio of Lmin/Lmax for each ring can be compared.​

​The rings on Mars are most likely caused by the same phenomenon. The objects casting the shadows are spherical with a layer that partially absorbs the light passing through it. The absorption layer absorbs more light at the periphery and causes a ring-like shadow on the ground.​​

NASA STScl-2003-11      NASA STScl-2002-25

This implies that the thickness is being regulated by some form of feedback mechanism that holds the relationship between shortest light path and longest light path to a set ratio.  Since a biological organism is the only thing imaginable that has a need of, or is capable of, controlling its size in order to regulate the absorption of sunlight, and because the shape and size of the absorbing spheres is so similar to earth trees, I will start referring to them as trees. 

7 Earth Analogy

To better visualize what a tree with a thin canopy would look like from space, a projected image was drawn to show how it might look.  Using the estimated MOC viewing angle and the solar incidence angle from MOC2-147, a tree with a height to width ratio of 50% was projected in the manner the MOC would image it.  

The canopy shadow is projected down to the ground at the sun angle, while the tree trunk is imaged directly at the angle of the MOC.  At the 3m/pixel resolution of this MOC image, details like branches blur into the background.​​

For a perfect sphere the sunlight through the center is always normal to the sphere and the path length through the absorption layer does not change.  For an angled profile, the path increases in length as the sun angle goes lower.

However, due to the shape of the tree, the derived height-to-width ratio can be considerably off.  To correct for the error in derived height the profile of the shape must be determined.

4 Graphical Method of Determining Profile

In calculating the height of an object by using sun angle and measured shadow length, the shape of the object can greatly influence the resulting derived height.  As shown in the figure below, a low elliptical object with a height-to-width ration of .5 will result in a derived height of 11.2 at an incidence angle of 79.7, but will result in a derived height of 15.4 at an incidence angle of 60.8.

To graphically determine heights H1 to H3 (reference figures below) an ellipse with width equal to the average width of rings in image SP2-49306 was sized (vertically) to fit within the parallel lines of the sun angle and the intersection with average shadow length (L1A & L1B).  Average shadow length and width measurements from image SP2-49306 were used as a starting point since the sun angle of 79.77 provides the closest measure of true height. ​

8 Thin Radiating Streaks

Earth satellites with similar resolutions to MGS MOC, such as IKONOS, are actively being used to monitor and catalog natural resources such as plant growth rates, type of vegetation, and year-to-year changes. An attempt was made to find images similar to the Mars ring-like shadows, but lacking a budget to access image catalogs nothing similar was found in public photos.   

Although no ring image was located, information regarding issues of radiating streaks in digital satellite imagery was found.  Objects such as trees and bushes cause thin radiating lines in unfiltered images.  Special algorithms are used to remove these high frequency spikes, while still retaining enough detail to delineate individual trees/shrubs.​

​Per "A Remote Sensing Tutorial for Natural Resource Managers" high frequency noise exhibited as thin radiating lines, can be a problem with remote sensing of trees and shrubs.  The ring shadows in MOC image SP2-50805 and E20-01114 also exhibit radiating streaks, which imply a structure similar to trees and shrubs on earth. ​

Assuming a profile slope of 38∞ for the absorption layer (this being equal to the maximum angle of sun), and setting thickness equal to one (1), then path length can be solved for angle q, where q equals sun angle + 38∞.  For the 60.86 and 75.05 angles of inclination in E20-01114 and SP2-53807, path lengths equal 1.252 and 1.085 respectively. 

Dividing Lmin values by the correction factor yields an Lmin/Lmax ratio for E20-01114.  This can be used to adjust the path length for Lmin in SP2-53807 and recalculate the ratio.  As can be seen in the plot, for ring thickness varying from 13 to 3 meters, the Lmin/Lmax ratio (blue line at top) is a consistent 64%.​

   MOC E20-01114                 MOC M20-00416               MOC SP2-50805               MOC SP2-53807              MOC M19-01495

In addition to an ample source of water, frozen CO2 is present at the poles, as well as sunshine 24 hours per day during the growing season.  Mars is considerably colder than Earth, but even Earth plants have adapted to extreme cold conditions.  In very hardy arctic woody plants, such as Betula, Populus and Salix their survival depends upon tolerating extracellular ice formation and cell dehydration (Sakai and Weiser 1973, George et al. 1982, Larcher 1982, Ashworth 1996).  These species can survive immersion in liquid nitrogen (-196 C) when fully cold acclimated (Sakai 1960).  

Everything needed for life is present and images returned by the Mars Global Surveyor Mars Orbiting Camera have enabled the following details of possible tree-like life to be observed or derived:  

∑        Location at base of dunes is similar to desert plant life on earth

∑        Tree shadows orient with sun direction

∑        Sand drifts are seen originating from base of leeward side of tree

∑        Tree trunks appear to slant with the wind

∑        Crown shadow distortions, caused by dune shape, match dune/tree model

∑        Trunk and crown size similar to trees on earth 

∑        Height to width ratio is 50%

∑        Profile of tree is somewhat like a diamond with 38∞ side angles

∑        Tree crown is hollow at the center, which causes ring shadows

∑        Absorption layer thickness is controlled by the relationship of light travel distance through the shortest and longest path

∑        The true crown diameter is approximately 1.67 times the visible ring shadow diameter

∑        When frosted CO2, the trees can reflect a bright white ìCî shaped ring

∑        Possible layered structure like Spruce and Balsam​

​The path length ratio variation is fairly small within MOC images, being 64.4% +/- 2.6% for rings in image E20-01114, and 73.9% +/- 3.1% for image SP2-53807.  The larger variation between images can be explained by the angle of solar inclination.  For one image it is 60.86∞ and the other it is 75.05∞.  If the shape were a perfect sphere, the angle of inclination would not affect the absorption path calculation.  But as determined in section 4 Graphical Method of Determining Profile, the profile is more likely to fit the form of Shape 3, and this shape does affect the path calculation.​

The diameter of the disturbance zones around five rings in the image above was measured.  This measure can be expressed as an average ratio of zone diameter to outer diameter of ring.  The result is a ratio of 1.67.  This relationship is important as it is needed to derive the absorption layer thickness.

6 Absorption Layer Thickness in Relation to Ring Diameter

In planetary nebulae there is a set volume of gas in the shell, or absorption layer, so as the nebulae expands the thickness of the absorption layer shrinks in relation to the diameter.  To determine if the absorption layer of the shadow rings was a constant thickness, random, or varied with diameter, the ring thickness of the clearest ring images were measured.  Although better tools for image analysis at this level of detail probably exist, for this study MOC image E20-01114 and image SP2-53807 were imported into Microsoft Excel and the line tool was used to measure inner and outer diameters.  

The change from dark to light at the boundaries of the ring shadow is a gradual transition rather than a discrete ON/OFF.  Lacking sufficient resolution to determine I.D. and O.D. by measuring image intensity and setting limits to a predetermined value, considerable tolerance uncertainty is unavoidable.  But by visually selecting the same shade of gray with care, every effort was made at consistency.

Results of 12 measured spots show thickness, Ω (O.D. ñ I.D.), having a wide variance, ranging from 3.40 to 13.12 meters.   Comparing ring thickness to ring O.D. shows some correlation within image set % to % for image E20-01114 and  % to  % for SP2-53807.  But if all 12 data points are considered together the result is % +/-  %, which seems to indicate there is no direct relationship between thickness and O.D.​

​Ring shadow length and width were measured in five of the NASA MOC images by inserting the images into Microsoft Excel and using the line tool.  With the incidence angle provided in the NASA ancillary data and the corresponding average shadow length, the average height of the mounds was derived (Shadow Length * TAN Incidence Angle) for each MOC image.  The results, shown in the chart below, indicate a fairly consistent height-to-width ratio with the average ranging from 53% to 67% for the five images.​

It is fortunate the MOC captured this reflected ring image because it provides additional insight into the nature of the trees.  For an object to reflect a ring requires a certain structure or organization.  A simple spherical crown would reflect a bright spot the way a dune reflects sunlight (note: bright area on dune in image above).  

One possible shape is a sphere with its top removed.  Since the ground is visible in many ring images, this could very well be the configuration.  Both these rings also have a section of the ring missing in the same location, which roughly coincides with the windward facing side of the dune. Perhaps wind has blown the frost loose or has distorted the tree in this zone; however, the biological advantage of a theorized "top removed" configuration seems rather unlikely.   Also, because the circle is flat, it would not strongly reflect sunlight up to the MOC. 

Another tree configuration that better satisfies all the visual clues is a layered structure similar to Balsa or Spruce.   Some layers would be at an angle that would trap sunlight, while other layers would be at an optimal angle to reflect light up toward the MOC.   

Plotted against an assortment of big tree record holders, the crown diameters of the Martian trees (blue bars) are average specimens, while only the Arizona Sycamore and the Baobab trees exceed the trunk/crown diameter ratio of the Mars trees.  

12 Sand Drifts Originating at Base of Tree.

All of the spots in image MOC2-169 exhibit a ray that points toward the upper left corner of the image.  Drifting sand rather than digital noise causes these rays.  Like snow drifting from fence posts and piling up against a snow fence, the dark sand in the MOC2-169 image displays similar traits.   The remaining streak and blotch portion of the spots do not have the characteristic ray-like sand drift shape of the upper-left pointing rays.  They also do not have the corresponding "snow fence" dark sand collections in the directions they are pointing. ​

​As can be seen in the ring images above, in each example the direction of orientation of the ring lines up with the direction of sunlight (yellow arrows).  Also the affect of dune slope on the shadows is very apparent as noted by blue arrows.

And in another similarity to planetary nebulae, the line of the dunes is visible through the center of the ring, as are stars through the center of planetary nebulae.

3 Height to Width Ratio

In the case of the ring shadows, the width, as well as shadow length, are large enough to be measured.  This allows the height-to-width ratio of the shape to be determined.​

​As can be seen below, the projected image is a close match to spots from the MOC2-147 image. ​

1 Introduction

The discovery that the gray spots around polar dunes on Mars are shadows cast by surface features, measuring upwards of 25 meters high and numbering over 11,000 in the 14 images analyzed, leads one to question the nature of these shadow-casting objects. How did they form? What are they composed of? Are they a mineral deposit or life form? This study proposes possible answers to those questions through the analysis of additional image details. One of the best clues providing insight into the nature of the features are the ring shadows observed in five MGS MOC images.​​

Through detail analysis of these rings and two other MOC images containing elliptical shadows, the height to width ratio, height above ground, internal structure, ring thickness to diameter relationship and several other features are characterized.  

2 Ring Shadows and Planetary Nebulae

As many planetary nebulae appear as rings, the phenomenon of ring images are familiar to astronomers.  In the case of these nebulae, the ring is actually a hollow shell of expanding gas that is emitting and absorbing light relatively uniformly.  But in the viewer's line of sight, more light is emitted along the periphery, so the outer ring is brighter.  And because this light must travel further through the shell of gas (see figure below) more of the short wavelength light is re-absorbed by the gas, resulting in the nebulae rings that often appear reddish in coloration.​​

Tree trunk and crown as seen in shadow

Signs pointing to dark sand drifting from trees:

  • In each and every instance, the starting point (thick end) of the rays originate from the base, or bottom, of the spots -- as would be expected if sand were drifting from the leeward side of tree trunks.
  • Dark lines (indicated by red lines below), which are presumably areas where dark sand falls out of the wind as it slows at the foot of the dunes, match up correctly with wind direction (red arrows).  
  • Slight variability in direction of rays.  Looking closely at the red arrows, it can be seen that local variation in ray direction matches surface features that would lead to local variation in wind direction. 

13 Summary

Based on images beamed back by every Mars Lander, life on Mars today seems like an impossible phenomenon.  But if we follow NASAís ìFollow the Waterî mantra, the Landers have all touched down in the parched Sahara, missing the water-logged Polar Regions.  As reported by the NASA 2001 Mars Odyssey spacecraft, the Polar Regions of Mars are believed to contain up to 50% water ice in the upper one meter of soil. [] The Mars Odyssey image bellow shows the distribution of water over the planet ñ deep blue being highest and red being lowest.  The locations of the Landers and the dune spots/rings have been added to the image.  As can be seen, the locations where the spots/trees have been found, all lie within the blue zone of high water concentration.​