What may be the first Martian topographic relief features ever identified from Earth, or from near Earth, has been found by Leonard Martin at Lowell Observatory's Planetary Research Center and Jeff Beish, Former A.L.P.O, Mars Recorder. These may be the first topographic features identified since the Viking Orbiter stopped sending data back to Earth in November 1982. Now, the Hubble Space Telescope has imaged the Red Planet near opposition in February 1995 containing an identifiable topographic feature on Mars (see Figures 1 and 2).

Figure 1. A CCD image of Mars with exploded view inset reveals a topographic relief in the exact position of Elysium Mons. The arrow points to a bright feature of the sunlit slope of the volcano with a shadow to the terminator side of Mars’ limb. Image taken from the left-hand image of PR95-17 ST Scl OPO HST labeled Tharsis Region 160° Longitude. Bottom inset is an image of Elysium Mons taken by the Viking Orbiter- I spacecraft during the Viking Space Mission to Mars during the period from July 1976 throughout November 1982. Image by NASA.

To illustrate that the HST image of the apparent volcano Elysium Mons Figure 1, bottom inset, reveals the volcano as seen from the U.S. Viking Space Mission to Mars during the period from July 1976 throughout November 1982. Notice the bright escarpment and following shadow towards the terminator of Mars.

To identify a topographic relief on another Solar System body, such as the Moon or Mars, one must be able to see sunlit wall, cliff, slope, escarpment, and/or a corresponding shadow. For instance, a Lunar crater with the right lighting conditions may show a bright Sunlit wall or rim and a shadow on the opposite side of the wall or rim and the reverse on the other side of the crater.

A Martian volcano would appear as a triangular slope with a corresponding shadow on the opposite side from the Sun. This would appear similar to a Lunar dome or a steep Lunar mountain, except domes are usually rounder in shape.

The 170-km (106-mi) wide Elysium Mons is located at 213.°5W, 25°N and stands 9-km (5.6-mi) high over the Elysium Planitia. This volcano has steeper slopes than the other large volcanoes, such as those found in the Tharsis region of Mars. The very large volcanoes Olympus Mons and Arcia Mons are very flat, appearing like giant cow patties and would not cause much of a shadow or sunlit slope. Ascraeus Mons and Pavonis Mons have steeper slopes and may be revealed in HST images in the next apparitions of Mars. The 23-km (14.3-mi) high volcano Ascraeus Mons, located at 104.°3W, 11°N, is 400-km (250-mi) wide and is one of the largest volcanoes in the Tharsis region of Mars.

The phase terminator appeared to be just right to identify these two volcanoes described above. During the next apparitions of Mars, the planet will be larger and we hope topographic relief of the Red Planet will be identified. The author feels that observing these features on Mars from ground-based telescope is impossible and will not be practical in our life times. The Hubble Space Telescope is the finest astronomical observing instrument ever constructed.

Figure 2. A CCD image of Mars with exploded view inset reveals a topographic relief in the exact position of Ascraeus Mons. The arrow points to a bright feature of what appears to be the sunlit slope of the volcano with a shadow to the terminator side of Mars’ limb.  However, this is a false assumption because what we see is the top of Ascraeus Mons poking through the cloud. Image taken from the center image of PR95-17 ST Scl OPO HST labeled labeled Valles Marineris Region 60* Longitude.

How long would the shadow of Olympus Mons (133.4° W, 18.1° N) appear on Mars? Using the 23 kilometer height of Olympus Mons relative to its surrounding area we then can find out how long the shadow would appear near the terminator of Mars.  Although the volcano is nearly 23 km high, it is over 20 times wider than it is tall. Thus, most of the volcano has a fairly gentle surface slope.

Remember that Olympus Mons is not like most volcanoes found on earth. The slope angle from the edge of its caldera is only is 2.5 degrees to about 50 miles out and then from there the grade increases to only 5 degrees and continues 162 miles to the very edge or scarp of the volcano.  Further shadowing would be seen to cover the slope from the 5-degree slope plane out to miles to the edge of the volcano.

Figure 3. Olympus Mons is classified as a shield volcano that stands 23 kilometers (14.3 miles) above the surrounding plains of the Tharsis region of Mars. It is Olympus Mons (27-Km or 16.8 miles) in altitude, 600 kilometers (373 miles) in diameter and is rimmed by a 6 kilometers (4 miles) high scarp. By comparison the largest volcano on Earth is the 9 kilometer (6 miles) high Mauna Loa that is 120 kilometers (75 miles) across.

Figure 4. Image of oblique view illustrates the shallow slope and scarp of Olympus Mons.  Oblique Olympus Mons, MGS MOC Release No. MOC2-479, 10 September 2003, Malin Space Science Systems.

A good example would be when the phase angle (i) of Mars reached 41.1° on September 05, 2005 at 0956UT.  The diameter of Mars was 14.6 seconds of arc and a Central Meridian (CM) of 146.7°.  Ed Grafton's image attached with inset is probably as good as one can capture that cow patty, Olympus Mons, and by me reckoning the image appears to be of a triangular, pointed feature with rills, canons, and a rather blunted and square "shadow" of what appears to me as a sheer straight wall on the evening side of the volcano "image."

Figure 5. Ed Grafton image taken on September 5, 2005 at 0956UT, CM 147°. Olympus Mons with accompanying shadow on the slope of the volcano.

We must first determine at what longitude the sub-Solar point or High Martian Noon is and then determine what angle the Sun is relative to Olympus Mons.  If Mars is positioned before opposition the sub-Solar longitude would be Central Meridian (CM) + phase angle (i) in degrees.  After opposition the sub-Solar longitude would be Central Meridian (CM) - phase angle (i).

On 2005-09-05 the sub-Solar longitude would be 146.7°+ 41.1° = 188° for pre-opposition Mars. The angle of the sun above Olympus Mons is now 187.8° - 133.4° or 54.4 degrees to the evening side of the Noonday sun, therefore we find the shadow length using the equation:  h tan a , where h = height of Olympus Mons, and a = angle of the Sun.   The angle of the Sun over Olympus Mons is: 54.4°, so, we find:  h tan a = 23 tan 54.4° = 23 x 1.397  = 32.1-Km (19.9 miles).

The 25-km high volcano would produce a shadow of 32.1-Km, a few degrees away from the terminator.  Since the average radius of the huge volcano 300 kilometers (186 miles) then around 11% of the slope of Olympus Mons will be in a shadow.  An hour later when the CM is 161.3° and Sub-Solar longitude at 202.4°, the shadow will be 202.4° – 133.4° = 69°.   With the cotangent of 21, then 23 tan 69 = 23 x 2.605 = 60-Km or around 20% of the slope in shadow, and so on until the shadow is engulfed in the terminator.

Any other shadowing would come from the accumulation of shadows from the hills and valleys of the irregular surface of the volcano slopes.  The drop off at the scarp edge, an 8-km high wall, will be seen as a ring like shadow at the parameters of Olympus Mons.  So, Olympus Mons is a huge, flat mountain and if I were standing on it the volcano would appear to me as the whole planet from horizon to horizon.  One way to look at it, that volcano is shaped like a giant cow patty.

Another example show how dark Olympus Mons is when seen under the noonday sun.

Figure 7.  LEFT: Don Parker’s image on July 31, 2001 at 0147UT.  CENTER: Damian Peach’s image on August 11,2007 at 0405UT. RIGHT: Larry Owen’s image of Mars on August 22, 2007 at 1018 UT .

Any other shadowing would come from the accumulation of shadows from the hills and valleys of the irregular surface of the volcano slopes.  The drop off at the scarp edge, an 8-km high wall, will be seen as a ring like shadow at the parameters of Olympus Mons.  So, Olympus Mons is a huge, flat mountain and if I were standing on it the volcano would appear to me as the whole planet from horizon to horizon.  One way to look at it, that volcano is shaped like a giant cow patty.

What I am attempting to say is that image processing tends to make features appear not as they are -- but as they are computed.  But, why does it appear like a full shadow cast by Olympus Mons when it is near the terminator?  If the slope was much steeper than space scientists say it is then it would indeed cast a dark shadow and would begin to be seen shortly after the volcano pasted through the Noon sky.  If the slope of Olympus Mons was more like 5 to 7.2 degrees slope, and not the 2.5 to 5 degrees, as space scientists say it is, then a shadow would be readily seen.   We could calculate this further until at some point the shadow would be long and dark enough to be considered observable.  Some shadowing can be explained due the cats the Mars’ limbs gets darker as you look closer to the terminator. However, at some point the so-called shadow and the slope of Olympus Mons would be in the terminator so it would be impossible to distinguish the shadow from the darkness of the terminator.  Remember, Mars fall into complete darkness at the terminator and throughout the night because there is nothing to cast light onto Mars except for the Sun.

Also, even the thin atmosphere of Mars reduces the contrast at or near the limbs, so in the absence of bright hazes the limbs will appear slightly darker due the optical depth of the atmosphere. Remember too that atmosphere can affect the appearance of volcanoes on Mars.  Many times we see orographic clouds that either are centered over the volcanoes or are on the lee side caused by winds.  Observers have reported seeing shadows cast by the dense orographic clouds over Olympus Mons and when close to the terminator will add to the cumulative shadowing, atmosphere opacity and the shadowing from the cloud to darken the area even more so.

While researching this problem I ran into an interesting paper that changed my thinking as far as the shadowing of Olympus Mons [ McGovern, et al , 2005].  Measurement confirms my suspicion that a shadow can be seen from ground-based observers.  Using WinJUPOS the crosshairs were placed in the center of the feature between the bright and darker areas and was found to be located at 133.4°W and 18.1°N. The center of that 6.9° long dark area, or shadow, to the left of Olympus Mons is centered 5.6 degrees to the east and measures ~380-Km (see figure 8 below).

Figure 8. Olympus Mons with accompanying shadow on the slope of the volcano measured by WinJUPOS and finding the longitude and latitude corresponding to published location of Olympus Mons.  Inset show ~380-Km dark area or shadow.  Ed Grafton image taken on September 5, 2005 at 0956UT, CM 147.2°.

While it is true that we can measure this feature and place it at the correct location of Olympus Mons, as we now know the giant volcano to be, would we be able to identify it as a maintain or volcano without prior knowledge?  One could conclude the feature would very well be a crater or a large dome similar those observed on our Moon.
 
 

Figure 9. Two spectacular earth-based images of Mars take by Dr. Donald C. Parker a week away from opposition 2003.  These images reveal the Tharsis regions of Mars where at least four huge volcanoes can be found.

Years ago someone asked me if a person could see the horizon of Mars from the top of Olympus Mons. That is not as simple as one may think. If Mars was a prefect sphere and the slopes of Olympus Mons were smooth then the distance to the horizon could be calculated fairly easy. However, the surface of Mars has hills and valleys, and Tharsis bulges out from the mean spherical surface of Mars several miles, so one must consider that as well.

Considering that this huge volcano is 16.8 miles (27-km) in altitude -- it only stands 14.3 miles (23 kilometers) above the surrounding plains of Tharsis. We know that the radius of Mars is ~2,106 miles, then find the line of sight distance to the horizon by the equation:

First, if we were 6 feet tall and stood at the base of Olympus Mons we would then be able to see the horizon some 2.19 miles away. Actually, Tharsis bulges out a few miles away from the mean surface datum or sphere of the planet – therefore compounding the ability to see over the first 50 miles of Olympus Mons -- but I will skip that little part. We would find that:

Second, if this same person stood at the top of Olympus Mons (88,704 ft) we would find that he or she could see a lot further to the horizon, some 265 miles away, if nothing was in the way:

At those angles a person 6 feet high would be looking down at an angle of 7.2 degrees in order to see the horizon of Mars, so they would surely look down into the hump positioned at 50 miles away or at the 2.5-degree grade of the slope instead of over it to the horizon.

Figure 10. LEFT: Diagram illustrating relationship of distance a person can see to horizon that is standing on surface of spherical object, such as Mars. CENTER: The height of Olympus Mons relative to diameter of Mars. RIGHT: The slope of Olympus Mons relative to the slope of the globe of Mars.

Yes, Olympus Mons is a huge, flat mountain and if I were standing on it the volcano would appear to me as the whole planet from horizon to horizon. One way to look at it, that volcano is shaped like a giant cow patty.
 

REFRENCE:  McGovern, P. J. and J. K. Morgan (2005),  “PREADING OF THE OLYMPUS MONS VOLCANIC EDIFICE, MARS, “ Lunar and Planetary Science XXXVI (2005), Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston TX 77058, USA, Department of Earth Science, Rice University, Houston, TX 77005, USA.  http://www.lpi.usra.edu/meetings/lpsc2005/pdf/2258.pdf