Can we identify topographic features on the planet Mars using Earth-based telescopes? This argument has gone on for years and probably will continue even after counter proposals are offered here. It centers on claims by a small number of observers who have seen and identified craters, mountain ranges, canyons, volcanoes, and other Earth-like feature on the Red Planet Mars.
One should not forget that we are dealing with personal opinions and are often predicated on some loose and untried theories. To render an opinion on what someone else sees or does not see is difficult at best; however, we must follow conventional wisdom and what is known about the nature of telescopic observations. Theories vary from those that become "laws of physics" to completely wrong ones that defy replication. In any event, the discussions should not stray far from known and accepted facts. Of course, in the subjective minds of humans, who can define what a fact really is?
Prior to the Mariner-4 Spacecraft passing by Mars during 14-15 July 1965 speculation about the existence of craters on this Red Planet was confined to a small group of astronomers. Well-known observers, E.E. Barnard and John Mellish, are credited with the supposition that Mars had craters even before spaceage technology took us out there for a closer look. The problem with their claim is; Mellish’s drawings and observing notes were destroyed when his house burned, or as the story goes.
Recently, Barnard's drawings and observation logs were recovered and from the preliminary reports no such evidence of Barnard’s crater sightings have been uncovered [Sheehan, 1995]. Without hard evidence, such as photographs, observational notes, or drawings with specific locations of these features, we cannot even begin to accept such claims.
Other notables have speculated that Mars was a cratered planet. In 1944 science writer D.L. Cyr, in the book Life on Mars, suggested craters on Mars. In the late 1940's and early 1950’s R.B. Baldwin, C.L. Tombaugh, and E.J. Opik independently predicted the possibility of Martian craters because of its close proximity to the asteroid belt. However, NASA and other space scientists questioned this. If being close to the asteroid belt was a major factor in the number of craters on Solar System objects then the crater density should have be significantly greater on Mars, more so than on the Moon -- something they did not find. [Glasstone,1968].
LIMITATIONS OF THE HUMAN EYE
Since the human eye is capable of resolving objects no smaller than about 62 seconds of arc we cannot identify objects such as craters on the Moon, the disks of planets or their satellites with the unaided eye [Sidgwick, 1980]. We can see gross albedo features on the Moon, such as the dark maria or bright areas; however, Lunar relief is just too shallow to be resolved with the human eye without an optical system to magnify them.
Planetary observers fantasize about being able to resolve Jupiter, Venus, or even Mars with their "naked" eyes, but it just isn't possible. Mars only reaches an apparent diameter of 25.1 seconds of arc during closest approach -- Jupiter and Venus only about 50 seconds of arc, we must use some instrument to magnify these objects. This is only common sense if we accept the conventional definition of resolution of the human eye [Sidgwick, 1980].
One interesting question should be asked; how do we identify a crater on another celestial body? The Moon has both craters and domes, so, how do we know which is a crater and which is a dome? When the Moon has a phase both features will have a bright side and a dark side. The obvious answer is to know the relative direction of Sunlight on the Moon or planet -- or find a mountain and remember which side is bright and which is dark. Then follow that convention to define craters and domes.
Adding to the difficulty of recognizing Martian craters is its atmospheric
activity. Ground-based telescopic observers regularly report clouds and
hazes in heavily cratered areas on Mars. Spacecraft data indicates the
planet's surface is nearly always covered by a dusty veil, further lowering
contrast and at times renders the surface completely featureless [Martin,
1994]. Unlike our Moon with its sharp crater boundaries, Mars has been
subjected to billions of years of wind erosion, leaving its crater walls
rounded and floors filled with dust.
Another important aspect must be considered -- contrast. Even if we could resolve such topography on Mars as described above, would there be enough contrast between the shadowed or Sunlit walls and the crater floor to be recognized by telescopic observers? Limb darkening, the ever-present dusty haze, and clouds also reduce the contrast of these features considerably. The extension of the atmospheric mass near the planet’s limb will also decrease the contrast of a surface feature. Numerous Martian craters have dark floors, so, how could a shadow of a crater wall be separated from the albedo of its floor?
Telescope Resolution Theory Discussed
Initially, we use the Dawes criterion (4.56"/aperture) to define the resolving power of optical telescopes. However, planetary observers often use a higher resolving power than allowed by the Dawes limit for the threshold for planetary details. Dawes criterion only applies to resolving or "splitting" equally bright double stars and would not take into account the color, intensity, and contrast of the features on extended objects, or the effect of irradiation of bright objects that reduces the acuity of the eye.
Irradiation of bright objects, especially planets in the eyepiece, is evidently a physiological effect, originating in the eye itself and occurs between adjoining areas of unequal brightness. The extent to which the bright area appears to encroach upon the fainter one is approximately proportional to their intensity difference. Equally important is whether the targeted feature is darker or brighter than its background [Sidgwick, 1980].
Experiments by well known planetary observers conclude that they can see planetary details in excess of the Dawes criteria and this limit may be as much as 5 to 14 times too low. Some observers have claimed they can detect black lines on a light background in moderately bright lighting conditions well below the limit of resolution for their instrument; however, they do not say that they actually resolve the line [Buchroeder, 1984]. Pickering and Steavenson found by empirical means that they could see black dots on a white background from 2.3 to 3 times smaller than the Dawes limit [Dobbins et al, 1987].
Did John Mellish See Martian Craters From Earth?
Complying with the Dawes limit a 40-inch telescope, such as that used by John Mellish in 1915 [Gordon, 1975], can resolve 0.114 seconds of arc. This yields only 31-km resolution of Mars' surface area when it is at 25.1 arcsec (largest apparent diameter). We can easily calculate this value by multiplying the diameter of Mars (6,787-km) by the image scale of the telescope: 6787 x 0.114 /25.1 = 30.8
However, when Mars is only 7.7 seconds of arc, as it was during Mellish's observations in 1915, the resolution of the giant Yerkes refractor would be reduced to only 100-km of surface area. Even believing we can resolve 14 times better than Dawes criterion with this giant telescope, that leaves us limited to 7 kilometers resolution. This resolution will not let you see even the highest crater walls on Mars.
How long are shadows on Mars? At the greatest phase angle (47º) the longest shadow would be only as long as the highest crater wall or about 2 to 3-km. The phase angle on 13 and 15 November 1915 was 38º and Declination of Earth (De) or Sub-Earth Point of 20º. To predict the length of the shadow we will use the equation h cot a , where h = height of crater wall, and a= angle of the Sun.
h cot a= 2 cot 52° = 2 x 0.781 = 1.56, where the cot 52° = 1 / Tan(52° )
Therefore, a 2-km high wall would produce a shadow of 1.56-km, only when the wall was positioned at the terminator and would decrease, as the wall is seen father away from the terminator.
From the available reports, Mellish observed in the early morning hours of November 13, 1915 (1400 U.T.) the visible illuminated face of Mars exposed longitudes from the terminator at 46º through the CM (98º) to the morning side at 188º. On the 15th the longitudes were from 27º through the CM (80º) to 170º. Anyone who has observed Mars for any time will realize that when this little planet is tilted 20º from Earth the region seen in the above longitudes shows the lowest contrast side of the entire planet. Some Mars observers call this region the "hard side" or "void," meaning the side of Mars with the fewest albedo features.
Also, the population of craters in that particular quadrant of Mars (Xanthe-Tharsis-Amazonis and Tempe-Arcadia regions) is lower than most other regions of the planet and the larger craters there only average about 60-km (1.5 degrees or less) in diameter. Since Martian craters are similar to those found on other celestial bodies and follow similar depth/diameter ratios, crater depths in those regions run from 1 to 3 kilometers [Strom, et al, 1992].
A PERSONAL EXPERIENCE WITH CRATERS ON MARS
While observing Mars in 1984 with Lowell's 24-inch Clark refractor, Charles F. ("Chick") Capen, an internationally recognized authority on Mars and highly respected telescopic observer, detailed to me his encounter with Martian "astroblemes" using that very telescope. The word "astroblemes," from the Greek word meaning "star wounds," was coined by crater scientist Robert Dietz in the August 1961 Scientific American to describe large terrestrial impact craters [Capen, 1986]. Capen also experienced "astroblemes" while using the 82-inch telescope at McDonald Observatory in 1969, and told me, "what appeared as iron filings or spider web like features covering its surface."
At that time Capen and I were photographing the Red Planet with the International Planetary Patrol Camera and could only observe visually at 830x. While we were not happy with the seeing, we encountered perfect seeing one morning and began to draw Mars. At one instant, I looked at the Red Planet -- God of War -- and saw "a bunch of" craters adjacent to the northwest corner of Syrtis Major! This wedged-shaped region is well known to Mars observers because it is very dark and easily recognized from charts (See Figure 2).
My mind flashed back to what must have struck others when confronted with this and to the interesting tales of my friend sitting next to me in that old historic dome! Chick understood. My imagination? Yes! The human psychomotor domain (taxonomy) is full of mysterious tricks [Harrow, 1972].
Was it possible that I really did see craters? Craters just like the
ones seen on the Moon? Will anyone admit seeing craters or mountains on
the Moon without at least some instrumentation? Not this observer.
Figure 2. CCD image of Mars showing Syrtis Major. IMAGE produced by the Hubble Space Telescope (HST) IMAGE covers a region of Mars that contains several large craters; A) Huygens (304ºW, 14ºS), B) Schiaparelli (343ºW, 03ºS), and C) Antoniadi (299ºW, 22ºN).
Hubble Space Telescope (HST)
Since the resolving power of a telescope can be degraded by poor optics and Earth's unsteady air we can eliminate these problems with the Hubble Space Telescope (HST) now that its optics have been corrected.
Because the Hubble Space Telescope (HST) is located outside the Earth's atmosphere it might be considered the best instrument available for resolving planetary details. The aperture of the HST is 2.4-meters (94 inches), so, using the above theoretical limit for angular resolution the Wide-Field and Planetary Camera (f/30) would give 0.043 seconds of arc resolution [Beatty, 1985]. When Mars appears to be 25.1 seconds of arc a telescope capable of resolving 0.043 arcsec would reveal a surface feature no smaller than 11.6-km.
For example, from the images of Pluto and Charon on page 14 of the November 1994 and page 25 of the February 1995 Sky and Telescope magazine it may be deduced that HST is probably capable of the above resolution. While Pluto's apparent size would be about 0.14 seconds of arc at that time, Charon appears about 2.5 times smaller than Pluto or a little more than 0.05 seconds of arc [NEWS NOTES, 1994], [Beatty, 1995]. Although it is well within the capability of amateur telescopes to reveal both Pluto and Charon as points of light they would be perceived as a double star [Sheehan, 1993].
Hubble Space Telescope will be imaging Mars during the third week of November 1994 and will capture images of Mars as it was exactly on November 15, 1915!
To identify a Martian crater wall of 1- or 2-km height from Earth would require a telescope with a plate scale of 0.004 - 0.007 arcsec, or an aperture between 16.5 to 33 meters (650 to 1,300 inches). We have not yet constructed such a monster.
We will never know what John Mellish really saw on November 13 and 15th of 1915. No drawings, descriptions, or locations of craters he claimed to have seen have ever been presented by the proponents of his so-called observations have been made [Goodman, 1992]. In my opinion, John Mellish saw only what he imagined as craters on Mars. According to observing records of E.E. Barnard, seeing at the Yerkes Observatory during 13 - 15 November 1915 was less than good and it is doubtful Mellish found a few mystical periods of time when seeing was perfect.
Some of the difficulties of observing topographic features on Mars are the limits in angular resolution of both the telescope and human eye. Also, the irradiation of this bright planet makes observing very difficult, especially without using proper filter techniques. Even using filters will not eliminate completely irradiation in the eye or reduce the effects of atmospheric diffusion enough to allow the surface of Mars to be seen as crisp and sharp as lunar details.
Mars has very little topographic relief and surface structures near the nighttime terminator are poorly lit. Transient albedo features also obscure shadows in red light, as hazes and clouds will do so in blue light.
The contrast of surface albedo features would make it very difficult if not impossible to separate from shadows crossing onto dark surface material. From the first images sent back from Mariner 4 (15 July 1965), Mariner 9 (1971), and Viking 1 & 2 (1976-82), Mars has been shown to be a relatively low contrast planet and required extensive computer enhancement and processing to bring out surface details.
Mars' atmospheric diffusion, hazes, clouds, and dust obscures its surface to a great degree. Dusty veils will leave large regions of Martian topography obscured. Observers regularly report clouds and hazes over and around the Tharsis volcanoes as confirmed by spacecraft orbiters. Wind and possibly water erosion rendered Mars’ crater walls smooth and less contrasty as those on the Moon.
Optical quality and ambient conditions to produce sharp and crisp images are very important. Very few optical instruments are capable of resolving even to their theoretical limits. Rarely do we encounter conditions good enough to see much of anything on Mars even if we are lucky to be using a high quality telescope at that exact moment a particular mountain or crater is positioned where we want it to be. Often we make mistakes in measuring images and waste time trying to prove something that is not real.
As a feature approaches the nighttime terminator of Mars it also decreases in brightness and would render shadows nearly the same intensity as the dark surface materials, how would anyone tell the difference.
After more than two decades of systematically observing this Red Planet, using a variety of different types and sizes of telescopes, I have come to the conclusion that we cannot recognize topographic features on Mars from Earth. Except for the large bright clouds hanging around inside Mars’ larger craters this author has never observed a Martian crater even using large professional equipment!
Because Mars has very little topographic relief I do not believe anyone can visually identify craters on its surface either, even with the largest and highest quality optical instruments on Earth. This also applies even after several noted astronomers had suggested the possibility of craters on Mars before space probes were sent there [Gordon, 1992 and 1993]. Of course, this proposition does not belong in the category of science; however, amateur observers may need to experiment in a scientific way to judge what can and cannot be seen.
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