Can We See Martian Craters From Earth? By: Jeff Beish (Revised 01/15/2019) INTRODUCTION 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. An excellent illustration of how to identify an impact crater on a celestial object can be found here and here. Most assuredly, if we are to compare the appearance of an impact crater on the Moon to one on Mars then the following criteria should apply: A crater should have a raised rim, walls, a floor, possible central uplift, ejecta and rays. 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 space age 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 clos e 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]. NOTE: Spacecraft images revealed new impact craters on Mars: see Malin Space Science Systems . 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. Figure 1. Cut away drawing of typical Martian crater. Drawing shows an average large Martian crater, such as Huygens (304ºW, 14ºS), with a depth of 3-km and diameter of 500-km. Maximum shadow for 47º phase defect = 3-km x sin 47º = 2.2- km. 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 limb of the planet 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? Accounts from various sources, mainly from the Journal of the British Astronomical Association (Sheehan, 1994) and others, claim to have letters to and from John Mellish alleging that he had observed craters on Mars in November 1915 using the 40-inch Clark refractor at Yerkes Observatory in Williams Bay, WI (long. 88ºW 33.4’, 42ºN34.2’) during Central Standard Times (CST – UT = -6). 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 25.13 arcsec (largest apparent diameter). We can easily calculate this value by multiplying the diameter of Mars (6,792-km) by the image scale of the telescope: 6792 x 0.114 /25.13 = 30.8-Km. 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. From various publications it is believed that John Mellish was observing Mars from 20 minutes to one hour before sunrise on November 13, 1915 and could have seen the crater Newton (154.5° – 161° W, 38.5° - 43.5° S) [Harris, 1995]. Let’s analyze this: The Sun rose at 1243UT (06:43CST) that day and Mars rose at 0444 UT (22:44 CST on 1915 Nov 12). That means that in order for Mellish to see and recognize the 6.5- degree wide crater Newton he would have to wait until the western wall of Crater Newton (161°W and 41°S) would appear on the southwest limb of Mars. Since the wall is at longitude ( ) = 161°W and latitude ( = 41°S, we find that the required Central Meridian (CM) to have to be 123.3°; mathematical proof: CM = / Cos - 90° = 161° / 0.75471 - 90° = 123.3°. Running the program WinJUPOS we find the western extent of Crater Newton (161°W and 41°S) would not appear on the limb until 1504 UT (09:04 CST) when the CM was 123.3° and the Ds = 6.7 and Phase Angle ( i ) = 38.3°.