The one and only image of Mars taken with the Hubble Space Telescope thus far during the 2001 apparition reveals what may be considered identifiable craters. HST took the image around the time of opposition and it will be hard to deny the apparent resemblance between those crater-like appearances on this image of Mars and images of lunar craters.

Figure 1. The one and only CCD image of Mars taken with the Hubble Space Telescope for the 2001 apparition. LEFT: Numerous apparent craters may be seen below the large crater Schiaparelli that is labeled on the above HST image. RIGHT: the craters 1) Cassini shown in upper right center, 2) Flaugergues, Bakhuysen, and Wislicenus. Shown below Shiaparelli. Many other craters can be picked out of these images if one looks carefully.

The similarities between Lunar craters and the crater-like appearances are striking on this image and several can be readily identified by comparing spacecraft derived maps and close up photographs to the HST image shown above (See Figure 1). Especially apparent is the crater Schiaparelli; a classical feature called "Edom," that displays telltale circular appearance with bright Sunlit and dark shadowed sloping walls. The crater-like features shown on this image is not unlike those seen on CCD images of the Moon taken with a ground-based telescope. While there are numerous craters-like features on this image at least two of the features are definitely artifacts. So, appearances can be deceiving.

Using image processing software to cut and paste the area the where crater Schiaparelli appears in the above figure this author scanned an image produced by the Viking spacecraft and made an overlay of the two images. After some aspect changes and size corrections the two images do indeed match almost exactly. So, I must conclude that this is proof that HST is capable of resolving craters on Mars, at least when Mars subtends an apparent angle of 20 seconds of arc or more. However, during the 1999 apparition when Mars was less than 20 seconds of arc images taken of the same regions on Mars were highly suggestive; the definition was simply not the same. Also, one can display an HST image from each apparition side by side for comparison and readily to see the difference. How would we compare the two images of Mars, or for that matter between images of Mars taken from ground-based using different apertures?

Using the best known estimates of the average height of a Martian crater to be 3-Km [Strom, et al, 1992], and the stated resolution of HST as 0.043 arcsec arc [Beatty, 1985], then the largest surface feature on Mars to be resolved by HST would be 6,779.8 x 0.043 /20.49 = 14.23-Km. However, it is apparent that several craters can be identified on the June 13, 2001 image so we can then establish a new resolving power for the HST as: (20.49 * 3) / 6,779.8 or 0.0091 seconds of arc. If we then applied the resolving power to HST as 0.0091 seconds of arc to sloping wall of a Martian crater then we very well identify it as a crater. 4.73 times the Dawe's limit of resolution.

HST image from the 2003 apparition of Mars proved even higher in detail and as one may see in the figure below the planets displays numerous craters, volcanoes, and other topographic features.
 

Figure 2. CCD images of Mars taken with the Hubble Space Telescope during August 27, 2003 – closest approach of Mars to Earth.   LEFT: Olympus Mons can be seen in upper center of Mars’ disk as a light yellowish tented circle. Numerous apparent craters may be seen embedded within the dark albedo features of the southern hemisphere of Mars.   RIGHT: the large craters Schiaparelli (Edom) within dark features Sabaeus Sinus-Meridiani Sinus as in Figure 1 and the largest crater, Huygens, as seen nestled between Sabaeus Sinus and Syrtis Major. Many other craters can be picked out of these images if one looks carefully.

As a general rule of thumb we use the Dawes criterion (4.56"/aperture) to define the resolution of an optical telescope. Dawes experimented with a fairly small aperture telescope to observe and record his impressions of how close he could separate equally bright double stars. His work as been used as a reference to define telescope resolution for over a century. However, his criterion does not take into account the color, intensity, and contrast of the features on extended objects.

Planetary observers often claim that they can resolve higher resolutions for planetary images than that of the Dawes criteria. Experiments by well known planetary observers have concluded that they can see planetary details as much as 14 times in excess of the Dawes criteria [Dobbins et al, 1987]. This is not easily proven. I doubt that observers have either a telescope of sufficient quality and aperture to resolve as well as HST. Also, the observing conditions for observing such high resolution is unlikely anywhere on the surface of Earth

Visual observing is highly subjective and some aspects of film or electronic imaging of planets is subjective as well. This is especially so when the images are processed using computer software. Image processing may lead one into the realm of producing pretty photographs and therefore resulting is a less scientifically informative image. While there is no device we know that can equal the resolution of the human eye it is difficult as best to reproduce the impressions to paper. So, if one can apply this to replicating their observations as closely as possible with the image that retains in their memory a drawing can be an important part of recording conditions on Mars. It takes practice but it is possible.

We can use the Dawes criteria to represent what optical telescopes are capable of resolving. For instance using the published equation for the Dawes "limit" or 4.56"/aperture, a 40-inch telescope can resolve 4.56/40 = 0.114 seconds of arc. We then find that when the Mars subtends an apparent angle of 25.11 seconds of arc, with a mean radius 3389.92 +/- 0.04 km [Kieffer et al, 1992], the 40-inch telescope would yield a 30.8-km. In other words, the smallest object or feature we can see on the 6,779.8-km diameter Mars will be 6,779.8 x 0.114 /25.11 = 30.8 kilometers.

If we further explore this theory and consider that some planetary observers claim to resolve extended objects 14 times better than Dawes limit allows then we can divide the 30.8-km resolution by 14 and find a result of 2.2 kilometers. We are getting closer to seeing those craters from Earth -- if we forget all the other factors such as the turbulent atmosphere of Earth that tends to degrade resolving power of our telescopes!

The aperture of the Hubble Space Telescope (HST) is 2.4-meters (94.5 inches) and using Dawes limit yields 0.048 seconds of arc resolution. The published angular resolution for the Wide-Field and Planetary Camera (f/30) has been reported to be 0.043 seconds of arc [Beatty, 1985], so this will be used in my example. Furthermore, to remove doubt that HST can resolve at this level one may look at the images of Pluto and Charon on page 14 of the November 1994 and page 25 of the February 1995 Sky and Telescope magazine. While Pluto's computed 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]. Using image-processing software one can simply enlarge and count the pixels on the image to determine his.

When Mars appears to be 25.11 seconds of arc a telescope capable of resolving 0.043 arcsec would reveal a surface feature no smaller than 11.6-km (6,779.8 x 0.043 /25.11 = 11.6). We are then presented with a problem of finding a surface feature that is at least 11.6 kilometers in size, such as a volcano caldera wall, sloping wall of a crater or a really big boulder! While we find many craters and volcano caldera on Mars in that size range or larger to resolve these from Earth telescopically there must be craters walls or the slopes that are at least that large to resolve. We have found that the average wall height for the known craters on Mars is only around 2 or 3 kilometers high [Strom, et al, 1992]. We now must find some other feature to test this resolution theory. Maybe a shadow from a mountain peak or crater wall will work.

How long would the slope of a crater wall or a shadow appear on Mars? Using then average height of crater walls on Mars of 3 kilometers we then can find out how long a crater wall shadow would appear near the terminator of Mars. A good example would be when the phase angle of Mars will reach 43 degrees on May 18, 2003. The diameter of Mars will then be 11.0 seconds of arc. To predict the length of the shadow we will use the equation:

where h = height of crater wall, and a = angle of the Sun (represented by the phase angle). The cotangent of a is 90 - 38 or 52°. So, we find:

Therefore, a 3-km high wall would produce a shadow of 2.34-km, only when the wall was positioned near, but not in the terminator. It will also decrease in length, as the wall is seen father away from the terminator. A bright sunlit slope to a crater wall would be longer so one might guess as to the slope angle and go from there. For example, if one counts the pixels of the bright arc in the north rim of Schiaparellie they would find an average of 6 pixels. The Mars image subtends 1,000 pixels across so the arc would be 6/1000 at 0.006. Since Mars is about 6780 kilometers in diameter and the huge crater is near the center of the globe then the bright sloped wall would be 0.006(6780) approximately 41 kilometers (~25miles). Be aware that during image processing these bright arcs or circular areas can be produced by over-processing and will appear larger than they would in a normally processed image.

In the table below several telescope apertures are listed along with the resolution of each according to the Dawes criteria. One may divide these values by 5, 14, or any other value to find the possible resolution according to well-known planetary observers -- or to suit your own preference. In any case it is not worthy of much consideration in the world of science; but it is interesting and a good review of apparent sized of stuff on other planets as seen with a telescope.
 

Table I. A table showing the apparent size, according to the Dawe’s criteria, of Martian features relative to telescope aperture when Mars will be 25.11 seconds of arc at closest approach in the 2003 Perihelic Apparition of Mars. The last row indicates the HST aperture and uses the Dawe’s criteria for resolution. Dividing these values by some subjective number will result in higher resolution.

This article will not introduce any startling news and will not resolve all the debates that have gone of for years over the issue of what observers can see or not see. It will continue for years to come. However, it does point out some mildly interesting points that may help those who wish to evaluate images of Mars and to help those who tend to over process images to back off a little. We are again going through a period when observers feel they have little to contribute to the study of Mars and this is just not the case. The value of visual observing and drawing at the telescope is good training observers so that of they do engage in high-tech telescoping they will at least have a feel for what their images actually are supposed to represent.

From the time when man evaded Mars with spacecraft loaded with instruments we started hearing this complaint from observers. During the 1978 apparition of Mars observers really began to lose interest and the ALPO Mars Section only collected a few hundred observations from a few astronomers. With the encouragement and careful guidance of our mentor, Chick Capen, we began a long but steady recovery that peaked out during 1988 when we received from 320 astronomers over 7,200 observations by the end of that apparition. Drawings and other visual observations accounted for 41% of the total then and the reason we got more photographs and CCD images is quiet simple: it takes a lot more time to produce an drawing that it does to snap a photo. The ratio of visual observers to image takers was about fifty to one, with visual observers in the majority. Participation is also part of our game plan.

Beatty, J. Kelly, (1985), "HST: Astronomy's Greatest Gambit," Sky and Telescope Magazine, Vol. 69, No. 5, pp. 409-414, May.

Beatty, J. Kelly, (1985), "HST: Astronomy's Greatest Gambit," Sky and Telescope Magazine, Vol. 69, No. 5, pp. 409-414, May.

Beatty, J. Kelly, (1995), "Hubble’s Worlds," Sky and Telescope Magazine, Vol. 89, No. 2, pp.25, February

Dobbins, Parker, and Capen, (1987), Introduction to Observing and Photographing the Solar System, Willmann-Bell, Inc., ISBN 0- 943396-17-4, pp.5-7.

Kieffer et al, (1992), "The Planet Mars:From Antiquity to the Present, Part 1 – Introduction," Mars, p. 29.

NEWS NOTES, (1994), "Zooming in on Pluto and Charon," Sky and Telescope Magazine, Vol. 88, No. 5, pp.14, November.

Strom, R.G., Steven K. Croft, and Nadine G. Barlow, "The Martian Impact Cratering Record," Mars, University of Arizona Press, ISBN 0-8165-1257-4, 1992.