Mars is the most earth-like planet in the solar system. However, there are significant differences between the two planets. Mars is a small world, about half the size of the Earth, and in the billions of years since its formation, most of its atmosphere leaked into space, leaving a relatively thin atmosphere predominantly of carbon dioxide. In its ancient past Mars certainly had shallow oceans, but now what little water is left exists seasonally in the two polar caps and what appears to be a planet-wide layer of frozen water beneath the Martian surface. A day on Mars is very close to 24 hours. The temperatures on this little world are far colder because Mars orbits further from the Sun than our own planet.
Mars is at the center of my personal fascination with planetary imaging and it is a very challenging subject. Getting sharp images of the surface of Mars is best done at opposition – the point of closest approach to Earth. Images don’t have to be obtained right at opposition, so, depending on the size of your telescope, useful images can usually be obtained for up to several months centered on the time of opposition. The Outer gas Giants of the solar system (Jupiter and Saturn) travel slowly in their orbits compared to the Earth, such that earth is in opposition with each of them on an almost annual basis. Mars is close enough to the earth that it actually takes the earth approximately two years to catch up with the planet between oppositions. What this means is that Jupiter and Saturn can generally be imaged for several months approximately once a year. In contrast Mars can only be imaged for a few months every two years. The planet is relatively small but it is bright. Because of its small size it is often desirable to apply use auxiliary lenses to increase the magnification of the telescope but this can be touchy if you don’t have really good seeing. Click on Mars on [NASA Solar System Exploration] page for a lot more information on the “Red Planet”.
Shown below are images from both the 2014 and 2016 oppositions. Different telescopes were used for each opposition and there has not been a Mars opposition since the addition of the OMC 200 telescope to the Observatory. In June and July 2018 it should be possible to try our luck with the new telescope, but the older images are included here to give you a sense of what can be accomplished.
During the 2014 apparition, the primary telescope at the Observatory was the very popular Celestron 8-inch (203 mm) Schmidt Cassegrain (SCT).The C8 had a native focal ration of f/10, which is too low a value for a reaslonable image scale. For this session the instrument was equipped with a 2.5X PowerMate for a working focal ratio of f/26.
Only one session produced really nice images, one of which is shown here. The small North Polar Cap, the low-albedo circumpolar ring is quite prominent, as are numerous surface features, including Syrtis Major toward the SW limb. The large white area to the extreme south is NOT the South Polar Cap. Note that it is offset from the Northern Polar Cap is is actually the Hellas Basin, a huge depression several kilometers below the average surface elevation. Hellas is often filled with clouds (as it is here). A very subtle feature can be seen along the western limb. What looks like a bluish-white haze is just that – high ice clouds typical of the night side of Mars (where temperatures commonly drop to more than 100 degrees below zero (Centigrade). The clouds here are still present after local sunrise but will disappear quickly as the surface warms to a balmy 50-60 degrees below zero!
Mars had an apparent diameter of 15.1 arc-sec. at opposition on 08 April that year. This image was obtained late in the month and the apparent diameter had dropped to just under 141arc-sec. The planet was at an elevation of 43 degrees – reasonably good conditions!
22 May was the date of opposition in 2016 and on 30 May the Earth made is closest approach to the Red Planet. Four days later, on the 3rd of June, I got the best image of the apparition. The 180 mm Maksutov (f/17) was used with a 1.6X Barlow lens on the camera to achieve a working focal ratio of f/28. This normally would have been too high a focal ratio, but for this session had exceptionally good seeing for a change. This image does not look as good as the one from 2014, but that is only because itg happened to cover a relatively “dull” area of Mars. In fact, surface detail is excellent for an instrument with an aperture of 180 mm/7 inches!
While the image on the third of June benefited from the use of an accessory Barlow to increase the image scale, this is not always the case. Above are two images from the 7th and 9th of June. The images on the left were obtained at the native or “prime focus” (f/17) of the 180 mm Maksutov, the images on the right were captured using the 1.6X Barlow, proving an effective focal ratio of f/28. While the f/28 images are larger, they have lower contrast and show less actual detail than the smaller f/17 images. On both 07 and 09 June, the seeing simply would not support the use of the higher focal ratio. Not only was the use of the Barlow a waste of time, it actually degraded the images compared to those captured at f/17! Additional magnification is of no use unless the seeing will support the magnification that is employed.
New Images will be posted here as Mars approaches the current opposition on 27 July 2018.
So why no new Mars images? Here is the main reason! As Mars began its close-in approach to opposition in May, a planet-wide dust-storm, something that occurs every six years or so, kicked up and the entire surface was obscured by a veil of atmospheric dust that completely obscured surface detail. This image, taken on 11 July 2018 is typical. Hints of some of the darker surface features, as well as traces of the polar caps, are faintly visible through the dust, but this is far from satisfactory when you have been waiting two years for what was supposed to be the best opposition in a long time! I will try some experiments with color filters as it is not like that this storm will subside until Mars has moved far enough from the Earth that imaging will not be practical.
The two images at the top of this composite were taken very close to the same time. The one on the right is a standard full-color view that shows that the dust storm still obscures almost all surface detail. In short, it provides little detail, but does indicate the basic color of the planet, In contrast, the grayscale image to the left was obtained through a red filter that gives a glimpse of the major features on the surface!
So what, you might ask, is the third image. Clif Ashcroft, an amateur astronomer from New Jersey, came up with the idea of combining the two images. The RGB image would provide color information, while the grayscale image would provide the luminance (L) information – essentially the details. The result is a synthetic LRGB reconstruction which mimics, to some degree, what the planet would have looked like without the dust storm! Not perfect, but a great idea. Who says you cannot fool Mother Nature!
The image above shows Mars just two days after opposition. The planet-wide dust storm is not over, but some clearing is notable in the southern hemisphere. The darker (low-albedo) surface features received a modest increase in contrast by taking the red channel data, converting it to a grayscale image, increasing the contrast, and then using it as the luminance channel in an LRGB reconstruction – a variation of the technique used for the earlier image from 25 July. In another week or so I intend to follow up with the use of a near-IR (685 nm) bandpass filter which should be even more effective in revealing surface detail beneath the atmospheric dust veil.
By the 13th the dust was definitely declining, at least in some areas. Imaging the planet through a 685 nm near-IR filter (left) does an excellent job of revealing ground features covered or diminished by the dust. Note however that although ground features are beginning to be visible through the airborne dust, their contrast is still fairly low in visible light as shown by the RGB image (center). The image on the right is a computer simulation of what Mars should look like at the time the images were obtained. The major, north-pointing feature to the east (right) side of thee simulation image is Syrtis Major. Note that it is wider in the simulation compared to either the near-IR or RGB images. This suggests that the extreme eastern margin of Syrtis Major is either covered by dust on the ground or suspended in the martian atmosphere.
It just occurred to me that I am tossing around a lot of Martian geography (areology) with no real reference. Doing some near-IR imaging on the 14th of August turned up a nice mix of high (bright) and low (dark) albedo features, a few of which I have labeled on the matching image to the right. If you are going to spend any time imaging Mars, a good Mars globe and/or map will be a necessity.
Just as was the case back on the 25th of July, the relatively poor RGB color image can be combined with the more detailed monochrome image (685 nm near-IR) to create an LRGB image that simulated what the planet would have looked like absent to dust storm.