Section Programme

This 2022 map of Mars, showing many of the classic telescopic names, was made by Martin Lewis with a 444 mm reflector. His near-opposition images were map-projected and combined. All Mars illustrations have south uppermost.

Martian oppositions and seasons

Mars is not the easiest of planets to observe, partly on account of its rather elliptical orbit, which brings the planet relatively close to the Earth for some oppositions, but far from it at others. Furthermore, to take the extreme cases, when Mars is at its nearest, during our N. hemisphere summers, its declination will be far to the south, and its altitude consequently will be low. The oppositions around the beginning of the year (as in 2025) see the planet high in northern skies, but its disk diameter will then be small.

Mars in the eyepiece shows a smaller disk than Jupiter, between 14 and 25 arcseconds at opposition, falling to less than 4 arcseconds near solar conjunction. The martian season occurring at opposition advances between successive oppositions, until after a period of 15 or 17 years, all the seasons will have been presented at opposition, and the Earth and Mars will have returned to approximately the same positions in their orbits. It is well worth observing visually if the planet’s diameter is above 6 arcseconds, while some skilled imagers are able to follow it even closer to conjunction. A typical martian apparition – this being the period of observability between successive conjunctions – can easily last for around 18 months, and if one can follow the planet over such an interval it is possible to monitor around 80 percent of a martian year.

Another problem with observing Mars compared to Jupiter is the fact that its axial rotation period is slow, and only about 37 minutes longer than Earth’s. This means that if we always observe at the same hour of the night, it would take five and a half weeks to be able to see all longitudes of the Red Planet on the Central Meridian. And this is where the value of a worldwide team comes in, for we need observers located over a wide range of terrestrial longitudes in order to keep the entire circumference of the planet under scrutiny simultaneously.

Like the Earth’s, the martian seasons are of unequal lengths, and the martian southern summer is shorter and hotter than the northern. A useful way to describe the seasonal date for this planet – with 687 terrestrial days in its year – is to refer to the value of what is called its areocentric longitude, abbreviated to Ls in the BAA Handbook. Ls = 0^o marks the start of N. spring (S. autumn), Ls = 90^o is the start of N. summer (S. winter), Ls = 180^o is the start of N. autumn (S. spring), and Ls = 270^o marks the beginning of N. winter (S. summer).

Perihelion occurs at Ls = 250^o and aphelion at Ls = 70^o. During the course of a martian year, the N. and S. poles of the planet are alternately tilted towards the Sun and Earth. The latitude of the apparent centre of the disk is denoted by the symbol De in the Handbook, which also contains tables for calculating the longitude of the planet’s Central Meridian at the time of observation.

What can we observe?

This image taken with the professional *Chilescope* by Damian Peach on 2018 August 9, in the aftermath of an encircling dust storm, shows bright dust fallout picking out the sinuous bends of the Valles Marineris canyon (centre right), as well as a general pallor. Much detail is seen in the S. polar cap.

The planet always shows one or other of its polar caps and a complex pattern dark markings, or albedo markings as they are called, while white clouds and dust storms may also be seen from time to time. The polar caps wax and wane like the Earth’s. Mars is a cold, dry planet with a surface pressure of around 7 mm of mercury, or 1/100^th of our own atmospheric pressure. Under these conditions the polar caps sublime, rather than melt, in the course of the martian summer seasons. Measuring the size of the cap upon images enables us to calculate the latitude of its edge, or the angular cap diameter. The caps dwindle to a minimum late summer diameter before being hidden by a polar ‘hood’ of cloud, which clears away just before the start of the following spring to reveal a newly deposited cap. The shrinking caps often reveal internal details, such as rifts and bright patches, and some outliers are left behind in the retreat.

As the polar caps sublime they release volatiles, and these may be visible as white water ice clouds in the dawn and dusk, and as specific orographic, or mountain-formed clouds over the great volcanoes of the Tharsis ridge, or over the slopes of the towering Olympus Mons. The seasonal dates for these phenomena are always worth recording. We also see, during the N. spring and summer, an ‘aphelion’ cloud belt around the equator, which is best seen from around Ls = 40^o till just before 140^o.

The warmer southern spring and summer seasons (raising stronger winds) are always a time for dust storm activity. Preferred locations are along the edges of the great equatorial martian rift valley, the Valles Marineris, and in the deep dusty basin of Hellas in the S. hemisphere. These clouds appear yellowish, and obscure the underlying albedo markings. Plotting the course of them over several days enables us to measure wind speeds and wind directions on Mars. Storms are classified either as local, where the long axis of the cloud is less than 3,000 km, Regional when they exceed that size, and Encircling when the entire circumference of Mars is swathed in dust. The latter category of event can persist for entire months, and occurs on average once in every three martian years.

A set of filter images by the late Don Parker on 2012 February 15 with a 355 mm Schmidt Cassegrain. Note the circular white orographic clouds on the afternoon side of the disk, which are much better defined in blue-violet light.

The systematic use of colour filters (whether for visual or imaging work) is very important, not least from a diagnostic viewpoint. White clouds will appear brighter in blue light than in red light, but contrarily dust clouds will be brighter in red, somewhat less bright in green, and invisible in blue. Filters therefore enable discrimination between white clouds and atmospheric dust. Any red, orange or yellow filter will also helpfully increase the contrast of the albedo markings. Nowadays most filters are of glass, and can conveniently be screwed into the barrel of an eyepiece, but a strip of photographic gelatine will also serve.

The basic outlines of the dark markings have been stable for centuries, but due to dust excavation and deposition, their outlines can vary, while temporary new albedo markings can appear. Thus the feature called Pandorae Fretum on telescopic maps can vary from broad and dark to completely invisible. When there is dust activity around Noachis (upon its southern boundary), dust-lifting can make Pandorae Fretum darken as dust is lifted off the surface, and the darker, underlying martian bedrock will be exposed. On the other hand, some well-known markings that were very prominent half a century or more ago, such as Nepenthes (a broad, curving band on the E. side of Syrtis Major) and Cerberus bordering the edge of the Elysium region (containing a shield volcano), are currently very faint due to net accumulation of dust in those places.

Following the planet-encircling dust storm at the 2018 opposition, the canal-like streak called Indus reappeared after many years of obscurity, and in 2024 it continues to be prominently visible. It follows that any general albedo map of Mars can never represent the planet with 100 percent accuracy, but reference to the latest available one will always enable features to be identified.

Equipment

A selection of near-opposition pencil drawings made by Richard McKim (410 mm Dall-Kirkham Cassegrain, x265 to x410) at the close opposition of 2020 during September to October. An orange filter was used to increase the contrast of the albedo markings.

As far as equipment goes, for visual work, drawings should initially be made to a scale of 50 mm for the polar diameter. Pencil drawings are the norm, but it is possible to tint sketches with watercolour or pastel, or of course, by means of computer software. Tips for drawing are available in an article by the writer posted upon the BAA website. Sectional drawings of small details such as those within the polar cap can also be made upon a larger scale.

Imaging of the planet can be done with any modern camera, even a mobile phone, provided the brightness of the display can be controlled. New cameras are coming onto the market all the time, and it is easy to discover which ones are favoured by the leading observers. The easiest choice is to buy a colour camera. Stacking of the best frames from a short video sequence of a minute or two (using suitable software) can produce excellent results. However, the most useful way to image Mars – if really serious scientific work is to be done – is to buy a monochrome camera, and to make separate exposures through red, green and blue filters. These images can then be combined into a single colour image using suitable software. The filter images should always be submitted in addition to the final colour result for they are actually the most important ones.

It is possible to obtain higher resolution and sharper results using the technique of derotation, where a longer video is processed with suitable software. However, this can easily be overdone with Mars, thereby greatly reducing the usefulness of images for scientific purposes (especially for time-variable phenomena at the limb). Moreover, it is important not to sharpen or enhance the result too much. There is a false but widely-held belief that Mars always shows well-defined markings, but when there is a large dust storm in progress, significant areas (and sometimes even all) of the planet will be hidden by dust clouds, leaving no obvious markings. Therefore a consistent processing approach is recommended.

Many commercially supplied blue filters have a rather broad passband, and will not be effective at picking out the white orographic clouds or the equatorial cloud belt. For this sort of work, a blue-violet filter is effective. It will, incidentally, show hardly any contrast between the desert areas and dark albedo markings.

The useful WinJUPOS software can generate library images of the albedo features at the time of observation, calculate the altitude of the planet, or show the positions of the little martian moons, Phobos and Deimos (which may be successfully imaged with around 200 mm aperture and upwards).

Reporting your work

Observations should be sent to the Director at frequent intervals, daily if possible. You may wish to post them upon your own BAA member page at the Association’s website, and/or to consider sending them to one of the internet planetary archiving sites.

The Director produces either Interim reports in the Journal during each apparition, or writes a regular online blog. At the end of the apparition, a final report is compiled for publication. The work of all observers is acknowledged in print. Over the last few oppositions we have had over 100 contributors each time, but beginners should not be discouraged either by this large number or by the remarkably detailed images that some skilled observers are able to make. The important thing is to be part of a team, and to be looking out for the unusual. It may be that the low-resolution image of a novice is the first and only record of a new dust storm.