Mars came to opposition in October 2020 and was the best it has been in UK skies for many years. Looking like a brilliant glowing coal, the Red Planet was unmistakable high up in the dark evening skies.
As planet Earth is now moving away from Mars, it continues to get smaller and fainter, and you might think that there is little left to explore.
In actual fact, Mars will remain high and bright in the evening skies for months to come; for many its appearance at a more convenient time in dark evening skies will be the time when it really grabs the attention.
Mars is a dynamic world, and over the next few months there will be some fascinating changes occurring on the Red Planet.
Where will Mars be in the night sky over the coming months?
By the end of 2020 Mars will have an apparent diameter of 11 arcseconds, still big enough to be noticeable to naked-eye and binocular observers.
Users of small to medium telescopes will find that the Red Planet’s respectable diameter will be more than enough to follow the seasonal changes that are coming up, while owners of large telescopes will be able to follow for considerably longer as Mars remains above 6 arcseconds in diameter until the end of March 2021.
Below we’ll highlight some of the seasonal changes on Mars you can expect to see with a telescope, and how best to go about observing them.
A year on Mars
Mars has an axial tilt of some 25° and so it experiences well-defined seasons of winter, spring, summer and autumn as it moves in its orbit around the Sun.
Just like Earth, opposite hemispheres experience opposite seasons: summer in the northern hemisphere means winter in the south.
Since many of the changes on Mars are due to the changes in seasons, it is important that we keep track of exactly where we are in the Martian calendar at any particular time.
Astronomers have devised a simple way of doing that – it’s called solar longitude, or Ls for short.
In the diagram above you can see that the Martian orbit is divided up into 12 intervals. Ls can be thought of as the angle made by Mars as it moves around the Sun.
Astronomers take the autumn equinox in the southern hemisphere to be the value Ls=0° and our first Martian month sees the value of Ls go from 0° to 30°, while the second month sees the Ls range go from 30° to 60° and corresponds to later summer in the south.
Meanwhile the northern hemisphere has passed into spring. We continue in this way until we have broken the Martian year up into 12 months, each spanning 30° of the Red Planet’s orbit.
Of course, Martian months are longer than Earth’s, since a Martian year is about twice that of our own.
By looking up the value of Ls, we can tell exactly where Mars is in its orbit and work out what the seasons should be in either the northern or southern hemisphere.
We can also mark out other important events: perihelion (when Mars is closest to the Sun) occurs at Ls=251°.
So, when opposition occurs close to this value of Ls, we know it will be a ‘perihelic’ opposition – as was the case in October.
You can find the value of Ls by using the free software WINJUPOS: it’s the value called ‘longitude of the Sun’ in the ‘Ephemerides’ tab. The Handbook of the British Astronomical Association also gives the values in its ‘Mars Section’.
How do the seasons change on Mars?
At the start of November, Ls has a value of around 310° and so it’s late summer in the southern hemisphere.
A number of changes have started to occur – the southern polar cap (SPC) has now shrunk and this has returned volatiles, such as dust and water vapour, back into the Martian atmosphere.
Watch out for brilliant white clouds that collect around the Tharsis volcanoes. Olympus Mons in particular can attract bright clouds; these indicate its position in small to medium telescopes – the Solar System’s largest volcano is otherwise invisible to such apertures.
Bright clouds and fogs also collect in the deep basins. In January 2021 keep an eye on the Hellas Basin, to the south of Syrtis Major.
It is not uncommon for Hellas to become filled with white clouds when it’s near the morning or evening limb and as a result, it can become very bright. Eridania, Electris and Chryse are regions also prone to attracting clouds.
Using filters to observe Mars
Visual filters are a great help when observing Mars. Most telescopes come with a set and they are very easy to use. You simply screw them into the thread at the base of your eyepiece.
Each filter is unique, and you can identify each one by looking at the W number on the side. This gives the filter’s ‘Wratten number’. If you have a small to medium sized telescope, try using a light blue filter (W80 or W80A) to increase the definition of white clouds.
If you do see fogs and white clouds, it is worth spending some time observing them as they can be quite dynamic, often forming and evaporating over the course of an hour or two.
It is particularly satisfying to watch bright clouds which have collected in the early morning chill of the Hellas Basin, slowly disperse during the course of an observing session.
When bright clouds occur on the limb of the planet, they can be really quite brilliant. If you have a larger telescope, a W47 violet filter will help bring fine details out further.
A W15 yellow filter will make the southern polar cap slightly easier to see, as it is now quite small and a challenge in small telescopes.
By January 2021 it is spring in the northern hemisphere and the vast north polar hood is extending quite a way south.
You should be able to glimpse it as a bluish-white haze on the northern limb, and it will become more prominent towards the year’s end.
Mars’s tilt will also start to change – from January onwards, the northern hemisphere will gradually be better placed for views of well-known albedo features like Acidalium and Elysium.
By March 2021, observers using larger scopes might be able to catch sight of the northern polar cap.
At Ls=0°, a phenomenon known as the ‘equatorial cloud band’ (ECB) should start to make an appearance.
The ECB takes the form of a thin white cloud along the equator; when it passes over darker regions like Syrtis Major, it can make the features appear bluish. Watch out for this from early February onwards.
The well-known dust storm season stars from about Ls=240°, so keep an eye out for them (more on this below).
These storms usually begin life as small orange clouds and they are easier to see if you use a red filter (W25) when observing Mars.
Dust storms can evolve quite rapidly and it is important to track them and any surface changes.
Mars’s changing surface
All of these seasonal effects result in long-term changes to the planet’s surface. Dust storms in particular can produce quite pronounced changes to the dark albedo features, as vast amounts of dust tend to be moved and deposited into new locations.
One only has to look back at a map of Mars drawn in the 1960s to see how a number of features like Syrtis Major and Solis Lacus have changed over time.
In the last few years the northern hemisphere albedo feature known as Acidalium has changed considerably and it will be important to track any new changes which result in future dust storms.
It is well worth recording your observations in a log book (read our guide on how to keep an astronomical log book) as this will allow you to keep track of all of the changes and seasonal weather patterns that you have observed.
It’s also a great way to become familiar with Martian geography, especially if you have several drawings of the same feature.
It is going to be some time before we have another splendid Mars apparition like the one we have at the moment. Take the chance to follow the Red Planet for as long as possible and keep track of all of the interesting developments occurring on it.
How to observe Mars dust storms
Although the Martian atmosphere is tenuous, it is quite capable of producing powerful dust storms. Winds of half the speed of sound have been recorded.
Typically, the dust storm season starts at about Ls =240° and it continues to Ls=0°. This means that we can expect to see them from now until early February 2021.
The Martian dust storm season has been studied in great detail, and there are three types of dust storm which occur:
Local: these dust storms are confined to very small regions like a corner of the Hellas Basin.
Regional: these dust storms may cover an entire region like Syrtis Major, or indeed a whole hemisphere.
Global: the largest, these dust storms cover the entire planet. During this time the entire globe can become featureless even to large telescopes.
Local and regional dust storms tend to be the most frequent. During the previous opposition in 2018, I was able to observe a regional dust storm at the Lowell Observatory, in Flagstaff, Arizona (pictured below)
Truly global dust storms are quite rare – two prominent ones occurred in 1975 and 1977, around the time that the Viking missions were approaching Mars.
There are a number of sites on Mars that are well known for producing storms, including the Hellas Basin, Solis Lacus, Noachis and Chryse, and you should survey these regions whenever you can.
Storms always start off as bright yellow or orange clouds. Local ones will require a 150mm telescope or larger to be seen, but regional ones can be visible in smaller instruments.
You’ll find a red (W25), orange (W21) or yellow (W15) filter will also help enhance dust clouds and make them easier to see.
If a large storm does erupt, it is a good idea to record its progress; you can do this by sketching the region and plotting the size and location of the dust storm as it changes over time.
Paul G Abel is director of the British Astronomical Society’s Mercury and Venus section. He is a theoretical physicist at the University of Leicester.
This guide originally appeared in the November 2020 issue of BBC Sky at Night Magazine.