The moons of Mars
The Red Planet’s two small satellites are within reach of amateur scopes
Mars has two tiny moons. The largest is Phobos, which is
17 x 22 x 18km in size and orbits the Red Planet at a distance of 6,000km, taking 7 hours and 39 minutes to complete each orbit. Deimos is smaller, at 15 x 12 x 11km, and orbits Mars once every 1.26 days at a distance of 23,460km. Both moons are dim but well within amateur observing range, Phobos appearing at mag. +10.7, Deimos at +11.8. The problem with seeing or imaging them is Mars itself, which is very bright. That said, the best time to try is when the Red Planet appears greater than 20 arcseconds across.
The best strategy is to wait for the moon you are after to be close to an elongation – its furthest apparent distance from the planet. As the orbital periods are relatively short, these occur quite frequently. Use a freeware program like WinJupos (jupos.org/gh/ download.htm) to determine when elongations take place and the best times to observe.
Using a high magnification or image scale is important to help see the moons. Their visibility may also be enhanced by using an occulting bar, a straight edge at the focal plane of your eyepiece (a piece of aluminium foil temporarily attached to this position is ideal). If you intend to catch the moons with a planetary camera, expect to overexpose the planet. However, you’ll need to judge this carefully to avoid Mars’s glare overpowering the scene.
lacks complexity; dark albedo features are in short supply, apart from the rabbit-shaped profile of Mare Sirenum in the southern hemisphere. Of course, this side of Mars is anything but boring, giving us a look at the Amazonis and Tharsis regions, which contain the huge shield volcanos Ascraeus Mons, Pavonis Mons, Arsia Mons and Olympus Mons. The subtle desert tones are fascinating, as is the way Martian weather interacts with the volcanic peaks. Bright orographic clouds, such as the Arsia ‘long cloud’, sometimes betray their positions. Most notable during the northern summer (Ls=120˚–160˚), the clouds appear to form a recurring W-shaped pattern as sunset approaches the volcanos. It will be interesting to see whether this pattern repeats during the southern summer, given the equatorial positioning of the volcanos. If conditions are very good, careful observation may reveal the central caldera of giant Olympus Mons as a defined spot surrounded by a wider ring – the volcano’s slopes
(at 01:00 BST, midnight UT) brings us face to face with the enigmatic
WEEK 4, STARTING ON 23 OCTOBER
includes Agathadaemon, Melas Lacus, Lus Lacus and Tithonius Lacus bordering the Ophir region to the north.
(at 00:00 UT), gives us a view of Margaritifer Sinus, with the complex ‘fingers’ of the Aurorae Sinus region following. The fingers appear to have darker spots at the end, which include Aurorae Fretum, Aromatum Promontorium and Juventae Fons.
WEEK 5, STARTING ON 30 OCTOBER
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So make sure you take advantage of this month’s opportunity to view the fascinating world of Mars, when it’s better presented than at any other time for years to come – and it’ll be visible in the coming months after opposition too. Just think, when you view it through a scope you could be looking at a world which one day may have humans living on, or beneath, its surface.
For more on Mars at opposition see page 48