Look up on a clear evening when Mars is in the sky and you will not mistake it for anything else. It sits there like a small ember among the stars, steady rather than twinkling, warmer in tone than Jupiter's pale cream. Everyone knows Mars is red. Far fewer people know why — and the answer turns out to explain a great deal about the planet itself.
The short answer: Mars is covered in rust
Mars's surface is rich in iron-bearing minerals, and iron has a habit of combining with oxygen. That reaction produces iron oxide — the same family of compounds as the rust on a garden gate. Billions of years ago, when Mars was wetter and its chemistry more active, the process got a head start. It has been ticking along ever since.
The particular mix matters. Martian dust contains haematite and other iron oxides alongside darker minerals, and the blend gives the planet its range of tones: pale butter-yellow near the limb, deep brick in the middle, almost grey in older, darker regions such as Syrtis Major.
Why the whole planet looks uniformly orange
On Earth, rust tends to stay where it forms. On Mars, it travels. The planet's dust is extraordinarily fine — much of it finer than talcum powder — and with a thin atmosphere and no rain to wash it out of the sky, individual grains can stay airborne for weeks.
Over time, wind has spread that dust across virtually the entire surface. Craters, plains and even the flanks of the giant volcanoes are coated in the same layer, which is why Mars reads as one colour through a telescope rather than a patchwork.
Dust storms that swallow the planet
Regional dust storms are routine, and every few Martian years one grows into a global event that hides the surface entirely. For observers in the UK, this is worth tracking: a planet you could see clearly last month can look like a featureless orange marble the next.
What you're actually looking at
Through a small telescope, Mars rewards patience. The main features to hunt for are:
- Polar caps. Bright white patches at the top and bottom of the disc. The northern cap in particular grows and shrinks with the seasons.
- Syrtis Major. A dark, roughly triangular marking, and one of the first surface features ever mapped.
- Hellas Planitia. A vast impact basin in the southern hemisphere that often appears brighter than its surroundings.
- Clouds and hazes. Thin white patches near the edge of the disc, usually water ice or fine dust.
- The phase. Near opposition, Mars shows a slightly gibbous disc — a useful check that you're looking at the right object.
Features you will not see visually from a UK garden include Olympus Mons, the largest volcano in the solar system, and Valles Marineris, a canyon system that would stretch across a continent on Earth. They are simply too low in contrast at this distance. Knowing they are there, though, makes those dark and light smudges far more interesting.
Why Mars changes size from year to year
Mars orbits the Sun further out than Earth, so we periodically catch it up. Roughly every 26 months the two planets line up on the same side of the Sun — an event called opposition — and Mars is at its brightest and best placed for viewing.
Not all oppositions are equal. Because Mars's orbit is noticeably elliptical, some encounters bring it far closer than others. A close opposition makes the disc several times wider than a distant one, which is the difference between a blank orange dot and a planet where you can trace dark markings.
Colour shifts too, and not only with distance. The shadowed side of the disc, seasonal changes in the polar caps and dust activity all alter what reaches your eye. Mars has a 25-degree axial tilt, so it has seasons much like ours — just about twice as long.
Best times to observe Mars from the UK
Distance gets the headlines, but altitude matters just as much from these latitudes. A Mars low in the south sits behind a thick wedge of our own atmosphere, and the image will boil and smear however good your telescope is. Aim for nights when Mars climbs well above the horizon, and pick a time near the meridian — the point where it is due south and at its highest.
- Check when the next opposition falls, then look in the weeks either side of it.
- Work out its highest point. Astronomy apps and free almanacs give altitude by the hour.
- Wait for a night of steady air rather than perfect transparency. A slightly hazy sky with a calm atmosphere often beats a crystal-clear, turbulent one.
- Let the telescope cool for at least twenty minutes before you start.
- Use higher magnification than you would for deep-sky objects, and keep looking. Detail on Mars comes and goes in seconds.
A red or orange filter deepens the dark markings; a blue one can help the polar caps and clouds stand out. Neither is essential. An undisturbed eye and twenty minutes at the eyepiece will teach you more than any accessory.
Between oppositions, Mars is not lost. It spends months as a bright morning or evening object, and even a small disc is worth a look. When it passes behind the Sun at conjunction, that is your cue for a break.
Getting started without the guesswork
Mars is the most forgiving planet to begin with, because you can identify it with nothing more than your eyes. Find the brightest warm-toned point of light along the ecliptic — the line the Sun and Moon trace across the sky — and check it against a planetarium app. If it is barely twinkling and sits roughly on that line, you have found it.
From there, a small refractor or a 130mm reflector will show the disc, the polar caps and the largest dark markings on a good night. Sketch what you see, date it, and keep the drawings. Over a season you will build a record of a rotating planet — and the rust that made it famous will stop being a trivia answer and start being a place.
Photo: WikiImages / Pixabay
