Knowing how barren and apparently lifeless modern-day Mars is, you would be forgiven for thinking it quite static and unchangeable.
Scientists have discovered piles of evidence that show ancient Mars was warmer and wetter, and have found signs of dried-up rivers and lakes that suggest the Red Planet was once much more like present-day Earth.
But today, Mars is a freezing, dry, arid, lifeless world. However, its surface is much more dynamic and changeable than you might think.
More on Mars
The European Space Agency's Mars Express probe has been orbiting the planet since 2003, and has revealed many of the secrets of its atmosphere, surface and moons.
These images from 2024 show a strange dark material passing over the surface of Mars, which wasn't there when the same region was photographed by NASA's Viking orbiters in 1976.
So what's going on?
The case of the dark cloud sweeping across Mars
Much of the surface of Mars has the rusty, reddish ochre colour from which the Red Planet gets its nickname.
But in this image from Mars Express's High Resolution Stereo Camera, the planet’s tan-coloured sands stand in stark contrast with the opposing large region of dark material.
The dark portion is covered in volcanic ash.
Yet when this region of Mars was seen by the Viking orbiters, the ash was less widespread than it is today.
Mars has no known active volcanoes today, but it did in its past.
The Red Planet has a known volcanic history, and even hosts the largest volcano in the Solar System, Olympus Mons, which is more than double the height of Mauna Kea, the largest volcano on Earth.
Planetary scientists say the dark material seen here was probably formed and distributed by volcanoes.
They say the volcanic material is rich in ‘mafic’ minerals, which form at high temperatures and give the layer of ash its dark colour.
But what has caused the spread of darkness across the Red Planet since 1976?
Scientists say there are two possible explanations: either it has been whisked around by powerful Martian winds, or the ochre dust that once covered the ancient dark ash has been blown away over time.
Either way, it shows how ever-changing the surface of even an apparently lifeless planet like Mars really is, and how planetary probes are constantly revealing the secrets of the Solar System worlds.
