Martian sand is unlike any other. Over billions of years, fierce winds blowing across the Red Planet picked up grains of Martian soil and pulverized them to a consistency closer to dust than sand. Even today, those winds continue. They kick up sandstorms on scales of hundreds of miles, and hurl fine particles dozens of miles into the atmosphere, where they remain suspended for weeks. Such activity has ensured that Mars has become covered in the fine, rusty sand that gives the planet its famous red hue.

This potato-sized gray meteorite launched the controversy over whether there was once life on Mars. The rock probably originated beneath the Martian crust when Mars formed some 4.5 billion years ago. Its name encodes its discovery: ALH stands for Allan Hills, the Antarctic ice field where it was the first meteorite found 1984 by the National Science Foundation's Antarctic Meteorite Program.

When spacecraft landed on Mars in the 1970s, they found that its "soil" was very different from that found on the Moon. The Moon is covered in regolith—a 15- to 45-foot-deep global layer composed of loose shards of shattered rock. This is the accumulation of billions of years of debris scattered by meteorite impacts. Mars has certainly endured its share of equally violent impacts, and so it too has a regolith. But the Red Planet has something that the Moon doesn't—wind. And on Mars, this wind has extensively modified that planet's soil and turned in into a sort of dusty sand.

Just as it does on Earth, the wind picks up loose layers of Martian soil and hurls them across the surface. The impact of airborne grains against obstacles grinds down the particles themselves and greatly reduces their size. The winds also whip up flakes of volcanic ash, common on Mars due to its strong volcanic history. So over the eons, the planet's regolith has been reduced to a powdery layer of rock and cinder. This, then, is the Martian "sand"—though "dust" would be a much better word. It is more than 100 times finer than the sand we find on Earth, and has a different composition. Most Martian dust is composed of the elements silicon and oxygen in the form of silicon dioxide, as are the rocks on Earth. But there are also significant quantities of metals like iron, and the individual particles are coated with iron oxide—in other words, rust. It is this rusty sand that gives Mars its distinctive color. And its fine size means hat is has spread around the globe because the winds lave been able to pick it up so easily.

As the wind whistles across the ground, it plucks up in grains and blows them across the surface in a skipping motion known as saltation. These grains then skip along and in turn throw up even smaller particles. At times on Mars, fierce gales can blow at speeds of more than 60 miles per hour. If the winds are this strong, the dust is then carried high into the atmosphere—dozens of miles above the surface—and can blanket the entire planet for weeks or even months. Such sandstorms, on scales of up to hundreds of miles, have actually been observed from Mars orbit. Another effect of this activity is that as the dust circulates, it sandblasts the surface and whittles it down. Evidence of such aeolian erosion is common on Mars—especially around raised structures such as crater rims—and enables astronomers to gauge the direction and speed of the Martian winds.

Also formed and shaped by the persistent winds, dunefields are common on Mars, even more so than on Earth, and with the dunes roughly the same height as those on our planet. They are the result of hundreds of thousands of years of global dust storms.