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Go to CuriosityStream.com/SciShowSpace to learn more and use the promo code “scishowspace” for a special offer on an annual subscription. [♪ INTRO]. Today, Mars is like the ghost of a planet; a desert world full of canyons and sand dunes.

Once, it was more like Earth, with a thicker atmosphere and liquid water. But those days are billions of years in the past. Now, looking at Mars, it's hard not to wonder, “What happened here?” And while there are many ways to try to answer that question, one place that scientists are looking for clues is in the planet's ancient sand dunes.

Sand dunes build up over time as strong winds pick up individual grains and then drop them when some obstacle breaks up the air flow. We see them on a few different planets and moons in the solar system, and the different forms they take can tell us a lot about the environments that created them. On Mars, scientists are especially interested in what are called fossil dunes.

Satellite images show that these dunes seem to be frozen in place, and may have been that way for billions of years, possibly because the wind in Mars' thin atmosphere no longer has the oomph to move them. As a result, these dunes are relics of a different time. And just like ordinary fossils, which preserve clues about ancient organisms, fossil dunes record clues about ancient wind patterns.

For example, in the Arkhangelsky crater in Mars' southern hemisphere, there are ancient dunes in the shape of crescents. And dunes like this, also known as barchan dunes, are common on Earth and Mars. They form in places where the wind blows in one direction, and the tips of the crescents point in the direction of the wind.

But in the Arkhangelsky crater, there seem to be multiple layers of dunes, and they don't all point in the same direction. The tips of the older crescents point toward the southwest, which tells us they were formed by a southwesterly wind. But newer, larger dunes that seem to have grown over them at a later time point in a different direction, indicating a northeasterly wind.

So together, the layers of dunes in this field appear to preserve the record of some ancient climate change. And these aren't the only dune features that hold clues to the planet's past. Mars also has what are called ghost dunes.

These are kind of like the opposite of dunes, actually: large pits that formed in places where lava or sediment once collected around a dune and hardened. Then, after the dune itself eroded away, their shapes remained. Ghost dunes can be especially valuable, because by looking at that sediment or lava that formed that hardened cast, scientists can figure out the age of the dunes.

That lets them estimate what the climate was like at a fairly specific point in time, which isn't always easy with a fossil dune. For example, ghost dunes in a region called Noctis Labyrinthus show that around 3 billion years ago, the wind was northerly, unlike today, when it's largely out of the northeast. That might seem like a small detail, but these subtle clues about things like wind direction, air pressure, and timing can help us reconstruct past climates.

And that, in turn, could help us understand things like whether or not Mars was ever hospitable to life, and how and when that changed. But not all of Mars' dunes are relics of the past, even though astronomers assumed they were when they first saw dunes on Mars in the 1970s. Since the late '90s, satellite imagery has revealed that some are still active.

Most seem to move pretty slowly, maybe only a few centimeters every thousand years, so compared to dunes on Earth, movement that takes only a few months here may take 100,000 years on Mars. But astronomers have noticed a few quicker movements, too, places where the wind was strong enough to move dunes around at Earth-like speeds. These active dunes may not reveal Mars' past, but they can help us see how it's evolving in the present, and how this ghost of a planet is actually continuing to change through processes like erosion.

For instance, some of the biggest shake-ups happen near boundaries in the landscape, like the edge of craters, where dramatic changes in elevation and temperature whip up super strong winds. Which not only tells us something specific about Mars, it also highlights the fact that Mars and Earth have important geological differences. So, while a lot of the factors that drive Earth's geology, like rain, underground water, and plant life, don't really affect Mars, the Red Planet still is shaped by certain things, like craters.

And those are things we might not have thought to consider if we hadn't looked at sand dunes. All this goes to show that, as much as dunes seem to be a relatively common phenomenon in our solar system, that doesn't mean they're not totally awesome. By looking at the dunes, we can better understand how Mars is similar to the Earth, how it's not, and what factors made the difference.

If you want to learn more about clues available on the surface of the Red Planet, you might like the documentary “Opportunity:A Life on Mars,” which is available on CuriosityStream. The Opportunity rover spent more than 14 years exploring Mars, starting in 2004, and it gave us some of our closest looks at Martian dunes and other features of the Martian surface. You can find this and thousands of other documentaries and nonfiction videos, by some of the world's best filmmakers, on CuriosityStream, an online streaming service that features content about science, nature, history, and more.

So, if you're spending more time at home than usual these days,. CuriosityStream can help you stay curious. [♪ OUTRO].