In a study posted online May 9, Nathan Bridges and colleagues analyzed data from an eye-in-the-sky called Mars Reconnaissance Orbiter. Using a high-resolution telescope, the researchers measured the movement of sand dunes over a 105-day span.

The fine-grained images showed that the dunes are indisputably on the move, says Bridges, a senior scientist at the Applied Physics Laboratory at Johns Hopkins University. “Even though Mars has a very thin atmosphere and high-speed winds are rare, the dunes are moving.”

The research group saw movement both in entire dunes, and in the ripples on their surface. Across one meter of dune front, they calculated an annual sand movement totaling about 2.3 cubic meters. “If you had a children’s sandbox, that would fill it with sand in a year,” Bridges says.

On Mars, as on Earth

And that, he adds, is within the range of movement seen in some Earthly dune fields. “We are not making the case that Mars has the fastest dunes, but they do move like some on Earth. Mars is an active planet, maybe not as active as Earth, but we are seeing significant movement.”

How much wind is needed to move sand when the atmosphere is less than one percent as dense as Earth’s? The grains would start moving in a wind of about 20 to 30 meters per second (40 to 50 miles per hour, measured at a height of 1 meter), Bridges says. “That is about 10 times what you need on Earth, due to the atmospheric density difference.”

Such winds do blow — rarely — on Mars, but once the sand starts moving, it’s easier to keep it rolling, he says. “Recent research by my colleagues has found … a lower-speed wind can sustain the movement.” Under the reduced gravity of Mars, a grain stays aloft longer, giving the wind more time to accelerate it. When the high-speed grain hits the sand bed, a high-energy collision impels more sand grains into motion.

Mars: A moving planet

At any rate, the discovery proves that wind needs no help from water in moving dunes, Bridges says. “We have seen dunes in images since the 1970s, but there was a question, were they currently active, moving? Mars has a very thin atmosphere and it would need high-speed winds to move sand, and those are very rare. So it’s been an open question, how much sand is moving now, and was more moving in the past?”

On Earth, water is highly erosive, but Mars has no liquid water, “so one agent of erosion on Earth is lacking,” says Bridges. “There is a lot of evidence for erosion — craters that appear to be filled in with dirt, and the primary mechanism is wind.”

And lasting sandblasting

Wind does not just move sand — it also creates sand, Bridges says. His group calculated that the natural Martian sandblaster sand would erode 1 to 50 microns off rock per year, about the same rate as in Victoria Valley.

That sandblasting would provide a source of the sand that litters so much of the red planet, Bridges says. “Erosion is occurring today, so wherever you have sand, and moderate winds, you are likely to get significant amount of erosion from rocks.” That could then create silt or more sand.

When we see all these eroded terrains, “you don’t have to evoke any past climate to explain this,” he says. “It’s a current process, and it was likely occurring for billions of years.”

Saturn in eclipse

Yes, this is a real picture. More accurately, it’s 165 pictures pasted together from NASA’s Cassini spacecraft’s flyby of Saturn as the planet between the probe and the sun. From this unique vantage point, the contrast of light and shadow enabled astronomers to discern new bands of ice and dust — perhaps the remnants of a shattered moon — between the inner and outermost edges of the ring system. This panorama reveals Saturn casting a massive shadow some 75,000 miles long over Cassini’s camera. Though the brightness in this image has been enhanced to reveal detail, the photo’s sharp contrasts owe foremost to the millions of square miles of orbiting ice crystals that ring the planet. The dark gaps between the bands are thought to be the result of gravitational forces exerted by the planet’s many moons, but these forces are not the only cause. The Encke Gap, among the largest of these vacuous rings, is maintained by Saturn’s innermost moon and “ring shepard,” Pan, which plows through the orbiting field of ice and dust.

Image courtesy CICLOPS team.

This deep, dark region retains heat from the hot gas and dust that formed Earth about 4.5 billion years ago.

The middle layer, the mantle, is solid rock, but it is hot enough to flow slowly, like Silly Putty. The movement, called convection, brings hot rock from the lower mantle to the surface. Cooler rock at the top of the mantle sinks.

The overall effect of convection is to create “conveyor belts” that transport the giant plates that form Earth’s crust. Mantle rock rises close to Earth’s surface along the mid-oceanic ridges. Some of the mantle rock melts, rises further, and, and where melt forms, rises, warms rock above it, which cools crystallizes to and to forms new ocean crust. As ocean the new crust moves away from a ridge, it cools and become denser, eventually sinking back into the mantle.

“As the continental plates are carried along on this conveyer belt, they may crash together (the Himalayas), slide past one another (California), or separate (Baja California),” says Goodwin. Over hundreds of millions of years, the continents have merged and re-separated in their continual movement around the globe. This movement explains why fossils of tropical animals are found in Antarctica, she says.