Category Archives: solar system exploration

Welcome to (another) Dune-ish World!

Intriguing Planetary Science Images

A radar scan of Titans surface, showing wind-whipped dunes. Courtesy NASA and the Cassini Mission Team. (Click to embiggen).
A radar scan of Titan's surface, showing wind-whipped dunes. Courtesy NASA/Cassini Mission Team. (Click to embiggen).

The Cassini mission keeps coming up with more surprises out at Saturn — and especially on Titan.  Normally we can’t see anything on Titan (at least optically) except its cloudy atmosphere. But, when Cassini turns on its radar mapper and scans the surface, an amazing wealth of surface features just leap out at us.

A closeup of dunes on Titan.  Courtesy NASA/Cassini Mission. (Click to embiggen.)
A closeup of dunes on Titan. Courtesy NASA/Cassini Mission. (Click to embiggen.)

A couple of weeks ago, the Cassini mission’s high-resolution radar system mapped a series of dune fields on Titan. These dunes are moved around by surface winds, and most likely are built-up piles of hydrocarbon sand grains that come from combinations of organic chemicals in Titan’s smoggy atmosphere. The winds blow them along, wrapping them around any surface features that stick up as obstacles in their paths.

From the radar images that Cassini sent back, planetary scientists were able to count up to 16,000 dune segments that act as a sort of weather vane, pointing the direction in which the winds blow on Titan.

As the winds change direction, the orientation of the dune fields also change. The winds come from several different directions, and the dunes reflect that in the way they line up and appear to be cutting across each other in some places. These dunes appear to be concentrated mostly around Titan’s equator. This is probably because weather conditions are drier there — which means there are more particles for the winds to pick up and scatter along their paths.

Conditions elsewhere on Titan are too “wet” because there are more lakes of liquid hydrocarbons. This makes it less likely that the climate will “dry out” enough to form these sand grains.

The Kuiseb river region in Namibia is bordered by wind-blown dune fields. Courtesy NASA. (Click to embiggen.)
The Kuiseb river region in Namibia is bordered by wind-blown dune fields. Courtesy NASA.

If you’re thinking this is all looking very familiar — it is.  Earth’s deserts and dune fields exist in dry climates, where winds can pick up grains of sand and dust and blow them around — forming traveling dunes.

Windblow dunes in and around a crater in the Syrtis Major volcanic region on Mars. Courtesy NASA/Mars Global Survery
Windblow dunes in and around a crater in the Syrtis Major volcanic region on Mars. Courtesy NASA/Mars Global Surveyor. (Click to embiggen.)

We see similar things on Mars, which is peppered with dune fields on its broad plains and inside some of its larger craters.

Anyone who has traveled in the American Southwest, for example, or in the deserts of North Africa, will be familiar with dunes. They move the same way on Earth as they do on Mars and now, Titan.

Images and discoveries like these of dunes on other worlds are all part of planetary science. Or, if you like, comparative planetology. Essentially when we look at other planets, we look for things we can explain and understand based on processes we see and usually understand here on Earth.

What are those processes?  Think of them in terms of what modifies a planet. What changes its surface or its atmosphere?  It’s easiest to think about what happens to Earth over time.  Geologists look at processes like volcanism (the action of volcanoes and volcanic flows), tectonism (faulting and folding of a planet’s outer layer), impact cratering (when projectiles slam into the surface and create craters), and atmospheric processes.  Geology is, in fact, a huge part of planetary science.

In the case of dune creation on Earth and other worlds, planetary scientists focus on the atmospheric processes that shape a planet. These can be things like rain or snow falling onto a surface and changing it in some way. Or, it can be chemical weathering — that is, the action of a chemical like (say) sulfuric acid falling as rain on a surface and eating away at it.  Or, we can see what’s called “aeolian” (wind-blown) changes to a surface. Sometimes this means that a surface is scoured clean by winds. Or, it can mean — as we’ve sen in these images of Titan, Earth, and Mars — that winds are taking what’s already present on the surface — piles of sand and dust — and moving them along, forming dunes as they go.

As we see more familiar processes occurring on other worlds, we can more easily explain them in terms of what we know about from what we see on Earth. These intriguing images, and many others taken by scores of spacecraft at other planets, are — in a very large sense — making us more at home in the solar system, even as they teach us about our own planet’s place in the hierarchy of worlds that orbit the Sun.

Venting on Enceladus

Cassini’s Cameras Hit the Target

Imagine flying all the way out to Saturn’s moon Enceladus to take images of a geyser vent the size of a small mountain from a distance of about 4,700 kilometers (about 2,900 miles) while whizzing past at a speed of 64,000 kilometers (40,000 miles) per hour. That’s exactly what the Cassini-Huygens mission did last week. In managing this feat, Cassini’s cameras pinpointed the origins of icy jets shooting material out from beneath the surface of this geologically active moon. The image at left shows the target areas and where Cassini spotted those vents.

These geyser-like spouts are in a region of “tiger stripe” terrain, named for the way the surface looks in earlier images of Enceladus. You can see fractures that are about 300 meters (980 feet) deep. The flanks of some of those fractures seem to have deposits of fine material that likely put there as plumes of vapor erupt through the vents. Blocks of ice tens of meters in size and larger (the size of small houses) surround the fractures.

Planetary scientists are really excited about this find because the mission is at last answering a lot of questions about Enceladus and its intriguing surface. “This is the mother lode for us,” said Carolyn Porco, Cassini imaging team leader at the Space Science Institute, Boulder, Colo. “A place that may ultimately reveal just exactly what kind of environment — habitable or not — we have within this tortured little moon.”

Enceladus has been a puzzle ever since the first Voyager 2 images showed its cracked, strangely repaved surface during a 1981 flyby. That evidence alone told scientists that something was going on with this little ice-covered moon, but they really needed high-resolution images. Voyager 2 was the reconnaissance mission to Cassini’s “in depth” followup.

So, how is Enceladus doing it?  What’s going on with those vents to cause what we see? It’s all in how material gets from relative warm areas deep inside Enceladus to the frigid surface. Imaging scientists suggest that once warm vapor rises from underground to the cold surface through narrow channels, the icy particles may condense and seal off an active vent. Because the material underneath is warmer and under some amount of pressure, it has to erupt somewhere, so eventually new jets probably appear elsewhere along the same fracture in fairly short order.

There’s more to come from the Cassini mission folks. To follow the action, check out new images at: http://www.nasa.gov/cassini, http://saturn.jpl.nasa.gov and http://ciclops.org.  Check it out!