Titan and Earth have much in common, but not when it comes to sand.
On Earth, sand grains form by breaking things down, but on Titan, the opposite may be true - with much of the sand a product of building things up.
That's one theory Cassini scientists are considering after studying Titan's massive sand dunes with the visible and infrared mapping spectrometer on the Cassini Saturn orbiter. The new observations raise the possibility that much of the sand grows from hydrocarbon particulates fallen from the sky that, once on the ground, join together and become sand grain-size particles.
Discovered with Cassini's imaging radar in 2005, Titan's windblown dunes of dark, organic material look like mountainous drifts of coffee grounds. Since then, Cassini scientists have been finding out why and how Titan's dunes resemble and differ from those on Earth, Mars and Venus. They are gaining new understanding into the fundamental physical processes that shape planet surfaces.
In the May 2008 issue of the journal Icarus Cassini scientists report that dunes contain less water ice than the rest of Titan. The dark brown sands appear to be made up of the same kind of complex organic chemicals that dominate Titan's smoggy atmosphere. If the dunes are made up of the same dark material on the inside as they have on the outside, then there's simply too much organic sand to have come from erosion alone.
The new findings may help explain how, once on the ground, hydrocarbon particulates the size of smoke particles might grow into sand grains through a process called "sintering" - a slight melting that welds particles together. It may be that sintering produces particles that are just the right size for sand grains - between 0.18-0.25 millimeters and no larger, perfect for blowing in the wind and drifting into dunes.
Scientist Jason W. Barnes of NASA's Ames Research Center, Moffett Field, Calif., answers some questions about recent studies of Titan's dunes:
A Q&A with Jason W. Barnes
Q. What are the erosional processes that might be producing Titan's sand - would they include erosion from flowing river channels similar to the sand-making processes we're familiar with on Earth?
Since Titan does have channels where liquid methane flows, the initial idea was that river erosion through Titan's icy crust would break down the bedrock into sand-sized grains that would then form the dunes. However, our data show that the sand is not made of ice, as this scenario predicts. The sand seems instead to be made of organic grains. So while river channel erosion may play a role in the formation of Titan's sand, the process is evidently more complicated than we had originally envisioned, and river erosion isn't the whole story. There's such a huge quantity of sand, if this mechanism did occur, it must be pretty common in order for there to have been enough erosion to create sand that covers the whole moon's equatorial region!
Q. So one theory is that the dunes might be smog particulates that fall to the ground and accumulate over eons to form the dunes?
A. Essentially, yes. The tricky part is that the particles in the atmosphere are tens of millions of times smaller than we think the sand grains are. So in this "bottom-up" hypothesis, the smog particles grow together into larger grains via a process called sintering. [In sintering, particles are warmed enough to stick together, making particles the size of sand grains.] The process is extremely slow, but it appears that Titan has been around for plenty long enough for it to have occurred!
Q. You call it "sand" but describe how it's the same and how it's different from sand that we're used to seeing in Earth's sand dunes and beaches?
A. To a geologist, sand is any set of particles with a typical diameter between 0.0625 and 2 millimeters. So while most sand here on Earth is made from silicates, there are also sand dunes made of gypsum in White Sands, New Mexico. An even greater diversity of sand compositions exists when it comes to beaches. Hawaii, for instance, has black sand beaches (made of dark volcanic basalt), pink sand beaches (made of coral), and even green sand beaches (made from olivene, a mineral usually found in Earth's mantle), in addition to the more common silicate sand beaches.
Based on our measurements, it seems that Titan's sand is probably made from organic molecules, looking something akin to giant 100-meter-high (328 foot) mountains of coffee grounds.
Q. You see dune spacing averaging about 2.1 kilometers (1.3 miles). Is that similar to Earth dune averages?
Considering Titan's exotic gravity, chemical environment, temperature, and atmospheric differences, the dune spacings on Titan are surprisingly similar to those in the Namib desert in southwest Africa here on Earth. The geological mechanisms that control dune spacings are not well understood, so we're not sure whether the similar spacing is a coincidence or whether we're looking at more significant underlying physics that controls dune formation.
Q. Describe the weathervane effects of the dunes.
A. Titan's dunes are a type of dune form known as "longitudinal" dunes. These dunes have their crests oriented parallel to the average wind direction. This is the same as the large sand seas on Earth: the Sahara, Namibia and Australia. However, it is different from nearly all of the dunes in our Western Hemisphere, so it is a bit unfamiliar to us here in the U.S. Dunes here have their crests perpendicular to the direction that the wind blows.
So, because we think that we understand the wind regime that forms longitudinal dunes, that allows us to infer the wind direction on Titan's surface from the orientation of the dune crests. This is actually a very important measurement -- so far the only surface winds measured were at the Huygens landing site. Present global circulation models, of admittedly low resolution, show ground winds the opposite of what the dune orientations show. So mapping the dune orientations allows us to measure what the winds are doing on the surface, but from space rather than from thousands of little landers.
Q. You also report that the visible and infrared mapping spectrometer saw "interdune" regions. What is the significance of being able to see the ground peeking through the troughs of the dunes?
A. That we're seeing substrate between the dunes allows us to know what terrain the dunes are on top of. Titan's surface is quite non-uniform, and the dunes are only on some parts of the surface near the equator. We don't yet know why. By correlating what types of terrain the dunes form on and which they don't, we are looking to constrain the larger-scale mechanisms for dune formation and evolution.
Q. What future Cassini measurements could provide more information on the composition of the substrate?
Now that we know what we're looking at, correlation of both past and future observations from the visible and infrared mapping spectrometer and Cassini's imaging radar will hopefully help us to nail down what types of terrain the dunes prefer, why, and what those preferences mean for Titan's global geology.