The recent images of Titan, Saturn’s largest moon, were captured by the Cassini spacecraft as it flew by Titan last October and are released this month in Science. They reveal striking sand dunes at Titan’s equator much like those in the Sahara desert ( Figure 1).
These images look remarkably similar to radar images of Namibia, as shown in Figure 2 for comparison. On Earth, all wind is ultimately a result of heat differences produced by sunlight that warms the planet unevenly. Scientists have long assumed Titan is too far from the Sun to have solar-driven surface winds powerful enough to cause sand dunes. But they have more recently learned that Saturn’s powerful gravity creates tidal effects in Titan’s thick atmosphere. This tidal force, almost 400 times greater than that of Earth’s moon tugging at our oceans, dominates near surface winds on Titan and sculpts dunes that are up to 100m high. The new images are evidence that these dunes were built from winds that blow in one direction before switching to another and then back to the first direction and so on. The tides cause wind to change direction as they drive winds toward the equator. This back and forth pattern cause the sand dunes to build up in long parallel lines. These tidal winds combined with Titan’s west-to-east zonal winds create dunes aligned nearly west-to-east everywhere except close to mountains, which alter wind direction. Before if was thought that the dark regions on Titan’s equator were in fact seas of liquid ethane that trap sand. But the images now reveal something different. Tidal winds might be blowing sand around the moon several times and working it into dunes at the equator. It is likely that tidal winds are carrying dark sediments from higher latitudes to the equator, forming Titan’s dark belt. The sand on Titan might have formed when liquid methane rained and eroded the ice bedrock. Although it doesn’t rain frequently on Titan, when it does rain it really pours. Energetic rain that triggers flash floods may be a mechanism for making sand. The sand could also have come from organic solids produced by photochemical reactions in Titan’s atmosphere.
Reference
- The Sand Seas of Titan : Cassini RADAR Observations of Longitudinal Dunes Lorenz, R. D., Wall, S., Radebaugh, J., Boubin, G., Reffet, E., Janssen, M., Stofan, E., Lopes, R., Kirk, R., Elachi, C., Lunine, J., Mitchell, K., Paganelli, F., Soderblom, L., Wood, C., Wye, L., Zebker, H., Anderson, Y., Ostro, S., Allison, M., Boehmer, R., Callahan, P., Encrenaz, P., Ori, G. G., Francescetti, G., Gim, Y., Hamilton, G., Hensley, S., Johnson, W., Kelleher, K., Muhleman, D., Picardi, G., Posa, F., Roth, L., Seu, R., Shaffer, S., Stiles, B., Vetrella, S., Flamini, E., West, R. Science, 2006, Volume 312, Issue 5774, pp. 724-727