Sand dunes discovered on Titan by Cassini radar
1er mai 2006
The radar on board the Cassini spacecraft (cooperative project between the European Space Agency and NASA) orbiting around Titan has send stunning images of the Saturne’s satellite. An article published in Science this month by an international team including researchers from Paris Observatory, show widespread regions (up to 1500 by 200 km) of near-parallel radar-dark linear features that appear to be longitudinal dunes similar to those seen in the Namib desert on Earth. Images at 2.17-cm wavelength show 100-meter ridges consistent with duneforms and reveal flow interactions with underlying hills. The distribution and orientation of the dunes support a model of fluctuating surface winds of 0.5 m/s resulting from the combination of an eastward flow with a variable tidal wind. The existence of dunes also requires geological processes that create particules of the same size as sand grains (100- to 300-microns) and a lack of persistent equatorial surface liquids to act as sand traps.
belet.jpg namib.jpg Figure 1 : Image of the Belet sand sea at about 12 degrees South and 100 degrees West on Titan. Credit : Science. Click on the image to enlarge it Figure 2 : Aerial view of the Namib desert from the Space Shuttle mission STS-107 at 283 kilometer altitude. Credit : NASA Johnson Space Center. Click on the image to enlarge it
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
Contact Pierre Encrenaz (Observatoire de Paris, LERMA)
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
Contact Pierre Encrenaz (Observatoire de Paris, LERMA)
Dernière modification le 4 mars 2013