Wednesday, March 7, 2012

Shifting Sands of Titan

Titan is an ideal environment for the formation of sand dunes as its low gravity and dense atmosphere allows sand particles to be transported even with low wind speeds. In the Solar System, Venus, Earth, Mars and Titan are the only known worlds where sand dunes can be found. Giant sand dunes cover one-fifth of Titan surface and they are confined to a belt within 30 degrees north and south of the equator. The sand dunes show remarkably uniform west-east orientation and based upon the manner in which the sand dunes divert around topographic obstacles, it is clear that they are deposited by winds that blow eastward. This is puzzling because the winds around Titan’s equator should be blowing westward, resulting in sand dunes with westward depositional patterns.

Figure 1: Titan’s golden, smog-like atmosphere and complex layered hazes appear to Cassini as a luminous ring around the planet-sized moon. (Credit: NASA/JPL)

On Earth, the surface transfers angular momentum to the overlying atmosphere by pulling the atmosphere with it as the Earth rotates. Since the Earth’s equator is the furthest part of the Earth from the spin axis, it has the most angular momentum. As a result, the atmosphere at the equator should rotate no faster than the surface and this produces the generally westward winds found around the Earth’s equator. These winds are also commonly known as the trade winds and for centuries, they have been used by sailing ships to cross the world’s oceans. Therefore, the sand dunes around Titan’s equator which generally have eastward depositional patterns are contrary to what is expected as the winds around Titan’s equator should instead be blowing westward.

Indeed, atmospheric models (Tokano 2010) show that like the Earth, the low latitude surface winds around Titan’s equator do blow westward. However, there is a brief episode which occurs twice every Titan year (29.5 Earth years) where stronger eastward winds appear in the low latitudes. This occurs near equinox when the inter-tropical convergence zone (ITCZ) crosses the equator of Titan and causes the winds to briefly reverse direction from westward to eastward. Since a threshold windspeed is required for sand particles to start moving, even though the winds blow westward most of the time, the sand particles only respond to the stronger but less frequent eastward winds.

Figure 2: Simplified global atmospheric circulation and precipitation patterns on Titan and Earth. (Credit: P. HUEY/SCIENCE)

On Earth, the ITCZ encircles the equator and it is where winds originating in the northern and southern hemispheres converge. However, Earth’s ITCZ does not migrate far and generally stays within the tropics. In contrast, Titan’s ITCZ migrates from pole to pole and rarely stays on the equator. This is due to Titan’s slow rotation of 15 days and 22 hours. Titan’s ITCZ carries with it methane rain clouds and it is where most precipitation occurs as air ascends as a result of converging surface winds from the northern and southern directions. Technically, this means that Titan has a ‘tropical climate’ spanning pole to pole, although its surface temperature is a frigid minus 180 degrees Centigrade.

Figure 3: An artist’s conception of Titan’s sand dunes. (Credit: Kees Veenenbos)

The sand dunes on Titan have similar geometries with those on Earth and some of the best terrestrial examples are the giant sand dunes found in the Namibian desert as they are directly comparable in size and spacing to those on Titan. Like those on the Earth, the sand dunes on Titan are typically one-third as wide as their crest-to-crest spacing. Titan’s sand dunes are generally spaced about 2.5 kilometres apart and have dune heights ranging from 100 to 150 metres. These dunes also stretch hundreds of kilometres in length. One fundamental difference between the sand dunes of Titan and those on Earth is that the sand dunes on Titan are not made of silicate sand particles like those on Earth. Instead, the sand dunes on Titan are made of solid particles of water ice or millimetre-sized aggregates of solid hydrocarbons that precipitate out of the atmosphere.

1. Tetsuya Tokano, “Relevance of fast westerlies at equinox for the eastward elongation of Titan’s dunes”, Aeolian Research 2 (2010) 113-127
2. R. D. Lorenz, et al., “The Sand Seas of Titan: Cassini RADAR Observations of Longitudinal Dunes”, Science 312, 724 (2006)