Titan Dunes

The moons of our Solar System are brimming with unusual landscapes. However, sometimes they look a little more familiar, as in this new radar image from the Cassini orbiter. The image shows dark streaks carved into dunes reminiscent of those we might find on a beach on Earth, or raked with flowing lines in a Japanese Zen garden — but this scene is actually taking place on Saturn’s moon Titan.

While our sand is composed of silicates, the ‘sand’ of these alien dunes is formed from grains of organic materials about the same size as particles of our beach sand. The small size and smoothness of these grains means that the flowing lines carved into the dunes show up as dark to the human eye.

These grains are shunted around by winds shifting over the moon’s surface. These winds aren’t particularly fast — only moving at around 1 m/s — but they blow in opposing directions throughout the year, causing Titan’s ‘sand’ to pile up in certain places over time.

Titan seems to be full of features and phenomena that are quite familiar to those found on Earth. Since Cassini arrived in the Saturn system in 2004, and dropped off ESA’s Huygens probe in 2005, scientists have been studying the similarities between Titan and Earth by exploring sand dunes, channels and lakes of liquid ethane and methane scattered across its surface.

While previous images have spotted these eerily familiar patterns on Titan’s dunes, this new image shows them in greater detail. The image was obtained by Cassini’s Titan radar mapper on 10 July 2013, by a team led by Steve Wall at NASA’s Jet Propulsion Laboratory in California, USA. The horizontal seam near the center is an artifact of radar image data processing.

Image credit: NASA/JPL-Caltech

Senkyo

The Cassini spacecraft once again dons its special infrared glasses to peer through Titan's haze and monitor its surface. Here, Cassini has recaptured the equatorial region dubbed "Senkyo." The dark features are believed to be vast dunes of hydrocarbon particles that precipitated out of Titan's atmosphere.

Titan, Saturn's largest moon, is 3,200 miles (5,150 kilometers) across. For more on Senkyo, see PIA08231.

This view looks toward Saturn-facing hemisphere of Titan. North on Titan is up and rotated 4 degrees to the left. The image was taken with the Cassini spacecraft narrow-angle camera on June 16, 2013 using a spectral filter sensitive to wavelengths of near-infrared light centered at 938 nanometers.

Image credit: NASA/JPL-Caltech/Space Science Institute

The Missing Waves of Titan

One of the most shocking discoveries of the past 10 years is how much the landscape of Saturn's moon Titan resembles Earth. Like our own blue planet, the surface of Titan is dotted with lakes and seas; it has river channels, islands, mud, rain clouds and maybe even rainbows. The giant moon is undeniably wet.

The "water" on Titan is not, however, H2O. With a surface temperature dipping 290 degrees F below zero, Titan is far too cold for liquid water. Instead, researchers believe the fluid that sculpts Titan is an unknown mixture of methane, ethane, and other hard-to-freeze hydrocarbons.

The idea that Titan is a wet world with its own alien waters is widely accepted by planetary scientists. Nothing else can account for the observations: NASA's Cassini spacecraft has flown by Titan more than 90 times since 2004, pinging the Moon with radar and mapping its lakes and seas. ESA's Huygens probe parachuted to the surface of Titan in 2005, descending through humid clouds and actually landing in moist soil.

Yet something has been bothering Alex Hayes, a planetary scientist on the Cassini radar team at Cornell University.

If Titan is really so wet, he wonders, "Where are all the waves?"

Here on Earth, bodies of water are rarely still. Breezes blowing across the surface cause waves to ripple and break; raindrops striking sea surfaces also provide some roughness. Yet on Titan, the lakes are eerily smooth, with no discernible wave action down to the millimeter scale, according to radar data from Cassini.

"We know there is wind on Titan," says Hayes. "The moon's magnificent sand dunes [prove] it."

Add to that the low gravity of Titan—only 1/7th that of Earth—which offers so little resistance to wave motion, and you have a real puzzle.

Researchers have toyed with several explanations. Perhaps the lakes are frozen. Hayes thinks that is unlikely, however, "because we see evidence of rainfall and surface temperatures well above the melting point of methane." Or maybe the lakes are covered with a tar-like substance that damps wave motion. "We can't yet rule that out," he adds.

The answer might be found in the results of a study Hayes and colleagues published in the July 2013 online edition of the journal Icarus. Taking into account the gravity of Titan, the low viscosity of liquid hydrocarbons, the density of Titan's atmosphere, and other factors, they calculated how fast wind on Titan would have to blow to stir up waves: A walking-pace breeze of only 1 to 2 mph should do the trick.

This suggests a third possibility: the winds just haven’t been blowing hard enough. Since Cassini reached Saturn in 2004, Titan’s northern hemisphere (where most of the lakes are located) has been locked in the grip of winter. Cold heavy air barely stirs, and seldom reaches the threshold for wave-making.

But now the seasons are changing. In August 2009 the sun crossed Titan’s equator heading north. Summer is coming, bringing light, heat and wind to Titan's lake country.

"According to [climate models], winds will pick up as we approach the solstice in 2017 and should be strong enough for waves," he says.

If waves appear, Cassini should be able to detect them. Radar reflections from wavy lake surfaces can tell researchers a great deal. Wave dimensions, for instance, may reveal the viscosity of the underlying fluid and, thus, its chemical composition. Also, wave speeds would track the speed of the overlying winds, providing an independent check of Titan climate models.

Hayes is excited about "bringing oceanography to another world. All we need now," he says, "are some rough seas."

Video credit: NASA

Titan's Senkyo Region and South Polar Vortex

The Cassini spacecraft peers through Titan's thick clouds to spy on the region dubbed "Senkyo" by scientists. The dark features include vast fields of dunes, composed of solid hydrocarbon particles precipitated out of Titan's atmosphere. And Titan's southern pole is shrouded in the recently formed polar vortex.

Titan, Saturn's largest moon, is 3,200 miles (5,150 kilometers) across.

For more on Senkyo, see PIA08231. For a color image of the south polar vortex on Titan, see PIA14919. For a movie of the vortex, see PIA14920.

Lit terrain seen here is on the Saturn-facing hemisphere of Titan. North on Titan is up and rotated 18 degrees to the right. The image was taken with the Cassini spacecraft narrow-angle camera on January 5, 2013 using a spectral filter sensitive to wavelengths of near-infrared light centered at 938 nanometers.

The view was obtained at a distance of approximately 750,000 miles (1.2 million kilometers) from Titan and at a Sun-Titan-spacecraft, or phase, angle of 79 degrees. Image scale is 4 miles (7 kilometers) per pixel.

Photo credit: NASA/JPL-Caltech/Space Science Institute

Dune Fields on Titan and Earth

Two different dune fields on Titan: Belet (top left) and Fensal (bottom left), as imaged by Cassini's radar. The image also shows two similar dune fields on Earth in Rub Al Khali, Saudi Arabia.

Fensal is at higher latitude and elevation than Belet and clearly shows thinner dunes with brighter and wider areas in between, suggesting less abundant dune material in this region.

Photo credit: NASA/JPL-Caltech/ASI/ESA and USGS/ESA

Note: For more information, see Cassini's Radar Observes Titan's Tropical Dune Fields; also, PIA15225: Dune Patterns.

Ksa Crater, Xanadu and Dunes on Titan

Three of Titan's major surface features-dunes, craters and the enigmatic Xanadu-appear in this radar image from NASA's Cassini spacecraft. The hazy bright area at the left that extends to the lower center of the image marks the northwest edge of Xanadu, a continent-sized feature centered near the moon's equator. At upper right is the crater Ksa, first seen by Cassini in 2006 (PIA09172). The dark lines running between these two features are linear dunes, similar to sand dunes on Earth in Egypt and Namibia.

The dune fields on Titan, Saturn's largest moon, nearly girdle the globe at latitudes from about 30 degrees north to 30 degrees south, with the notable exception of Xanadu. In this image, the dunes overlap Xanadu only slightly. They are also more widely separated and discontinuous at the boundary, a characteristic typical of dunes on Earth where the sand supply is limited. The dunes also either wind their way around or terminate at other, smaller features, including Ksa.

Cassini's Titan Radar Mapper acquired this synthetic-aperture radar (SAR) image, centered at 11 degrees north latitude and 74 degrees west longitude, on June 21, 2011. The image covers an area 350 kilometers (217 miles) high by 930 kilometers (578 miles) wide, with resolution of about 350 meters per pixel. North is at the top, and the image is illuminated from the top. Incidence angle varies from 15 to 30 degrees.

Photo credit: NASA/JPL