Author: Fraser Cain

Titan’s sand dunes. Image credit: NASA/JPL. Click to enlarge
When they first noticed the dark equatorial regions on Titan, researchers thought they could be looking at oceans of liquid methane. But new radar images taken by NASA’s Cassini spacecraft has provided the answer: sand dunes. The images show enormous dunes that run parallel to each other for hundreds of kilometers. Saturn’s powerful gravity causes gentle winds on Titan, possibly transporting sand from across the moon and depositing it around the equator.

Until a couple of years ago, scientists thought the dark equatorial regions of Titan might be liquid oceans.

New radar evidence shows they are seas — but seas of sand dunes like those in the Arabian or Namibian Deserts, a University of Arizona member of the Cassini radar team and colleagues report in Science (May 5).

Radar images taken when the Cassini spacecraft flew by Titan last October show dunes 330 feet (100 meters) high that run parallel to each other for hundreds of miles at Titan’s equator. One dune field runs more than 930 miles (1500 km) long, said Ralph Lorenz of UA’s Lunar and Planetary Laboratory.

“It’s bizarre,” Lorenz said. “These images from a moon of Saturn look just like radar images of Namibia or Arabia. Titan’s atmosphere is thicker than Earth’s, its gravity is lower, its sand is certainly different — everything is different except for the physical process that forms the dunes and resulting landscape.”

Ten years ago, scientists believed that Saturn’s moon Titan is too far from the sun to have solar-driven surface winds powerful enough to sculpt sand dunes. They also theorized that the dark regions at Titan’s equator might be liquid ethane oceans that would trap sand.

But researchers have since learned that Saturn’s powerful gravity creates significant tides in Titan’s atmosphere. Saturn’s tidal effect on Titan is roughly 400 times greater than our moon’s tidal pull on Earth.

As first seen in circulation models a couple of years ago, Lorenz said, “Tides apparently dominate the near-surface winds because they’re so strong throughout the atmosphere, top to bottom. Solar-driven winds are strong only high up.”

The dunes seen by Cassini radar are a particular linear or longitudinal type that is characteristic of dunes formed by winds blowing from different directions. The tides cause wind to change direction as they drive winds toward the equator, Lorenz said.

And when the tidal wind combines with Titan’s west-to-east zonal wind, as the radar images show, it creates dunes aligned nearly west-east except near mountains that influence local wind direction.

“When we saw these dunes in radar it started to make sense,” he said. “If you look at the dunes, you see tidal winds might be blowing sand around the moon several times and working it into dunes at the equator. It’s possible that tidal winds are carrying dark sediments from higher latitudes to the equator, forming Titan’s dark belt.”

The researchers’ model of Titan suggests tides can create surface winds that reach about one mile per hour (a half-meter per second). “Even though this is a very gentle wind, this is enough to blow grains along the ground in Titan’s thick atmosphere and low gravity,” Lorenz said. Titan’s sand is a little coarser but less dense than typical sand on Earth or Mars. “These grains might resemble coffee grounds.”

The variable tidal wind combines with Titan’s west-to-east zonal wind to create surface winds that average about one mile per hour (a half meter per second). Average wind speed is a bit deceptive, because sand dunes wouldn’t form on Earth or Mars at their average wind speeds.

Whether the grains are made of organic solids, water ice, or a mixture of both is a mystery. Cassini’s Visual and Infrared Mapping Spectrometer, led by UA’s Robert Brown, may get results on sand dune composition.

How the sand formed is another peculiar story.

Sand may have formed when liquid methane rain eroded particles from ice bedrock. Researchers previously thought that it doesn’t rain enough on Titan to erode much bedrock, but they thought in terms of average rainfall.

Observations and models of Titan show that clouds and rain are rare. That means that individual storms could be large and still yield a low average rainfall, Lorenz explained.

When the UA-led Descent Imager/Spectral Radiometer (DISR) team produced images taken during the Huygens probe landing on Titan in January 2005, the world saw gullies, streambeds and canyons in the landscape. These same features on Titan have been seen with radar.

These features show that when it does rain on Titan, it rains in very energetic events, just as it does in the Arizona desert, Lorenz said.

Energetic rain that triggers flash floods may be a mechanism for making sand, he added.

Alternatively, the sand may come from organic solids produced by photochemical reactions in Titan’s atmosphere.

“It’s exciting that the radar, which is mainly to study the surface of Titan, is telling us so much about how winds on Titan work,” Lorenz said. “This will be important information for when we return to Titan in the future, perhaps with a balloon.”

An international group of scientists are co-authors on the Science article, “The Sand Seas of Titan: Cassini Observations of Longitudinal Dunes.” They are from the Jet Propulsion Laboratory, California Institute of Technology, U.S. Geological Survey – Flagstaff, Planetary Science Institute, Wheeling Jesuit College, Proxemy Research of Bowie, Md., Stanford University, Goddard Institute for Space Studies, Observatoire de Paris, International Research School of Planetary Sciences, Universita’ d’Annunzio, Facolt di Ingegneria, Universit La Sapienza, Politecnico di Bari and Agenzia Spaziale Italiana. Jani Radebaugh and Jonathan Lunine of UA’s Lunar and Planetary Laboratory are among the co-authors.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington. The Cassini orbiter was designed, developed and assembled at JPL.

Original Source: UA News Release

Dreamy colours of Saturn. Image credit: NASA/JPL/SSI. Click to enlarge
With solid planets, like the Earth and Mars, it’s easy to track the length of their days. Just watch for a surface feature to rotate into view again. With gas giants, however, it’s a tricky business. Scientists have used features of Saturn’s magnetic field to act like objects on its surface; tracking the amount of time it takes for that point in the magnetic field to rotate around again. Cassini has determined that Saturn’s day is 10 hours, 47 minutes, 6 seconds (+- 40 seconds).

We all know Earth rotates every 24 hours, but scientists have long had difficulty pinpointing how long the day is on Saturn. The magnetometer onboard the Cassini spacecraft has, for the first time ever, measured a periodic signal in Saturn’s magnetic field, key information to finally understanding the length of a Saturn day and the evolution of this gaseous planet.

The latest research suggests a Saturn day is 10 hours, 47 minutes, 6 seconds (plus or minus 40 seconds). That’s 8 minutes slower than NASA Voyager results from the early 1980s, and slower than previous estimates from another Cassini instrument. The magnetometer results provide the best estimate of the Saturn day to date, because it can see deep inside Saturn. These Cassini results are in the May 4 issue of the journal Nature.

“Measuring the rotation period of a rocky planet like Earth is easy, but measurements for planets made of gas, such as Saturn, pose problems,” said the lead author of the paper, Dr. Giacomo Giampieri, a researcher at NASA’s Jet Propulsion Laboratory, Pasadena, Calif.

Planets rotate around their “spin” axes as they orbit about the sun. Rocky planets like Earth and Mars have rotation periods that are easy to measure because we can see surface features as they go by, such as the continents as viewed from space. Gaseous planets do not have a solid surface to track.

The magnetic field is generated deep inside Saturn’s liquid metallic core by flowing electric currents. By measuring the field, researchers can determine the length of the day on Saturn.

“Making this measurement has been one of the most important science goals for the mission,” said Professor Michele Dougherty of Imperial College London. “Finding a distinct periodic rhythm in the magnetic field helps us understand the internal structure of Saturn that in turn will help us understand how it formed.”

Knowing the length of a day or how fast the planet rotates is critical to understanding the internal structure of the planet and modelling the weather patterns on Saturn.

On approach to Saturn, Cassini’s radio and plasma wave instrument measured radio signals and predicted that the day on Saturn was 10 hours, 45 minutes, 45 seconds. That was considered a very good estimate at the time.

Since the Voyager days scientists have been seeing changes in the period of radio observations. They knew that it was virtually impossible to slow down or speed up a mass as large as Saturn. As Cassini’s measurements of the rhythms of natural radio signals from the planet continued to vary, scientists began to realize these signals were probably not a direct measurement of the internal rotation rate. Suddenly the length of Saturn’s day became uncertain. Measurements of the magnetic field help scientists “see” deep inside Saturn and may have finally solved this puzzle.

“Our magnetic field measurements have remained constant since Cassini entered orbit almost two years ago, while radio measurements since the Voyager era have shown large variability. By monitoring the magnetic field over the rest of the mission, we will be able to solve this puzzle,” Giampieri.

In addition to Giampieri the other authors are: Michele Dougherty, from Imperial College, London; Edward Smith also from JPL; and Christopher Russell from the University of California, Los Angeles.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA’s Science Mission Directorate, Washington. The Cassini orbiter was designed, developed and assembled at JPL. The magnetometer team is based at Imperial College in London, working with team members from the United States and several European countries.

For images and more information, visit: http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov.

Original Source: NASA News Release

Titan’s surface. Image credit: ESA. Click to enlarge
Researchers from NASA, ESA and the University of Arizona have put together a new animation that shows what the Huygens probe saw as it landed on Titan on January 14, 2005. The 5-minute video was put together with data collected by Huygen’s Descent Imager/Spectral Radiometer instrument. The scene below the lander is a mosaic, updated piece by piece as the instrument captured new images.

New views of the most distant touchdown ever made by a spacecraft are being released today by NASA, the European Space Agency and the University of Arizona. The movies show the dramatic descent of the Huygens probe to the surface of Saturn’s moon Titan on Jan. 14, 2005.

The movies were put together with data collected by the Descent Imager/Spectral Radiometer instrument during the probe’s 147-minute plunge through Titan’s thick orange-brown atmosphere to a soft sandy riverbed. The Descent Imager/Spectral Radiometer was funded by NASA.

The data were analyzed for months after the landing and represent the best visual product obtained from the Huygens mission. It is the most realistic way yet to experience the Huygens probe landing. The movie “View from Huygens on Jan. 14, 2005,” provides in 4 minutes and 40 seconds of what the probe actually “saw” during the 2.5 hours of the descent and touchdown.

“At first, the Huygens camera just saw fog over the distant surface,” said Erich Karkoschka, team member at the University of Arizona, Tucson, and creator of the movies. “The fog started to clear only at about 60 kilometers [37 miles] altitude, making it possible to resolve surface features as large as 100 meters [328 feet],” he said. “But only after landing could the probe’s camera resolve little grains of sand millions and millions of times smaller than Titan. A movie is a perfect medium to show such a huge change of scale.”

For the second movie, scientists used artistic license and added sound to represent the different data sets collected. They re-created a scientifically accurate representation of the mission life in less than five minutes.

“These movies really demonstrate that the Huygens camera was very well designed for the job,” said Jean-Pierre Lebreton, Huygens project scientist and mission manager at the European Space Agency. “They show so many different details of a landscape that covers only a tiny fraction — one-thousandth — of Titan’s surface. This makes me dream of what a possible future mission to Titan may return of this wonderful and fascinating Earth-like world,” he said.

The Huygens probe was delivered to Saturn’s moon Titan by the Cassini spacecraft, which is managed by NASA’s Jet Propulsion Laboratory, Pasadena, Calif. NASA supplied two instruments on the probe, the Descent Imager/Spectral Radiometer and the Gas Chromatograph Mass Spectrometer.

The Cassini spacecraft continues orbiting Saturn in its second year of its four-year tour. Cassini’s next Titan flyby is on May 20, 2006. Twenty-two flybys of Titan are planned this year by Cassini, with 45 total flybys of Titan in the full tour.

The new movies and images are available at: http://saturn.jpl.nasa.gov, http://www.nasa.gov/cassini, http://saturn.esa.int and http://www.lpl.arizona.edu/DISR/.

The Cassini-Huygens mission to Saturn and Titan is a joint mission of NASA, the European Space Agency and the Italian Space Agency. ESA supplied and manages the Huygens probe that descended to Titan’s surface. NASA’s Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate in Washington. NASA funded the Descent Imager/Spectral Radiometer, which was built by Lockheed Martin. University of Arizona Lunar and Planetary Laboratory scientist Martin Tomasko leads the Descent Imager/Spectral Radiometer team. Team members are based throughout the United States and Europe.

Original Source: NASA/JPL/SSI News Release