Winds Measured On Titan To Help Robot Probe

When combined with previous observations, new research with the Heterodyne Instrument for Planetary Wind And Composition (HIPWAC) joined to the large aperture of the Subaru telescope supports the model that Titan has currents or jet streams at high latitudes racing through its upper atmosphere (stratosphere) at speeds of approximately 756 km/hour (470 miles/hr.). The new observations reveal that the wind travels in the same direction as Titan's rotation, and that the stratospheric winds are milder (about 425 km/hr. or 264 miles/hr.) near the equatorial regions, as the jet stream model predicts. HIPWAC was designed and built at NASA's Goddard Space Flight Center in Greenbelt, Md. The Subaru telescope is operated by the National Astronomical Observatory of Japan.

Wind direction on Titan is difficult to measure remotely because Titan's upper atmosphere consists of an orange haze of hydrocarbons (molecules of hydrogen and carbon) with no global features that show movement.

The observations were originally encouraged by the Cassini mission, an international mission of NASA, the European Space Agency (ESA) and the Italian Space Agency (ASI) that will employ a large robotic spacecraft to explore Saturn and its system of 31 known moons beginning this July. The Huygens probe, built by ESA, is attached to the Cassini spacecraft and will separate in December on a 22-day course ending with a plunge into Titan's atmosphere. NASA's Jet Propulsion Laboratory in Pasadena, Calif., manages the Cassini mission for NASA.

"Our observations will complement local wind measurements by the Huygens probe during its descent, because we offer a global view. Acquiring the direction and speed of global winds is important for understanding the dynamics of planetary atmospheres, particularly dynamics of those bodies that rotate slowly on their axes. Titan's 'day' is 16 Earth days," said Dr. Theodor Kostiuk of NASA Goddard.

"We hope to be able to repeat our success during the probe's descent so we can have detailed local information from Cassini and the Huygens probe and a global portrait from HIPWAC and Subaru from the same time," said Professor Hiroshi Karoji, Director of the Subaru telescope, Mauna Kea, Hawaii.

Titan, the second largest moon in the solar system, is larger than the planet Mercury and is the only known moon with a thick atmosphere, actually 1.5 times more dense than Earth's. Because it is far from the Sun, Titan is extremely cold (surface temperature of about minus 178 Celsius (minus 289 Fahrenheit), allowing a hydrocarbon rain that may form gasoline-like seas. Scientists are eager to explore Titan because its atmosphere may resemble the Earth's atmosphere shortly after our planet's formation, when it was rich with hydrocarbon molecules that became the building blocks of life.

HIPWAC can measure wind speed and direction on Titan, even though the moon's atmosphere lacks apparent features, because the instrument relies instead on the faint, infrared glow of the hydrocarbons in Titan's atmosphere. Infrared light, invisible to the human eye, can pass through Titan's hydrocarbon haze and is detectable by special instruments. HIPWAC measures the very slight color (frequency) change of the hydrocarbon's infrared light caused by the motion of these molecules as they are carried by Titan's winds. This is called a Doppler shift, and is similar to the change in tone of an ambulance siren as it races by. Since the hydrocarbons are moved along by Titan's winds, the Doppler shift of their emitted light gives the wind velocity.

To measure such miniscule Doppler shifts, HIPWAC must be capable of distinguishing among infrared colors, or frequencies, to a very fine degree. This is called spectral resolution, and HIPWAC possesses a spectral resolution 200 times better than any instrument in regular use today. It also must measure specific infrared frequencies very accurately, and HIPWAC can identify a frequency to one part in a hundred million.

Subaru telescope brings to HIPWAC the light gathering power of a modern large aperture telescope. Subaru's 8.2-meter (27- foot) diameter mirror is the largest single-piece mirror in the world that is currently in regular operation. Since HIPWAC achieves its high spectral resolution by finely dividing light into different frequencies, the more light it has to work with the better. Other institutions contributing to this research include the Challenger Center for Space Science Education, University of Maryland, University of Hawaii, and the University of Cologne, Germany.

Joint Subaru release: http://www.naoj.org/Pressrelease/2004/06/29/index.html