NEWS FROM SPACE

STORM ON SATURN

Credit: Reta Beebe (New Mexico State University), D. Gilmore, L. Bergeron (STScI), and NASA.

This Hubble Space Telescope image of Saturn shows a rare storm that appears as a white arrowhead-shaped feature near the planet's equator. The storm is generated by an upwelling of warmer air, similar to a terrestrial thunderhead. The storm is about equal to the diameter of the Earth (about 7900 miles). Hubble is providing new details about the effects of Saturn's prevailing winds on the storm. The new image shows that the storm's motion and size have changed little since its discovery in September 1994.

The storm was imaged with the Wide Field/Planetary Camera 2 in the wide field mode on December 1, 1994, when Saturn was 904 million miles from the Earth. The Hubble images reveal Saturn's prevailing winds as a dark wedge that eats into the western (left) side of the bright central cloud. The planet's strongest eastward winds (estimated to be 1000 miles per hour from analysis of Voyager spacecraft images) are at the latitude of the wedge.

To the north of this arrowhead-shaped feature, the winds decrease so that the storm center is moving eastward relative to the local flow. The clouds expanding north of the storm are swept westward by the winds at higher latitudes. The strong winds near the latitude of the dark wedge blow over the northern part of the storm, creating a secondary disturbance that generates the faint white clouds to the east (right) of the storm center. The storm's white clouds are ammonia ice crystals that form when an upward flow of warmer gases shoves its way through Saturn's frigid cloud tops. Hubble observed a similar, though larger, storm in September 1990, which was one of three major Saturn storms seen over the past two centuries. Although these events were separated by about 57 years (approximately 2 Saturnian years) there is yet no explanation why they apparently follow a cycle, occurring when it is summer in Saturn's northern hemisphere.

TOPEX/POSEIDON CONFIRMS RETURN OF STRONGER EL NINO

TOPEX/POSEIDON spacecraft

The El Nino phenomenon is back and getting stronger, according to scientists studying data from the ocean-observing TOPEX/POSEIDON satellite. El Nino is a climatic event that can bring devastating weather to several parts of the world, including the recent heavy rains and flooding in California, and the warmer-than-normal winter in the eastern United States.

"The satellite has observed high sea-surface elevation, which reflects an excessive amount of unusually warm water in the upper ocean," said Dr. Lee-Lveng Fu, JPL TOPEX/POSEIDON project scientist. "The associated excess of heat creates high sea-surface temperatures, which affect the weather worldwide by heating the atmosphere and altering the atmospheric jet streams."

Jet streams are high-level winds, five to ten miles above the Earth's surface, created when warm and cold air masses meet. Shifts in the location of jet streams change temperatures and precipitation zones at the surface.

El Nino begins when the westward trade winds weaken and a large warm water mass, called a Kelvin wave, is allowed to move eastward along the equator in the Pacific Ocean. Data from the radar altimeter onboard TOPEX/POSEIDON, recorded from October through December 1994, reveal a new Kelvin wave moving toward the western coast of South America.

"This wave is currently occupying most of the tropical Pacific Ocean. It will take another month or two before the wave disperses. Compared to the El Nino condition of the winter of 1992-93, the present one appears somewhat stronger and might have stronger and longer lasting effects," Fu said.

TOPEX/POSEIDON, a joint program of NASA and the Centre Nationale d'Etudes Spatiales, uses a radar altimeter to precisely measure sea-surface height. Scientists use the data to produce global maps of ocean circulation. Launched August 10, 1992, the satellite has provided oceanographers with unprecedented global sea-level measurements that are accurate to better than 2 inches (5 centimeters). Sea surface height data are essential to understanding the role oceans play in regulating global climate, one of the least-understood areas of climate research.

"The global sea-surface elevation information provided by TOPEX/POSEIDON is unique because it is related to the amount of heat stored in the upper ocean, which is important for long-range weather forecasting. The speed and direction of ocean currents also can be determined from the elevation information, providing another piece of critical information about the ocean, which is the key to climate change," Fu said.

HUBBLE IMAGES A COMPLEX PLANETARY NEBULA

Credit: J. P. Harrington and K. J. Borkowski (University of Maryland), and NASA

This color picture, taken with the WideField/ Planetary Camera-2, is a composite of three images taken at different wavelengths (red, hydrogen-alpha; blue, neutral oxygen, 6300 angstroms; green, ionized nitrogen, 6584 angstroms). The image was taken on September 18, 1994.

This Hubble Space Telescope image shows one of the most complex planetary nebulae ever seen, NGC 6543, nicknamed the Cat's Eye Nebula. It reveals surprisingly intricate structures including concentric gas shells, jets of high-speed gas, and unusual shock-induced knots of gas. Estimated to be 1000 years old, the nebula is a visual "fossil record" of the dynamics and late evolution of a dying star.

A preliminary interpretation suggests that the star might be a double-star system. The dynamical effects of two stars orbiting one another most easily explains the intricate structures, which are much more complicated than features seen in most planetary nebulae. (The two stars are too close together to be individually resolved and appear as a single point of light at the center of the nebula.)

According to this model, a fast stellar wind of gas blown off the central star created the elongated shell of dense, glowing gas. This structure is embedded inside two larger lobes of gas blown off the star at an earlier phase. These lobes are pinched by a ring of denser gas, presumably ejected along the orbital plane of the binary companion.

The suspected companion star also might be responsible for a pair of high-speed jets of gas that lie at right angles to this equatorial ring. If the companion were pulling in material from a neighboring star, jets escaping along the companion's rotation axis could be produced.

These jets would explain several puzzling features along the periphery of the gas lobes. Like a stream of water hitting a sand pile, the jets compress gas ahead of them, creating the curlicue features and bright arcs near the outer edge of the lobes. The twin jets are now pointing in different directions than these features. This suggests the jets are wobbling, or precessing, and turning on and off episodically.

The image was taken with the WideField/Planetary Camera-2 on September 18, 1994. NGC 6543 is 3000 light years away in the northern constellation Draco. The term planetary nebula is a misnomer; dying stars create these cocoons when they lose outer layers of gas. The process has nothing to do with planet formation, which is predicted to happen early in a star's life.

A GLIMPSE OF TITAN'S SURFACE

Global projection of the HST Titan data. The surface near the poles is never visible to an observer in Titan's equatorial plane because of the large optical path.

The first images of surface features on Titan have been made by Peter H. Smith, University of Arizona's Lunar and Planetary Laboratory, and a team using the Hubble Space Telescope in October 1994. They mapped light and dark features over the surface of Saturn's large, haze-shrouded moon for nearly a complete 16-day rotation. A prominent bright area is apparently a surface feature 2500 miles across, about the size of Australia.

Slightly smaller than Mars, Titan is the only body in the solar system besides Earth that may have oceans and rainfall, though the oceans and rain are ethane and methane rather than water. Scientists suspect that Titan's environment, although colder than -289 F, might be similar to Earth's billions of years ago, before life began pumping oxygen into the atmosphere. Titan's atmosphere, about four times as dense as Earth's, is primarily nitrogen laced with methane and ethane. This thick, orange, hydrocarbon haze was impenetrable at wavelengths used by cameras onboard Pioneer and Voyager spacecraft. The haze is formed as methane in the atmosphere is destroyed by sunlight, producing a hydrocarbon smog similar to smog found over large cities on Earth, but much thicker.

Smith's group used the Wide Field/Planetary Camera-2 at near-infrared wavelengths (between 0.85 and 1.05 mm), where Titan's haze is transparent enough to allow mapping of surface features by reflectivity. Only the polar regions could not be mapped because of viewing geometry and the thick haze near the edge of the disk. Image resolution is 360 miles.

Scientists have long suspected that Titan's surface is covered with a global ethane-methane ocean. The new images show that there is at least some solid surface, although what the light and dark regions might represent (continents, oceans, impact craters?) can't be determined.

The images may help scientists target "landing" sites for the Huygens Probe carried by the Cassini mission, a 7-year robotic spacecraft journey to Saturn in October 1997. The probe will parachute to Titan's surface.