Hundreds of scientists around the world continue to pore over images and data from the Cassini spacecraft following its successful July 1st insertion into Saturn orbit and its October 26th close flyby of the cloud-enshrouded moon Titan. The latest results, presented on November 8th at the American Astronomical Society's Division of Planetary Sciences meeting in Louisville, Kentucky, include a mysterious circular feature on Titan and mountains on Iapetus that might be the highest in the solar system. "It's an embarrassment of riches," says Carolyn Porco (Space Science Institute), principal investigator of Cassini's camera system.
Some of the highlights include:
A belt in Saturn's upper atmosphere at 36° south latitude has spawned several major storm systems in the past few months, leading to the moniker "storm alley."
Scientists have confirmed that major storm systems in Saturn's atmosphere spark powerful electrostatic discharges in other words, lightning. Experts have puzzled over the origin of the lightning since the Voyager flybys of the early 1980s.
A huge cloud of hydrogen atoms extends 45 Saturnian radii from the planet's equator. A rapidly variable cloud of oxygen huddles closer in. The atoms come from ice particles knocked off the rings and moons by the incessant pounding of micrometeorites and collisions between moonlets in the rings. It seems that Saturn's radiation belts break the water molecules apart into their constituent hydrogen and oxygen atoms.
A major brightening of auroral activity in Saturn's polar region was recorded on July 25th, just as a solar wind shock front blew past the ringed planet.
Cassini has spotted two new tenuous rings so far. The first shares the same orbit as the small moon Atlas, which lies in the narrow region between the A and F rings. The second discovery was also located in the same region between A and F, just inside the orbit of Prometheus.
The diminutive moon Pan, which carves the Encke Division, follows an eccentric orbit. Gravitational perturbations from Pan cause strange scalloped features in the A ring. By analyzing these undulations, Porco refined Pan's mass. Images suggest a diameter of about 25 kilometers, which allowed her to calculate the moon's mean density: about 0.5 gram per cubic centimeter. This value is similar to the densities of small, inner moons such as Janus and Epimetheus. Observations of Atlas and its perturbations on the rings also suggest a density of about 0.5. These low densities suggest porous bodies that are possibly rubble piles rather than solid water ice.
During its close approach to Saturn on July 1st, Cassini discovered a new collection of ring phenomena at small scales never seen before. Several unusual types of features seem to be related to perturbations caused by various moons. Bizarre wisps in the outer edge of the Keeler Division strongly suggest the existence of an unseen moonlet in the middle of the gap.
Scientists continue to see no evidence in images or in radar data for bodies of liquid hydrocarbons on Titan's surface. Some scientists speculate that liquid hydrocarbons might be mixed in with surface ices to form slush, or it might be stored in underground reservoirs.
Images and radar data have failed to turn up obvious impact craters on Titan. This could indicate that the surface is young. A radar map from the October 26th flyby shows a 100-kilometer-wide circular feature that appears raised, so it is probably not an impact crater. Instead, it appears similar to Venus's "pancake domes." If this interpretation is correct, the feature was probably formed by upwelling water- or ammonia-rich ices (known as cryovolcanism). But Robert M. Nelson (NASA/Jet Propulsion Laboratory) and other scientists argue that the circular features are palimpsests, the remnants of ancient craters that have been filled in by the slow flow of material from the crater walls. This interpretation would suggest an older surface.
Cassini's Composite Infrared Spectrometer has registered various gaseous hydrocarbons in Titan's north polar hood that are not found in the southern hemisphere. These variations indicate seasonal effects on Titan. Studying seasonal effects may provide crucial information about the origin and evolution of Titan's dense atmosphere.
Images of the medium-sized moon Iapetus taken from more than 100,000 kilometers away show several white peaks poking high above dark terrain on the moon's leading hemisphere. Preliminary analysis by Tillman Denk (Freie University, Germany) and colleagues suggests some of these peaks might be 10 to 20 kilometers high, which would rival or perhaps even exceed Mars's giant volcano Olympus Mons for being the highest mountains in the solar system. Scientists will have to wait until the September 2007 close Iapetus flyby to nail down the altitudes.
Three large impact basins also appear in the Iapetus images. Material raining back down to the surface from one or more of these impacts may be responsible for coating the moon's leading hemisphere black, explaining the mystery of why the leading hemisphere is as dark as fresh asphalt while the trailing hemisphere is as reflective as water ice. Other scientists still believe the dark material on Iapetus's leading hemisphere came from Phoebe and other small outer moons that orbit Saturn in a retrograde direction.
The overall structure of the rings has changed very little since Voyager 1 and 2 flew by Saturn in 1980 and 1981. But ongoing interactions and small-scale features within the rings indicate they are relatively young, perhaps only a few hundred million years old. This supports the theory that the rings are remnants of a moon that was blasted apart by an impact or an external body that ventured too close to Saturn and was ripped apart by tidal forces.
During an occultation of the 4th-magnitude star Xi Ceti on October 6th and 7th, Cassini's ultraviolet imaging spectrograph recorded sharp edges to some of the gaps. The starlight turned totally "on" to "off" in a span of just 30 meters. Gravitational influences of moons must be keeping these sharp edges in place. The occultation experiment also revealed previously unseen density waves in the rings that are induced by resonances with various moons.
Scientists are still digging through gigabytes of data taken by Cassini's suite of a dozen instruments, and the spacecraft has four more years and 76 orbits to go before the end of the primary mission. Despite the "embarrassment of riches," the mission is just getting started. Cassini will fly within 2,400 kilometers of Titan on December 13th, and the European Space Agency's Huygens probe will parachute through Titan's atmosphere on January 14, 2005.