While touring the Saturn system in 2008, Cassini enabled great scientific studies and observations. Below is captured the Top 10 Science Highlights of the year as selected by the science teams.
- Identification of liquid ethane in a lake on Titan
- Polar storms on Saturn
- Strong inference of a liquid water layer in Titan's interior
- The likelihood of dusty rings around Rhea
- The possibility of plate-tectonic-like spreading in the Enceladus south polar region
- Water vapor jets inside the plume of gas leaving Enceladus
- Moonlet population in and around the F ring
- New insights into Saturn's aurora
- Three belts of sub-moonlets in the A ring (propellers)
- Six month-old lightning storm shatters record for longevity
1. Identification of liquid ethane in a lake on Titan
NASA scientists have concluded that at least one of the large lakes observed on Saturn's moon Titan contains liquid hydrocarbons, and have positively identified the presence of ethane. This makes Titan the only body in our solar system beyond Earth known to have liquid on its surface.
Scientists made the measurements using data from an instrument aboard the Cassini spacecraft. The instrument identified chemically different materials based on the way they absorb and reflect infrared light. Before Cassini, scientists thought Titan would have global oceans of methane, ethane and other light hydrocarbons. More than 40 close flybys of Titan by Cassini show no such global oceans exist, but hundreds of dark, lake-like features are present. Until now, it was not known whether these features were liquid or simply dark, solid material.
2. Polar storms on Saturn
Cassini scientists revisited the north polar hexagon last year (see the March 27, 2007 press release). This huge polygonal pattern in the clouds was first seen by Voyager in 1980. In 2008, Cassini found that the aurora glows at infrared wavelengths at the same latitude as the hexagon, suggesting a connection over a huge range of altitudes (see Saturn's North Pole Hexagon and Aurora, Saturn's Polar Aurora, and the related Nov. 12, 2008 press release.)
In addition, Cassini found a hot spot resembling the eye of a hurricane, but it is locked to the north pole at the center of the hexagon, with swirling cyclonic winds signifying a low pressure center (see Saturn's North Pole Hot Spot and Hexagon). The hot spot is confined to latitudes above 88 degrees, while the corners of the hexagon are at 75 degrees (see The Persistent Hexagon and Infrared Images of Saturn’s Poles).
The north polar hot spot resembles one in the south that was imaged in exquisite detail in 2008 (see the Oct. 13, 2008 press release, The Yet Yawning Gulf, and Convection in Saturn's Southern Vortex). These findings cast light on how large vortices - swirling masses of gas - behave in planetary atmospheres throughout the solar system.
The new-found cyclone at Saturn’s north pole is only visible in the near-infrared wavelengths because the north pole is in winter, thus in darkness to visible-light cameras. At these wavelengths, about seven times greater than light seen by the human eye, the clouds deep inside Saturn’s atmosphere are seen in silhouette against the background glow of Saturn’s internal heat.
Peak winds exceed 450 kilometers per hour (280 mph, or 130 m/s) near 88 degrees latitude. New measurements by Cassini show that clouds within the hexagonal feature located near 77 degrees north latitude zoom around the “race track” of the hexagon at this same high speed -- 460 kilometers per hour (127 meters per second, or 285 mph) -- while the hexagonal “race track” itself stays nearly stationary in Saturn’s atmosphere.
Dyudina et al., (2008), Dynamics of Saturn's South Polar Vortex, Science 319, 1801
3. Strong inference of a liquid water layer in Titan's interior
NASA's Cassini spacecraft has found evidence that points to the existence of an underground ocean of water and ammonia on Saturn's moon Titan. The findings, made using radar measurements of Titan's rotation, appeared in the March 21 issue of the journal Science.
"With its organic dunes, lakes, channels and mountains, Titan has one of the most varied, active and Earth-like surfaces in the solar system," said Ralph Lorenz, lead author of the paper and Cassini radar scientist at the Johns Hopkins Applied Physics Laboratory in Laurel, Md., "Now we see changes in the way Titan rotates, giving us a window into Titan's interior beneath the surface."
Subsequent data has suggested that these radar observations may be related to precession, only indirectly related to the presence of an ocean, but other geophysical evidence continues to point to a subsurface ocean. Titan continues to amaze and confound!
An Ocean Runs Through It
4. The likelihood of dusty rings around Rhea
A broad debris disk and at least one ring appear to have been detected by a suite of six instruments on Cassini specifically designed to study the atmospheres and particles around Saturn and its moons.
Saturn's Moon Rhea Also May Have Rings
Jones et al., The Dust Halo of Saturn's Largest Icy Moon, Rhea, published in Science, March, 2008.
5. The possibility of plate-tectonic-like spreading in the Enceladus south polar region
The closer scientists look at Saturn's small moon Enceladus, the more they find evidence of an active world. The most recent flybys of Enceladus made by NASA's Cassini spacecraft have provided new signs of ongoing changes on and around the moon. The latest high-resolution images of Enceladus show signs that the south polar surface changes over time.
Close views of the southern polar region, where jets of water vapor and icy particles spew from vents within the moon's distinctive "tiger stripe" fractures, provide surprising evidence of Earth-like tectonics. They yield new insight into what may be happening within the fractures. The latest data on the plume -- the huge cloud of vapor and particles fed by the jets that extend into space -- show it varies over time and has a far-reaching effect on Saturn's magnetosphere.
6. Water vapor jets inside the plume of gas leaving Enceladus
Scientists continue to search for the cause of the geysers on Saturn's moon Enceladus. The geysers are visible as a large plume of water vapor and ice particles escaping the moon. Inside the plume are jets of dust and gas. What causes and controls the jets is a mystery. The Cassini spacecraft continues to collect new data to look for clues.
At the heart of the search is the question of whether the jets originate from an underground source of liquid water. Some scientists working on the Cosmic Dust Analyzer (CDA) have suggested that the sodium found present in the E ring can be traced back to liquid in Enceladus. Some other theories offer models where the jets could be caused by mechanisms that do not require liquid water. Painstaking detective work by Cassini scientists is testing the possibilities to get closer to an answer.
Enceladus' Water Vapor Plume
7. Moonlet population in and around the F ring
Saturn's F ring has long been of interest to scientists as its features change on timescales from hours to years and it is probably the only location in the solar system where large scale collisions happen on a daily basis. Understanding these processes helps scientists understand the early stages of planet formation.
Models have been developed which clearly imply a population of perhaps hundreds of unseen objects with sizes between 100 meters and 1 kilometer or so, both lying within and also criss-crossing through the narrow F ring core and causing havoc in the orbits of its particles.
Stellar occultations also found a number of clumps and opaque objects, measuring their size directly to be in the few hundred meter size range.
(Murray et al. [2008, Nature] and Esposito et al. [2008, Icarus]).
8. New insights into Saturn's aurora
Saturn has its own unique brand of aurora that lights up the polar cap, unlike any other planetary aurora known in our solar system. This odd phenomenon revealed itself to one of the infrared instruments on NASA's Cassini spacecraft.
Stallard et al., Complex structure within Saturn’s infrared aurora, published in Nature, November 2008.
9. Three belts of sub-moonlets in the A ring (propellers)
Tiscareno et al. (2008, Astron. J.).
10. Six month-old lightning storm shatters record for longevity
Often a year goes by when there are no discharges, during which time the ISS does not see the storms. Then suddenly the radio signals begin and a new storm appears in the atmosphere. One such storm was identified on November 27, 2007, probably within a day of its birth. Unlike past storms, which lasted for a few weeks at best, this one lasted for 7.5 months (see Hissing Storm, Saturn's Long-lived Storm, Continuing Storm, and the April 29, 2008 press release "NASA Spacecraft Tracks Raging Saturn Storm") and thereby set a new record for longevity of lightning storms throughout the solar system.