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Cassini visits Enceladus:
New light on a bright world

(Released July 2006)

 
  by Salvatore A. Vittorio  

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Cassini sheds new light on Enceladus

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Internal structure model. Prior to the Cassini mission, relatively little was known about the interior of Enceladus. However, results of recent Cassini flybys have provided much needed information for models of Enceladus's interior. These include a better determination of the mass and triaxial ellipsoid shape, high-resolution observations of the surface, and new insights on Enceladus's geochemistry [1]. The figure and caption below gives a proposed model that summarizes some of these findings.

cutaway view showing Enceladus's core  
Left: Model of the interior of Enceladus based on recent Cassini findings. The inner silicate core is shown in brown, the outer water-ice-rich mantle is shown in white, and the yellow and red colors in the mantle and core, respectively, represent a proposed diapir under the south pole. [16]
 

Cryovolcanism, geysers, and the feeding of the E Ring. During the three close encounters with Enceladus by Cassini on February 17, March 9, and July 14, 2005, cryovolcanism was discovered on Enceladus. On the February 17th encounter, data from the magnetometer on board Cassini provided the first hints when it found evidence for an atmosphere on the moon. An increase in the power of ion cyclotron waves near Enceladus was detected by the magnetometer. The ion cyclotron waves were produced by ionization of particles near Enceladus [1]. They showed, by way of the frequency of the waves, that the particles consisted of ionized water vapor [13]. The next two encounters showed that the gases in Enceladus's atmosphere are concentrated over the South Polar Region, with the density greatly decreasing with distance away from the pole [13]. The Ultraviolet Imaging Spectrograph (UVIS) on board Cassini confirmed this result by observing two stellar occultations during the February 17th and July 14th encounters [1].

shining sliver behind dark moon
Plumes above the limb of Enceladus feeding Saturn's E Ring. [NASA/JPL photo}

On July 14, 2005, the Cassini spacecraft made its closest flyby yet of Enceladus. The flyby came within 175 km (109 miles) of the moon's icy surface and flew through the gas cloud emanating from the South Polar Region. Instruments such as the Ion and Neutral Mass Spectrometer (INMS) and the Cosmic Dust Analyzer (CDA) allowed for the direct sampling of the plume of gases. The INMS detected water vapor and minor components like molecular nitrogen (N2), methane (CH4), and carbon dioxide (CO2) [14]. The CDA confirmed the satellite as the primary source for the E Ring when it detected a large increase in the number of particles near Enceladus [1]. Cassini revealed an unexpected hot spot on the satellite's south pole. The finding revealed that Enceladus is geologically active, and that what was supposed to be the moon's coldest region turned out to be its warmest. Because of its orientation to the sun, scientists expected the moon's south pole to be about -198 C (-324 F), but what Cassini revealed instead was that it was over 110 K (-261 F). Since solar heating alone could not account for this difference, scientists think that internal heating is occurring. [9]

Images taken by Cassini in November 2005 by the Imaging Science Subsystem (ISS) instrument visually confirmed the February 17th and July 14th measurements and observations by showing streams of fine icy particles rising from the moon's south pole [12], suggesting they originated from warm zones in the region. Cassini passed through the plume, which stretched up to 300 miles above Enceladus's surface [10], nearly as great a distance as Enceladus's diameter. The instruments aboard Cassini measured the plume's makeup and found water vapor and icy particles [10]. A team of researchers led by NASA Cassini scientist Candice Hansen-Koharcheck at the Jet Propulsion Laboratory in Pasadena, California, believe the plume may have been erupting continuously for 15 years, and appears to replenish the E Ring with material and provide the source of oxygen and hydrogen permeating Saturn's neighborhood. "It's definitely the water, there's no doubt about it," said Hansen-Koharcheck.

rainbow ascends from Enceladus
Cassini's image of Saturn's moon Enceladus backlit by the Sun shows the fountain-like sources of the fine spray of material that towers over the South Polar Region. The image is greatly enhanced and colorized. [Credit: NASA/JPL/Space Science Institute]

In addition to the hot spot found on the south pole of Enceladus, Cassini also revealed that "icy veins" found on the moon's surface are actually a series of fractures. What is more, the fractures appear to be active, violently spewing a slushy jet of warm water and ice into space. Together, the venting fractures and hot spot provide strong evidence for geologic activity on Enceladus. If true, the findings could explain how Saturn's E Ring becomes continuously supplied with microscopic particles of ice and dust [9].

Before Cassini's findings, Enceladus was thought to have no geological activity (i.e., to be geologically dead), and so there could be no spouting fountains of water and ice on the moon's surface. It was therefore thought that impacts on the surface were the most likely source of particles for the E Ring. The discovery of the hot spot on the moon's south pole is critical because it provides the moon with an engine to drive the geysers and volcanoes [9] that could sustain the E Ring with a constant supply of ice and dust particles. According to Linda Spiker, Cassini's deputy project scientist, "If you turned Enceladus off, you would probably turn off the E Ring." [11]. The discovery also puts Enceladus with Jupiter's Io and Neptune's Triton in the class of geologically active moons [10].

It is currently unclear why the south pole of Enceladus is so active, but one theory is that radioactive material left over from the moon's formation billions of years ago is acting as a heat source, according to Spiker. Another theory is that a change in the moon's spin (rotation) rate caused fractures to form on its surface. If, at one time, Enceladus had moved in closer to Saturn, it would have had to spin more quickly in order to synchronize its rotation period with its orbit around Saturn, which is a natural tendency for planetary satellites. Spiker says, "that change in spin might have caused the cracking seen at the south pole." It is still unknown how or when this event might have occurred. [9]

Astrobiological implications. In March 2006 NASA announced that the high-resolution Cassini images of Enceladus show icy jets and lofty plumes expelling large quantities of particles at high speed. Scientists think the jets spout from near-surface pockets of liquid water. The very strong evidence for the presence of liquid water has led to the tantalizing question of whether so small and cold a body as Enceladus could provide conditions suitable for living organisms [15]. According to Chris McKay, a planetary scientist at the NASA Ames Research Center in Moffett Field, California, there are three reasons why Enceladus is very interesting for astrobiology. The moon's activity suggests that there must be some sort of subsurface energy source, possibly gravitational tidal effects, and that this energy source is concentrated and not a global diffuse heating. This may imply liquid water below the surface. When added up, (1) a concentrated energy source, (2) liquid water, and (3) perhaps evidence of methane makes Enceladus a key target for astrobiology. According to McKay, further observations of the plume and more detailed measurements of the dust emitted would be ideal. He adds that the detection of carbon- and nitrogen-containing particles would be key factors. McKay explains, "clearly the Saturn system with organic rich Titan and water-active Enceladus is on the first rank of targets for astrobiology. We should start planning a combined Enceladus lander and Titan balloon mission." [15]

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