土星的卫星Enceladus （有图）

tibet

哈哈，看完这个，理解大半了。但没有讲潮汐。Blobs of warm ice that periodically rise to the surface and churn the icy crust on Saturn's moon Enceladus explain the quirky heat behavior and intriguing surface of the moon's south polar region, according to a new paper using data from NASA's Cassini spacecraft.
"Cassini appears to have caught Enceladus in the middle of a burp," said Francis Nimmo, a planetary scientist at the University of California Santa Cruz and a co-author of the new paper in Nature Geoscience. "These tumultuous periods are rare and Cassini happens to have been watching the moon during one of these special epochs."
The south polar region captivates scientists because it hosts the fissures known as "tiger stripes" that spray water vapor and other particles out from the moon. While the latest paper, released on Jan. 10, doesn't link the churning and resurfacing directly to the formation of fissures and jets, it does fill in some of the blanks in the region's history.
"This episodic model helps to solve one of the most perplexing mysteries of Enceladus," said Bob Pappalardo, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., of the research done by his colleagues. "Why is the south polar surface so young? How could this amount of heat be pumped out at the moon's south pole? This idea assembles the pieces of the puzzle."
About four years ago, Cassini's composite infrared spectrometer instrument detected a heat flow in the south polar region of at least 6 gigawatts, the equivalent of at least a dozen electric power plants. This is at least three times as much heat as an average region of Earth of similar area would produce, despite Enceladus' small size. The region was also later found by Cassini's ion and neutral mass spectrometer instrument to be swiftly expelling argon, which comes from rocks decaying radioactively and has a well-known rate of decay.
Calculations told scientists it would be impossible for Enceladus to have continually produced heat and gas at this rate. Tidal movement – the pull and push from Saturn as Enceladus moves around the planet – cannot explain the release of so much energy.
The surface ages of different regions of Enceladus also show great diversity. Heavily cratered plains in the northern part of the moon appear to be as old as 4.2 billion years, while a region near the equator known as Sarandib Planitia is between 170 million and 3.7 billion years old. The south polar area, however, appears to be less than 100 million years old, possibly as young as 500,000 years.
Craig O'Neill of Macquarie University in Sydney, Australia, and Nimmo, who was partially funded by the NASA Outer Planets Research program, adapted a model that O'Neill had developed for the convection of Earth's crust. For Enceladus, which has a surface completely covered in cold ice that is fractured by the tug of Saturn's gravitational pull, the scientists stiffened up the crust. They picked a strength somewhere between that of the malleable tectonic plates on Earth and the rigid plates of Venus, which are so strong, it appears they never get sucked down into the interior.
Their model showed that heat building up from the interior of Enceladus could be released in episodic bubbles of warm, light ice rising to the surface, akin to the rising blobs of heated *** in a lava lamp. The rise of the warm bubbles would send cold, heavier ice down into the interior. (Warm is, of course, relative. Nimmo said the bubbles are probably just below freezing, which is 273 degrees Kelvin or 32 degrees Farenheit, whereas the surface is a frigid 80 degrees Kelvin or -316 degrees Farenheit.)
The model fits the activity on Enceladus when the churning and resurfacing periods are assumed to last about 10 million years, and the quiet periods, when the surface ice is undisturbed, last about 100 million to two billion years. Their model suggests the active periods have occurred only 1 to 10 percent of the time that Enceladus has existed and have recycled 10 to 40 percent of the surface. The active area around Enceladus's south pole is about 10 percent of its surface.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate in Washington. The Cassini orbiter was designed, developed and assembled at JPL.






These drawings depict explanations for the source of intense heat that has been measured coming from Enceladus' south polar region. These models predict that water could exist in a deep layer as an ocean or sea and also near the surface. Credit: NASA/JPL

tibet

这部分是讲Cassini探测卫星的


In 2005, the Cassini spacecraft began to acquire additional data on Enceladus, answering a number of the questions opened by the Voyager spacecraft and starting a few new ones. Cassini performed several close flybys of Enceladus, revealing the moon's surface and environment in greater detail. In particular, the probe discovered a water-rich plume venting from the moon's south polar region. This discovery, along with the presence of escaping internal heat and very few (if any) impact craters in the south polar region, shows that Enceladus is geologically active today. Moons in the extensive satellite systems of gas giants often become trapped in orbital resonances that lead to forced libration or orbital eccentricity; proximity to the planet can then lead to tidal heating of the satellite's interior, offering a possible explanation for the activity.

misskahi

您太及时了~感谢！！！

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补充一，wikepedia有两段关于狗狗中讲到两个成因，tide heating和固态冰下有大量水存在。http://en.wikipedia.org/wiki/Enceladus_(moon)
Mass estimates from the Voyager program missions suggested that Enceladus was composed almost entirely of water ice.[26] However, based on the effects of Enceladus's gravity on Cassini, its mass was determined to be much higher than previously thought, yielding a density of 1.61 g/cm³.[4] This density is higher than Saturn's other mid-sized icy satellites, indicating that Enceladus contains a greater percentage of silicates and iron. With additional material besides water ice, Enceladus's interior may have experienced comparatively more heating from the decay of radioactive elements.
Castillo et al. 2005 suggested that Iapetus, and the other icy satellites of Saturn, formed relatively quickly after the formation of the Saturnian sub-nebula, and thus were rich in short-lived radionuclides.[50] These radionuclides, like aluminium-26 and iron-60, have short half-lives and would produce interior heating relatively quickly. Without the short-lived variety, Enceladus's complement of long-lived radionuclides would not have been enough to prevent rapid freezing of the interior, even with Enceladus's comparatively high rock-mass fraction, given Enceladus's small size.[51] Given Enceladus's relatively high rock-mass fraction, the proposed enhancement in 26Al and 60Fe would result in a differentiated body, with an icy mantle and a rocky core.[52] Subsequent radioactive and tidal heating would raise the temperature of the core to 1000 K, enough to melt the inner mantle. However, for Enceladus to still be active, part of the core must have melted too, forming magma chambers that would flex under the strain of Saturn's tides. Tidal heating, such as from the resonance with Dione or from libration, would then have sustained these hot spots in the core until the present, and would power the current geological activity.[53]
In addition to its mass and modeled geochemistry, researchers have also examined Enceladus's shape to test whether the satellite is differentiated or not. Porco et al. 2006 used limb measurements to determine that Enceladus's shape, assuming it is in hydrostatic equilibrium, is consistent with an undifferentiated interior, in contradiction to the geological and geochemical evidence.[4] However, the current shape also supports the possibility that Enceladus is not in hydrostatic equilibrium, and may have rotated faster at some point in the recent past (with a differentiated interior).[52]
Possible water ocean
In late 2008, scientists observed water vapor spewing from Enceladus's surface. This could indicate the presence of liquid water, which might also make it possible for Enceladus to support life.[54] Candice Hansen,[55] a scientist with NASA's Jet Propulsion Lab in California headed up a research team on the plumes after they were found to be moving at ~2,189 kilometres per hour (1,360 miles per hour). Since that speed is unusual and is usually attained when water is involved, they decided to investigate the compositions of the plumes.[56]
Evidence from the Cassini probe points to a possible global liquid ocean beneath the frozen surface.[57] Particles of ice analysed by Cassini revealed that the ice was of salt water which could, it is surmised, only occur in a large liquid body of water; as such Enceladus is a candidate for the harbouring of extraterrestrial life.[58] An alternative interpretation of the results is of large water filled caverns.
On August 13, 2009 scientists announced that analysis of the vapour spewing from Enceladus' south pole contain unusually high levels of salt in the ice grains. Additionally, Cassini found traces of organic compounds such as carbonates and dust grains. All these together strengthen evidence that an ocean does exist under the moon's surface. The dust particles may be able to provide details that would normally require drilling to obtain.
The presence of liquid water under the crust means there has to be an internal heat source. Scientists now believe it is a combination of radioactive decay and tidal heating[59][60], as tidal heating alone is not enough to explain the heat. Mimas, another of Saturn's moons, is closer to the planet and has a much more eccentric orbit, meaning the moon should be exposed to far greater tidal forces than Enceladus, and yet the object seems to be geologically dead judging from the old and scarred surface[61].

ann110726

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感谢lz！