Marvelous Puzzle: Enceladus South Pole Surface Less Than 1,000 Years Old
Enceladus, a moon of Saturn smaller than the British isles (comparison image), has a region at the south pole that is less than 1,000 years old, and maybe only 10 years old. This conclusion, announced at Cassini science briefings in London August 30, is based on multi-instrument observations taken July 14 during the closest flyby ever of Enceladus (08/09/2005, 07/14/2005). Crystalline ice has been found in four 80-mile-long parallel canyons dubbed “tiger stripes” due to their appearance. Water ice has been observed venting into a plume of small particles from these cracks, which are noticeably warmer than the surrounding regions. Measurements from all the instruments aboard the Cassini spacecraft converge on the conclusion that Enceladus’ southern polar surface is young, probably active today.
Two instruments detected icy particles coming from the south pole. The Ion and Neutral Mass Spectrometer (INMS) and the Cosmic Dust Analyzer (CDA) measurements reached a peak at closest approach (graph, flyby chart), showing that material is being emitted now, and probably accounts for at least some of the fresh material replenishing the E-ring around Saturn. The Magnetometer (MAG) confirmed the existence of this plume by watching its asymmetric influence on the magnetic field lines around Enceladus; the Ultraviolet Imaging Spectrometer (UVIS) arrived at the same conclusion. The Virtual and Infrared Mapping Spectrometer (VIMS) (picture) and Composite Infrared Spectrometer (CIRS, see article and picture) showed a temperature rise across the tiger-stripe cracks up to -279°F, almost 40° warmer than expected. Apparently this is warm enough to cause sublimation of ice from the surface. The crystalline nature of the ice constrains its age to 1000 years or less. Crystalline ice rapidly changes to an amorphous structure when exposed to solar radiation. Another odd thing about the tiger stripes is that each of them curves into a question mark shape at one end, all facing the same way (picture 1, picture 2). Could this be a rotational effect, or is it due to a massive flow from the west?
The closest-approach image (picture and zoom-in movie) revealed boulders about 20m in size that New Scientist said could be ejecta from eruptions. That Enceladus should be erupting material today is puzzling to the planetary scientists for several reasons. For one, the north pole is heavily cratered and therefore looks much older; why would the south pole be active, when normally the equatorial regions are the warmest? (compare prediction vs. observation). Another puzzle is the comparison with Mimas, a moon of similar size. Mimas is heavily cratered with no activity, even though it was pummeled by a colossal impact at some time in the past and is subject to greater tidal stresses due to its proximity to Saturn. Finally, small bodies cool the fastest. A moon the size of Enceladus should long ago have lost all its internal heat and remained forever frozen solid.
To account for the heat and resultant activity (03/04/2005), planetary geologists are constructing models combining radiogenic heat from a rocky interior and tidal heating from interactions with Saturn and other moons. So far, however, these energy sources seem to come up short by an order of magnitude or two (see New Scientist). This is a “marvelous puzzle,” said one scientist; another said that “Enceladus is constantly evolving and getting a makeover.” Enceladus joins a “short list of bodies in our solar system where scientists have found internal activity,” the press release said. That list includes Io (05/04/2004), Titan (06/09/2005, 05/18/2005, 04/08/2005), Triton (05/30/2002), Earth, and Venus (see “Earth’s Ugly Sister Can’t Get a Date,” 08/16/2004). Most other solid planets and moons exhibit surface features that, while not active today, appear young (06/05/2003 commentary). The Cassini team expected that Enceladus would prove one of the prima donnas of the Saturn system. It appears that she delivered a “stunning surprise” of a performance.
Sources: JPL, Cassini press release, NASA Cassini page, New Scientist, BBC News, the Planetary Society, EurekAlert and the Cassini Imaging Team. The latter contains polar projection maps, a graph and flyby chart, and three models attempting to explain the heating process.
Scientists love a good puzzle, and being surprised is fun; we can all share the excitement of a new and baffling phenomenon. But what none of them seems to be asking is the obvious question: how could this moon be anywhere near 4.6 billion years old? Look at the size of those canyons – 80 miles wide and 50 miles apart – they speak of large-scale processes at work, not just minor eruptions. This revelation is just the latest in a long string of discoveries that challenge the consensus view of the age of the solar system. For all the flash and color of the model diagrams, New Scientist says that tidal heating and internal radioactivity are not anywhere near sufficient to drive the activity. Why not consider the possibility that Enceladus – as well as the solar system that contains it – is young.
Don’t miss the zoom-in movie; it is really cool. It puts the imaging capabilities of Cassini into perspective. The color mosaic is also a beautiful sight. Here are the tiger stripes up close. For a complete catalog of Enceladus images, go to Cassini multimedia page and select “Enceladus.” The Planetary Photojournal is the NASA repository for all solar system images. Simply click on Saturn, then on Enceladus.