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Enceladus continues to intrigue scientists


Further results from the Cassini spacecraft’s July flyby of Saturn’s moon, Enceladus, reveal more detail about the features and processes going on. Speaking at a press conference at Imperial College, London this morning (30th August) the Cassini scientists spoke about the intriguing south pole area which has a surprising hotspot and “tiger stripe” features on the surface.

Following the two distance flybys of Enceladus in February and March this year results from the Magnetometer instrument showed a flowing of the magnetic and plasma field from Saturn towards and then around Enceladus. Effectively Enceladus was acting as an obstacle to the flow – indicating the presence of an atmosphere. In addition there was an indication of an internal signature.

Professor Michele Dougherty from Imperial College is the lead scientist for the Magnetometer instrument. She takes up the story.

“On the basis of the results obtained from the Magnetometer instrument from the two first flybys we persuaded the Cassini team to take a much closer look at Enceladus. As a result on 14th July we were able to get within 173 km of the surface enabling all of the instrument teams to get data which is starting to build up a really surprising picture of the processes at work on this moon. We were able to confirm the presence of an atmosphere following the third flyby. Whilst it was found around the whole body is was concentrated at the south pole indicated by the presence of a cloud of water vapour.”

Images from the Imaging Science Subsystem (ISS) team on Cassini revealed Enceladus in incredible detail – with the south pole region being of particular interest because of its relatively smooth terrain indicating that it is relatively young. Cassini’s Visual and Infrared Mapping Spectrometer recently discovered that the long, cracked features dubbed “tiger stripes” are very young – between 10 and 1,000 years.

These findings support previous results that the moon’s southern pole is currently active and has undergone episodes of geologic activity as recently as 10 years ago. These cracks, which are approximately 80 miles long are roughly parallel to one another and are spaced about 25 miles apart. The cracks act like vents. They spew out vapour and fine ice water particles that have become ice crystals. The crystals can be dated, which helped scientists pin down the age of the features.

Cassini’s visual and infrared mapping spectrometer shows water ice exists in two forms on Enceladus: pristine, crystalline ice and radiation-damaged amorphous ice. When ice comes out of the “hot cracks at the south pole – the “tiger stripes” – it forms as fresh crystalline ice. As the ice near the pole remains cold and undisturbed it ages and coverts to amorphous ice. Since this process is believed to take place over decades or less, the tiger stripes must be very young.

“One of the most fascinating aspects of Enceladus is that it’s so very small as icy moons go, but so very geophysically active. It’s hard for a body as small as Enceladus to hold onto the heat necessary to drive such large-scale geophysical phenomena, but it had done just that,” said Dr Bob Brown, team leader for the visual and infrared mapping spectrometer from the University of Arizona. “Enceladus and its incredible geology is a marvellous puzzle for us to figure out.”

Enceladus and its south pole – the story so far

  • the presence of an exotic atmosphere – concentrated at the south pole
  • large crevasse features at the south pole dubbed “tiger stripes”
  • an intriguing hot spot at the south pole – particular in the area of the tiger stripes – showing similarities to features seen at Jupiter’s moon, Io. Temperatures warmer than expected.
  • presence of “orderly” water ice at the south pole, especially within the tiger stripe features, indicating that it must have been very hot, be very young or both!
  • presence of simple organics along the fractures
  • indication that water vapour and fine material are originating from the “hot” polar cap region. The production of water vapour and ejection of fine material are connected as they are in a comet – suggesting that these materials are coming from the tiger stripes.

Dr. Torrence Johnson from NASA’s Jet Propulsion Laboratory is a member of the ISS team. He said, “Enceladus has certainly thrown up more surprises than anticipated – which is great for planetary science. We are piecing together the results from the various instrument teams to build up more of a complete picture of what is happening on Enceladus. With more data analysis and further flybys planned we are beginning to understand more about this icy satellite.”

Gill Ormrod | alfa
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