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Enceladus - the strange moon of Saturn

Using the instrument aboard Cassini probe, scientists have confirmed that the 500km-wide Saturn moon Enceladus has an atmosphere. They have also seen a "hotspot" at the icy moon’s south pole, which is riven with cracks dubbed "tiger stripes".But they could not yet explain fully the energetic processes driving all the activity on Enceladus.

Enceladus orbits the ringed planet at a distance of approximately 237,400km and is described as the most reflective object in the Solar System; its icy surface throws back about 90% of the sunlight that hits it.

High-resolution imagery shows the southern polar region to be relatively smooth - usually a good indicator of recent activity - but cut by a number of long, dominant cracks. These are the so-called tiger stripes. They are about 130km long and roughly parallel to one another, spaced about 40km apart.

Cassini’s composite infrared spectrometer shows the region to be much warmer than expected.

Whereas temperatures near the equator are a frigid 80 Kelvin (minus 193C), the south polar average reaches 85K (minus 188C). Small areas of the pole, concentrated near the tiger stripe fractures, are even warmer: well over 110K (minus 163C) in some places. This is only the second place in the Solar System beyond Earth that we’ve seen signs of heat coming out of the interior - the other being Jupiter’s moon Io. The scientists think the cracks may act like vents, spewing out water vapour and very fine water-ice particles. Some have suggested there could be ice geysers and even ice volcanoes at the stripe locations - but these have not been imaged directly.

The puzzle for researchers is how to explain such an energetic system on Enceladus. As the moon moves around an eccentric orbit of Saturn, gravitational forces should subject the tiny world to some tidal heating. Radioactive isotopes in its rocky core may also be a source of some warming. But scientists are struggling to make the numbers add up and are frankly baffled as to why the activity they see should be so concentrated in just the one region. 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.

The finding moves Enceladus from being a small denizen of the outer Solar System - a frozen iceberg - to something that’s more of an active type world that we’re interested in exploring."

Cassini discoveries at Enceladus include:

• presence of a strange atmosphere concentrated at the south pole
• atmosphere mostly (91%) water vapour, but with some nitrogen, carbon dioxide and other simple carbon-based molecules (organics)
• large crevasse features at south pole dubbed tiger stripes
• intriguing hotspot at south pole - anomalous warmth in the area of the tiger stripes
• presence of "orderly" water-ice at south pole, especially within tiger stripe features, indicates region 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 being ejected from tiger stripes
• fine ice material is probably the significant and sustaining source of ice particles that make up Saturn’s outermost ring - its E ring.

The $3.2bn Cassini-Huygens mission is a joint venture between the US space agency (Nasa), the European Space Agency (Esa) and the Italian Space Agency (Asi).

posted by: kyawoo at 19:22 | link | comments |
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