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More Excitment from Cassini

More Excitment from Cassini


As far as our solar system is concerned, astronomers have grown accustomed to expecting the unexpected. Certainly nobody expected liquid water spewing from a small moon of Saturn.

An article published in Science (March 10, 2006) describes how the Cassini spacecraft discovered Yellowstone-like geysers of water vapour and ice particles shooting 430 kilometres into space from Enceladus. One scientist confided to the media that this discovery is a “big deal”. Who would ever expect a tiny body with a diameter a mere 500 km, to be warm enough still to contain liquid water? It is not as if this moon is substantially warmed by the sun; it is not.

If this small satellite of Saturn has been around for billions of years (as most astronomers presume), then it should have lost its original heat energy long, long ago. What we see however is enough energy to force water to explode out of vents in the moon’s surface.

There are two possible explanations for the liquid water on Enceladus. Firstly the planet may be very young. Assuming that the moon was once hot, then evidently it has not completely cooled. That would imply a very short time interval for such a small object.

Alternatively some process may be operating, as yet unobserved, which causes the moon to retain hot water. The moon is so small however that there seems little scope for such a presumed process. The obvious conclusion is that the moon is very young. Most scientists ignore that possibility, preferring instead to speculate that the scene could be ideal for origin of life processes.

During this past year, the Cassini spacecraft also surveyed Saturn’s largest moon Titan. This too is a body which scientists consider a potential host for extraterrestrial life. Based on their belief in the old age of this moon, and on their hope for an origin-of-life friendly environment, scientists expected to find oceans of liquid methane on the surface. The Huygens lander, which descended to the moon’s surface on January 14, 2005, would hopefully confirm that prediction.

The actual scene on Titan was so contrary to expectations that scientists could not believe their eyes. One NASA scientist, reported Nature (January 20, 2005 p. 181), declared that if these scenes were really from Titan, then they were all “in big trouble.” At first these scientists believed that they were viewing old footage from Mars, with its barren boulder- strewn landscape. But there was no mistake! The solid objects however were of pebble size rather than the big rocks of Mars. The surface thus turned out to be a big surprise. Its texture was like moist sand. This news represented “big trouble” for scientists’ hopes for Titan.

The problem is that there is considerable (about 5%) methane in the atmosphere around Titan. Since methane in the atmosphere is expected to completely break down after a mere 10 to 20 million years, there should not be any of the gas left on a satellite that is a billion or more years old. Since there is methane in Titan’s atmosphere, either the planet must be young, or some thus far undetected process must be adding new methane to the atmosphere. This is what the methane oceans would have accomplished.

The secular scientists seek a weather cycle which could keep methane circulating into and out of Titan’s atmosphere. Unfortunately there does not seem to be a realistic source of energy to drive such a cycle. A news item in Nature (Dec. 1, 2005) reports: “The sun provides very little light, and radioactive decay and internal movement caused by Titan’s orbit around Saturn don’t seem to add much power.” Another possibility is volcanoes, but there is little support for this idea also. Thus the mystery of the methane in Titan’s atmosphere is an unexpected legacy of the Cassini program.

Indeed everywhere that space probes go in our solar system, startling situations, best explained as recent in origin, are observed. So hurray for Cassini and the other probes. Long may they explore.

Margaret Helder
April 2006

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