The Sky at Night Phần 3 doc

22 6 News from the Planets

This is not at all. Cassini showed that a high mountain ridge runs for a long

distance round Iapetus, making it look rather like a table-tennis ball which has been

broken in half and then unskilfully glued together. The ridge is high, rising to a

maximum of 8 miles above the surrounding terrain, running for 800 miles almost

among the geographical (should it be the Iapetographic?) equator. It is unlike any￾thing else known in the Solar System, so how was it formed? Could it be that it is

due to icy material which welled up from below and then solidified? Could it be

that, as suggested by Paulo Frerie of Arecibo observatory, Iapetus once grazed the

outer edges of the ring system, and later retreated to its present distance? It has even

been suggested that Iapetus itself may have had a ring – a ringed satellite orbiting

a ringed planet. Less plausibly, some UFO enthusiasts have claimed that Iapetus

itself is artificial, put together by the usual nebulous aliens from afar. Certainly, it

may be a popular sight for future interplanetary tourists because its orbit is inclined

to the plane of Saturn’s equator by almost 16°, and travellers will see the rings well

displayed – while the inner satellites, including Titan, orbit almost in the equatorial

plane so that seen from them the ring system will always be edgewise-on.

Perhaps, the greatest surprise of all came from Enceladus, discovered in 1787 by

William Herschel. It is a mere 310 miles across (about the distance between

London and Penzance) and was expected to be icy and inert. This is certainly true

of the even smaller Mimas, discovered by Herschel at the same time; incidentally,

these were the first of the few important results coming from Hershel’s largest

telescope, the 40-foot focus reflector with its 49-in. mirror. Mimas is dark with one

vast crater, which had led to its being compared with Darth Vader’s “Death Star”.

Enceladus has the highest albedo of any Solar System body; there are no large

craters and wide areas where there are no craters at all. This must mean that these

areas are young, and have been resurfaced in comparatively recent times.

When Cassini flew past Enceladus on 17th of February 2005, at a range of

725 miles, it detected a tenuous but appreciable atmosphere – totally unexpected for

a world with so weak a gravitational pull; In fact no atmosphere could be retained

for long, and so there must be continual replenishment from below. Next came the

discovery of ice geysers spouting from the south polar region; the jets rise to hundreds

of miles above the ground. At NASA, they caused great excitement. To quote

Carolyn Porco, head of the Cassini imaging team: “I think this is important enough

to see a redirection in the planetary exploration programme. We’ve just brought

Enceladus to the forefront as a major target of astrobiological interest.” The readings

from Enceladus’ geyser plumes indicate that all of the prerequisites for life as we

know it could exist below Enceladus’ surface. “Living organisms require liquid

water and organic materials, and we know we have both on Enceladus now”.

A few tens below the surface the temperature and pressure may be sufficient to

keep water in a liquid state. Further evidence comes from the so-called “tiger

stripes”, which indicate cracks. The ice here is a more amorphous and virtually

crater-free, so that it must have welled up comparatively recently. The geysers rise

upward for several 100 miles, so that they are violent – and violence was the last

thing to be expected on a world as small as Enceladus. Most of the ice crystals fall

back as snow, but some break and free altogether to become part of the wide, thin