tailieunhanh - The Sky at Night Phần 3

Đây không phải là tất cả. Cassini cho thấy rằng một sườn núi cao chạy cho một Iapetus vòng khoảng cách dài, làm cho nó trông giống như một quả bóng bóng bàn đã bị phá vỡ trong một nửa và sau đó unskilfully dán lại với nhau. | 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 anything 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