Dark Terrain on Saturn's Iapetus
影像提供： Cassini Imaging Team, SSI, JPL, ESA, NASA
Explanation:
Why are vast sections of Iapetus as dark as coal? No one knows for sure. Iapetus, the third largest moon of Saturn, was inspected again as the Saturn-orbiting robot Cassini spacecraft swooped past the enigmatic world again late last year. The dark material covers most of the surface visible in the above image, while the small portion near the top that appears almost white is of a color and reflectance more typical of Saturn's other moons. The unknown material covers about half of the 1,500 kilometer wide moon. The material is so dark that it reflects less than five percent of incident sunlight, yet overlays craters indicating that it was spread after the craters were formed. Iapetus has other unexplained features. The bright part of Iapetus is covered with unexplained long thin streaks. The orbit of Iapetus is also unusual, being tilted to the plane of Saturn's orbit by an unusually high fifteen degrees. A strange ridge about 13 kilometers high crosses much of Iapetus near the equator and is visible near the bottom. Oddly, this ridge is almost exactly parallel with Iapetus' equator. The exact shape of Iapetus remains undetermined, but images indicate that it is quite strange -- something like a walnut. Research into the formation and history of mysterious Iapetus is active and ongoing.
說明:
為什麼雅特佩斯空曠的大地看來黑的和一團煤球一樣？沒人肯定的知道。雅特佩斯，土星的第三大衛星，再度被繞土運行的卡西尼號無人太空船重新調查，時間在去年對這個充滿迷團的衛星的一次接近觀測裡。在可見地表覆蓋的黑暗物質如上圖所示，在頂端一部份的白色是從土星其他衛星反射過來的光。這種未知的物質覆蓋這顆 1,500 公里大小的衛星，大約一半的地表。這種物質是這麼的黑暗，因此只反射不到 5％的入射光，而覆蓋在隕石坑上也指出這種物質是在隕石坑形成後才散佈的。 亞特佩斯還有另一個難以解釋的特徵。亞特佩斯的亮部覆蓋了難以解釋的細長條紋。 亞特佩斯的軌道也很奇怪，比土星的軌道面高出不尋常的 15度。一座大約 13公里高，在赤道附近的奇異山脈幾乎橫跨整個亞特佩斯，在影像的下方可以看到。 怪的是，這個山脈幾乎完全與亞特佩斯的赤道平行。亞特佩斯的確切形狀仍未決定，不過影像指出它相當的怪異 -- 有時像個胡桃。 關於神秘亞特佩斯的形成與歷史的研究正方興未艾的進行中。
資料來源: Scientific American
Department of Physics, NCKU
