Astronomy Picture of the Day—20190223

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2019February 23

Phobos:Doomed Moon of Mars

ImageCredit: Viking Project, JPL, NASA; Mosaic Processing: Edwin V. Bell II(NSSDC/Raytheon ITSS)

Explanation:This moon is doomed. Mars, the red planet named for the Roman god of war, hastwo tiny moons, Phobos and Deimos, whose names are derived from the Greek forFear and Panic. The origin of the Martian moons is unknown, though, with aleading hypothesis holding that they are captured asteroids. The larger moon,at 25-kilometers across, is Phobos, and is indeed seen to be a cratered,asteroid-like object in this false-colored image mosaic taken by the roboticViking 1 mission in 1978. A recent analysis of the unusual long grooves seen onPhobos indicates that they may result from boulders rolling away from the giantimpact that created the crater on the upper left: Stickney Crater. Phobosorbits so close to Mars - about 5,800 kilometers above the surface compared to400,000 kilometers for our Moon - that gravitational tidal forces are draggingit down. The ultimate result will be for Phobos to break up in orbit and thencrash down onto the Martian surface in about 50 million years. Well before that-- tomorrow, in fact, if everything goes according to plan -- NASA's roboticInSight lander will touch down on Mars and begin investigating its internalstructure.

注定灭亡的火卫一

解释：这颗卫星注定灭亡。火星，这颗红色行星以罗马战神的名字命名，它有两颗小卫星火卫一和火卫二，名字分别来源于希腊语中的害怕和恐慌。火卫的起源仍然未知，一种流行的假设认为它们是被捕获的小行星。火卫一是较大的火卫，直径约25公里。在1978年海盗1号拍摄的这幅伪彩色拼接影像中，它看上去确实像是一颗遍布陨石坑的小行星状天体。对火卫一上这条极长沟槽的分析表明，它们可能来自在左上方产生斯蒂科尼陨石坑的巨大撞击中滚离的巨石。火卫一非常靠近火星，距离火星表面仅约5800公里，潮汐力正把火卫一往下拖拽，最终将导致火卫一在轨道上解体，然后在大约5千万年之后坠毁于火星表面。