实心球和篮球扔起来感觉一样吗?我们生活中判断物体空心和实心通常有两种方法去辨别,分别是测密度、敲打听声。
首先是测密度,NASA官网上的月球密度数据为3.344克/立方厘米,大于地球上岩石沙子的主要成分——二氧化硅。也就是说月球比同大小的实心石球还重,所以怎么可能是空心?顺便说一下比石头重是因为有大量金属包括表面丰富的铝镁钛钙铁等更重的物质拉高密度。
阿波罗月球土壤样本元素丰度,当然不同区域有差别
不过其他行星的月球(天然卫星)倒是可能真的是空心的。例如土卫七(不是土卫六)密度0.544克/立方厘米,低于冰。被认为有较高的孔隙率,也就是说是一堆碎石、碎冰松散的堆叠没压紧,某种程度上可以认为是空心的。
海绵般的土卫七
至于怎么测密度,先测直径,如果太远看不清则可以先测反照率(反射光的能力)和星等(亮度)然后推算出,根据直径得出体积。然后再根据和附近天体的运行轨道(可以说是观测引力影响)情况得出质量,然后就能算密度了。
Earth's Moon - NASA Solar System ExplorationNASA月球数据页面
Hyperion - NASA Solar System ExplorationNASA土卫七数字页面
其次除了测密度了解实心空心外外我们还会通过敲击物体听声音来探测内部是空心还是实心。我们知道声波是振动,阿波罗计划中把地震仪带上了月球,通过检测陨石撞击月球产生的震波来了解月球内部结构,就像是我们敲罐子听响一样。探测结果进一步表明月球是实心的,有熔融的地幔跟坚固的核心。
阿波罗宇航员安置在月球上的地震仪
https://www.lpi.usra.edu/lunar/missions/apollo/apollo_15/experiments/ps/NASA关于阿波罗15号月震学研究结果说明The Passive Seismic Experiment produced several important scientific results:
Knowledge of Lunar Interior Structure. Like Earth, the Moon has a crust, mantle, and core. The lunar crust is rich in the mineral plagioclase and has an average crustal thickness of 60-70 kilometers, which is about 3 times the average crustal thickness on Earth. The lunar mantle lies between the crust and the core and consists mostly of the minerals olivine and pyroxene. The core is probably composed mostly of iron and sulfur and extends from the center of the Moon out to a radius of no more than 450 kilometers; i.e., the core radius is less than 25% of the Moon's radius, which is quite small. In comparison, Earth's core radius is 54% of Earth's radius. However, the size of the lunar core is not well constrained by existing seismic observations. Better constraints come from the laser ranging retroreflector and magnetometer experiments.
Distribution of Lunar Seismic Sources. More than 1700 meteoroid impacts were recorded by the seismometer network, with impactor masses estimated to be between 0.5 and 5000 kilograms. Most moonquakes occur at depths of 800-1000 kilometers. These occur at monthly intervals at about 100 distinct sites, indicating that these moonquakes are caused by stresses from changes in lunar tides as the Moon orbits the Earth. These moonquakes are quite small, mostly with Richter scale magnitudes of less than 2. The amount of energy released by earthquakes in a typical year is about 10 million times larger than that released by moonquakes in a year. Only a few near-surface moonquakes were detected.
Attenuation of Seismic Waves. Meteoroid impacts cause heavy fracturing in the upper 20 kilometers of the lunar crust. These fractures in turn cause scattering of seismic waves in these regions. Below 20 kilometers, seismic-wave scattering decreases as a result of either closure of these fractures due to increasing pressure or of a change in chemical composition of the crust. In the mantle, seismic waves are attenuated much less on the Moon than they are on Earth. Seismic-wave attenuation is enhanced at high temperatures and in the presence of water, and the low attenuation on the Moon indicates a cold, dry interior. Because the Moon is smaller than Earth, it is expected to have cooled more rapidly, producing a cold interior. The absence of water may be due either to the failure of the Moon to accumulate water when it formed or to subsequent loss of water to space. Below 1000 kilometers depth, seismic-wave attenuation increases, possibly indicating the presence of a small amount of molten rock. 好了有疑问请找NASA怼,我只是搬运工,上面都给出链接了。当然相信空心说的估计还认为阿波罗登月都是假的,宁可信路边社、朋友圈啥的谣言阴谋论也不愿信NASA,反正人蠢最后还是自己倒霉,例如容易被忽悠诈骗。
被收录了,补充一张地震波探测天体内部结构的示意图
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