土星环很小，很快就会消失

Cassini mission data suggests that Saturn’s rings are young, possibly only a few hundred million years old, and could disappear in a similar timescale. The rings’ mass, purity, and debris accumulation rates indicate their relatively young age and short lifespan. Two studies show that the rings formed relatively recently and are rapidly losing mass, while a third predicts their disappearance within the next few hundred million years.

While no human could ever have seen Saturn without its rings, in the time of the dinosaurs, the planet may not yet have acquired its iconic accessories – and future Earth dwellers may again know a world without them.

Three recent studies by scientists at NASA’s Ames Research Center in California’s Silicon Valley examine data from NASA’s Cassini mission and provide evidence that Saturn’s rings are both young and ephemeral – in astronomical terms, of course.

The new research looks at the mass of the rings, their “purity,” how quickly incoming debris is added, and how that influences the way the rings change over time. Put those elements together, and one can get a better idea of how long they’ve been around and the time they’ve got left.

Although all four giant planets have ring systems, Saturn’s is by far the most massive and impressive. Scientists are trying to understand why by studying how the rings have formed and how they have evolved over time. Three recent studies by NASA researchers and their partners provide evidence that the rings are a relatively recent addition to Saturn and that they may last only another few hundred million years. Credit: NASA/JPL-Caltech/Space Science Institute

The rings are almost entirely pure ice. Less than a few percent of their mass is non-icy “pollution” coming from micrometeoroids, such as asteroid fragments smaller than a grain of sand. These constantly collide with the ring particles and contribute debris to the material circling the planet. The rings’ age has been hard to pin down, because scientists hadn’t yet quantified this bombardment in order to calculate how long it must have been going on.

Now, one of the three new studies^[1] 它可以更好地了解非冰川物质的总到达率，因此，自环形成以来它一定“污染”了多远。 这项由科罗拉多大学博尔德分校领导的研究还表明，微陨石并不像科学家认为的那样快，这意味着土星的引力可以更有效地将物质拉入环中。 这一证据继续表明，光环暴露在这场宇宙冰雹中的时间不会超过几亿年——这只是土星和太阳系 46 亿年的一小部分。

支持这个结论的是第二篇论文，^[2] 由印第安纳大学领导，该大学对小太空岩石不断敲打环采取了不同的角度。 研究作者确定了研究中基本上被忽视的两件事。 具体来说，他们正在研究控制环长期演化的物理学，发现两个重要的组成部分是微陨石的轰击和这些碰撞产生的碎片在环内的分布方式。 考虑到这些因素表明，环可能在几亿年内达到目前的质量。 结果还表明，由于它们还很年轻，它们很可能是在土星系统内不稳定的引力摧毁了一些冰冷的卫星时形成的。

“土星标志性的大环可能是我们太阳系最近的一个特征的想法一直存在争议，”埃姆斯研究所的研究员、最近一篇论文的合著者杰夫考西说，“但我们的新发现补充了三重使这一发现难以避免的卡西尼测量结果。” 考西还在卡西尼号土星环任务中担任跨学科科学家。

那么，土星在采用现在的形式之前可能已经超过 40 亿年。 但是，您能指望戴上我们今天所熟知的漂亮戒指多久呢？

卡西尼号任务发现，随着物质从行星更深的区域掉落，这些环正在迅速失去质量。 第三篇论文^[3] 此外，在印第安纳大学的领导下，首次确定环状物质向这个方向漂移的速度有多快——陨石再次发挥了作用。 它们与现有环状粒子的碰撞以及由此产生的碎片向外抛出的方式结合在一起，形成了一种运动传送带，将环状物质运向土星。 通过计算所有这些流动的粒子对它们最终消失在地球上意味着什么，研究人员得出了土星的一些坏消息：它可能会在接下来的几亿年内失去它的光环。

“我认为这些结果告诉我们，所有这些外星碎片的持续轰炸不仅会污染行星环，而且随着时间的推移还会削弱它们，”艾姆斯研究所研究员、所有三项研究的合著者保罗埃斯特拉达说。 “或许[{” attribute=””>Uranus’ and Neptune’s diminutive and dark rings are the result of that process. Saturn’s rings being comparatively hefty and icy, then, is an indication of their youth.”

Young rings but – alas! – relatively short-lived, as well. Instead of mourning their ultimate demise, though, humans can feel grateful to be a species born at a time when Saturn was dressed to the nines, a planetary fashion icon for us to behold and study.

References:

“Micrometeoroid infall onto Saturn’s rings constrains their age to no more than a few hundred million years” by Sascha Kempf, Nicolas Altobelli, Jürgen Schmidt, Jeffrey N. Cuzzi, Paul R. Estrada and Ralf Srama, 12 May 2023, Science Advances.
DOI: 10.1126/sciadv.adf8537

“Constraints on the initial mass, age and lifetime of Saturn’s rings from viscous evolutions that include pollution and transport due to micrometeoroid bombardment” by Paul R. Estrada and Richard H. Durisen, 9 May 2023, Icarus.
DOI: 10.1016/j.icarus.2022.115296

“Large mass inflow rates in Saturn’s rings due to ballistic transport and mass loading” by Richard H. Durisen and Paul R. Estrada, 9 May 2023, Icarus.
DOI: 10.1016/j.icarus.2022.115221