在几百万年前的一段短暂的时间里，地球可能已经脱离了太阳的等离子体保护层，称为日球层，这里描绘的是星际空间背景上的深灰色气泡。根据一项新的澳门威尼斯人注册网站研究，这可能会使地球暴露在高水平的辐射中，并影响气候。图片由Opher等人提供，《自然天文学》

日球层

太阳系可能在200万年前穿过稠密的星际云，改变了地球的气候

在一篇由bu主导的新论文中，天体物理学家计算了地球暴露在寒冷、严酷的星际云中的可能性，这是地质气候模型中以前没有考虑过的现象

大约200万年前，地球是一个非常不同的地方，我们早期的人类祖先与剑齿虎、乳齿象和大型啮齿动物生活在一起。而且它们可能很冷：地球陷入了深度冻结，直到大约12000年前才经历了多次冰河时代。科学家们认为，冰河时代的发生有很多原因，包括地球的倾斜和旋转、板块构造的移动、火山爆发和大气中的二氧化碳水平。但是，如果像这样的剧烈变化不仅是地球环境的结果，也是太阳在银河系中的位置的结果呢？

在《自然天文学》杂志上发表的一篇新论文中，由哈佛大学领导的澳门威尼斯人注册网站研究人员发现了证据，表明大约200万年前，太阳系遇到了一个星际云，它的密度如此之大，以至于可能干扰了太阳的太阳风。他们认为，这表明太阳在太空中的位置可能比之前认为的更能塑造地球的历史。

我们整个太阳系都被太阳发出的等离子体保护层所覆盖，这层保护层被称为日球层。它是由带电粒子流——太阳风——组成的，这些粒子流一直延伸到冥王星之外，将行星包裹在美国宇航局所说的“一个巨大的气泡”中。它保护我们免受可能改变DNA的辐射和银河射线的伤害，科学家们认为这是地球上生命进化的部分原因。根据最新的论文，冷云以这样一种方式压缩了日球层，使地球和太阳系中的其他行星暂时处于其影响之外。

“这篇论文是第一次定量地表明太阳和太阳系外的东西之间的相遇会影响地球的气候，”波士顿大学空间物理学家Merav Opher说，他是日球层专家，也是这篇论文的主要作者。

她的模型确实塑造了我们对日球层的科学理解，以及太阳风是如何通过推动星际介质（我们银河系中恒星之间和日球层之外的空间）形成气泡的。她的理论是，日球层的形状像一个蓬松的羊角面包，这个想法震动了空间物理学界。现在，她对日球层以及太阳在太空中运动的位置如何影响地球大气化学有了新的认识。

“Stars move, and now this paper is showing not only that they move, but they encounter drastic changes,” says Opher, a BU College of Arts & Sciences professor of astronomy and member of the University’s Center for Space Physics. She worked on the study during a yearlong Harvard Radcliffe Institute fellowship. 

Opher and her collaborators essentially looked back in time, using sophisticated computer models to visualize where the sun was positioned two million years in the past—and, with it, the heliosphere and the rest of the solar system. They also mapped the path of the Local Ribbon of Cold Clouds system, a string of large, dense, very cold clouds mostly made of hydrogen atoms. Their simulations showed that one of the clouds close to the end of that ribbon, named the Local Lynx of Cold Cloud, could have collided with the heliosphere. 

If that had happened, says Opher, Earth would have been fully exposed to the interstellar medium, where gas and dust mix with the leftover atomic elements of exploded stars, including iron and plutonium. Normally, the heliosphere filters out most of these radioactive particles. But without protection, they can easily reach Earth. According to the paper, this aligns with geological evidence that shows increased 60Fe (iron 60) and 244Pu (plutonium 244) isotopes in the ocean, Antarctic snow, and ice cores—and on the moon—from the same time period. The timing also matches with temperature records that indicate a cooling period.

“Only rarely does our cosmic neighborhood beyond the solar system affect life on Earth,” says Avi Loeb, director of Harvard University’s Institute for Theory and Computation and coauthor on the paper. “It is exciting to discover that our passage through dense clouds a few million years ago could have exposed the Earth to a much larger flux of cosmic rays and hydrogen atoms. Our results open a new window into the relationship between the evolution of life on Earth and our cosmic neighborhood.”

The outside pressure from the Local Lynx of Cold Cloud could have continually blocked out the heliosphere for a couple of hundred years to a million years, Opher says—depending on the size of the cloud. “But as soon as the Earth was away from the cold cloud, the heliosphere engulfed all the planets, including Earth,” she says. And that’s how it is today. 

It’s impossible to know the exact effect the cold cloud had on Earth—like if it could have spurred an ice age. But there are a couple of other cold clouds in the interstellar medium that the sun has likely encountered in the billions of years since it was born, Opher says. And it will probably stumble across more in another million years or so.

Opher and her collaborators are now working to trace where the sun was seven million years ago, and even further back. Pinpointing the location of the sun millions of years in the past, as well as the cold cloud system, is possible with data collected by the European Space Agency’s Gaia mission, which is building the largest 3D map of the galaxy and giving an unprecedented look at the speed stars move. 

“This cloud was indeed in our past, and if we crossed something that massive, we were exposed to the interstellar medium,” Opher says. The effect of crossing paths with so much hydrogen and radioactive material is unclear, so Opher and her team at BU’s NASA-funded SHIELD (Solar wind with Hydrogen Ion Exchange and Large-scale Dynamics) DRIVE Science Center are now exploring the effect it could have had on Earth’s radiation, as well as the atmosphere and climate. 

“This is only the beginning,” Opher says. She hopes that this paper will open the door to much more exploration of how the solar system was influenced by outside forces in the deep past. 

This research was supported by NASA.