星际影响：美国国家航天局（NASA）研究揭示太阳的古老历史如何塑造地球

在太阳系（solar system）中心，太阳影响着每一颗绕其运行的行星（planet）。在两项近期由美国国家航天局（NASA）资助的研究中，科学家们揭示了太阳历史上的一些古老事件可能如何帮助创造了地球独特的环境，并驱动了此前无法解释的气候变化。

At the center of our solar system, the Sun influences every planet that orbits it. In two recent NASA-funded studies, scientists uncovered how ancient events in the Sun’s history may have helped create Earth’s unique climate and driven previously unexplained climatic shifts.

在新的研究中，NASA SHIELD（太阳风与氢离子电荷交换及大规模动力学）中心——NASA DRIVE（多元化、实现、整合、创业、教育）科学中心之一——的科学家们追踪了日球层（heliosphere）的轨迹。日球层是由太阳创造并包裹着太阳系的巨大气泡，它在银河（Milky Way）中移动时影响了地球的气候。在另一篇论文中，一位NASA科学家及其合著者研究了年轻且暗淡的太阳如何通过播种强效温室气体的产生来加热地球。

In new research, scientists at NASA’s SHIELD (Solar Wind with Hydrogen Ion charge Exchange and Large-Scale Dynamics) center — one of NASA's DRIVE (Diversify, Realize, Integrate, Venture, Educate) Science Centers — trace the trajectory of the heliosphere, the massive bubble created by our Sun that envelops our solar system, as it moved through our galaxy and influenced Earth’s climate along the way. In another paper, a NASA scientist and coauthors investigate how the younger, dimmer Sun managed to heat Earth by seeding the production of potent greenhouse gases.

移动中的太阳

A Sun on the move

在过去几千万年（million years）里，地球气候经历了显著变化，包括全球平均气温暂时下降几度的显著冰期（ice age）。在这些时期，更频繁的气候波动导致地球变暖和变冷。为了解释这些变暖和变冷时期，科学家们研究了地球内部的因素，包括轨道变化、温室气体和冰。但新的研究表明，太阳环境的变化可能是理解地球气温波动的关键。

Over the last tens of millions of years, Earth’s climate has undergone significant shifts, including notable ice ages in which the global average temperature temporarily dropped by several degrees. During these periods, more frequent climate swings led Earth to warm and cool. To explain these periods of warming and cooling, scientists looked to factors internal to Earth, including orbital changes, greenhouse gases, and ice. But new research suggests changes to the Sun’s environment may be key to understanding Earth's temperature swings.

正如我们的行星被大气层包裹一样，我们的整个太阳系也被太阳创造的一种“大气层”所包裹。这个被称为日球层的保护性气泡，是由太阳向四面八方喷射的荷电粒子（charged particle）组成的持续太阳风（solar wind）形成的。

Just as our planet is encased by an atmosphere, so our entire solar system is encased inside a kind of “atmosphere” created by the Sun. This protective bubble, known as the heliosphere, is formed by a continuous solar wind of charged particles streaming out from the Sun in all directions.

我们的日球层绕着银河中心运行。在太阳46亿年的存在过程中，我们的日球层穿过了银河系内的不同区域。在8月21日发表于《天文学和天体物理学年评》（Annual Review of Astronomy and Astrophysics）的一篇论文中，NASA SHIELD的研究人员利用计算机模型（computer model）对日球层穿过银河系的路径进行了逆向工程，揭示了它所经过的环境可能触发了地球上的变化。

Our heliosphere orbits around the center of our galaxy, the Milky Way. Throughout the Sun's 4.6-billion-year existence, our heliosphere has traversed various regions within our galaxy. In a paper published on Aug. 21 in Annual Review of Astronomy and Astrophysics, researchers at NASA’s SHIELD used computer modeling to reverse-engineer the path of the heliosphere through our galaxy, revealing that the environments it passed through may have triggered changes on Earth.

波士顿大学SHIELD的首席研究员Merav Opher和她的团队运行了模拟，显示太阳在过去几百万年里至少三次遇到了寒冷的气体和星际尘埃（interstellar dust）区域。在这些情况下，巨大的星际“冷云”挤压日球层，使其缩小到比地球轨道还小，从而将我们的行星置于太阳保护盾之外。

Merav Opher, SHIELD’s principal investigator at Boston University, and her team ran simulations that showed the Sun has encountered frigid expanses of gas and dust at least three different times in the past few million years. In these instances, massive interstellar “cold clouds” pushed against the heliosphere to such an extent that it shrank to smaller than Earth’s orbit, stranding our planet outside the Sun’s protective shield.

这些暴露事件——分别发生在约200万至300万年前、600万至700万年前以及1300万至1400万年前——使地球大气层暴露在完全不同的环境中。模拟结果与地质证据相吻合：在这些时间段内，深海沉积物岩芯样本、南极雪和月球样本中出现了星际尘埃中常见的元素。

These exposures — approximately 2 to 3 million years, 6 to 7 million years, and 13 to 14 million years ago — would have exposed Earth’s atmosphere to totally different surroundings. The simulation results match geologic evidence: Elements prevalent in interstellar dust appear in deep-sea sediment core samples, Antarctic snow, and lunar samples during these timelines.

这些日球层坍缩事件也可能解释地球上古老的气候模式。在模拟中，当地球大气层暴露在寒冷、致密的银河氢云（hydrogen cloud）中时，它增加了水蒸气含量并改变了高层大气的动力学，最终改变了地表的条件。总之，我们的日球层在银河系较冷区域的旅行可能是驱动地球一些古老气候变化（包括可能的冰期）的关键因素。

These heliosphere collapse events may also explain ancient climatic patterns on Earth. In the simulations, when Earth’s atmosphere was exposed to a cold, dense galactic hydrogen cloud, it increased water vapor content and shifted upper-atmospheric dynamics, ultimately altering the conditions at the surface. In summary, our heliosphere’s trips through colder regions in our galaxy may be a key factor in driving some of Earth's ancient changes in climate, including possible ice ages.

日球物理学研究的新前沿

Next frontier in studying heliophysics

SHIELD中心是NASA资助的旨在开启下一代日球层研究的多个中心之一。作为一个DRIVE科学中心，SHIELD建立了一个由具有不同专业知识、方法和观点的研究人员组成的团队，以开发日球层的模型或“数字孪生”，帮助揭示日球层如何与其周围环境（包括致密的星际云）相互作用。了解我们独特且宜居的太阳系将有助于解开地球生命演化的奥秘，并可能发现其他宜居的恒星系统（star system）。

The SHIELD center is one of several that NASA funds to unlock the next generation of heliospheric research. As a DRIVE Science Center, SHIELD builds a team of researchers with differing expertise, approaches, and opinions to develop a model, or “digital twin,” of the heliosphere that helps reveal how the heliosphere interacts with its surroundings, including dense interstellar clouds. Understanding our unique, habitable solar system will help unravel the mysteries of life’s evolution on Earth and potentially uncover other habitable star systems.

年轻的太阳

Young Sun

在另一篇论文中，马里兰州格林贝尔特NASA戈达德航天中心（Goddard Space Flight Center）的科学家Vladimir Airapetian专注于一个长期存在的谜团：古老的太阳是如何加热早期地球（early earth）以维持生命的。30亿年前，年轻的太阳亮度仅为今天的70%。在这些较暗淡的条件下，地球本应完全冻结。然而，地质证据表明，早在那个时候，稳定的液态水就已经存在了。这个谜题——在较冷、较暗淡的太阳下地球却气候温和——被称为“黯淡太阳悖论”。

In another paper, Vladimir Airapetian, a scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, focuses on a long-standing mystery of how the ancient Sun warmed early Earth enough to sustain life. Three billion years ago, the young Sun was 70% as bright as it is today. Under these dimmer conditions, Earth should have been frozen solid. Yet geological evidence shows stable liquid water already existed long before that. This puzzle — a balmy Earth under a cooler, dimmer Sun — is known as the Faint Young Sun paradox.

解决该悖论的一条线索来自银河系中其他年轻的类太阳恒星（Sun-like star）。这些“蹒跚学步”的恒星容易发脾气。具体来说，来自NASA已退役的开普勒（Kepler）空间望远镜（space telescope）的数据显示，年轻的类太阳恒星经常爆发巨大的超级耀斑，每天向四面八方抛射高能粒子（high-energy particle）。Airapetian提出，如果我们的年轻太阳像这些恒星一样，那么高能太阳粒子的轰击可能触发了对加热早期地球至关重要的化学反应。

One clue to resolving the paradox comes from young Sun-like stars elsewhere in the galaxy. These “toddler” stars are prone to throwing fits. Specifically, data from NASA’s retired Kepler space telescope shows that young Sun-like stars regularly erupt with massive superflares, flinging high-energy particles in all directions on a daily basis. If our young Sun was like these other stars, Airapetian proposes, the barrage of high-energy solar particles could have triggered chemical reactions that were key to warming early Earth.

Airapetian的团队在一个密封舱中模拟了早期地球的大气层，混合了分子氮、氨、二氧化碳（carbon dioxide）和一氧化碳（carbon monoxide）。然后，他们向混合物中发射质子，模拟超级耀斑带来的粒子冲击。这种质子轰击触发了多种变化，包括产生了一氧化二氮，这是一种比二氧化碳强300倍的温室气体。该研究发表在《天体物理学报通信》（Astrophysical Journal Letters）上。

Airapetian's team simulated early Earth's atmosphere in a sealed chamber, mixing molecular nitrogen, ammonia, carbon dioxide, and carbon monoxide. They then fired protons into the mixture, simulating the onslaught of particles from superflares. This proton bombardment triggered several changes including the production of nitrous oxide, a greenhouse gas 300 times more potent than carbon dioxide. The research was published in Astrophysical Journal Letters.

这些一氧化二氮（nitrous oxide）可能有助于地球保持热量。但并非所有的一氧化二氮都能留存下来。年轻太阳强烈的紫外辐射（ultraviolet radiation）会分解其中一部分，将分子重新分裂为氮气和氧气。然而，Airapetian团队的计算机模拟（computer simulation）证实，即使实验中观察到的一氧化二氮只有10%存留下来，它依然能将地球赤道地区加热到约41华氏度（5摄氏度），即高于水的冰点。这种较少量的一氧化二氮甚至可能加速前生物合成：研究发现，略高于冰点的温度比更温暖的温度在构建复杂的氨基酸（amino acid）链方面效率更高。

This nitrous oxide could help Earth hold onto heat. But not all the nitrous oxide would last. The young Sun’s intense ultraviolet radiation would break some of it down, splitting the molecule back into nitrogen and oxygen. But even if only 10% of the nitrous observed in the experiment survived, Airapetian's team’s computer simulations confirmed, it would still warm Earth’s equatorial regions to about 41 degrees Fahrenheit (5 degrees Celsius), above water’s freezing point. This smaller amount of nitrous could even accelerate prebiotic synthesis: just-above-freezing temperatures have been found to be more efficient for building complex chains of amino acids than warmer temperatures.

发掘我们恒星-行星系统的秘密

Unearthing secrets of our star-planet system

这两项研究共同表明，太阳可能对地球产生令人惊讶的影响。虽然我们的行星在许多方面是独特的，但它是在恒星-行星系统（star-planet system）中形成并始终作为其一部分存在的。理解这种独特的关系有望为地球以及维持其生命的恒星提供新的见解。

Together, these two studies show that the Sun can lead to surprising implications for Earth. While our planet stands alone in many ways, it was formed and has always existed as part of a star-planet system. Understanding that unique relationship promises new insights about both Earth and the star that sustains it.

作者：Desiree Apodaca 和 Miles Hatfield

By Desiree Apodaca and Miles Hatfield

美国国家航天局（NASA）戈达德航天中心（Goddard Space Flight Center），马里兰州格林贝尔特。

NASA’s Goddard Space Flight Center, Greenbelt, Md.