大西洋月刊：星际访客的冰冷秘密 [美国媒体]

The Icy Secrets of an Interstellar Visitor

星际访客的冰冷秘密

The mysterious space object 'Oumuamua may harbor ice under a crust hardened by cosmic radiation.

这个神秘的太空物体“乌穆阿穆阿”可能在其被宇宙辐射硬化的外壳之下藏匿着一块冰体。



To telescopes, ‘Oumuamua, the interstellar asteroid that made itself known to Earth in October, looks like a point of light in the dark, much like a star in the night sky—a perhaps underwhelming picture of a significant discovery.

在望远镜上，乌穆阿穆阿这颗在10月被发现的星际小行星所地球上的人们所知，它看起来就像是在黑暗中发光的一个小点，很像是夜空中的一颗星星——这也许是一幅无法给人留下深刻印象的重大发现的画面。

But for astronomers, the tiny speck—the sunlight reflected by the asteroid—can reveal a trove of information. They can break down the light from an object into a spectrum of individual wavelengths, from which they can infer the object’s shape, chemical composition, and other properties. As astronomers like to say, if a picture is worth a thousand words, then a spectrum is worth a thousand pictures.

但是对于天文学家来说，由小行星反射的微小的斑点可以反映出大量的信息。他们可以将一个物体的光分解成一张拥有各种波长的光谱，从中可以推断出物体的形状、化学成分和其他的性状。正如天文学家所喜欢说的，如果一张图片胜过千言万语，那么一张光谱就胜过了一千张图片。

The astronomy community has spent weeks sorting through these pictures of ‘Oumuamua, captured by telescopes around the world as the asteroid sped away from the sun and faded from view. The earliest analysis of the light from ‘Oumuamua, conducted by its discoverers in Hawaii, revealed a strange, fast-spinning, cigar-like object unlike anything they’ve ever seen. The latest analyses continue to produce tantalizing results, further challenging long-standing predictions for the first visitor to our solar system.

天文学团体花了几个星期的时间来整理这些图片，这些照片是由世界各地的天文望远镜在小行星转离太阳后逐渐消失的的过程中拍摄的。由夏威夷的发现者进行的最早的分析发现它是一个奇怪的、快速旋转的、类似于雪茄的物体，这与他们之前所见过的任何东西都不一样。最新的分析继续发现了令人激动的结果，从而进一步挑战了对第一个到我们太阳系的访客的存在已久的预测。

‘Oumuamua has a thick crust of carbon-rich material, hardened by years of exposure to cosmic radiation in interstellar space, that could be protecting an icy interior, according to a new analysis in Nature Astronomy of the object in visible and near-infrared wavelengths. The coating could explain why ‘Oumuamua shows no signs of being a comet, the kind of object scientists long expected would coast into our solar system.

根据一项自然天文学中对可见光和近红外波长光谱的分析，乌穆阿穆阿有一层厚厚的富含碳的物质，它在星际空间的宇宙辐射中暴露了多年，可能起到了保护一个冰冷内部核心的作用。这种涂层可以解释为什么乌穆阿穆阿没有显现出彗星的迹象——科学家们长期以来一直认为进入我们的太阳系中的物体会是彗星。

When the asteroid was first spotted by the Pan-STARRS telescope in Hawaii, ‘Oumuamua was already speeding away from the sun and quickly fading from view. Scientists had only about two weeks to deploy telescopes to soak up the reflected light before the interstellar visitor got too far away. They looked for a coma, a stream of evaporated particles that trails comets as they pass near the sun and their icy contents become heated. ‘Oumuamua had made a fairly close pass to the sun—about a quarter of the distance between the sun and Earth—and telescopes were prepared to spot as little as a sugar cube’s worth of material flying off the object every second.

当第一次被位于夏威夷的泛星计划望远镜发现时，乌穆阿穆阿已经在远离太阳了，并且很快就从人们的视线中消失了。科学家们只有大约两周的时间来使用望远镜观测反射而来的光线，直到星际游客走得实在太远。他们寻找的是彗尾，这是一股蒸发的粒子，当彗星在靠近太阳的地方经过时，它们的冰冷的物质会被加热。乌穆阿穆阿与太阳的距离一度很近，大约是太阳和地球之间距离的四分之一，望远镜也准备好了观测每秒从这个物体上飞离的一块方糖大小的物质。

“We had data that matched pretty closely with what we’d expect for a body out there,” said Alan Fitzsimmons, an astronomer at Queen’s University Belfast who led the new analysis. “And yet we saw no sign of ice being heated and ejected into space.”

“我们的数据与我们对出现在那里的物体的期望值非常接近，” 领导了这项新的分析工作的贝尔法斯特女王大学的天文学家Alan Fitzsimmons如道，“但是我们没有看到任何冰被加热并被喷射到太空中的迹象。”

‘Oumuamua didn’t show signatures of ice or minerals found in rock, which means it’s neither icy nor rocky, at least not exactly. But it did show signs of carbon compounds. Fitzsimmons said previous studies have revealed that when carbon-rich, comet-like objects are exposed to the radiation that would be found in interstellar space, the material forms a crust that acts as insulation. If ‘Oumuamua has ice, as a comet would, it may be hiding beneath a mantle half a meter thick, formed after hundreds of millions—perhaps even billions—of years of bombardment by high-energy particles.

乌穆阿穆阿没有显示在岩石中发现的冰体或矿物的特征，这意味着它既不是冰体，也不是岩石，至少不完全是。但它确实显露出了碳化合物的迹象。Fitzsimmons说，之前的研究表明，当碳含量高的类似彗星的物体暴露在星际空间中时，这些物质会形成一种类似绝缘体的外壳。如果乌穆阿穆阿就像彗星一样存在冰体，在经过几亿年甚至是数十亿年的高能粒子轰击后，它可能隐藏在半米厚的地幔之下。

Fitzsimmons and his colleagues say ‘Oumuamua’s crust may have been able to prevent heat from the sun from penetrating the surface and vaporizing ice particles. According to their thermal models, any ice buried 30 centimeters (12 inches) deep would have remained intact even as the surface of the asteroid reached temperatures of about 600 degrees Kelvin (620 degrees Fahrenheit) during its pass of the sun.

Fitzsimmons和他的同事们说，乌穆阿穆阿的地壳可能能够阻止太阳的热量穿透表面，从而使冰粒子蒸发。根据他们的热模型，即便是在这颗小行星从太阳旁通过，其表面温度达到大约600绝对温度(620华氏度)时，任何被埋在30厘米(12英寸)深处的冰都将保持完整。

“Comets are absolutely excellent insulators,” said Karen Meech, one of ‘Oumuamua’s discoverers at the University of Hawaii, who was not involved in this new analysis.“They’re very fluffy and porous, like a down jacket.”

“彗星是非常好的绝缘体，”，夏威夷大学中乌穆阿穆阿的发现者之一Karen Meech——他没有参与这项新的分析工作——说道，“它们很蓬松，很多孔，就像一件羽绒服。”

Scientists will likely never know for sure what lies inside ‘Oumuamua. When astronomers study the light reflected from the asteroid, they’re only examining the top few microns of its surface, a width smaller than that of a human red blood cell. “We can infer only so much about these little dots of light,” said Michele Bannister, an astronomer at Queen’s University Belfast and a coauthor on the new study.

科学家们可能永远也不会确切地知道乌穆阿穆阿内部是什么。当天文学家研究小行星上反射的光线时，他们只是检查到了其顶层几微米厚的范围，这比人类红细胞的宽度都要小。“我们可以推断出这些小点的光，”贝尔法斯特女王大学的天文学家、也是这项新研究的合着者Michele Bannister说道。

Astronomers also have a finite amount of data to work with. Most of their ground-based telescope observations were carried out in the two weeks after ‘Oumuamua was discovered, when it was still visible enough to get good measurements. The majority of research on ‘Oumuamua—the attempts to explain its weird shape, its nonstop spinning, its unexpected hardiness—will come out of this data set.

天文学家可以处理的数据非常有限。他们的大部分地面望远镜观测工作都是在乌穆阿穆阿被发现后的两周内进行的，当时它仍然足够显眼，可以获得不错的测量数据。大多数关于乌穆阿穆阿的研究成果——它们试图解释它怪异的形状、它不停旋转的特征，它意想不到的硬度——都将从这个数据中产生。

The rest may come from yet-to-be published observations by space telescopes like the Hubble, which has tracked ‘Oumuamua to help astronomers better understand its trajectory and where it came from. The asteroid is now about twice the distance between the Earth and the sun from our planet. Even to Hubble, the producer of countless, radiant images of distant stars and galaxies, ‘Oumuamua, a small object about 400 meters long, will look like a small point of light.

剩下的部分可能就要来自尚未公布的太空望远镜——如哈勃望远镜——的观测结果了，它追踪了乌穆阿穆阿，以帮助天文学家更好地了解它的轨迹和它的起源。这颗小行星现在与地球的距离是地球和太阳距离的两倍。即使是在拍摄了无数的遥远恒星和星系的辐射图像的哈勃望远镜看来，乌穆阿穆阿这个大约400米长的小物体看起来也就像是一个小光点。

“It could be icy inside, and we’ll never know,” Meech said.

“里面可能存在冰体，但我们永远也不会知道，”Meech如此说道。