小行星:太阳系的时间胶囊
2024-08-01 06:17阅读:

想象一下,我们打开一个时间胶囊,希望了解远古时代。但这次它不是一个盒子或箱子,而是一颗小行星。它可以让我们了解地球上生命的开端。
这就是使用先进光源设施的研究人员所面对的情况。先进光源是美国能源部科学办公室的设施。在那里工作的团队看过很多不同寻常的事物。但哪怕对这个团队来说,来自小行星的样本也是不同寻常的。幸运的是,先进光源设施提供的创新工具能够支持科学家深入研究这些太空岩石的历史。
就像研究地球上的岩石可以让我们了解地球早期历史一样,研究小行星、陨石和彗星等原始小天体可以让我们了解太阳系的历史。
球粒状陨石是一种特别有用的陨石。它们未分化,从化学角度看很原始。其内的岩石可以追溯到早期太阳系的尘埃和小颗粒,它们聚集在一起形成了一个巨大的母体。
某种类型的球粒状陨石(被称为碳质球粒状陨石)保存了相对丰富的易蒸发化学物质,包括碳和水。它们是构成地球上生命的要素。通过研究这些被保存下来的物质,科学家可以探究人类面对的一个基本问题:“我们从哪里来?”
使用先进光源设施的团队研究了采集自碳型小行星“龙宫”表面的样本。他们估计这颗小行星与碳质球粒状陨石类似。与位于火星和木星之间的主小行星带内的小行星相比,“龙宫”距离地球比较近。
科学家假设“龙宫”是一颗“碎石堆”小行星。他们认为,它的形成原因是,一个物体撞击它的母体,之后喷射出来的岩石重新合并成一颗新的小行星。在这个过程之后,这颗小行星从主小行星带移动到了近地轨道。
2019年,日本宇宙航空研究开发机构的“隼鸟2”号小行星探测器从“龙宫”表面的两处地点采集了样本,并于2020年将其带回地球。该机构共带回了5.4克样本。该机构将一小部分样本分配给“隼鸟2”号初始分析团队。该团队由全球约400名科学家组成。
小行
星颗粒的超薄切片被送到先进光源设施。先进光源设施使科学家能够精确地识别材料中的元素和分子。它使用粒子加速器产生异常明亮的X射线束。它们揭示了物体内容物的信息。
首先,研究团队用X射线仔细地扫描了水平地排成一行行的样本。通过测量扫描过程中X射线的变化,科学家可以识别出小行星样本内的单个有机物颗粒。这些颗粒的大小仅为脱氧核糖核酸链的100倍。
一旦科学家识别出令他们感兴趣的颗粒,他们就用X射线来揭示有机碳颗粒中化学键的类型。这一次,研究人员利用这个过程将样本中的各种元素和官能团(原子的特定排列方式)绘制成图。
基于这一分析结果,科学家发现了四种不同类型的碳化合物和不同类型的结构。在识别出这些物质后,科学家将它们与类似的陨石进行比较。后者的历史是他们已经知道的。
将所有这些数据拼凑在一起后,他们得以勾勒出这颗小行星在太阳系早期的大致历史。样本含有有机碳表明,“龙宫”的有机物是以下过程的产物:其前体在与该小行星母体上的液态水发生化学反应时产生了变化。
样本中的碳同位素反映出,这些有机物前体来自极冷的太空环境(温度约为零下200摄氏度)。该团队首次证明,碳质小行星中的有机物与原始碳质球粒状陨石中的类似有机物之间存在直接联系。
研究来自“龙宫”小行星的物质不是科学家第一次、可能也不是最后一次使用先进光源设施来仔细研究太空岩石。研究人员曾用它来分析美国国家航空航天局(NASA)“星尘”探测器2006年从维尔特二号彗星采集的尘埃颗粒。
这些研究展示了一些工具和技术。事实已经证明,这些工具和技术对分析其他任务(比如NASA“冥王”号探测器)采集的样本也很有用。“冥王”号采集了小行星“贝努”的样本,并于2023年秋天将样本带回地球。
With X-Ray Analysis, an Asteroid Provides a View into Our
Solar System's Past
Shannon Brescher Shea
Imagine opening a time capsule, hoping to learn about the ancient
past. Except, instead of a box or a chest, it’s an asteroid that
could provide insights into the very dawn of life on Earth. That
was the situation that researchers using the Advanced Light Source
(ALS) faced. As the ALS is a Department of Energy (DOE) Office of
Science user facility, the team that works there sees a lot of
unusual items, from materials for solar cells to particles
influenced by wildfires. But even for this crew, a sample from an
asteroid was unusual. Fortunately, the innovative tools available
at the ALS allowed them to support scientists digging into the
history of these rocks delivered from space.
Just like studying rocks on Earth can tell us about Earth’s early
history, studying primitive small bodies such as asteroids,
meteorites, and comets can tell us about our solar system’s
history. Chondrites are a particularly useful type of meteorite.
They are undifferentiated and chemically primitive. The rocks in
them trace back to dust and small grains in the early solar system
that came together to form a large parent body. A certain type of
chondrites (called carbonaceous chondrites) preserve relatively
abundant chemicals that are easily vaporized, including carbon and
water. These are the building blocks of life on Earth. By studying
these preserved materials, scientists can investigate one of
humanity's fundamental questions: “Where did we come from?”
The team using the ALS examined a sample from the surface of a
carbonaceous-type asteroid, Ryugu. They expected this asteroid to
be similar to carbonaceous chondrite meteorites. Ryugu is
relatively close to Earth, compared to asteroids in the main belt
between Mars and Jupiter. Scientists hypothesize that Ryugu is a
rubble-pile asteroid. They think that it formed when an object hit
its parent body and then the rocks that were ejected re-coalesced
into a new asteroid. After that process, the asteroid moved from
the main belt to near-Earth orbit.
The Japan Aerospace Exploration Agency (JAXA)’s spacecraft,
Hayabusa2, collected samples from two locations on the surface of
Ryugu in 2019 and returned them to Earth in 2020. The curatorial
work at JAXA found a total of 5.4 g of sample. The agency allocated
a small portion of the sample to the Hayabusa2 initial analysis
team, consisting of about 400 scientists around the world. Hikaru
Yabuta at Hiroshima University led one of six sub-teams of the
initial analysis team.
Ultrathin sections of the asteroid particles arrived at the ALS at
DOE’s Lawrence Berkeley National Laboratory. The ALS allows
scientists to precisely identify the elements and molecules inside
materials. It uses a particle accelerator to produce
extraordinarily bright X-ray beams. Like the X-rays at a doctor’s
office, they reveal information about what is inside an object. But
instead of just highlighting bones, these X-rays allow scientists
to probe the chemical and structural properties of the matter
itself.
First, the team carefully scanned the sample in long horizontal
rows—like text in a book—with X-rays. By measuring how the X-rays
change as the scanning happens, scientists could identify
individual grains of organic material in the asteroid sample. These
grains were tiny – only 100 times bigger than a strand of DNA.
Once the scientists identified grains of interest, they used X-rays
to reveal the type of chemical bonds in the organic carbon grains.
In this case, the researchers used the process to map out the
various elements and functional groups (specific arrangements of
atoms) in the sample.
Based on this analysis, the scientists found four different types
of carbon compounds as well as different types of structures. After
identifying these materials, the scientists compared them to
similar meteorites that they already knew the history of.
Piecing together all of this data allowed them to outline a broad
history of the asteroid during the early solar system, which formed
about 4.6 billion years ago. The chemical compositions of the
organic carbon in the samples indicated that Ryugu’s organic matter
resulted from the precursors to that matter changing during a
chemical reaction with liquid water on the asteroid’s parent body.
The isotopes of carbon in the samples reflected that the organic
precursors came from the extremely cold environment of space (about
-200 °C). The team was the first to prove the direct link between
organic matter in the carbonaceous asteroid and the similar organic
matter in primitive carbonaceous chondrites (meteorites).
There was one type of material notably missing – graphite. Graphite
is a familiar form of carbon used in pencil leads. In asteroids,
graphite or graphite-like material is a sign that the carbon was
formed by radiogenic heating in parent bodies for several million
years. The lack of it suggests that the sample collected from the
asteroid was never exposed to heat above 390 °F (200 °C).
Studying the material from Ryugu wasn’t the first or likely the
last time that scientists will use the ALS to take a close look at
rocks from space. Researchers used the ALS to analyze dust
particles from the comet 81P/Wild 2 collected by NASA’s spacecraft
Stardust in 2006. They found that the comet dust contained organic
matter. This matter was composed of nitrogen- and oxygen-bearing
chemical bonds as well as types of organic matter similar to that
observed from the asteroid Ryugu and other chondritic
meteorites.
These studies demonstrated tools and techniques that have proven
useful for analyzing samples like those from NASA’s OSIRIS-REx
mission. This mission collected samples from the asteroid Bennu. In
the fall of 2023, it returned them to Earth. The agency recently
released a catalog of samples for scientists to study.
The ALS and other light sources allow us to draw lines from the
earliest history of our solar system to today. Through shedding
light on the objects in our current solar system, the DOE Office of
Science scientists and user facilities may one day help us better
understand how Earth became habitable.