The closest planet to our sun is shrinking. Here’s what that means

Telltale wrinkles that cover Mercury, the closest planet to the sun, suggest it was once a larger world that has shrunk in size — and it may be shrinking more than scientists realized.

The smallest planet in our solar system is about 36 million miles (58 million kilometers) from the sun on average and completes one orbit around our star every 88 days, according to NASA .

The rocky world formed from swirls of gas and dust roughly 4.5 billion years ago. The early days of our solar system were chaotic, marked by violent interactions as a multitude of space rocks orbiting the sun crashed into other objects.

Newly formed planets are already hot, much like volcanic lava, but the energy from such collisions generate even more heat. As the heat escapes these worlds over time, the interiors of planets such as Mercury shrink.

Mercury’s cracked surface bears scars from such shrinkage — scientists call these ridges and cliffs shortening structures. Some of the cliffs can reach up to 1 mile (1.6 kilometers) in height and can extend for hundreds of miles in length.

But asteroids and comets have also bombarded Mercury, riddling it with impact craters, much like the pockmarked surface of Earth’s moon.

Debris from the impacts might have hidden just how much Mercury has contracted over time, according to a new study published Thursday in the journal Geophysical Research Letters .

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Understanding Mercury’s shrink rate can shed light on how the planet has evolved and reveal secrets about its mysterious interior.

“There are lessons about the formation and evolution of large, rocky bodies like our Earth, that can be learned on Mercury better than anywhere else in our Solar System,” wrote Dr. Hannes Bernhardt, who was not involved in the study, in an email. Bernhardt is an assistant research scientist in the department of geological, environmental and planetary sciences at the University of Maryland, College Park.

The new study also highlights just how many questions linger about this tiny world — some of which could soon be answered by an unprecedented mission that will release two orbiters around Mercury in November.

As Mercury shrank, cracks likely appeared across its surface in a consistent way. But craters and debris from billions of years of impacts covered many of these fissures, said lead study author Gaku Nishiyama, a planetary scientist at the German Aerospace Center’s Institute of Space Research in Berlin.

Mercury’s proximity to the sun has made it a challenging planet to study and explore. Only two missions have ventured into this harsh environment in previous decades: NASA’s Mariner 10 in 1974 and the agency’s MESSENGER mission in 2011.

Mariner 10 conducted flybys of the planet that revealed Mercury’s intriguing wrinkles, while MESSENGER provided a more global portrait of the planet’s features, said Bernhardt, who has also studied Mercury’s shrinking.

Nishiyama and his team used data from MESSENGER to create a global map of Mercury’s surface roughness. They spotted a trend of fewer wrinkles in the roughest parts of Mercury, compared with smoother regions of the surface.

“It made us think that there’s a process obscuring shortening structures,” Nishiyama said, such as debris cloaking signs of contraction on the surface.

The team estimated how many wrinkles might be present across Mercury’s surface if they weren’t hidden by debris. The computations determined that the planet may have shrunk 10% to 30% more than previously believed, or a radius change of about 7.2 miles (11.6 kilometers). Previous studies have suggested a change in radius between 0.6 to 1.2 miles (1 to 2 kilometers) and up to 4.3 miles (7 kilometers).

Dr. Paul Byrne, associate professor of Earth, environmental and planetary sciences at Washington University in St. Louis, said it’s plausible that previous studies of Mercury’s shrinking, including one of his own, missed geological cracks on the planet because they were hard to see in rough areas or because they hadn’t formed at all due to Mercury’s extremely rugged crust. Byrne was not involved in the latest study.

“Yet if we can accurately measure how much Mercury has contracted, we can make better estimates of its interior layering, the size and make-up of its core, its tectonic and volcanic histories, how its magnetic field is generated, and a whole lot more,” wrote Byrne, also the director of the NASA Planetary Data System Geosciences Node, in an email.

Mercury is the second densest planet in our solar system after Earth, mainly due to its large metallic core.

Calculating how much the planet has shrunk could provide a window into Mercury’s core and what has transpired inside the planet as it cooled.

“More shrinking means Mercury could have a larger metal core, less light elements like silicon mixed into the metal core, or a higher starting temperature,” Nishiyama said.

The BepiColombo mission, which recently began its arrival at Mercury after traveling for nearly eight years, will position two orbiters near the planet at the end of this year.

Topography data that the mission collects could enable scientists to estimate more precisely how much Mercury has contracted over time and showcase the planet’s cliffs, craters and ridges like never before, said Nishiyama, who is a member of BepiColombo’s science team.

BepiColombo will make the first comprehensive global measurements of Mercury’s topographic properties, which is something MESSENGER couldn’t do, Byrne said.

“Those new data will be key to testing whether this and other global contraction papers are right,” Byrne said. “But I suspect we’ll find that they are, and that we still have so very much to learn about the tiny innermost planet.”

While MESSENGER collected high-resolution data for Mercury’s northern hemisphere, BepiColombo’s Mercury Planetary Orbiter will also measure topography and surface roughness across the southern hemisphere, said Dr. Kelsey Crane, geologist at Seres Engineering and Services in Charleston, South Carolina. Crane was not involved in the new study but previously analyzed Mercury’s surface features using MESSENGER data.

“The instruments onboard will detect gravity anomalies, improve estimates of crustal thickness and interior structure, and detect elemental and mineral compositions of the surface,” Crane said. “All these types of data complement each other, helping scientists tell a more complete story of planetary evolution and spurring new questions for future missions to address.”

Mercury has likely shrunk more than any other planet because its core is much larger relative to the planet’s size than other rocky worlds in our solar system, Bernhardt said.

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While he found the methodology and results of the paper to be compelling and sound, Bernhardt said he believes that such a dramatic decrease in Mercury’s size would have initiated visible large-scale buckling across the planet’s surface.

Bernhardt and his team are working on their own research to determine how much the planet has shrunk, but said orbital data alone might not answer the question.

“With all the things that government money can buy, maybe supporting our best engineers and scientists to put robotic boots into Mercury’s scorched dust is one of the most inspiring ones,” he said.

水星是距离太阳最近的行星，其表面的皱纹表明它曾经是一个更大的星球，后来体积缩小了——而且它的缩小速度可能比科学家们意识到的还要快。

据美国宇航局称，我们太阳系中最小的行星距离太阳平均约 3600 万英里（5800 万公里），每 88 天绕太阳公转一周。

这个岩质世界大约在45亿年前由气体和尘埃的旋涡形成。我们太阳系的早期十分混乱，大量绕太阳运行的太空岩石相互碰撞，引发了剧烈的相互作用。

新形成的行星本身就很热，就像火山熔岩一样，但碰撞产生的能量会产生更多的热量。随着时间的推移，热量从这些行星散失，导致像水星这样的行星内部空间逐渐缩小。

水星龟裂的表面留下了这种收缩的痕迹——科学家称这些山脊和悬崖为缩短构造。有些悬崖高达1英里（1.6公里），绵延数百英里。

但小行星和彗星也曾轰击水星，使其表面布满撞击坑，就像地球的月球表面一样坑坑洼洼。

根据周四发表在《地球物理研究快报》上的一项新研究，撞击产生的碎片可能掩盖了水星随着时间的推移究竟收缩了多少。

科学家称，在撒哈拉沙漠发现的两颗陨石可能来自太阳系研究最少的岩质行星。

了解水星的收缩速度可以揭示这颗行星的演化过程，并揭开其神秘内部的秘密。

“关于像地球这样的大型岩质天体的形成和演化，在水星上能学到比太阳系其他任何地方都多的知识，”汉内斯·伯恩哈特博士在一封电子邮件中写道。伯恩哈特博士并未参与这项研究。他是马里兰大学帕克分校地质、环境和行星科学系的助理研究科学家。

这项新研究还强调了关于这个微小世界还有多少疑问悬而未决——其中一些问题可能很快就会通过一项前所未有的任务得到解答，该任务将于 11 月释放两个绕水星运行的轨道器。

随着水星体积缩小，其表面很可能出现了一系列裂缝。但数十亿年来撞击形成的陨石坑和碎片覆盖了许多这样的裂缝，该研究的主要作者、柏林德国航空航天中心空间研究所的行星科学家西山岳（Gaku Nishiyama）表示。

水星距离太阳很近，这使得它成为一颗难以研究和探索的行星。在过去的几十年里，只有两项任务冒险进入了这颗环境恶劣的星球：美国宇航局1974年的“水手10号”探测器和2011年的“信使号”探测器。

水手10号探测器飞掠水星，揭示了水星引人入胜的褶皱，而信使号探测器则提供了水星特征的更全面的图像，伯恩哈特说道，他还研究了水星的缩小现象。

西山和他的团队利用信使号探测器的数据绘制了水星表面粗糙度的全球地图。他们发现，与表面较为光滑的区域相比，水星表面最粗糙的部分褶皱较少。

西山说：“这让我们想到，可能存在某种过程掩盖了缩短的结构”，例如碎屑掩盖了地表的收缩迹象。

研究团队估算了如果水星表面没有被碎片遮蔽，可能会出现多少褶皱。计算结果表明，水星的体积可能比之前认为的缩小了10%到30%，半径变化约为7.2英里（11.6公里）。此前的研究表明，水星半径的变化范围在0.6到1.2英里（1到2公里）之间，最大可达4.3英里（7公里）。

圣路易斯华盛顿大学地球、环境与行星科学系副教授保罗·伯恩博士表示，此前关于水星缩小的研究，包括他自己的一项研究，很可能忽略了水星上的地质裂缝，原因可能是这些裂缝在崎岖不平的区域难以观察，或者由于水星地壳极其崎岖，根本没有形成。伯恩博士并未参与这项最新研究。

“然而，如果我们能够准确测量水星的收缩程度，我们就可以更好地估算其内部分层、其核心的大小和组成、其构造和火山历史、其磁场的产生方式等等，”伯恩在一封电子邮件中写道。伯恩同时也是美国宇航局行星数据系统地球科学节点的负责人。

水星是太阳系中密度第二高的行星，仅次于地球，这主要是由于其巨大的金属内核。

计算出这颗行星缩小了多少，可以让我们了解水星的核心以及这颗行星冷却过程中内部发生了什么。

西山表示：“水星体积进一步缩小意味着它的金属核心可能更大，金属核心中混入的硅等轻元素更少，或者初始温度更高。”

历经近八年的飞行，BepiColombo 任务最近开始抵达水星，并将于今年年底将两颗轨道器部署在水星附近。

BepiColombo 科学团队成员西山表示，该任务收集的地形数据可以让科学家更精确地估算水星随着时间的推移收缩了多少，并以前所未有的方式展示这颗行星的悬崖、陨石坑和山脊。

伯恩说，BepiColombo 将首次对水星的地形特征进行全面的全球测量，这是 MESSENGER 无法做到的。

“这些新数据对于检验这篇以及其他关于全球行星收缩的论文是否正确至关重要，”伯恩说。“但我怀疑我们会发现它们是正确的，而且我们对这颗最小的内侧行星仍然有很多需要了解的地方。”

据南卡罗来纳州查尔斯顿市塞雷斯工程服务公司（Seres Engineering and Services）的地质学家凯尔西·克莱恩博士（Dr. Kelsey Crane）介绍，信使号（MESSENGER）探测器收集了水星北半球的高分辨率数据，而贝皮科伦坡号（BepiColombo）的水星行星轨道器也将测量水星南半球的地形和表面粗糙度。克莱恩博士并未参与这项新研究，但她此前曾利用信使号的数据分析过水星的表面特征。

克莱恩说：“机载仪器将探测重力异常，改进对地壳厚度和内部结构的估算，并探测地表元素和矿物成分。所有这些类型的数据相互补充，帮助科学家更完整地讲述行星演化的故事，并激发未来任务需要解决的新问题。”

伯恩哈特说，水星的体积缩小程度可能比其他任何行星都大，因为与其他太阳系中的岩质行星相比，水星的内核相对于行星本身的大小要大得多。

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伯恩哈特表示，虽然他认为该论文的方法和结果令人信服且合理，但他相信水星体积如此剧烈的缩小会在行星表面引发可见的大规模弯曲。

伯恩哈特和他的团队正在进行自己的研究，以确定这颗行星缩小了多少，但他们表示，仅凭轨道数据可能无法回答这个问题。

他说：“政府的钱能买到很多东西，但或许支持我们最优秀的工程师和科学家将机器人靴子放入水星灼热的尘埃中是最鼓舞人心的事情之一。”