Chinese names shining on Mercury

Mercury is the smallest of the eight planets in the solar system and the planet closest to the sun. So far, humans have named nearly 500 landforms on Mercury, more than 20 of which are from China. These immortal names represent the marks of ancient Eastern civilization and remain in space forever. Let's get to know them and try to explore the planetary stories behind them.

Mercury and Earth▏Image source: Internet

Mercury and its exploration history: Like other terrestrial planets, Mercury has a silicate crust and a metallic core. However, Mercury also has many unique features. For example, compared with other planets, its surface composition is extremely rich in sulfur and carbon but poor in iron. Its internal structure is "thin skin and large filling" and has a huge iron core, thus constituting a unique end member among terrestrial planets. Exploring Mercury can help us understand important scientific issues such as the origin of the solar system, the formation and evolution of planets, and the habitability of planets.

Due to the huge gravitational force of the sun and the extreme surface environment, Mercury exploration is very difficult. So far, only the Mariner 10 launched by the United States in 1973 and the MESSENGER launched in 2004 have conducted close observations of Mercury. Mariner 10 used three flybys to cover 45% of Mercury's surface at a resolution of 1 km. MESSENGER is an orbiting spacecraft that has completed global coverage with a resolution of 250 meters. The BepiColombo jointly launched by Europe and Japan is on its way and is expected to arrive in 2025.

Mariner 10 (top) and MESSENGER (bottom) probes ▏Image source: NASA

BepiColombo probe ▏Image source: Internet

Naming conventions for planets and Mercury landforms Mariner 10 and the MESSENGER probe unveiled the secrets of Mercury, which has a diverse range of landforms including impact craters, volcanic basins, and various linear structures, recording a long and complex geological evolution process.

To facilitate scientific research and mapping, scientists will give various characteristic landforms catchy names.

In theory, anyone can submit an application to name a Mercury feature. The submitted name must follow the principles of simplicity, clarity, harmlessness, non-repetition, and consideration of cultural diversity and national balance. For the same feature, there are usually candidate names from multiple cultures or countries. After a special working group under the International Astronomical Union (IAU) votes and approves it, it will be recognized by the scientific community.

IAU Planetary Nomenclature Bulletin Home Page

Seeing this, I guess many people are eager to try. However, according to IAU rules, if you want to name a landform, you have to consider the following issues: first, the naming should be based on scientific research needs, otherwise it will not be considered; second, for landforms smaller than 100 meters, unless they have special scientific significance, they are generally not named.

In order to ensure sufficient sources of names, the IAU has established independent naming systems for different planets and satellites, each following its own set of rules. For example, impact craters on Mercury are mainly named after deceased famous artists, musicians, painters and writers, while those on Venus are named after women. The light spots on the surface of Mercury are named after names related to "snakes" in different languages, while those on Ganymede are named after places in ancient Egyptian mythology.

If all goes well, you will usually get a decision 4 to 6 weeks after submitting your naming application.

Chinese names on Mercury So far, there are 21 Mercury craters named after Chinese people, and another one is a bright spot on the surface of Mercury, which is taken from the Chinese word Yinshe. Among these names, ancient painters and poets dominate, such as the well-known Li Bai and Du Fu, and some are from modern people, such as Lu Xun and Qi Baishi.

Judging from the age, the naming was mainly completed before the 1980s, and the English spelling used the Wade-Giles system, which looks a bit strange. Since 2008, new names have gradually appeared, and the English spelling follows the familiar Chinese phonetic system.

A search of the IAU records revealed that apart from former Purple Mountain Observatory Director YC Chang and a few Chinese scientists from the MESSENGER team who were included in the naming committee in 1985, no other Chinese participated in the naming. The Wen Tianxiang crater and the Silver Snake spot, which were newly named in 2020, came from the team of Professor David Rothery of The Open University in the UK.

List of Mercury landforms named after Chinese people ▏ Content extracted from the IAU website

Mercury landforms named after Chinese people (please watch in horizontal mode) ▏Image source: NASA

The Mercury stories behind the Chinese names The owners of these twenty or so Chinese names all have wonderful stories, but unfortunately, they are just used as synonyms for landforms here. Only those landforms with important scientific significance will be paid attention to and invited to the center of the stage.

Below we will introduce several names with important scientific significance and the scientific stories behind these landforms based on the progress of Mercury research.

〇 Mercury's global thermal contraction

The drift and collision between different plates are the main mechanisms that shape the surface of the Earth. Unlike the Earth, there is no plate movement on Mercury. The main driving force behind the surface morphology of Mercury is the high-speed impact of other celestial bodies and global thermal contraction.

Here we first introduce global thermal contraction. Although Mercury is small in size, it has a seemingly disproportionately large metal core, much larger than other terrestrial planets in the solar system. After its formation, the metal core slowly cooled and solidified, causing Mercury's radius to shrink by 7 kilometers. This radial contraction caused Mercury's crust to be horizontally squeezed, forming a large number of thrust faults and related landforms.

Cai Wenji is a well-known talented woman in the Eastern Han Dynasty. It is said that the famous ancient guqin music "Hujia Eighteen Beats" was composed by her. The impact crater named after her presents us with the two main mechanisms of meteorite impact and global thermal contraction.

The Caiwenji crater, with a diameter of about 124 kilometers, is located in the northern hemisphere of Mercury. It has a slightly degraded rim and a prominent central peak. A linear structure related to the thrust fault runs through the north and south of the crater. The Caiwenji crater was formed about 3.5 billion to 1 billion years ago. The fact that the thrust fault-related structure cuts the crater indicates that the fault was formed later and the global thermal contraction of Mercury may have lasted for a very long time.

A partial view of the Qing Dynasty Huayan painting "The Return of Lady Wenji to Han" (above) and the Cai Wenji impact crater (below) ▏Image source: Internet

０ Water ice at Mercury's poles Water, as an important indicator of the existence of life, is an eternal topic in the exploration of extraterrestrial planets. Compared with other terrestrial planets, Mercury's extreme thermal environment and lack of atmosphere make it difficult to imagine the existence of water ice on its surface. However, in 1992, using the now-collapsed Arecibo radio telescope in Puerto Rico, scientists discovered radar wave bright spots at the bottom of the impact crater at the south pole of Mercury, which is out of reach of sunlight, and may be related to water ice.

The polar water ice has been confirmed by the MESSENGER probe. Its advanced instruments, such as the neutron spectrometer, laser altimeter, and dual-lens imaging system, strongly prove that water ice exists in the permanent shadows of polar impact craters. It has also been found that, unlike the polar water ice on the moon, the polar water ice on Mercury is of high purity and thick, reaching several meters or even thicker. At the same time, scientists have found that this water ice seems to be relatively young, indicating that it arrived on the surface of Mercury late or has undergone redeposition. However, scientists are still continuing to study the origin, duration, and storage of this water ice.

Zhao Mengfu was a famous official, calligrapher, painter and poet during the Song and Yuan dynasties. His outstanding artistic achievements and ups and downs in life are eternal topics in Chinese cultural history. The impact crater named after him is located at the South Pole of Mercury, with a diameter of about 140 kilometers. The geographic South Pole of Mercury is located on the edge of the Zhao Mengfu crater. The bottom of the crater cannot receive direct sunlight and is a permanent shadow area, so the temperature is extremely low, which is an ideal location for preserving water ice.

Zhao Mengfu's self-portrait (above) and a partial view of "Ten Bundles of Running Script" (below) ▏Image source: Internet

Zhaomengfu Crater is also the largest one among all the impact craters at the two poles of Mercury with a high radar echo signal area, indicating that it has a large amount of water ice. This largest impact crater at the South Pole of Mercury with a Chinese name may be the best place to explore water ice on Mercury in the future.

Mercury's South Pole topography (top) and radar reflectivity map (bottom). The largest crater is the Zhao Mengfu crater.[5]

〇 Volcanic Activity and Volatiles on Mercury MESSENGER confirmed the existence of volcanic activity on Mercury in 2008, opening a window for scientists to understand the elemental composition and evolutionary history of its interior from the surface.

Similar to Earth, Mercury's volcanic activity can be divided into overflow type and explosive type. The former has more lava but less gas content, while the latter has smaller lava volume and rich gas. The widespread flat plains on Mercury are likely related to overflow type volcanic activity. Explosive type volcanic activity usually forms sunken irregular calderas and surrounding associated pyroclastic deposits.

Although these pyroclastic deposits are thin, they are usually distributed over hundreds of kilometers. At the same time, because the albedo is higher than other materials on Mercury, they are very conspicuous in remote sensing data and are called "faculas". Because faculas are closely related to volcanic activity, they can be used to estimate the content and composition of volatile components in the volcanic source area, as well as the time of volcanic activity.

"Silver Snake" is a famous image used by Mao Zedong in his poem "沁园春·雪" to describe the magnificent mountains and rivers in the snowy scenery of the north. The Silver Snake light spot is located in the southern hemisphere of Mercury, with a diameter of about 41 kilometers. It is one of the 25 named light spots on Mercury.

Silver snake spots▏Image source: NASA

The light spot is significantly brighter and redder than the surrounding pyroclastic deposits. There is also a concave, irregular crater near the center, providing an ideal sample for scientists to study the history of Mercury's volcanic and volatile activity.

０ Young impact craters on Mercury The mutual collisions between celestial bodies in the solar system have never stopped, and a large number of new impact craters are still being formed, shaping the shape of the planets and injecting foreign matter.

Mercury also has a large number of young impact craters, one of their characteristics is the striking radiation lines. These radiation lines are formed by high-speed impacts, and they appear bright due to high albedo, but they will gradually dim and disappear due to space weathering over time. Some scientists believe that the water ice at Mercury's poles comes from those water-rich asteroids or comets that recently hit the surface of Mercury.

Xiao Zhao was a famous court painter in the Southern Song Dynasty, who was good at painting landscapes, figures, and unusual pines and rocks. The Xiaozhao crater named after him is located near the equator of Mercury and has a small diameter of only 24 kilometers. However, it has spectacular radial patterns that are much larger than the actual diameter of the crater.

Xiao Zhao's famous work "View from the Mountainside Tower" (above) and the Xiao Zhao impact crater (below) ▏Image source: NASA

There is a similar crater called Hangan in the southern hemisphere. Like Xiaozhao crater, it is a rare crater on Mercury with a small diameter but abnormally developed radial patterns. It is an ideal object for studying the history of young impact craters on Mercury. I don’t know if it is a coincidence that these two craters named after famous painters in the Tang and Song dynasties have also been heavily outlined on the distant surface of Mercury.

It is worth mentioning that Chinese scientists have also discovered dark spots with extremely low albedo in the Xiaozhao crater, which are likely formed by strong degassing of sulfides. They were formed later than young craters, indicating that degassing may have occurred in the interior of Mercury hundreds of millions of years ago, and may even continue to this day.

Conclusion The right to name indicates the boundaries of civilization's influence. On the maps of terrestrial planets including Mercury, Chinese names that show the imprint of Chinese civilization are still sparse. my country has not yet announced a Mercury exploration plan, but there is reason to believe that as my country's scientific and technological strength gradually increases, the corresponding exploration plan will be put on the agenda. More and more domestic scientists will turn their attention to this peculiar planet closest to the sun, contributing Chinese strength to human exploration of the mysteries of Mercury. In the future, more great names from Chinese civilization will be engraved on the map of Mercury.

Acknowledgements

We thank Dr. Tenielle Gaither of USGS and Dr. Zhiyong Xiao of Sun Yat-sen University for their support during the writing process.

References:

[1] Harmon JK and Slade, MA (1992). Radar mapping of Mercury: Full-disk images and polar anomalies. Science, 258(5082), 640–643.

[2] Moore P. (2006). Moore on Mercury: The Planet and the Missions. Springer Science & Business Media.

[3] Head JW, et al. (2011). Flood volcanism in the northern high latitudes of Mercury revealed by MESSENGER. Science, 1853–1856.

[4] Xiao ZY, et al. (2013). Dark spots on Mercury: A distinctive low-reflectance material and its relation to hollows. JGR: Planets, 118, 1752–1765.

[5] Chabot NL, et al. (2018). Investigating Mercury's south polar deposits: Arecibo radar observations and high-resolution determination of illumination conditions. JGR: Planets, 123, 666–681.

[6] Deutsch AN, et al. (2019). Age constraints of Mercury's polar deposits suggest recent delivery of ice. EPSL, 520, 26–33.

[7] Xiao ZY, et al. (2021). Recent dark pyroclastic deposits on Mercury. GRL, 48.9: e2021GL092532.

[8] Rothery DA, et al. (2021). On the asymmetry of Nathair Facula, Mercury. Icarus, 355: 114180.

About the Author

Liao Shiyong

Associate researcher at the Astrochemistry and Planetary Science Laboratory, Purple Mountain Observatory, Chinese Academy of Sciences. Research interests: meteorite and astrochemistry.

Rotating Editor-in-Chief: Wang Ying

Editor: Wang Kechao