Editor's note: "There are still many unknown mysteries to be explored in the vast space", to this end, the Voice of the Chinese Academy of Sciences and the National Space Science Center of the Chinese Academy of Sciences jointly opened a "calling space" science column to tell you interesting stories and introduce some knowledge related to space science and spaceflight.
Mercury, also known as Chenxing in ancient China, is the smallest and closest of the eight planets in the solar system. Mercury is only 4880 km in diameter, which is only one-third the diameter of the Earth. The average distance from the sun is only 58 million kilometers, and the surface lacks atmosphere, the temperature difference between day and night is extremely large, and the temperature of the sunny side can reach up to +432 °C, and the temperature of the back sun can be as low as -172 °C. Because of its proximity to the Sun, the Sun's appeal to Mercury is strong, so Mercury is also the fastest planet to orbit the Sun, with only 88 Earth days to orbit the Sun.
Figure 1 Mercury (Image source: nasa)
Relative to the history of more than a hundred explorations of the Moon, humans know very little about Mercury. NASA has sorted out the top ten major scientific questions about Mercury: (1) Why is Mercury so dense? (2) What are mercury's magnetic field characteristics? (3) What is the geological evolution of Mercury? (4) Is there water ice at the poles of Mercury? (5) What are the volatile components of Mercury's outer layer? (6) In the absence of an ionosphere, how does the interplanetary magnetic field interact with the solar wind? (7) What are the main components of Mercury's atmosphere? (8) What is the composition of Mercury's magnetosphere? What role do heavy ions play? (9) What is the structure and state of Mercury's nucleus? (10) What is mercury's shell made of?
As early as 3000 BC, Mercury was discovered by the Semitic people. In 1631, the English astronomer Thomas Harriot and the Italian astronomer Galileo Galilei observed Mercury using a newly invented telescope, and in the same year, the French scientist Pierre Gassendi used a telescope to observe Mercury transiting.
In 1991, astronomers detected unusually bright radar spots at the poles of Mercury through ground-based radio telescopes, and the results of radar reflectance and polarization characteristics were similar to those of the Polar Crown of the South Pole of Mars, due to the small inclination of Mercury's axis, there was a permanent shadow area in the polar region that the sun could not illuminate, and water ice may exist at the bottom of the polar craters, so astronomers speculated that there may be water ice at the bottom of Mercury's north and south pole craters.
Figure 2 Radar image of Mercury's North Pole obtained by the Arecibo Observatory. Yellow areas indicate areas with high radar reflectivity (Image: nasa)
Due to the harsh space environment for the detection of Mercury, only the Mariner 10 and Messenger probes have so far reached Mercury.
In November 1973, NASA successfully launched the Mariner 10 probe, the first to probe two planets in a row. After detecting the surface of Venus by flying over Venus, he continued to fly with the help of Venus's gravitational attraction to change the speed and direction of motion, and finally entered the heliocentric orbit of perihelion in mercury's orbit to carry out overflight detection of Mercury. Between 1974 and 1975, Mariner 10 made three overflights of Mercury, sending back more than 5,000 photographs and exploring about 45 percent of Mercury's surface. Mariner 10's findings suggest that Mercury's surface is similar to that of the Moon, made of rocks, and that it has been hit by meteorites and is densely packed with large and small craters, basins, and craters. Mariner 10 also identified Mercury's global dipole magnetic field, which is 1% stronger than Earth's. It was detected that Mercury's extremely thin exodus layer contained trace amounts of he, ar, ne and other elements. Mariner 10's detection data fills a gap in humanity's understanding of Mercury.
Mariner 10 launch on November 3, 1973 (Credit: nasa)
Photo taken by Mariner 10's first flyby of Mercury (Credit: nasa)
30 years after Mariner 10 completed its first exploration of Mercury, NASA identified the Messenger mission as one of the Discovery missions for deeper exploration and understanding of Mercury.
The Messenger probe, known in English as messenger, is taken from the acronyms of mercury surface, space environment, geochemistry, and ranging, translated as Mercury's surface, space environment, geochemistry, ranging. Launched on August 3, 2004 on a Delta II carrier rocket at Canaveral Air Force Base in Florida, USA, it flew about 7.9 billion kilometers and made five orbit corrections over the course of more than six years. After six planetary overflights, it successfully entered the orbit of Mercury and conducted a probe in March 2011, becoming the first space probe to orbit Mercury.
The main mission of the Messenger probe is to study mercury's chemical composition, geology and magnetic field, and obtain data such as mercury's three-dimensional images, surface chemical characteristics, internal magnetic field and geometry.
Schematic flight diagram of the Messenger probe (Image source: nasa)
On August 3, 2004, the Messenger probe was launched (Image source: nasa)
The Messenger probe made five orbit corrections and six overflights to mercury (Credit: nasa)
To accomplish these tasks, the Messenger probe is equipped with a total of 7 scientific payloads, including: magnetometer (mag), γ-ray and neutron spectrometer (grns), X-ray spectrometer (xrs), Mercury Atmosphere and Surface Composition Spectrometer (MASCS), High Energy Particle and Plasma Spectrometer (EPPS), Mercury Dual Imaging System (mdis), mercury laser altimeter (mLA).
The Messenger probe is equipped with a total of 7 payloads (Image source: Edited according to NASA data)
Courier probe payload configuration layout (Image source: Edited according to NASA data)
The key scientific problems, scientific objectives and scientific payload detection targets to be addressed by the Courier are as follows:
The six key scientific questions of the Messenger probe correspond to scientific objectives and scientific payload detection targets (Image source: According to Reference 2, ed.)
The Messenger probe conducted a four-year comprehensive exploration of Mercury in orbit, fundamentally changing our understanding of Mercury, the main objects of detection include Mercury's surface layer, core, escape layer, magnetic field, magnetosphere and interplanetary space, obtained a large amount of scientific exploration data and rich scientific output, including: Mercury's north pole contains water ice and organic compounds, volcanoes play an important role in the formation of Mercury's surface form, Mercury is a volatile-rich planet, Mercury's surface has obvious chemical inequities, etc., and has completed tasks such as mapping the global distribution of material composition in Mercury, 3D model modeling of Mercury's magnetosphere, topographic profile of Mercury's northern hemisphere and gravity field detection map.
Maps of magnesium/silicon and aluminum/silicon illustrate significant chemical inequities in Mercury's surface (Credit: NASA)
Detailed topographic map of Mercury's northern hemisphere drawn by the Messenger probe MLA payload (Image source: NASA)
Among them, the scientific data provided by the Messenger probe for the presence of water ice in Mercury's polar regions is a landmark discovery. The γ-ray and neutron spectrometer (grns) carried by the Courier detected the poles of Mercury, and the scientists' detection data showed that the presence of hydrogen had indeed been detected in areas with anomalous polar reflectance of Mercury observed by ground-based radar, and there was a "hydrogen-rich layer" in these areas, which was located below the surface layer of Mercury with a thickness of less than 10 to 20 centimeters. At the same time, the Mercury laser altimeter (mla) and the Mercury Dual Imaging System (mdis) were used to observe mercury's north pole region for the first time, and information such as spectral changes in the Arctic polar region and detailed terrain were obtained, which further confirmed the detection results of the Polar Region by the Arecibo Telescope and the Mercury γ Ray and Neutron Spectrometer. This discovery provides strong support for the hypothesis of the presence of water ice in the permanent shadow of the early Mercury polar region.
The red area is the permanent shadow area measured by the Messenger, and the yellow bright spot is the ground-based radar observation, and this mosaic shows that the parts of the polar reflectivity anomaly of the ground-based radar are located in the permanent shadow area measured by the messenger. (Image source: nasa)
Messenger probe data suggests that there is water ice in the polar region (yellow part) (Image source: nasa)
On 30 April 2015, the Courier ran out of propellant and finally hit Mercury's surface as planned, ending nearly 11 years of missions, leaving a crater about 15 metres in diameter at Mercury's north pole close to the Janácek crater in Suisei planitia (latitude 54.44°N, longitude 210.12°E).
The Messenger probe ended its mission by hitting Mercury on April 30, 2015 (Credit: NASA)
As the innermost planet in the solar system, Mercury exploration helps to reveal major scientific issues such as the origin and evolution of the solar system, and after the only two Mercury exploration missions by mankind - Mariner 10 and Messenger, ESA and the Japan Aerospace Exploration Agency (JAXA), the "Bepi Colombo" is in the process of "running water", "Bepi Colombo" includes two probes: the Mercury Planet Orbiter and the Mercury Magnetosphere Orbiter, It is expected to enter the orbit of Mercury in December 2025 and carry out a coordinated exploration mission, and the scientific data of "Bepi-Colombo" will greatly promote human understanding of Mercury. It is expected that the probe developed by China will also appear on the road of "running water" in the future.
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Source: National Space Science Center, Chinese Academy of Sciences, General Department of Lunar and Deep Space Exploration, Chinese Academy of Sciences