How is the internal structure of the planet measured?

The solar system is our home. Besides the earth, there are seven planets revolving around the sun. According to the distance from the sun, these eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, and they are divided into terrestrial planets and woody planets according to their physical properties.

Comparison between the Sun and the Eight Planets in the System

Earthlike planets include Mercury, Venus, Earth and Mars. They are close to the sun, with high surface temperature, similar size to the earth, small volume, high density and similar structure. They are composed of three layers: core, mantle and shell, in which the shell and mantle are mainly composed of silicate rocks and the core is mainly composed of iron. Earth-like planets are similar to the earth, and are composed of rocks containing metals such as oxygen, silicon and iron. They have a solid rock surface and can be studied by geological methods.

Comparison of terrestrial planets (from left to right: Mercury, Venus, Earth and Mars)

Woodlike planets, including Jupiter, Saturn, Uranus and Neptune, have large mass and low average density. They are similar to Jupiter, with faster rotation, more satellites and rings, lower temperature and denser atmosphere. Woodlike planets are mainly composed of light elements such as hydrogen, helium and nitrogen, and the abundance of volatile elements is high.

Comparison of Woodlike Planets (from left to right: Jupiter, Saturn, Uranus and Neptune)

We can't directly measure the material composition and structure of terrestrial planets, but we can establish a series of planetary material composition models and internal structure models through a series of physical and chemical constraints to estimate their internal structures. Such as planetary gravity field, planetary density, planetary seismic wave velocity and propagation characteristics, planetary magnetic field, solar system element abundance, solar system chemical evolution theory, planetary geological survey and mineral composition analysis. Studying the internal structure of terrestrial planets is helpful for us to understand the formation and evolution of terrestrial planets themselves and even the solar system.

Comparison of various properties of terrestrial planets

Here we will introduce the common geophysical methods for measuring the internal structure of planets: seismology, geodesy and electromagnetism. Assuming that the planet is in hydrostatic equilibrium, it is divided into a series of concentric spherical shells radially outward from the center of the sphere. According to the physical model, a series of differential equations of internal pressure, mass and gravity are established, and numerical integration is carried out by using boundary conditions. Or carry out Bayesian inversion according to seismology, geodesy and electromagnetism data to obtain the internal structure model.

Structure and magnetic field distribution of mercury

Seismology is a comprehensive science to study the law of earthquake occurrence, seismic wave propagation and macro-consequences of earthquakes in solid earth media. Through the measurement and analysis of seismic waves, we can obtain the occurrence process of earthquakes, the physical changes of elastic properties, velocity and density of underground media, and the information of various earthquakes related to the earth.

For example, the use of seismometers all over the world has made us have a clearer understanding of the internal structure of the earth, and obtained the commonly used one-dimensional velocity models of the earth: PREM, AK 135, IASP9 1. These models not only have clear first-order velocity discontinuities: Moho surface (the interface between crust and mantle), Gutenberg surface (the interface between mantle and core) and Lyman surface (the interface between core and outer core), but also have detailed internal velocity discontinuities such as mantle transition zone.

PREM model

Geodesy, the full name of geodesy, its basic goal is to determine and study the position, gravity and its changes with time of earth space points. The constraints of geodesy mainly include planetary mass, size, moment of inertia, love number k2 of solid tide, gravity field and so on. Among them, the gravity field reflects the information of material and density distribution in the planet. When the distribution of matter inside the planet is unbalanced and there is density anomaly, gravity anomaly will be detected, which is one of the important means to explore the internal structure of the planet. There are two common methods to measure the gravitational field of planets: directly carrying a gravimeter or accelerometer on the orbiter can directly obtain the spatial gravitational field distribution of celestial bodies, such as CHAMP and GRACE satellites over the earth and GRAIL satellites over the moon, which can directly obtain the spatial gravitational field distribution of the earth and the moon; For celestial bodies that do not directly measure the gravity field, the spatial gravity field distribution of celestial bodies can be calculated by accurately measuring the orbit of spacecraft around celestial bodies and its changes. In the exploration of terrestrial planets, due to the limitation of load and scientific purpose, the second method is often used to measure gravity field.

In addition to topographic mapping, the measurement of magnetic field is also very important to the constraint of internal structure. Electromagnetism can determine the conductivity of the corresponding layer by measuring the magnetic field of the planet, and then constrain the internal structure according to the relationship between the conductivity and physical quantities such as material composition, temperature and pressure, which is especially suitable for exploring the shallow structure of celestial bodies.

Magnetic field distribution of vertical component of lithosphere (CHAMP and Swarm satellite integrated model

The seismic measurement of the planet surface can determine the elastic properties, density, the position of the core-mantle boundary and so on, and can judge the state of the planet's inner core. With the development of space technology, more and more planetary orbit probes have been successfully launched. At present, there are seismic observation data of Mars, Venus and the moon, which is of great significance for obtaining the internal structure of extraterrestrial bodies.

Main earthquake signal of Wenchuan earthquake in 2008 recorded by Beijing station

GL-CS60 three-component small broadband seismometer

Lunar earthquake record (lunar earthquake)

The seismograph that Apollo planned to take to the moon.

Fire and earth instrument records (red is Martian wind, green is fire shock event, and light green is mechanical arm vibration)

InSight's Mars Seismograph (Fire Seismograph)

But there are no seismographs on most planets at present. Mercury is the closest planet to the sun. Its radius at the equator is 2439.7km, but its density is 5.427g/cm 3, which is the second highest in the solar system, second only to the earth's 5.5 15g/cm 3. Mercury consists of about 70% metal and 30% silicate material. At present, only Mariner 10 and Messenger have successfully detected Mercury, among which Mariner 10 has only made three overflights of Mercury.

Sailor 10 and its scientific instruments

Mariner 10 is equipped with near infrared radiometer, ultraviolet spectrometer, magnetometer, video photography, charged particle telescope and plasma detector.

The main geological structure on the surface of Mercury detected by Mariner 10 (a folded ridge; Leaf cliff; C high convex floating ridge)

Messenger is equipped with laser altimeter (MLA) and gamma-ray neutron detector (gamma-ray&; GRS & amp； NS), X-ray spectrometer (XRS), magnetometer/magnetometer (MAG), Mercury Atmospheric and Surface Composition Spectrometer (MASCS), Mercury Dual Imaging System (MDIS) and High Energy Particle and Plasma Spectrometer (EPPS).

Venus is most similar in size to Earth, and the internal structures of the two planets may be similar. There is no direct data about the internal structure of Venus. According to the theory of planetary model, Venus should have a movable nickel-iron core in its center, the outer core is molten and the inner core is solid. The mantle consists of molten upper mantle and solid lower mantle. The crust and lithosphere are very thin.

Since 196 1, the Soviet Union and the United States have successively launched more than 30 probes to visit Venus, among which the Magellan spacecraft launched by the National Aeronautics and Space Administration (NASA) in 1989 was more successful. Magellan probe carried high-resolution synthetic aperture radar (SAR), and obtained valuable information such as Venus surface image, global gravity field, surface topography and the number of craters, which greatly improved people's understanding of Venus.

Five perspectives of Magellan probe to detect Venus

A crater on the surface of Venus.

Mars is the fourth planet in the solar system from the inside out. Its diameter is about half that of the earth, its surface area is equivalent to the land area of the earth, and its density is far less than that of the other three terrestrial planets (Mercury, Venus and Earth). The rotation period of Mars tilted from its axis is similar to that of the Earth, while that of period of revolution is about twice that of the Earth. Its orange-red appearance is due to the surface covered with hematite (iron oxide). Mars was once considered as the planet with the greatest possibility of extraterrestrial life in the solar system, which made it the planet with the highest degree of exploration and research among celestial bodies in the solar system except the earth. The detection methods range from flying over to remote sensing detection around, and then to in-situ detection of unmanned lander/rover, which has accumulated a lot of scientific data. In 2020, China, the United States and the United Arab Emirates will all launch Mars probes, and Mars/Phobos sampling return and Mars manned exploration may also be realized in the next 10~20 years.

Comparison between the Earth and Mars

The Mars Global Explorer (MGS) is one of the successful devices for Mars exploration. It is equipped with five scientific instruments, namely Mars orbit camera, Mars orbit laser altimeter, thermal radiation spectrometer, magnetometer, electronic reflector and ultra-stable Doppler measuring oscillator. After 9 years of Mars exploration (1996 165438+20061October 7th165438+1October 2nd), MGS has successfully acquired the terrain, gravity field, magnetic field, surface image and image of Mars.

Curiosity is the world's first rover powered by nuclear energy. Its mission is to explore the life elements on Mars, investigate the climate and geology of Mars, evaluate whether Mars has ever provided favorable environmental conditions for microorganisms, and prepare for human beings to explore the habitability of planets. Scientific instruments include: MASTECamera, MastCam), sample analysis of Mars, SAM), chemistry and mineralogy, CheMin), α -particle X-ray spectrometer (apxs), radiation evaluation detector (RAD), environmental monitoring station of Mars (REMS), neutron dynamic albedo (DAN) and so on.

Curiosity's selfie on Mars

InSight is the first lander to explore the internal structure of Mars. Insight carries three core laboratories, SEIS, which are used to detect fire and meteorite impact activities and to detect the internal structure of Mars. The heat flow and physical properties package (HP3) is used to measure the ground temperature/gradient, thermal conductivity and physical properties from the ground to a depth of 5m; Rotation and internal structure experiments (RISE), as well as geodesy for sub-decimeter accurate tracking of planetary rotation.

Mars insight

China's planetary exploration started late, with a high starting point. In 2020, China's mission to mars "Tian Wen No.1" plans to achieve three major tasks of "circling the earth", "landing" and "patrolling" in its first launch, which is unprecedented in the world space history. The scientific instrument carried by Tian Wen 1: medium and high resolution camera, which is responsible for imaging the surface of Mars and studying the topography and geological structure of the surface of Mars; Mars magnetometer is mainly responsible for detecting the space magnetic field environment of Mars. The Martian mineral spectrum analyzer is used to analyze the composition and distribution of Martian minerals, study the overall chemical composition and chemical evolution history of Mars, and analyze the resources and distribution areas of Mars.

Structure and size of Tianwen No.1

Planets in the solar system are considered to have the same cosmic origin, but today's planetary environment is completely different. Therefore, the detection and comparison of planets is conducive to a deeper understanding and understanding of the evolution of the earth and the formation of a livable environment. At present, planetary physics is one of the most important means to study extraterrestrial stars. Planetary physics integrates geophysics, space science, atmospheric science, physics and other disciplines, and its research scope covers planetary multi-layer physical processes such as planetary space environment, atmospheric environment, surface environment and internal structure. The research and development of planetary exploration has not only become the wrestling field of the comprehensive national strength of all countries in the world at present, but also become an important index to measure the country's future sustainable comprehensive development ability. We expect more people with lofty ideals to join the team of planetary exploration.

China planet detection mark

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