In three years, what did Tianwen-1 discover on Mars?

July 23, 2023 marks the third anniversary of the successful launch of my country's first Mars probe, Tianwen-1. In the past three years, Tianwen-1 has achieved great success in both engineering and science, and has exceeded its mission.

The lander and the rover took a group photo. The rover drove to about 10 meters south of the landing platform and released the detachable camera installed on the bottom of the rover. The rover then retreated to the vicinity of the landing platform. The detachable camera took photos of the rover's movement and the rover and the landing platform. The images were transmitted to the rover via wireless signals, and then relayed back to the ground by the rover through the orbiter. Image source: China National Space Administration

"Kill three birds with one stone" creates miracles

On July 23, 2020, my country's Tianwen-1 Mars probe was launched, and after 202 days of flight, it successfully entered the orbit of Mars on February 10, 2021. On May 15, 2021, its lander successfully landed on the surface of Mars; on May 22 of the same year, the Zhurong Mars rover safely left the landing platform, arrived on the surface of Mars, and began patrol exploration; on June 11, the National Space Administration announced the first batch of scientific images taken by the Zhurong Mars rover after landing on Mars, which marked the complete success of my country's first Mars exploration mission.

This is the first mission of my country's planetary exploration project. It has achieved the three goals of orbiting, landing and patrolling Mars through a single launch, which is unprecedented in the history of Mars exploration. This "three-in-one" Mars exploration method has the characteristics of a high starting point and high efficiency, but also great challenges. Its success has enabled my country's deep space exploration capabilities and level to achieve leapfrog development, making it the third country in the world to land on Mars and the second country to patrol Mars.

Based on the main features of Mars exploration technology and the lessons learned from space exploration in China and abroad, my country has set engineering and scientific goals for Tianwen-1 that are both in line with my country's national conditions and can achieve leapfrog development. Now, Tianwen-1 has successfully achieved these two goals.

Project objectives:

Breakthrough on Mars braking capture;

entry/descent/landing;

Long-term self-management;

Long-distance measurement and control communication;

Key technologies such as Mars surface patrol;

Realize Mars orbital and patrol exploration;

Obtaining scientific data from Mars exploration

Realize a technological leap in my country's deep space exploration.

The project objectives have now been completed.

Scientific objectives:

Study the morphology and geological structure of Mars;

Investigate the soil characteristics and water ice distribution on the Martian surface;

Analyze the composition of Martian surface materials;

Measure the Martian atmosphere, ionosphere, surface climate and environmental characteristics;

Explore the physical field and internal structure of Mars, and detect the characteristics of the Martian magnetic field.

This scientific target has also been achieved.

Tianwen-1 consists of an orbiter and a lander. The lander consists of an entry capsule and a rover. The entry capsule is used to complete the mission of entering, descending and landing on Mars. The total mass of Tianwen-1 is about 5 tons (including fuel), of which the orbiter is 3.6 tons (the fuel weight accounts for most of the total weight), the Zhurong rover is 240 kilograms, and the rest is the mass of the entry capsule.

Schematic diagram of the tail of the Tianwen-1 orbiter

Surrounding detection has made great achievements

The orbiter of Tianwen-1 is designed to last for one Martian year (687 Earth days). It will carry a lander to Mars, mainly completing tasks such as Earth-Mars transfer, Mars braking and capture, and orbit adjustment, providing three months of data relay support services for the Mars rover, and conducting scientific exploration of Mars for about one Martian year through the scientific payload it carries, realizing a global survey and local detailed survey of Mars.

The surround has three functions:

• Aircraft;
• Communicator;
• detector.

Its mission consists of five main phases:

• Earth-fire transfer;
• Mars capture;
• Deorbit and landing;
• Relay communication;
• Scientific exploration.

The main scientific missions of the orbiter are:

• Take China's first full map of Mars;
• Detect the distribution and structure of Martian soil types and detect water ice on the Martian surface and underground;
• Detect the topographic features and changes of Mars;
• Investigate and analyze the composition of Martian surface materials;
• Analyze the Martian atmosphere and ionosphere and detect the interplanetary environment.

To this end, the orbiter carries 7 scientific instruments:

• A medium-resolution camera for obtaining global remote sensing images of Mars;
• a high-resolution camera for imaging the landing zone and areas of high scientific interest;
• Subsurface radar for detecting the subsurface structure of the Martian surface and ice layers in the polar regions;
• A mineral spectrometer for detecting the types, contents and spatial distribution of minerals on the surface of Mars;
• A magnetometer for detecting the magnetic field environment of Mars space;
• the Ion and Neutral Analyzer for measuring the energy, flux, and composition of ions and neutral particles in the solar wind and in space on Mars;
• An energetic particle analyzer used to obtain energy spectral flux and elemental composition data of energetic particles in the Martian space environment.

High-definition image of Phobos obtained by the Tianwen-1 orbiter. Image source: National Space Administration

The orbiter is currently still "serving beyond its service life" in Mars orbit because it can obtain more scientific results.

A Superior Tour to Mars

The main exploration mission of the orbiter is to conduct a global and comprehensive survey of Mars, while the main exploration mission of the Zhurong Mars rover is to conduct a high-precision, high-resolution detailed survey of local areas of scientific research value. The Zhurong is 1.85 meters tall and weighs about 240 kilograms. It is used to conduct patrol exploration in the landing area and has a designed working life of 3 Martian months (92 Earth days).

Its main scientific tasks are:

• Detect the surface elements, minerals and rock types in the Mars patrol area;
• Explore the soil structure and water ice in the Mars patrol area;
• Detect the atmospheric physical characteristics and surface environment of the Mars patrol area, and detect the morphology and geological structure of the Mars patrol area.

To this end, the Zhurong Mars rover carries 6 scientific instruments:

• The navigation/terrain camera is used to provide navigation and positioning basis for the Mars rover and obtain high-resolution three-dimensional images of the landing area and patrol area;
• The multispectral camera is used to detect the distribution of material types on the Martian surface;
• Subsurface radar is used to detect the subsurface geological structure of the patrol area;
• The surface composition detector is used to obtain high-resolution spectral characteristic information from ultraviolet to near-infrared spectrum;
• The surface magnetic field detector is used to detect the local magnetic field in the patrol area;
• Meteorological measuring instruments are used to detect meteorological conditions such as ambient temperature in the patrol area.

Zhurong is the world's first Mars rover to use active suspension technology, which can raise the chassis of the entire vehicle when encountering complex terrain. Since the six wheels of Zhurong Mars rover can steer and move independently, it can also walk sideways like a crab.

New ways to get energy

Because it is far away from the sun, the intensity of sunlight on the surface of Mars is only 40% of that on Earth, so the Mars rover needs larger and more efficient solar wings. For this reason, my country's Mars rover adopts a novel four-fold solar wing. When unfolded, the four solar wings on the Mars rover look like a blue butterfly.

Zhurong mainly relies on the electricity generated by solar panels. But at night, the rover can only use the stored electricity to continue working. Because the efficiency of converting light energy into electricity is 30%, solar panels alone cannot meet the demand. For this reason, a binocular-like heat-collecting window is equipped on the top of Zhurong, which can directly absorb solar energy and store energy using a substance called n-undecane. When the temperature on Mars rises during the day, this substance absorbs heat and melts. When the temperature drops at night, this substance releases heat energy during the solidification process. The efficiency of this energy conversion method can reach more than 80%.

Zhurong Mars rover solar wing

Heat preservation adopts black technology

Since the temperature on Mars reaches below zero at night, the night temperature varies at different latitudes and seasons, and the coldest can reach -100℃. Therefore, the Mars rover must be kept warm at night. Zhurong uses a new type of thermal insulation material, high-performance nano aerogel, to cope with the two harsh environments of "extreme heat" and "extreme cold" on Mars, and its ultra-light characteristics greatly reduce the burden on the Mars rover, allowing it to run faster and farther. The density of nano aerogel is the lightest solid in the world; its thermal conductivity is only half of that of still air, making it the solid with the lowest thermal conductivity. Ultra-low-density nano aerogel insulation panels can be used to block the extremely cold environment of the Martian surface as low as -120℃, and can also block the high-temperature heat flow of up to 1200℃ generated by the landing engine. Its density is only 1/10 of that of conventional aerogel materials.

The Zhurong rover was photographed by a navigation camera, with the lens pointing to the rear of the rover. The rover's circular heat-collecting window can be seen in the picture, and the solar panels and antennas are deployed normally. The surface texture of Mars is clear and the landform information is rich. Image source: China National Space Administration

Overcoming all difficulties and landing on Mars

The implementation of my country's Tianwen-1 Mars exploration mission includes six stages: launch, Earth-Mars transfer, Mars capture, Mars parking, deorbiting and landing, and scientific exploration. The most difficult of these is the deorbiting and landing of the lander from the Martian orbit.

Tianwen-1 innovatively "took" a selfie at a distance of about 2.2 million kilometers from Mars. It adopted a "separate monitoring solution", that is, "throwing" a light camera to take pictures of Tianwen-1 under suitable lighting conditions, and transmitting the images to the probe in real time, and then transmitting them back to Earth.

At about 2 a.m. on May 15, 2021, Tianwen-1 entered the landing window in the Mars parking orbit, and then carried out the orbit reduction maneuver and the separation of the orbiter and the lander. The lander entered the Martian atmosphere at an altitude of 125 kilometers from the surface of Mars, and completed the deployment of the trim wing (a new technology in my country), the opening of the supersonic parachute (a new technology in my country), the separation of the large bottom, the separation of the back cover, the power deceleration, the hovering, the obstacle avoidance and the slow descent, the landing cushion and other actions, and finally made a soft landing on the surface of Mars.

After landing, Zhurong unlocked and separated from the landing platform. At 10:40 on May 22, 2021, Zhurong left the landing platform, arrived on the surface of Mars, and began patrol exploration.

Simply put, the specific landing process of the lander can also be divided into the following four stages. The first is the aerodynamic deceleration stage, that is, relying on the resistance of the Martian atmosphere to reduce its speed from 4.8 km/s to 460 m/s. The second is the parachute deceleration stage, that is, using a parachute to reduce its speed from 460 m/s to 95 m/s. The third is the power deceleration stage, that is, using a high-thrust engine to reduce its speed to 3.6 m/s. The fourth is the landing buffer stage, that is, through the buffering effect of the four landing legs on the lander, it softly lands on the surface of Mars.

During the entire landing process, the distance between the Earth and Mars was very far, resulting in a one-way delay of more than 20 minutes for the communication between the Earth and Mars. During the landing process, the lander and the ground "command center" were in a "lost connection" state. After entering the Martian atmosphere, it had to complete more than 10 actions autonomously within 9 minutes. Each action was done in one go, linked together, and there was no room for error. However, my country's landing rover was successful in one fell swoop.

"Chinese Imprint" map. The Zhurong Mars rover drove to the landing platform about 6 meters away at 60° east-southeast of the landing platform and took an image of the landing platform. The image shows that the landing platform is shining, the national flag is bright red and square, and the surface topography is rich in details. Image source: China National Space Administration

Choose your landing spot carefully

Choosing a suitable landing site for the Mars rover is not an easy task. It must meet two basic conditions: one is that it is feasible in engineering; the other is that it is valuable in scientific research. After comprehensive consideration of various factors, my country's Mars landing area is located in the Utopia Planitia at 5° to 30° north latitude on Mars. The Utopia Planitia is relatively flat and has good sunlight conditions. In addition, the Utopia Planitia is likely to be the location of the ancient ocean on Mars. Landing there is conducive to exploring and studying whether there is life on Mars, a hot issue in current Mars exploration, so it has high scientific value.

In addition, since my country's Mars rover is powered by solar cells, the light sensors it carries for navigation and obstacle detection also require good lighting conditions. For this reason, Zhurong landed on the Utopia Plain at a latitude less than 30°, because there is plenty of sunshine and the temperature difference between day and night is small, which is conducive to the work of Zhurong.

Because Mars is very far away from the Earth, the strength of the transmitted signal is inversely proportional to the square of the distance, and the diameter of the antenna is directly proportional to the detection distance, so a ground-based deep space tracking and control network with a large antenna diameter must be used.

my country's deep space tracking and control network includes a 66-meter diameter antenna tracking and control station in Jiamusi, and a 35-meter diameter antenna tracking and control station in Kashgar and Argentina. In addition, my country's high-performance receiving antenna with a main reflector diameter of 70 meters was put into use in Wuqing, Tianjin in February 2021. It is currently the largest single-aperture antenna in Asia, operating in the S, X and Ku bands, and is mainly responsible for receiving scientific data sent back by the Mars rover.

High-resolution images of the landing area of ​​my country's first Mars exploration mission Tianwen-1. The left picture is before landing, and the right picture is after landing. Image source: National Space Administration

Fruitful results have been achieved

On August 15, 2021, after completing the 90-day mission, the Zhurong Mars rover continued to carry out the expansion mission. So far, it has patrolled and explored for 358 days, traveled 1,921 meters, and is currently in a dormant period. As of June 29, 2022, the orbiter has achieved global remote sensing detection. It is currently in good condition and continues to carry out scientific exploration and accumulate raw data in the remote sensing mission orbit.

my country's first Mars exploration mission has been successfully completed. By April 24, 2023, the 13 payloads carried by the Tianwen-1 mission have accumulated 1,800GB of raw scientific data and formed standard data products. The scientific research team has achieved a number of original scientific results through the study of first-hand scientific data. For example, my country used the sub-meter resolution terrain data of the landing area obtained by the high-resolution camera on the orbiter to conduct a comprehensive study on typical landforms such as concave cones, barrier craters, and grooves distributed in the landing area, revealing the important connection between the formation of the above landforms and water activities.

Full-color image of Mars taken by the Tianwen-1 orbiter. Image source: National Space Administration

By studying the image data of the rover's ruts obtained by the camera on the Zhurong, my country obtained mechanical parameters such as soil cohesion and bearing strength in the landing area, revealing the physical characteristics of the landing area surface. By studying the underground stratification information of the landing area obtained by the dual-frequency full-polarization radar of the Mars rover, my country discovered that there are two sets of upward-fining sedimentary sequences about 30 meters and 80 meters below the several-meter-thick wind-blown dust on the surface of Mars, revealing the surface modification events and geological processes of Mars related to multiple periods of water activities since 3 billion years ago.

Through a comprehensive analysis of data obtained by the Mars rover's navigation terrain camera, the Mars surface composition detector and the Mars meteorological instrument, my country has discovered evidence of brine activity and modern water vapor circulation dating back about 760 million years in the patrol area.

The above original results have been published in authoritative academic journals at home and abroad such as Nature, Nature Astronomy, Nature Geoscience, Science Advances and National Science Review.

On April 24, 2023, the National Space Administration and the Chinese Academy of Sciences jointly released China's first global image map of Mars for Mars exploration, with a spatial resolution of 76 meters, which provides a better quality base map for Mars exploration projects and scientific research on Mars. This is a global color image of Mars obtained by processing 14,757 image data obtained by the orbiter's medium-resolution camera by the ground application system. The scientific research team used it to identify a large number of geographical entities near the landing site. According to relevant rules, the International Astronomical Union named 22 of the geographical entities after Xibaipo, Yangliuqing, Zhouzhuang and other historical and cultural villages and towns in China with a population of less than 100,000. The biggest highlight of this global map of Mars is its high resolution and true color.

Distribution map of 22 geographical entities named by the International Astronomical Union. Image source: National Space Administration

Recently, an international research team led by Professor Xiao Long from the School of Earth Sciences of China University of Geosciences (Wuhan) has discovered petrological evidence of marine sedimentary rocks on the surface of Mars for the first time by comprehensively analyzing scientific data obtained by the multispectral camera on the Zhurong rover, proving that an ocean once existed in the northern part of Mars. The relevant research results were published in the comprehensive authoritative journal National Science Review under the title "Evidence of Marine Sedimentary Rocks on Utopia Planitia: Observations of the Zhurong Mars Rover".

On July 6, 2023, the new findings of Zhurong on Mars were published online in the journal Nature. Based on the observation data of Zhurong, an international research team led by Chinese researchers found sedimentary sequence evidence of changes in the ancient wind field on Mars in the Zhurong landing area, confirming that the wind and sand activities recorded the changes in the ancient environment of Mars with the rotation axis of Mars and the ice age.

Zhurong undergoes tests

After setting foot on Mars, the Zhurong rover went through multiple tests. After completing the patrol and exploration mission of 90 Martian days, the Zhurong rover passed the solar eclipse phase. Due to unstable communication between the rover and the ground, the rover suspended its scientific work during the solar eclipse.

After the solar eclipse ended, the "over-service" Zhurong continued to carry out extended patrol and exploration missions, obtaining topographic images of the patrol area, magnetic field information on the driving path and underground profile structure information, composition information on typical landforms such as rocks and sand dunes, and first-hand scientific data such as temperature, air pressure, wind direction, and wind speed meteorological information, searching for clues to the mystery of the origin and evolution of Mars.

In May 2022, the patrol area of ​​the Zhurong Mars rover has entered winter. According to measurements, the maximum temperature at noon has dropped to -20°C, and the ambient temperature at night has dropped to below -100°C. In addition, due to the presence of sandstorms, the light intensity has further weakened, affecting the power generation capacity of the Mars rover's solar panels. To this end, energy balance has been achieved by taking measures such as rotating the solar panels to adjust the light angle and reducing the number of daily work items and duration.

Mars sand dunes photographed by the Zhurong Mars rover. Image source: China National Space Administration

In order to cope with the reduced power generation capacity of the solar panels caused by sandstorms and the extremely low ambient temperature in winter, the Zhurong Mars rover entered hibernation mode on May 18, 2022, in accordance with the design plan and flight control strategy. In fact, in order to safely survive the cold winter on Mars, sandstorms and other extreme weather, the Zhurong rover has designed working modes such as autonomous hibernation. When the energy is reduced to a certain level, it will automatically enter the hibernation mode, and when the environmental conditions gradually improve, it will resume the normal working mode. During this period, the orbiter continued to carry out remote sensing detection. As of hibernation, the Zhurong Mars rover has worked for 358 Martian days and has traveled a total of 1,921 meters.

Zhurong was originally scheduled to wake up at the end of 2022, but it has not yet woken up. Zhang Rongqiao, chief designer of my country's planetary exploration project, believes that the most likely reason for the failure to wake up autonomously is the unpredictable accumulation of dust on Mars, which has reduced the rover's power generation capacity, making it insufficient to wake it up. Zhurong's automatic awakening requires two conditions to be met at the same time: one is that the temperature in the cabin must be higher than -15°C; the other is that the solar wing power generation must meet the minimum power consumption of the rover on that day. Therefore, the best opportunity for Zhurong to wake up is in the summer on Mars. If it hasn't woken up in the summer, there will be no chance. In fact, it doesn't matter whether Zhurong wakes up or not, because it has completed its scheduled mission. But we still hope that Zhurong can wake up in the summer and make new contributions.

Zhurong's route map. Image source: Beijing Aerospace Flight Control Center

Planning and production

Source: China National Astronomical Service

Author: Pang Zhihao, Chief Scientific Communicator of National Space Exploration Technology, Editorial Board Member of Space Exploration Magazine, and Editorial Board Member of China National Astronomy Magazine

Editor: Cui Yinghao

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