In February 2021, The first Mars probe independently developed by China, Tianwen-1, successfully reached Mars after more than 6 months and spanning 465 million kilometers. The main scientific mission of Tianwen-1 includes investigating the surface environment of Mars to understand the evolution of the Martian atmosphere. Before the successful launch of China's "Tianwen-1", the international Mars satellite exploration mission was mainly dominated by large aerospace agencies in the United States, the former Soviet Union and Europe.
Their research results show that about 4 billion years ago, there were lakes and oceans on the surface of Mars, but today's Martian surface is incomparably dry, and its surface morphological characteristics are similar to those of Wind Erosion landforms such as Yadan in Gansu, Qinghai, Xinjiang and other places in China. Why are there still oceans on the surface of the Earth and the lakes of Mars gone? Could Earth's future be as dry as Mars is today? These questions about "where the liquid water on the surface of Mars is going" have become a top priority for scientists.
Figure 1: Comparison of photos of alluvial fans of Mars and Earth
Credit: NASA (left), ESA (right)
If you compare Mars with Earth, you can find many differences, of which two points are particularly remarkable:
Mars has a smaller mass and a gravity of only 0.38% of Earth's;
Mars lacks an intrinsic magnetic field, unlike Earth's geomagnetic field, which extends beyond 50,000 kilometers above the surface.
The gravitational force is smaller, which means that after the water and dry ice on the surface of Mars enter the Martian atmosphere through evaporation and sublimation, it is easier to spread to the sky, ionized by solar radiation, and decomposed into H, O, CO, CO and other charged particles; and Mars lacks a "global geomagnetic field" similar to earth, and these charged particles in the high altitude can enter the solar wind at an altitude of 400 to 600 kilometers on the sunny side of Mars, accelerate by the solar wind farm, and eventually escape to interplanetary space. This process, known as "ion escape," is one of the biggest causes of water composition and atmospheric loss on the Martian surface, and is the main scientific target of the European Space Agency's Mars Express (2003) and NASA's Mars Atmospheric and Volatile EvolutioN (MAVEN, 2013) missions.
Figure 2: OBSERVATION OF MAVEN FUGITIVE IONS
Credit：NASA
The main sources of energy that drive ion escape are solar radiation and solar wind. When solar wind conditions increase, the ion escape rate of the Martian magnetosphere also increases exponentially: for example, a 2011 study by Researcher Wei Yong of the Institute of Geology and Geophysics of the Chinese Academy of Sciences showed that when the solar wind dynamic pressure increased by 2 to 4 times, the global ion escape rate of Mars increased by an order of magnitude. In addition, when the solar activity level is strong, it is often accompanied by high-intensity solar flares and coronal material ejections that greatly enhance solar radiation and solar wind dynamic pressure at Mars. However, both Mars Express and MAVEN are single satellites orbiting Mars to make in situ measurements, and their satellites have an orbital period of up to 4 to 6 hours, which is much longer than the response time of the ion escape phenomenon in the Martian magnetosphere to solar eruption events.
Figure 3: Schematic diagram of the orbit of the MAVEN and EscaPADE satellites
In addition, a single satellite can not detect changes in the solar wind and the Martian magnetosphere at the same time during the process of circumnavigating Mars - when the probe is in the solar wind, it loses information about the magnetosphere ions; and when the spacecraft enters the magnetosphere, it cannot monitor the changes in the solar wind, and it cannot present the real-time response of the Martian magnetosphere to changes in solar wind parameters, so in order to make up for the detection limitations of a single satellite, NASA plans to carry out "Escape and Plasma Acceleration and Kinetic Explorer" in 2024 and Plasma Acceleration and Dynamics Explorers) two-satellite Mars exploration mission, or EscaPADE for short.
This will be NASA's second Mars "dual satellite" mission after 1969, and it will also be the world's first long-term ring fire exploration mission for the Martian space environment. The main scientific objectives of the Escape Explorer include: to further understand the main controlling factors of the Martian magnetosphere and the influence of these driving sources on the magnetosphere ion flow; to further understand the energy and momentum transfer process of the solar wind to the Martian magnetosphere; and to further understand the energy and material exchange process of escape/sedimentation particles in the Martian atmosphere.
To put it more intuitively, "EscaPADE" plans to establish a quantitative relationship between the energy injection of the solar wind and solar radiation and the escape rate of Martian ions through the synchronous exploration of two satellites inside and outside the Martian magnetosphere, try to explore the specific amount and physical mechanism of water and carbon dioxide escape losses in the Martian atmosphere during the solar calm period and solar storm, and finally, use the current observations to reverse the "youth" sun to Martian ions and even global water ice in the past 4 billion years. The evolution of atmospheric escape.
The single detector of the "EscaPADE Mission" weighs less than 90 kg, and the orbital near-fire point is 200 kilometers, and the far-fire point is 7000 kilometers and 5660-8685 kilometers according to the different stages of the mission, and the satellite orbit period is about 4 to 6 hours.
Figure 4: "EscaPADE" dual satellite exploration program
Credit:Lilis et al. 2020
In order to overcome the previous shortcomings of a single satellite can not detect multiple points at the same time, the two "EscaPADE" satellites are expected to carry out two flight modes:
"Double star companion flight" in the same orbit one after the other;
The "high and low combination" of the height difference between the two tracks is about 3,000 kilometers.
The "binary satellite companion flight" model can not only observe the magnetospheric dynamics process with a space scale smaller than the satellite spacing at the same time, but also obtain short-term time scale information spanned by two satellites in turn at the same location, so this model can effectively distinguish the spatial distribution and temporal evolution of observation phenomena. Another mode: the "high-low match" flight mode can enable the two probes to obtain observation data from the same period in the solar wind, magnetosphere and ionosphere of the upstream of Mars, respectively, and obtain real-time responses from the near-fire space environment to changes in solar wind conditions.
Figure 5: "Double Star Companion Flight" (left), "High and Low Match" (right)
Each EscaPADE satellite is scheduled to carry 3 scientific detection payloads:
Magnetometer (EscaPADE Magnetometer, EMAG);
Electrostatic Analyzer (EscaPADE Electrostatic Analyzer, EESA);
EscaPADE Langmuir Probe (ELP).
These payloads will be measured around magnetic fields, thermal and superthermal ion/electron spectra, plasma density, 55-130 nanometer solar EUV radiation intensity, and spacecraft potential in the Martian space environment.
Since the Soviet Union tried to launch the first Mars exploration satellite in 1962, human beings have carried out Mars flyby or in situ exploration missions for nearly 60 years, with a total of 49 explorations. Of these exploration programs, with the exception of the 1969 Mariner 6 & 7 mission, which was a dual-satellite mission, the rest were single-satellite or single-satellite landers. The "Mariner 6 & 7" dual-satellite exploration program more than 50 years ago was only two satellites flying by Mars in conjunction with each other, failing to make long-term observations of the Martian space environment.
The EscaPADE twin-satellite fire exploration program, which is expected to orbit Mars for up to two years, is not only the first two-satellite mission in human history to conduct long-term detection of the Martian space environment, but also brings new breakthroughs to a series of problems such as Martian atmospheric escape and martian evolution history.
Figure 6: The 2018 "BepiColombo" two-satellite Mercury exploration mission
Credit：JAXA
In addition, the "BepiColombo" Mercury twin satellite exploration program, launched by the European Space Agency in cooperation with the Japan Aerospace Exploration Agency (JAXA), was successfully launched in 2018 and is expected to reach Mercury in 2025. "BepiColombo" will also be the first Mercury "two-satellite" exploration mission in human history. It is foreseeable that in the near future, the era of multi-satellite joint exploration of the three terrestrial planets in the inner solar system according to specific scientific issues such as: the escape of the atmosphere of Mars, the "greenhouse effect" of Venus, and the magnetosphere of Mercury is just around the corner.
Source: Science Exploration Award
Edit: Paarthurnax