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Investigating the origins and evolution of Earth by Exploring Mercury Hajime Hayakawa Project Manager, Mercury Exploration Mission BepiColombo
BepiColombo is a joint Japanese-European exploration mission to Mercury. Until now, few direct observations of Mercury have been possible, due both to its harsh environment and to the enormous amount of fuel required to put an explorer into the Hermean orbit. Mercury is the closest planet to the Sun and, like Earth, is composed of rock and metal.  When it is situated directly under the Sun, Mercury's temperature can reach up to 400ºC.  BepiColombo will investigate the origin and evolution of this Earth-type planet by studying Mercury's magnetic fields, its magnetosphere, and the interior and chemical composition of the planet's surface.
Tracking the Mysteries of Mercury
Q. What kind of planet is Mercury?

Dr. Giuseppe Colombo (1920-1984) (Courtesy of ESA)
Dr. Giuseppe Colombo (1920-1984) (Courtesy of ESA)

Thought to be the youngest planet in the solar system, Mercury is the closest planet to the Sun. The distance between the Sun and Earth is known as one astronomical unit (AU). Mercury's orbit is 0.31 AU at its closest point to the Sun, and 0.47 AU when it's farthest from the Sun, so the orbit is very elliptic. In addition, Mercury's orbital period - the time it takes to make one revolution around the Sun - is 88 Earth days, while its rotation period is 59 Earth days. In other words, Mercury rotates only three times in the time it takes to travel around the Sun twice - i.e. one Mercury day equals to 2 Mercury years. This synchronization between the revolution of Mercury and its rotation at a ratio of 2 to 3 was resolved by the renowned Italian astronomer Giuseppe Colombo. The Mercury exploration project is named after Dr. Colombo, whose nickname was Bepi. Dr. Colombo has another close connection to Mercury, he proposed Mariner 10 orbit which used multiple swing-by and achieved multiple observation of Mercury.

Q. What did Mariner 10's exploration of Mercury reveal?

Mariner 10, which explored Mercury and Venus (Courtesy of NASA)
Mariner 10, which explored Mercury and Venus (Courtesy of NASA)
Passing by Mercury three times, Mariner 10 revealed something nobody had ever expected: Mercury has a magnetic field and magnetosphere. Mercury's diameter is about 4,880 km. It is a very small planet, less than half the size of Earth, and because of that scientists had believed that its interior was solid rock, and that it had no magnetic field. In order for Mercury to have a magnetic field, dynamo theory requires a part of the planet's core must be molten, with thermal circulation taking place in the core.
Mariner's discovery forced us to correct the existing theory of magnetic field generation, although we still don't know why Mercury has a magnetic field. Mariner 10 also observed high-energy electrons. These electrons accelerate to high energy in a period of only a few seconds, but how this acceleration occurs remains a mystery.
To understand the formation and evolution of the planet, it is very important to understand the topography and mineral composition on the Hermean surface. However, Mariner 10 was able to image less than half of its surface area, and the planet's chemical composition has barely been understood. The BepiColombo project hopes to fill in these gaps in our knowledge.

Q. Can you tell us the main objective of BepiColombo?

Mercury (Courtesy of NASA)
Mercury (Courtesy of NASA)

The surface of Mercury (Courtesy of NASA)
The surface of Mercury (Courtesy of NASA)

BepiColombo's main objective is to make precise observations of Mercury's magnetic field, magnetosphere, interior and surface. Earth and Mercury are the only planets in the solar system that have their own magnetic fields. The magnetosphere around the Earth has been observed by satellites for about 50 years now. But in order to find out how universal this phenomenon is, the observation data has to be compared to that of other planets. In comparison to Earth, Mercury has the following characteristics: 1) Its magnetosphere is rather small; 2) The solid planet occupying large portion of its magnetosphere; 3) It has no ionosphere; 4) The contribution of particles that are much heavier than hydrogen, such as Sodium, is expected to be important to Mercury's magnetosphere. By observing Mercury's magnetosphere with BepiColombo and comparing it to Earth's, we can find out which phenomena are universal and which are specific to Mercury. This will allow us to learn the roles of Earth's ionosphere and of the heavy ions involved in plasma phenomena in its magnetosphere.
By looking at the particle acceleration point, we would like to investigate how the large number of high-energy electrons observed by Mariner 10 could be generated in such a short time span. We also expect to generate new knowledge about particle acceleration in astronomical phenomena by observing shock waves that are created by very high-speed plasma flows. These plasma flows are related to the explosion phenomena on the Sun's surface known as solar flares.
It is said that Mercury doesn't have an atmosphere. Actually Mercury has an atmosphere of about 500 km, equivalent to Earth's altitude. Observations by Mariner 10 and from Earth prove that one of the constituents of Mercury's atmosphere is sodium, and that the circulation of sodium changes a great deal with time. Other important objectives include understanding how this thin atmosphere is generated, how it spreads and lost, and what influence it has on the phenomena in the magnetosphere.
The interior of a planet is strongly related to its magnetic field. Mercury has a huge central nucleus that comprises up to three quarters of its radius. But how does it affect the generation of a magnetic field? We would first like to understand how Mercury's magnetic field is generated, and then discover the planet's internal structure. Mariner 10 observed Mercury's topography about 30 years ago, but at that time only about 45 percent of the surface was mapped, and also its mineral composition remains unknown. It is very important to know whole surface topography to discuss the formation and evolution of the Mercury. For example, on Earth, the proportion of land and oceans is different in the northern and southern hemispheres; the surface topography is very different on the near and far sides of the Moon, and in the northern and southern hemispheres of Mars. Since planets have different profiles in different areas, it is very important to observe the entire area of a planet to understand its formation and evolution. By understanding the global circulation of the surface topography and mineral composition, I think we can gain important knowledge about the formation and evolution of Mercury.
We would like to track the mysteries of Mercury by making multilateral observations of the planet using two orbiters. As a result, we expect to gain new knowledge about the processes of the formation and evolution of a solid planet and cosmic plasma physics.
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