There are also auroras on Mercury, but they haven’t emitted light yet!

Produced by: Science Popularization China

Author: Wang Yuqi (Institute of Geology and Geophysics, Chinese Academy of Sciences)

Producer: China Science Expo

The aurora is the only space physical phenomenon visible to the naked eye .

The aurora on Earth is a luminous phenomenon caused by charged high-energy particles from the magnetosphere and solar wind being guided into the Earth's atmosphere by the Earth's magnetic field and colliding with atoms in the upper atmosphere. Because it often appears near the North and South Poles, it is called the aurora.

Did you know that auroras exist not only on Earth, but also on other planets?

Auroras don’t just exist on Earth.

The auroras come in various shapes, generally band-shaped, arc-shaped, curtain-shaped, radial , etc. These shapes are sometimes stable and sometimes change continuously.

Aurora phenomenon

(Photo source: Veer Gallery)

The color of the aurora depends on the type of atmospheric atoms at the altitude where the aurora is generated and the energy they absorb . The most common are green and red auroras, but there are also blue or purple auroras. Occasionally, there are pink or yellow auroras, which are a mixture of green and red in proportion.

As early as thousands of years ago, the magnificent aurora phenomenon was recorded by civilizations around the world, witnessing mankind's exploration of the natural world and the evolution of human civilization.

Atmospheric glow observed by the Visible and Infrared Survey Telescope (VISTA) at the Paranal Observatory in Chile

(Image source: (LCO)/ESO)

With the rapid development of modern science and technology, deep space observation technology provides us with a unique perspective, enabling us to observe and study auroral phenomena on celestial bodies other than the Earth.

The Hubble Space Telescope captured Jupiter's auroras as early as the 1990s, and the Cassini probe's narrow-angle camera captured the flickering auroras over Saturn's northern sky between October 5 and 8, 2009.

Auroras have also been observed on Venus and Mars. Because Venus and Mars do not have global dipole magnetic fields like Earth, Jupiter, and Saturn, their auroras are more dispersed and not concentrated in the polar regions .

NASA released a photo of Jupiter's aurora taken by the Hubble Space Telescope in January 1998

(Image credit: NASA)

In addition to these planets, are there any other planets with auroras?

The answer is: Yes.

An article recently published in Nature Communications titled "Direct evidence of substorm-related impulsive injections of electrons at Mercury" takes us to Mercury , the planet closest to the sun in the solar system, and reveals discoveries and explorations related to the auroras on this planet.

Screenshot of the paper

(Image source: Nature Communications)

This research may prove the possibility of auroras or aurora-like phenomena on Mercury.

Mercury: A new perspective on understanding auroras

Mercury, the planet closest to the sun, has always been the focus of scientists' research.

Mercury has a weak magnetic field, which interacts with the solar wind to form a small magnetosphere. Mercury's magnetosphere is a dynamic place , affected by the interaction with the solar wind, generating a complex series of physical processes.

Mercury's magnetosphere is more compact than Earth's, about 5% of its size and located about 1.45 times the radius of Mercury above the planet's surface.

This causes Mercury's magnetosphere to undergo rapid reconfiguration and change, with a key process known as the Dungey cycle , in which plasma is accelerated, transported, lost, and circulated, resulting in a variety of "mysterious" phenomena, such as the ring currents outside Mercury, Mercury substorms, and more.

Schematic diagram of Mercury's magnetosphere

(Image source: Zong Qiugang's team at Peking University)

However, we know very little about these processes on Mercury, and whether auroras exist on Mercury remains a mystery.

It was not until October 1, 2021 that the BepiColombo mission, a joint exploration mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA), flew over Mercury for the first time, providing new insights into our understanding of the space environment of this mysterious planet.

During BepiColombo's first Mercury flyby, scientists observed a striking compression feature in Mercury's magnetosphere .

The interaction of Mercury's magnetosphere with the solar wind causes it to be compressed to a pressure estimated to be more than twice the average observed by previous MESSENGER missions.

This means that Mercury's magnetosphere is changing rapidly under the influence of the solar wind. The emergence of this phenomenon provides important background information for subsequent observations.

BepiColombo observes electrons emitted to the surface of Mercury during its first flyby of Mercury

(Photo credit: Sae AIZAWA)

Soon after BepiColombo entered Mercury's magnetosphere, scientists observed energy-time dispersion of high-energy electrons (particles of different energies are separated) , a feature that has never been observed on Mercury before. At the same time, electrons in the energy range of 100eV to several keV showed a clear flux enhancement with clear periodic fluctuations with a period of 30-40 seconds.

Observed energy-dispersive electrons (red arrows indicate periodic fluctuations in electron flux)

(Photo credit: Sae AIZAWA)

These features indicate that the electrons are accelerated in the near-magnetotail region of Mercury's magnetosphere, then drift rapidly to the dayside region, are injected onto closed magnetic field lines, and finally settle to the surface.

This process is called "electron injection" and provides the "green channel" for the generation of auroras.

On Earth, high-energy particles enter the channel and excite atoms in the atmosphere, eventually producing auroras. However, unlike Earth, Mercury has almost no atmosphere, and electrons have nowhere to exert their force after entering the channel, so auroras similar to those on Earth have not yet been observed on Mercury .

Schematic diagram of the Earth's electron injection

(Image source: Yang Jian's team from Southern University of Science and Technology)

Conclusion

Although this study did not directly observe the auroras on Mercury, it observed a series of physical phenomena closely related to the generation of auroras. This provides us with a new perspective to uncover the mysteries behind the wonders of planetary auroras, and allows us to have a more comprehensive understanding of the generation mechanism and evolution process of planetary auroras, as well as the intricate interactions between different celestial bodies and the sun.

With the continuous advancement of space exploration technology, we believe that Mercury will continue to bring us more surprises and discoveries, revealing the mysteries of the light of the planets in the solar system.