Shulou(Shulou.com)11/24 Report--

This is an image taken on the Spanish island of La Palma on April 27, 2022. At the top of the picture is the famous Pleiades star cluster. The object with a tail below is not a comet, but Mercury. If we take a closer look at this picture, we will find that Mercury's tail is yellow, as students from the periodic table know that this is a phenomenon caused by the excitation of sodium, and there is a familiar term that sums up this phenomenon-flame reaction.

Figure 1. La Palma Island, located in the Canary Peninsula, is the second largest astronomical observation site after Hawaii. The Herschel Telescope and the image of the sea of clouds raise two questions: why does Mercury have a tail? And why is it sodium tail? We can solve these two problems very simply. Like comets, Mercury has a tail because it is blown by the solar wind, especially as the nearest planet to the sun, which is more violent, while the sodium tail indicates that Mercury is rich in sodium on its surface, thus forming a sodium tail. So why is it rich in sodium? This is a good question, and it will eventually point to the origin of Mercury. To solve this problem, we need to know something about Mercury.

The scientific history of Mercury before the invention of the telescope, little was known about Mercury, not even the transit of Mercury, which was first observed by Gassandi in 1631. In the 20th century, with the use of large telescopes and some instruments, people learned more about Mercury. Scientists have found that there are many similarities between Mercury's surface and the moon: using a photometer to measure Mercury's brightness, scientists finally analyzed some characteristics of Mercury's surface soil, which are very similar to the moon. Mercury's soil is even darker. By studying Mercury's soil with a polarizer, Mercury has evidence of craters. In addition, there is one feature of Mercury that the moon does not have, and beyond imagination-Mercury has an atmosphere: astronomers have tried many ways to discover the existence of Mercury's atmosphere, and Mariner 10 finally provided the strongest evidence in 1976. But this atmosphere is not the atmosphere we are accustomed to perceive, but an exosphere: matter is constantly leaving Mercury, and sodium is naturally one of them.

Surprisingly, Mariner 10 also found that Mercury has a stable magnetic field, and it is a global magnetic field, with an intensity of 1% of the Earth's magnetic field. Mercury has a molten core of metals such as iron and nickel, with a diameter of about 3x4 of Mercury and a volume of about 57% of Mercury (Earth is 2 and 17%, respectively). In addition, Mercury is only slightly less dense than Earth, but if the effects of gravitational compression are excluded, Mercury can be called the densest planet. Such a special situation goes beyond the normal state of the planets in the solar system, and scientists ask: what is the reason?

Figure 2. Comparison between Mercury's inner core and Earth's inner core the most widely accepted theory of Mercury formation hypothesis is the giant collision hypothesis [1]. This hypothesis holds that Mercury is no different from other Earth-like planets in the early days of its formation, but was hit by a planet with a mass of Mercury 1. 6, due to Mercury's low escape velocity and high relative velocity between colliding planets in Mercury's orbit, the impact stripped most of Mercury's crust and mantle, resulting in Mercury's huge core share. The big collision theory of Mercury further points out that Mercury may have initially been in the position of today's asteroid belt and then turned into today's orbit after being collided. In this sense, the similarity between Mercury and the moon is further improved, because the moon's most extensive origin hypothesis is also the collision theory.

Figure 3. Moon: do I really have a relative? Another hypothesis is that Mercury may be affected by the primitive solar nebula, and its position temperature may be as high as 2500-3500 °K later in the constant nebula, which can melt most of the rocky mantle and take them away [2]. There is also a hypothesis that the sun hindered Mercury's accretion in the early stages of Mercury's formation, which prevented Mercury from getting a thick enough crust. and the Messenger probe orbiting Mercury in 2011-2015 found higher-than-expected levels of potassium, sulfur and other substances on the surface of Mercury, which is a challenge to both the big collision and the nebula hypothesis. It may be the victory of the third hypothesis in the end, but further discussion requires new detectors, and this is how science moves forward in constant self-denial.

An important relationship between the prediction and discovery of sodium tails is that the composition of Mercury's exosphere is related to the material composition of the earth's crust, so scientists can infer the chemical composition of Mercury's crust from the study of outer space, and vice versa. Some phenomena in Mercury's space can also be explained by its chemical composition. That's why scientists are so interested in Mercury's atmosphere.

In 1985, Drew Potter and Tom Morgan discovered the presence of sodium in the Fraunhofer spectrum of Mercury's atmosphere [7]. How does this sodium come from? Some scholars believe that this comes from the material produced between Mercury's shell and the meteorite impact, and Mercury's own high metal content has also become an important source of sodium. The injection of sodium into the escape layer is the result of a combination of thermal evaporation, photon excitation, impact evaporation and ion sputtering.

Figure 4. Solar spectrum and Fraunhofer lines in visible light, of which two D lines are the famous "sodium double lines". However, the interesting part is that each of these processes has its own space-time dependency. and if most of them can't accelerate sodium to escape velocity, will it form a sodium tail? The two scholars simulated the effects of solar radiation on the trajectories of sodium atoms occurring from different surfaces and concluded that as long as the initial velocity was greater than 2 km / s, the sodium atoms could get enough energy from the radiation acceleration to form a sodium tail similar to a comet.

Figure 5. Sodium brightness distribution at 2km, which is an illustration from the 1986 paper. In 2000 and 2001, they finally observed the predicted sodium tail of Mercury. The reason why it is so difficult to observe is that Mercury is near the sun all the year round and can only be seen within half an hour before and after sunrise in a very small period of time. They found that the acceleration of solar radiation on the sodium tail eventually led to a speed of up to 11 km / s at the end of the sodium tail and a lateral speed of 2-4 km / s. According to their estimates, the total flux of sodium in the tail is about 1 × 10 atoms per second, corresponding to 1% of the estimated total sodium productivity on Earth. Obviously, this is a very high value. The sodium tail escaping from Mercury alone reaches this level. At first, the shooting of sodium tail was rather blurred, and with the optimization of scientific instruments, we got this beautiful picture today.

Fig. 6 A sodium-tailed photo taken in May 2020, it seems that all good photos have a strong later stage, but students who have built a science building in their last life know that there may be only two dark clouds above their heads at first, but it may suddenly be dark clouds all over the sky. The temporal and spatial distribution of sodium shows a rich diversity (low EQ: complex and even seemingly random); as an ion, it is closely related to Mercury's magnetosphere; in addition, scientists also found that radiation acceleration does not seem to provide complete energy; and its relationship with ion sputtering, meteorite evaporation, and so on. (there are also a lot of dazzling things that are difficult to organize language.)

Because of our restrictions on Mercury's access to data, scientists can only make more connections through limited data, which may be a way to fight against inward rolls.

We seem to know a lot about Mercury now, but we don't seem to know anything.

References

[1] Benz W, Slattery W L, Cameron A G W. Collisional stripping of Mercury's mantle [J]. Icarus, 1988, 74 (3): 516-528.

[2] Cameron A G W. The partial volatilization of Mercury [J]. Icarus, 1985, 64 (2): 285294.

[3] https://www.csmonitor.com/Science/2011/0929/Messenger-s-message-from-Mercury-Time-to-rewrite-the-textbooks

[4] https://www.chemistryworld.com/news/messenger-sheds-light-on-mercurys-formation/3002463.article

[5] Hunten D M, Morgan T H, Shemansky D E. The mercury atmosphere [J]. Mercury, 1988: 562-612.

[6] Killen R, Cremonese G, Lammer H, et al. Processes that promote and deplete the exosphere of Mercury [J]. Space science reviews, 2007, 132 (2): 433,509.

Smyth W H. Sodium Atmosphere of Mercury. (Title only) [C] / / Bulletin of the American Astronomical Society. 1985, 17: 712.

[8] Potter An E, Killen R M. Observations of the sodium tail of Mercury [J]. Icarus, 2008, 194 (1): 1-12.

This article comes from the official account of Wechat: stone popular Science Studio (ID:Dr__Stone), written by Xingsi, edited by Liu Yuling

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.