A comet that appears once every 50,000 years reappears. Why does this comet look green?

Produced by: Science Popularization China

Author: Yang Yi (Institute of Geochemistry, Chinese Academy of Sciences)

Producer: China Science Expo

On March 2, 2022, astronomers Frank Masci and Bryce Bolin discovered an object they initially thought was an asteroid using the Zwicky Transient Facility (ZTF) at the Palomar Observatory in Southern California, USA. Subsequent observations showed that it was a comet with a nucleus diameter of 1-1.6 kilometers. At the time, the comet was about 643 million kilometers from the sun, near the orbit of Jupiter.

The comet we discovered at that time was named C/2022 E3. It is currently the brightest comet predicted for 2023. February 1-2 is the best time to observe it in the Northern Hemisphere. It will gradually become visible in the Southern Hemisphere after February 3, and due to the influence of the full moon, it will only be suitable for observation in the Southern Hemisphere from February 9 to 15. In fact, this comet passed through the Earth's orbit for convenient observation as early as 50,000 years ago. This encounter is more like a romantic reunion.

This comet is easy to locate, but not bright enough

We mentioned that this is the brightest comet predicted so far in 2023. As early as November 3, 2022, C/2022 E3 was observed to have a brightness of 10.2, which was nearly 1 magnitude brighter than predicted at the time.

On January 12, 2023, the comet passed perihelion, with a perihelion distance of 1.11 AU (Astronomical Unit, referred to as AU, is based on the average distance from the Earth to the Sun, which is 14,959,787,0700 meters); on February 2, the perigee distance was only 0.28 AU (about 41.89 million kilometers). After passing perihelion, the brightness of the comet increases rapidly due to its close distance to the Earth.

As the comet gets closer to Earth and brighter, it will become easier to observe. The forecast brightness is 8th magnitude in early January and will brighten to nearly 5th magnitude by the end of the month, making it a good time for observation and photography.

In January, the comet moved from Corona Borealis through Bootes, Draco, Ursa Minor to Camelopardalis, and moved very quickly. By the end of January, the comet had moved near the north celestial pole, and its brightness would be around magnitude 5 between January 28 and February 4, Beijing time. On January 30, it was only 10 degrees away from the north celestial pole, and the comet was located directly above the North Star, making it easy to find. The comet reached perigee on February 2, Beijing time, and February 1-2 was the most suitable time period for observation in the Northern Hemisphere. It would gradually become visible in the Southern Hemisphere after February 3, and due to the influence of the full moon, it would be suitable for observation in the Southern Hemisphere only from February 9 to 15. On February 11, the comet, with an apparent magnitude of 6.5, would pass near Mars, which had an apparent magnitude of 0.

Figure 1 The comet's path between January 28 and February 4, Beijing time, calculated based on Stellarium software.

(Image source: self-made by the author)

From the previous description, it seems that the brighter the comet, the easier it is to observe. So since this comet is the brightest forecast for 2023, it must be easy to observe, right?

In fact, it is not. The above mentioned the apparent magnitude of comets and their surrounding celestial bodies, and the apparent magnitude is a scale for observing the brightness of celestial bodies. For every increase of 5, the brightness decreases to 1% of the original value. The brightness of Vega observed from the earth is defined as 0 magnitude.

That is to say, the larger the apparent magnitude value, the dimmer the observed celestial body appears. The concept of magnitude was first proposed by the ancient Greek astronomer Hipparchus. At that time, the dimmest celestial body observable by the human eye was about 6th magnitude. If observed with ordinary binoculars, 9-10th magnitude stars could be observed.

Figure 2: The comet was photographed with a mobile phone on January 30, Beijing time. It was barely visible after about 30 seconds of exposure.

(Image source: self-made by the author)

Practice has found that even with telescopes, long-exposure cameras and other auxiliary tools, the comet can only be easily observed in a clear night sky with weak light pollution and no haze. For dim celestial bodies, in order to obtain excellent shooting effects, in addition to telescopes, long-exposure cameras and other auxiliary tools, an equatorial mount is also needed to track the target and overcome the influence of the earth's rotation.

Figure 3 Comet Iketani-Seki (magnitude -10) which returned in 1965 is also clearly visible during the day.

(Image source: National Astronomical Observatory of Japan)

Compared with some bright comets in the last century, such as Comet Hale-Bopp (magnitude -1.8) returning in 1997, Comet Hyakutake (magnitude -0.3) returning in 1996, Comet Iketani-Seki (magnitude -10, clearly visible during the day) returning in 1965, and Comet Halley (magnitude -3.3, clearly visible during the day) returning on April 20, 1910, Comet C/2022 E3, which reached its brightest in late January to early February 2023 and was only about magnitude 5 at its brightest, is much inferior.

A harbinger of disaster? Atmospheric burning? Ancient misunderstandings about comets

my country has a long history of comet observation and also has the world's earliest observation records of comet periodic returns.

In ancient China, comets were called "bo stars" and "sweeping stars". The "Book of Jin·Astronomy" in the "Spring and Autumn Annals" (613 BC) records that "so-called sweeping stars are stars, but comets. The small ones are a few inches long, or they reach the sky. When they are seen, wars and floods will occur. They are responsible for sweeping away the old and bringing in the new. They have five colors, each according to the essence of the five elements. According to the historian, comets have no light, but shine with the sun. Therefore, they point to the east when seen in the evening and to the west when seen in the morning. They point to the north and south of the sun, and their rays are blunted, sometimes long and sometimes short." This accurately describes the shape of comets.

However, ancient people often believed that the appearance of comets would bring bad luck. Even when Halley's Comet returned in 1910, there was still a rumor that "a comet appears in the southeast, and the Qing Dynasty will last for two and a half years." At that time, it was the end of the Qing Dynasty, and many people still believed that comets brought disasters. This is a superstitious idea and a completely unfounded belief. Influenced by the views of scholars such as Aristotle, ancient Europe simply believed that comets were produced by combustion in the upper atmosphere, which was also a limited understanding due to the crude observation methods.

Composition and structure of comets

A comet consists of three main parts: the nucleus, the coma, and the tail.

(Photo source: Veer Gallery)

1. Although the comet nucleus is small, it concentrates the essence of the comet's material.

The volume of a comet nucleus is generally smaller than that of the planets and dwarf planets in the solar system, with a diameter of 100 meters to 10 kilometers. The largest known comet nucleus has a diameter of no more than 160 kilometers. The comet nucleus generally accounts for more than 99% of the total mass of the comet.

Comet nuclei may be one of the sources of "meteoric ice". The dirty snowball model of comet nuclei proposed by Whipple (1950) vividly reflects that the main components of comets are porous solid water ice, carbon dioxide, carbon monoxide, ammonia, methane, hydrogen cyanide mixture, and a small amount of micron- to millimeter-sized, dust-like solid silicate and sulfide minerals. It can be seen that meteoric ice from comet nuclei is likely to be highly toxic and cannot be regarded as a "panacea" as some ancient superstitious sayings believe.

The color of a comet nucleus is not a single color, but a variety of colors. Some comets are grayish white because of the abundance of water ice on their surfaces; some comets are black because of the abundance of carbon and hydrocarbons on their surfaces; some long-period or non-periodic comets may also contain earthy solid methane, ethane or methane hydrates, which appear red under sunlight.

This red soil-like substance has its own name, which is called tholin. Kuiper (1951) believed that the components that make up tholin can only be gathered and formed under the cold conditions of the outer solar system. In addition to comets, it can also be seen on some icy satellites, centaurs, dwarf planets farther than 30 astronomical units from the sun, and other celestial bodies. For example, there are landforms composed of red tholin on Pluto, Triton, and Titan.

Later, thousands of celestial bodies whose main components include water ice, solid methane, ammonia ice, nitrogen ice, and solid cyanide were discovered in the area beyond the orbit of Neptune. This area rich in icy celestial bodies is called the Kuiper Belt. Scientists have also confirmed that the home of most comets is the Kuiper Belt and the Oort Cloud by analyzing the similarity of components and the geometric relationship of orbits.

2. The coma adds color to the comet.

In many photos taken by astronomy enthusiasts, the comet C/2022 E3 appears bright green. However, its nucleus itself is not green, and it will not emit strong enough green light out of thin air.

So where does the green light come from? The green halo outside the front of the nucleus of this "green comet" is the coma. Similarly, on April 1, 2017, Comet 41P flew past the perigee, with the closest distance being only 0.142 AU. This time, a green coma was also observed during the comet's return.

The green light of the coma comes from the emission spectrum of C2 molecules in the visible light band after being ionized under ultraviolet light. The cause is that the mixed material on the solid comet nucleus undergoes sublimation or evaporation due to the sun's light, thermal radiation, and solar wind, and then chemically decomposes to produce C2 molecules. The C2 molecules are further ionized and stimulated to emit blue-green fluorescence under the irradiation of ultraviolet light, thus forming a spherical shell-shaped green coma around the front end of the comet nucleus's movement direction. However, this process is mainly limited to the front end of the comet nucleus's movement direction and disappears before reaching the comet's tail.

In addition, it is necessary to add that when the comet nucleus is about one astronomical unit away from the sun, the diameter of the coma is about 100,000 kilometers. At this time, the gas that forms the coma is rapidly ejected from the surface of the comet nucleus, and the relative speed of the comet nucleus is about 0.5 kilometers per second on average. If the comet nucleus moves away from the sun, the diameter of the coma tends to decrease, and vice versa.

3. Comet tail, which is very large in size and very low in concentration.

A comet's tail is a thin plume of gas, dust, and plasma that is usually seen at the rear of the comet's nucleus when sunlight, heat, and solar wind particles act on the nucleus and coma. It usually only appears near perihelion. Similar to the coma, the size of a comet's tail also tends to increase with the distance between the nucleus and the sun.

Although the comet tail is thin, it is thousands to tens of thousands of kilometers wide and hundreds of millions of kilometers long. It includes plasma comet tails and dust comet tails. Plasma comet tails are parallel to the rays in the direction away from the sunlight, and generally appear blue; dust comet tails are mostly yellow or orange-red, generally facing away from the direction of the comet nucleus, and are composed of dust with a diameter of about a few microns to a few millimeters. Its mineral composition is similar to the original material of the solar system that has not been metamorphosed and differentiated - chondrites.

Figure 6 Schematic diagram of the comet tail of Comet Hale-Bopp

(Image source: https://www.wikiwand.com/en/Comet Saturation and contrast have been adjusted)

Some comets (such as the aforementioned "Hale-Bopp Comet") have "sodium tails" containing neutral sodium atoms, which are generally invisible to the naked eye and can only be identified using spectroscopic instruments or detectors. The formation mechanism is still controversial. Some scholars believe that it is photochemical decomposition, while others believe that it is the instantaneous high temperature caused by the friction and collision of comet dust that leads to the evaporation of sodium salts.

The "doomsday" commotion caused by the comet's tail

When Halley's Comet was about to return in 1910, French scientist Flammarion and researchers from the Yerkes Observatory in the United States and others conducted spectral observations on its tail and believed that the tail contained highly toxic gases such as cyanide, hydrogen cyanide, and carbon monoxide.

This conclusion caused a stir at the time, and many people believed that the comet's return would bring a large amount of highly toxic gases into the Earth's atmosphere. For a time, gas masks and detoxification pills became hot items, and some superstitious groups even took the opportunity to launch the "comet doomsday theory" or promote "detoxification witchcraft" and "detoxification amulets" to make a fortune.

However, when Halley's Comet arrived as expected, the "poisonous disaster" predicted earlier did not appear - this is because the comet tail material is extremely thin, which is different from the comet nucleus mentioned above, which can account for more than 99% of the total mass of the comet. The cyanide in the comet nucleus is "the essence of concentration". Spectroscopic observations are highly sensitive, even if the comet tail contains highly toxic gases that do not reach the concentration that can widely poison Earth's life, it can be observed.

In fact, although some comets contain a considerable proportion of highly toxic substances, their nuclei are generally several kilometers in diameter, and the sum of the amount of gas and dust substances that make up the entire comet tail is about tens of millions of moles, among which the molar percentage of toxic volatiles is as high as about 15% (mainly including hydrogen cyanide, cyanide gas, and carbon monoxide). However, due to the high vacuum of space, the gases will dissipate quickly, resulting in the average maximum concentration of toxic gases in the entire comet tail being only about 2×10-11 mol/m3.

The lowest concentration of hydrogen cyanide that is toxic to humans is about 10-7 mol/m3, which is nearly 5,000 times the concentration of toxic gases in comet tails - that is, the concentration of toxic gases in comet tails is only one-fifth of the concentration that is harmful to humans or even lower. Therefore, when watching comets from Earth, as long as the comet does not collide with the Earth, there is no need to worry about poisoning (but if a comet really hits the Earth, the worry may not be about poisoning).

Conclusion

With the advancement of human science, many rumors about comets have been exposed, and we are gradually unveiling the truth of nature. Seeing this comet again, which had already met the Earth 50,000 years ago, we now have the conditions to use modern scientific methods to understand the romantic starry sky.

Editor: Guo Yaxin

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